The 2010 Watermark Awards
Simplicity and balance are the winning combination in this year’s kitchen and bath competition.
A “less is more” philosophy prevails in these recessionary times, and nowhere is this shift more evident than in kitchens and baths. Contemporary spaces continue to gain ground, even in homes that are traditional on the outside, but there’s a difference between streamlined and sterile. The projects that received highest marks from the jurors this year illustrate a deft understanding of that distinction. Clean, bright white—be it in the form of painted cabinets, marble countertops, or lacquered storage units—seems once again to be the designers’ look of choice. But this time around, it’s more likely to be paired with something warm, such as natural wood, a veiny stone, or oil-rubbed bronze, than with cold stainless steel or chrome. In fact, what made many winners stand out was their artful interplay of warm and cool, light and dark, texture and fl at plane.
Another trend worth noting (and this is a biggie) is a clear movement away from the upper cabinets that have conventionally dominated kitchens. Many designers have found clever ways to redistribute storage space and eliminate those bulky masses in order to bring in more light, preserve views, and create a feeling of openness.
And yet, for the second year in a row, the project that received highest honors from the judges was not a kitchen, but a bath—and a traditional one at that. Take one look at this beguiling space, and you’ll see that it’s not so much the style that matters as the execution of it. It’s clean, original, contextual, and balanced.
Victorian Master Bath Minneapolis
Bath of the Year and Grand, Best master bath in a remodeled home
FINE VINTAGE
Victorian styling can easily be- come fussy and over-accessorized, but this renovated master bath strikes an elegant tone. The owners wanted a serene spot that was stylistically consistent with their period farmhouse. Rehkamp Larson Architects delivered just that.
The design leaves the room’s existing walls, stained oak beams, and windows intact, focusing on natural light and classic finishes to lend a sense of tailored style. “We were careful to create a balance of things feeling open and closed,” says architect Jean Rehkamp Larson, noting that open towel shelving in the vanity is counterbalanced with leaded glass cabinet doors that hide clutter.
High contrast lights and darks are similarly juxtaposed for balance. Painted white cabinets, frosted glass, and Calcutta gold marble tiles (in the tub surround and floor) are off set by dark, oil-rubbed bronze plumbing fixtures, drawer pulls, cabinet feet, and wall sconces.
A custom shower enclosure burnished to match those bronze finishes serves as a fi ne focal point and unifying element. “We wanted something that felt kind of like a vintage phone booth,” says Larson. “We wanted to max out the size of the shower, so we didn’t want [solid] walls boxing it in.”
The expressive curves of the shower’s ornamental scrolls are repeated in smaller, more subtle features, such as the calligraphic brackets underneath the makeup and sink vanities—a motif that Larson says was inspired by the legs of beautiful old piano benches.
Fairfield House Kitchen Austin, Texas
Grand, Best kitchen in a custom home—less than 3,000 square feet
HOVER CRAFT
he owners of this modest home wanted a bright, open kitchen to make the most of a small footprint and views to the outdoors. The solution is a culinary space that feels airy and uncluttered, thanks to an un-conventional layout of its storage units. Cabinets in the main galley are reduced to just two rows of undermount drawers running the perimeter of the workspace and across the peninsula. Custom designed by the architects at Webber + Studio, the cantilevered drawers hover above the floor, enhancing the feeling of openness and light.
That feeling also comes by way of what is absent at eye level. Eschewing the usual vent hood (which, surprisingly, is not a lo-cal code requirement) and blocky upper cabinets, the owners opted for open display shelving with accent lighting to allow cleaner lines and unobstructed views. Large south- and east-facing windows overlooking the backyard flood the space with natural light from morning to late afternoon. A taller bank of cabinets between the picture window and patio slider doors houses the refrigerator/freezer, a matching pantry, broom closet, hidden microwave and glassware cabinet, and double ovens. Each tall cabinet provides deep storage up top for larger items.
Spare by design, this kitchen is less about color than it is about texture. The floors, paneling, and drawers are pecan, a light Texas hardwood with a characteristically dramatic grain variation, brushed with a soy-based Velvit Oil Golden Honey stain and a zero-VOC seal. The countertop surfaces are Carrara statuary marble with delicate, interesting veining.
Penthouse Kitchen Columbus, Ohio
Grand, Best multifamily kitchen
HIGH STYLE
One of the reasons this pent-house was among the last units to sell in a 26-story high-rise, despite its killer river views, is that its kitchen was over the top—in a bad way. The original layout had two islands, a superfluous luxury that ate into the living area. “We were trying to be prudent ... but we did have to rip out some of the rough-ins to improve the design,” says architect John Behal, whose simplified layout features a single island with a tighter workspace.
The openness of the plan did require some careful integration, though. Variations in ceiling heights, combined with furniture-like storage, both define and connect the cooking area to adjacent spaces. The building’s structural concrete ceiling is exposed high above a circular dining area, but in the kitchen the ceiling drops down and “floats” above the island, placing task lighting closer to prep surfaces.
To create the illusion of rails in the full-overlay cabinets, the wood-worker used fi r and reconstituted fir, turning the veneer perpendicular to create a perimeter border around each cabinet face. “The warm gray wash has a softness that contrasts nicely with the stainless steel, marble, and glass tiles,” Behal says. The biggest challenge was working around fixed structural and mechanical elements. A tall bank of cabinets next to the balcony contains a pantry but also conceals a structural column. And connecting the vent hood required some slight re-jiggering be-hind the walls. “Being in a high-rise, the placement of the ventilation system is not flexible,” Behal explains. “We did have a little trouble connecting to the shaft when we shifted the placement of the hood exhaust a bit.”
Focal Point Kitchen Weston, Conn.
Grand, Best kitchen in a remodeled home—2,000 to 3,000 square feet
PURPLE REIGN
The general openness of this 1970s contemporary home made it wonderful for parties. But its cramped “condo kitchen in an otherwise upscale house” meant that the chef was removed from the action, notes architect Alex Esposito.
Charged with creating a more handsome and functional layout for casual entertaining, Esposito borrowed space from an ad-joining area and expanded the kitchen to accommodate a large stainless island. He then introduced a sculptural countertop bar that serves as a transitional element connecting the kitchen to the adjoining communal spaces. A curved drop in the ceiling above the bar is clad in alder veneer to match rich alder cabinetry in the kitchen, dining room, and living room. This same rounded form is echoed in the base of the bar—a sleek half cylinder of wood, steel, and glass—which intersects the fl at plane of a vibrant, grape-colored wall. Talk about a focal point.
But that’s not the only dramatic element in this bold redesign that emphasizes symmetry and balance. The dining room, with its vaulted ceilings and skylights, is outfitted with a rolling library ladder providing access to tall storage cabinets (also clad in alder). Its artsy glass tabletop, with color streaks resembling rock striations, was custom designed by the owner, an industrial designer, and fabricated by a local glass shop.
Similar glass features appear in the bar and kitchen backsplashes, complemented by Labradorite granite countertops flecked with electric blue and brown.
Pliaconis Residence El Segundo, California
Grand, Best master bath in a custom home
LIGHT AND HEIGHT
The design solution for this ethereal bath started with exterior massing considerations at the outset of a whole-house renovation. “City zoning code doesn’t allow big massive boxes,” ex-plains designer Daryl Olesinski. “At least 25 percent of the façade had to step back, per code.”
This mandate ended up being rather fortuitous, in that pushing the second floor back about 4 feet provided a perfect opportunity for a stretch of south-facing clerestory windows running the entire length of the second level. The design team added a few skylights and ended up with a space that was all air and light with views of sky and trees, yet completely private.
Capitalizing on the home’s newly stepped roofline, the master bath is completely open (its only enclosure is a central toilet closet) and incorporates two ceiling heights. The lower section is capped at 9 feet, creating a feeling of intimacy, while in the taller part, water cascades from a ceiling-mounted showerhead suspended 14 feet above the floor.
Ever budget conscious, the designers specified a basalt stone in matte charcoal as the predominant substrate in the floor and shower wall. “That al-lowed us to use a more expensive material on the opposite wall—a tumbled marble tile cut in a coarse pattern—which provides a nice texture,” Olesin-ski explains. “Then we balanced everything out with teak cabinets and shelves, simple white basin sinks, and Vola faucets.”
Interestingly, the remodel was engineered without a single piece of steel in the house, “which is very unusual,” Ole-sinski says. “If you provide enough wall surface, you don’t need steel for lateral support for earthquakes. This renovation was achieved for about $245 per square foot, which is extremely low for the area.”
Grussing Renovation Saint Louis Park, Minn.
Grand, Best kitchen in a remodeled home—less than 2,000 square feet
STOVE LOVE
Measuring just shy of 2,000 square feet, this 1940s bungalow had neither the frame, nor the personality to sup-port a large, overblown kitchen. But the owner did want something a little bigger than the postage-stamp–sized nook she’d been cooking in. The solution, which lives happily inside a modest addition, is a culinary space as quirky and charming as the rest of the house, with appliances cast in leading roles in the design. The touchstone is a vintage, refurbished Roper stove, capped by a custom metal range hood with scalloped edging. “The Roper has a pretty grounded, mechanical expression to it,” notes architect Jean Rehkamp Larson. “Before electronics made every-thing sleek and smooth, appliances had their pieces and parts on display, which the client really liked.”
Complementary elements include a retro-faced Elmira fridge and matching dishwasher, metal grille cabinet doors topped by a farmhouse sink, a handy butcher-block island top, Eastvold custom Shaker-style cabinets, Caesar stone polished countertops, oak flooring, and a liberal allotment of marble sub-way tiles.
This is old school design at its fi nest, minus the kitschy avocado and harvest yellow color scheme. Instead, the clean lines and neutral tones allow whimsical details such as bead board soffits and cut-out millwork in the island base to stand out. The dropped bead board soffits hide a jog in the ceiling plane and reduce the scale of the room to create a feeling of intimacy.
1024 Glen Oaks Pasadena, California
Grand, Best kitchen in a custom home—less than 3,000 square feet
POINT OF VIEW
To understand this kitchen configuration, one must first understand that it is part of a house designed in homage to California’s post-and-beam case study houses of the 1950s. Al-though the original 1,300-square-foot home (built in 1956) on the steeply cascading slope was too decrepit to salvage, D.S. Ewing Architects maintained its spirit by repeating the same pivoting (at a 12 percent angle) floor plan shape in the new design. An exact replica was impossible, however, given that the new house had to conform to more restrictive hillside ordinances and stringent fire codes.
The resulting two-story structure is engineered with three terraced levels of aluminum grating decks extending down the hillside. To maintain privacy from the street, its western façade is clad in red cedar with high clerestory windows, while the east side of the house is all glass (framed with 6-by-8 foot posts at 8 feet on center). Interior spaces spill onto those fireproof decks with spectacular views of Pasadena and the Rose Bowl.
Perched on the top floor like a tree house, the kitchen enjoys one of the most panoramic vantage points. To preserve those sight lines, the architects specified open bamboo shelving on its view side, enhancing the illusion of space. Rich natural materials make up for in quality what the modest galley lacks in square footage. Those include Honduran mahogany cabinets, wenge counters, custom stainless steel detailing, concrete tile flooring, and tailored lighting.
West Lake Residence Austin, Texas
Grand, Best kitchen in a remodeled home—over 3,000 square feet
SHOW OFF
Drama was a priority for the owners of this high-contrast kitchen, with functionality a bonus. “They wanted a showpiece for events,” says architect Kevin Alter. “They wanted the opposite of a family kitchen.”Located in a 1971 home with little to offer except for a nice location on a hill, the existing galley kitchen was small, dark, and ugly with brown tiles, dated laminate, and little connection to the rest of the house.
Alter and fellow architect Ernesto Cragnolino totally reorganized the middle portion of the house where the kitchen was located, razing walls and distilling the new space to its barest essence. A large is-land topped with striking Calcutta Gold marble is now complemented by walnut base cabinets and a white terrazzo floor.
Meanwhile, the refrigerator, a pantry, and additional storage cabinets were relocated to an adjacent breakfast area. “We liked the idea of not having up-per cabinets,” Alter explains, although their absence necessitated something else to draw and hold the eye. The duo opted for a black wall/backsplash fabricated from lacquer-painted medium-density fiber-board. Despite its hue, the wall element does not make the space seem dark. “It has a huge amount of glass nearby as well as a skylight above that brings light down into the space, so we felt good about using it.”
Two Creeks Ada, Mich.
Grand, Best kitchen in a custom home—over 5,000 square feet
ENGLISH LANGUAGE
This 7,148-square-foot house was designed to “bring English country style across the pond,” ex-plains architect Wayne Vis been. It isn’t hard to imagine enjoying afternoon tea or a nip of sherry in its kitchen, which is at once gracious and cozy. Warmth is the operative word.
Some designers might be averse to mixing multiple wood species, but Vis-been and interior designer Donna Cohen did so to lovely effect, satisfying the owner’s penchant for autumnal colors. A distressed alder wood island and ochre-painted cabinets are off set by a cherry plank ceiling and walnut floors. Granite countertops flecked in brown and gold, a copper island prep sink, and oiled bronze faucets add even more richness and complexity to the mix.
Earthy though its palette may be, this kitchen is more refined than rustic, thanks to furniture-style moldings and raised panel cabinet faces, upholstered chairs, antique pendant lights, and a graceful range hood with display shelving for china. A leaded-glass pocket door connects to an adjacent pantry.
Perhaps the most important ingredient is light, which keeps all that heavy wood and ornate detailing from feeling too dark. Although the vaulted ceiling above the is-land reaches 13 feet 6 inches, the cove that surrounds it drops to 10 feet, creating a perimeter soffit that conceals lighting. And during most hours of the day, sunlight streams in and dances off the white apron sink, tile backsplash, and molding.
Rocky River Kitchen Austin, Texas
Grand, Best kitchen in a custom home—3,000 to 5,000 square feet
TOE TO HEAD
Before this kitchen could begin to take shape, architect Bob Wet-more and his wife, Glenda, had to resolve some rather divergent vernacular tastes. He liked mid-century modern. She? Not so much. The Arts & Crafts aesthetic of this broad-shouldered house proved a good compromise that felt modern enough, but still warm and homey.
From there, the kitchen design was built from the ground up (literally) starting with the concrete floor. “When you do scored and stained concrete floors, it takes about 28 days for the colors to cure, so you really shouldn’t make your other selections until the floor is set,” Wetmore explains. Case in point: The olive green they thought they had originally specified in the floor later turned brown. And, in a happy accident, a copper patina stain on the concrete ended up creating a striated effect that looked like cut stone. Steamed beech wood cabinetry (an economical choice stained to resemble cherry) with black walnut detailing came next, coupled with granite countertops and earth-tone tiles.
Framed by an arched wall, this big, welcoming family space is both handsome and functional. “If someone isn’t sleeping, there’s a good chance they are in the kitchen, and the design takes that into consideration,” Wetmore says. A massive 6-by-12–foot island seats six comfortably, while allocating ample space for food prep at the other end. Casual entertaining and every-day meal prep are made easy with an adjacent pantry, scullery, and wet bar.
And let’s not forget the finishing touches up top. One of the space’s most subtle and ingenious features is a “monorail” lighting scheme that hides rope lighting between bisected ceiling beams. The effect is an uncluttered ceiling that gives off an ambient glow.
Los Altos Hills House Los Altos Hills, Calif.
Grand, Best kitchen in a custom home—over 5,000 square feet
ACCENTUATE THE POSITIVE
If you’re building on a lot with an enviable position high in the hills, your No. 1 priority should be to capture the panoramic views. Architect Mark English and his team of Masha Barmina and Andrea Sessa did just that in the design of this simply detailed modern home over-looking the Santa Clara valley. But they also wanted the kitchen to stand out. “Our goal was to create a serene home environment that would allow for focus on the tremendous views,” English says. “On the other hand, the kitchen, as the hub of the home, needed to be warm, striking, and cheerful.”
Indeed. The design team achieved warmth (perhaps even heat) with a brightly colored island that sets the figurative tone for the space. “Orange has an optimistic feel about it,” English declares. The color plays off of the abundant light filtering in via large adjacent sliding doors and strikes a bold contrast to the white countertops and base cabinets, and smoked and clear glass.
At 500 square feet, the kitchen is the perfect size for entertaining guests. But it feels even larger thanks to the open floor plan that allows the adjacent spaces to merge. “Partly shielded behind a partition wall, the kitchen still preserves lines of sight to the adjoining piano room, sitting room, and great room,” the firm says. Bamboo flooring installed throughout living areas adds warmth, while also helping to meld the rooms into a cohesive whole.
The Egyptians used 8,362 men to erect an obelisk – not including the 900 who died
In 1585, Pope Sixtus V decided to make an urban and architectural move that had been under consideration for over a century. He would shift the Egyptian obelisk near the Vatican in Rome 83m from the side of the Basilica of St Peter to directly in front.
It was a project of the utmost symbolic power. While taming a heathen icon and creating a centre piece to an axially planned and richly ornamented Rome (which continued under Sixtus), it would prove Renaissance engineering skill matched that of the Romans and the Egyptians before them.
Many architects will sympathise with what happened next. Sixtus convened a ‘congregazione’, basically a client committee, made up of cardinals, the heads of the local government, magistrates, a treasurer-general (the 16th-century version of quantity surveyors) and a tax collector. Then they put out a call or entries to ‘men of letters, mathematicians, architects, engineers and other valiant men’ to enter a competition to decide who would undertake the work.
Over 500 people from all over Italy entered, flooding Rome with working models, drawings and treatises about how the task could be achieved. Then, the pope gave the job to an old friend.
Forty-two-year-old Domenico Fontana had been responsible for building a palace for Sixtus when he was a cardinal, and seemed to be in the box seat for the job the whole time. But it was an inspired choice. Not only did he achieve the feat with the use of only 907 men and 75 horses (compared to 8,362 men used by the Egyptians to move and erect an obelisk in 1150BC, not including the 900 who died in the process), but thereafter became a kind of expert subcontractor, raising further fallen obelisks in Rome, and even installing one for the Medici family in Florence.
This book, by four historians, tells this story and many more in compelling detail from the 4,500-year history of obelisk raising. They take the narrative from Old Kingdom Egypt, to Rome, then Paris, London and New York, tracing the symbolic power of these Egyptian monuments that endured and transformed through the centuries. The book is very readable, and at its best has the effect of making the reader understand these mysterious columns as fundamental architectural objects, created often as memorials, but symbolising power and majesty through their form, inscription, and the awe-inspiring technology of their making.
The authors begin with an account of how Egyptian workmen would literally bash the obelisks out of the bedrock using harder, hand-held rocks, and then transport them on vast obelisk ships to their intended sites. They cover the inconclusive debate about how the Egyptians erected them, given their lack of iron pullies and winches. Later, we travel to Augustan Rome, where engineers had to work out how to move them across the Mediterranean. The Romans used obelisks to symbolise how a great power (Egypt) had been brought under the sway of the empire, and also incorporated their symbolic, religious meaning, integrating the gods Isis and Osiris into their pantheon. All but the Vatican obelisk eventually fell, but many were resurrected by Sixtus and later popes, sanctified and capped with crosses.
Later, Western powers found obelisks just the things for their pretensions. The Luxor Obelisk in Place de la Concorde in Paris was erected in 1833 by engineer Jean-Baptiste Lebas. The authors compellingly describe this as a kind of sanitising urban move, placing an ancient and politically neutral (but impressive) object in the square where Louis XVI and Marie Antoinette had lost their heads. It was transformed into an object of cold scientific enquiry, the decoration around its base focused on the engineering methods Lebas used to move it.
By 1878, the arrival of Cleopatra’s Needle in London roused little political interest and, like its cousin in New York’s Central Park, seems to have been integrated into the consumer economy through advertising and media, rather than playing a symbolic role. This was the end (for now?) of journeying obelisks, and these days arguments are more about repatriation of heritage than the symbolic power of the stones.
While this book is not at its best when trying to be analytical, its meticulous storytelling shows us the symbolic power of architecture in the starkest way possible. It is the story of moving the Vatican obelisk that feels like the centre piece of this fascinating history – when ritual, technology, history, politics and urbanism met in one, very large piece of Aswan granite from a quarry in the south of Egypt.
You focus on what is close by, plus the silhouettes of people against the white windows
Imagine a gallery that has closed, its windows whitewashed, the whole place empty except for some crates that presumably used to protect artworks. Galleries have an eeriness anyway, but that feeling is amplified when they’re empty. The gallery becomes generic again, ready to open up if a new use can be found.
The Neue Nationalgalerie in Berlin feels like that right now. Perhaps Mies van der Rohe’s most iconic project (if not his best), the building has profound symbolism for Berlin, built on a site near the wall in former West Berlin’s Kultur forum. Today, the complex of late modernist landmarks sits somewhat uncomfortably in the shadow of the massive Potsdamer Platz/Leipziger Platz development. The older buildings suffer from mild neglect, with weeds growing up the Richard Serra sculpture outside Hans Scharoun’s Philharmonie concert hall.
Imi Knoebel’s installation, Zu Hilfe, zu Hilfe, is profoundly strange and defamiliarises a building that many will feel they know well. You stand inside Mies’ famous steel-table-on-a-plinth and expect to have the usual view of the city. Instead, the windows obscured, you find yourself contemplating the strange quality of light filtering through whitewashed windows, and the rudeness of that technique on a building that is the image of abstract perfection.
The title (meaning ‘Help, help’) is taken from the first line of Mozart’s Magic Flute, cried by Tamino as he is pursued by a serpent, just before he passes out. But the work of Knoebel on Mies’ windows has a numbing, strangely comforting sensation. The three shades of whitewash flatten the usually dramatic light. You focus on what is close by, plus the silhouettes of people against the white windows. It is a quite unique experience of the building, a simple gesture that attains a referential complexity.
Behind the two pavilion-like cloakrooms of the gallery stand piles of strange plywood shapes, mostly Euclidean solids, which look like abstracted packaging, or objects gathered together in readiness for removal men.
Knoebel currently has another show, of his abstract paintings, at the Deutsche Guggenheim. Give that one a miss and head for the Neue Nationalgalerie. You may know this building well, but I promise you, nothing can prepare you for this subversive gesture of rude beauty.
The Morris Arboretum, Philadelphia
Treetops are all the rage these days. Ecotourism destination include a tree house restaurant in Auckland and a teahouse in Japan. And for the stir-crazy office worker there are even tree house office popping up in the Pacific Pennsylvania’s Morris Arboretum in Philadelphia.
Out on a Limb, a new permanent installation at the arboretum, invites you to literally experience trees as the birds and squirrels do. Visitors move from solid ground into the canopy through a series of gently sloping boardwalks, wooden gateways, and human sized habitat structures that reach heights of up to 50 feet. Constructed of galvanized steel and locally harvested hardwoods, the branching walkways wind through woodland habitat vignettes rising out of some of of the oldest tree specimens in the collection.
Out on a limb, brings visitors into the forest canopy
The stark contrast of steel with the organic texture and structure of the surrounding hardwoods is jarring at first, but lead architect Alan Metcalfe says it was intentional. The designers at Metcalfe Architecture & Design, working with arboretum director Paul Meyer, CVM Engineers, and Forever Young Tree houses, wanted to remain honest about the fabricated character of the exhibit while still reflecting and responding to the natural architecture of the surrounding mature forest and providing space for learning and connection.
tree collection from an unusual vantage point
One of the most popular spaces in out on a limb is a convergence of boardwalks, affectionately known as the “Squirrel Scramble” where two oversized hammocks hang several stories above the ground. Metcalfe says the intent of the “Scramble” was to provide the feeling that one is suspended, literally, in the canopy. Adults and children alike can be reclining in the dappled sunlight, others actively exploring the space, climbing and playing as squirrels or chipmunks might.
it’s a people perch
Other points of interest include a giant nest woven of grapevine at the end of a swinging suspension bridge and a teahouse pavilion that provides an outdoor classroom space and board views into the surrounding woodland. For hours, admission, and program information visit the Morris Arboretum online at www.morrisarboretum.org.
Structural Materials - Concrete
Concrete, which is a composite of stone fragments (aggregate) and cement binder, may be regarded as a kind of artificial masonry because it has similar properties to stone and brick (high density, moderate compressive strength, minimal tensile strength). It is made by mixing together dry cement and aggregate in suitable proportions and then adding water, which causes the cement to hydrolyse and subsequently the whole mixture to set and harden to form a substance with stone-like qualities.
Plain, unreinforced concrete has similar properties to masonry and so the constraints on its use are the same as those which apply to masonry. The most spectacular plain concrete structures are also the earliest – the massive vaulted buildings of Roman antiquity.
Concrete has one considerable advantage over stone, which is that it is available in semi-liquid form during the building process and this has three important consequences. Firstly,it means that other materials can be incorporated into it easily to augment its properties. The most important of these is steel in the form of thin reinforcing bars which give the resulting composite material (reinforced concrete) (Pic. 1) tensile and therefore bending strength as well as compressive strength. Secondly, the availability of concrete in liquid form allows it to be cast into a wide variety of shapes. Thirdly, the casting process allows very effective connections to be provided between elements and the resulting structural continuity greatly enhances the efficiency of the structure.
Pic. 1. In reinforced concrete, steel reinforcing bars are
positioned in locations where tensile stress occurs
Reinforced concrete possesses tensile as well as compressive strength and is therefore suitable for all types of structural element including those which carry bending-type loads. It is also a reasonably strong material.Concrete can therefore be used in structural configurations such as the skeleton frame for which a strong material is required and the resulting elements are reasonably slender. It can also be used to make long-span structures and high, multi-storey structures.
Although concrete can be moulded into complicated shapes, relatively simple shapes are normally favoured for reasons of economy in construction (Pic. 2). The majority of reinforced concrete structures are therefore post-and-beam arrangements of straight beams and columns, with simple solid rectangular or circular cross-sections,supporting plane slabs of constant thickness.The formwork in which such structures are cast is simple to make and assemble and therefore inexpensive, and can be re-used repeatedly in the same building. These non-form-active arrangements are relatively inefficient but are satisfactory where the spans are short (up to 6 m). Where longer spans are required more efficient ‘improved’ types of cross-section and profile are adopted. The range of possibilities is large due to the mould ability of the material. Commonly used examples are coffered slabs and tapered beam profiles.
Pic. 2. Despite the mouldability of the material,
reinforced concrete structures normally have a relatively
simple form so as to economise on construction costs. A
typical arrangement for a multi-storey framework is shown
The mouldability of concrete also makes possible the use of complex shapes and the inherent properties of the material are such that practically any shape is possible.Reinforced concrete has therefore been used for a very wide range of structural geometries.Examples of structures in which this has been exploited are the Willis, Faber and Dumas building, where the mouldability of concrete and the level of structural continuity which it makes possible were used to produce a multi-storey structure of irregularly curved plan with floors which cantilevered beyond the perimeter columns,and the Lloyd’s Building, in London, in which an exposed concrete frame was given great prominence and detailed to express the structural nature of its function. The buildings of Richard Meier and Peter Eisenman are also examples of structures in which the innate properties of reinforced concrete have been well exploited.
Sometimes the geometries which are adopted for concrete structures are selected for their high efficiency. Form-active shells for which reinforced concrete is ideally suited are examples of this (see Fig. 1.4). The efficiency of these is very high and spans of 100m and more have been achieved with shells a few tens of millimetres in thickness. In other cases the high levels of structural continuity have made possible the creation of sculptured building forms which, though they may be expressive of architectural meanings, are not particularly sensible from a structural point of view. A well-known example of this is the roof of the chapel at Ronchamp by Le Corbusier, in which a highly individual and inefficient structural form is executed in reinforced concrete. Another example is the Vitra Design Museum by Frank Gehry. It would have been impossible to make these forms in any other structural material.
The Pantheon Revisited
The Pantheon is one of the most celebrated and most carefully studied buildings of Western architecture.In the modern age,as it had been in the Renaissance,the Pantheon is a crucible of critical thinking.Preservation of the Pantheon had been undertaken in the seventeenth century and continued in the eighteenth during the pontificate of Clement XI.Floodwater stains had been removed and some statues placed in the altars around the perimeter.Antoine Derizet, professor at Rome’s official academy of arts,the Accademia di San Luca,praised Clement’s operation as having returned the Pantheon “to its original beauty.”A view of the interior painted by Giovanni Paolo Panini recorded the recent restorations.From a lateral niche,between two cleaned columns, Panini directs our vision away from the Christianized altar out to the sweep of the ancient space.The repeated circles of perimeter,marble paving stones,oculus,and the spot of sunlight that shines through it emphasize the geometrical logic of the rotunda.Panini’s painted view reflects the eighteenth-century vision of the Pantheon as the locus of an ideal geometrical architectural beauty.
Engraving from Vedute di Roma, c. 1748
Not everything in Panini’s view satisfied the contemporary critical eye,however.The attic,that intermediate level above the columns and below the coffers of the dome,seemed discordant—ill proportioned, misaligned, not structurally relevant.A variety of construction chronologies were invented to explain this “error.”The incapacity of eighteenth-century critics to interpret the Pantheon’s original complexities led them to postulate a theory of its original state and,continuing Clement XI’s work, formulate a program of corrective reconstruction.
In 1756,during the papacy of Benedict XIV, the doors of the Pantheon were shut,and behind them dust rose as marble fragments from the attic were thrown down.What may have started as a maintenance project resulted in the elimination of the trouble some attic altogether. The work was carried out in secret;even the pope’s claim of authority over the Pantheon,traditionally the city’s domain,was not made public until after completion. Francesco Algarotti, intellectual gadfly of the enlightened age, happened upon the work in progress and wrote with surprise and irony that “they have dared to spoil that magnificent,august construction of the Pantheon....They have even destroyed the old attic from which the cupola springs and they’ve put up in its place some modern gentilities.”As with the twin bell towers erected on the temple’s exterior in the seventeenth century, Algarotti did not know who was behind the present work.
The new attic was complete by 1757.Plaster panels and pedimented windows replaced the old attic pilaster order,accentuating lines of horizontality.The new panels were made commensurate in measure to the dome’s coffers and the fourteen“windows”were reshaped as statue niches with cutout figures of statues set up to test the effect.The architect responsible for the attic’s redesign,it was later revealed,was Paolo Posi who,as a functionary only recently hired to Benedict XIV’s Vatican architectural team,was probably brought in after the ancient attic was dismantled. Posi’s training in the baroque heritage guaranteed a certain facility of formal invention.Francesco Milizia, the eighteenth century’s most widely respected architectural critic, described Posi as a decorative talent, not an architectural mind.Whatever one might think of the design, public rancor arose over the wholesale liquidation of the materials from the old attic.Capitals,marble slabs,and ancient stamped bricks were dispersed on the international market for antiquities. Posi’s work at the Pantheon was sharply criticized,often with libelous aspersion that revealed a prevailing sour attitude toward contemporary architecture in Rome and obfuscated Posi’s memory.They found the new attic suddenly an affront to the venerated place.
Reconsidering Posi’s attic soon became an exercise in the development of eighteenth-century architects in Rome. Giovanni Battista Piranesi,the catalytic architectural mind who provided us with the evocative engraving of the Pantheon’s exterior,drew up alternative ideas of a rich,three-dimensional attic of clustered pilasters and a meandering frieze that knit the openings and elements together in a bold sculptural treatment.Piranesi,as we will see in a review of this architect’s work,reveled in liberties promised in the idiosyncrasies of the original attic and joyously contributed some of his own.Piranesi had access to Posi’s work site and had prepared engravings of the discovered brick stamps and the uncovered wall construction,but these were held from public release.In his intuitive and profound understanding of the implications of the Pantheon’s supposed “errors,”Piranesi may have been the only one to approach without prejudice the Pantheon in all its complexity and contradiction.
The polemical progress of contemporary architectural design in the context of the Pantheon exemplifies the growing difficulties at this moment of reconciling creativity and innovation with the past and tradition. History takes on a weight and gains a life of its own.The polemic over adding to the Pantheon reveals a moment of transition from an earlier period of an innate,more fluid sense of continuity with the past to a period of shifting and uncertain relationship in the present.The process of redefining the interaction of the present to the past,of contemporary creativity in an historical context,is the core of the problem of modern architecture in Italy and the guiding theme of this study.
Rome of The Nolli Plan
The complex layering found at the Pantheon was merely an example of the vast palimpsest that is Rome itself,and there is no better demonstration of this than the vivid portrait of the city engraved in1748. The celebrated cartographer Giovanni Battista Nolli and his team measured the entire city in eleven months using exact trigonometric methods.At a scale of 1 to 2,900,the two-square-meter map sacrifices no accuracy:interior spaces of major public buildings,churches,and palazzi are shown in detail; piazza furnishings,garden parterre layouts,and scattered ruins outside the walls are described with fidelity.Buildings under construction in the1740s were also included:Antoine Derizet’s Church of Santissimo Nome di Maria at Trajan’s Column,the Trevi Fountain, Palazzo Corsini on Via della Lungara. In the city’s first perfectly ichno graphic representation Nolli privileges no element over another in the urban fabric. All aspects are equally observed and equally important.Vignettes in the lower corners of the map,however,present selected monuments of ancient and contemporary Rome:columns,arches,and temples opposite churches,domes,and new piazzas. Roma antica and Roma modern a face one another in a symbiotic union.
The Nolli plan captures Rome in all its richness, fixing in many minds the date of its publication as the apex of the city’s architecturals plendor. It is an illusory vision,however,as Rome,like all healthy cities,has never been in stasis. Nolli’s inclusion of contemporary architecture emphasizes its constant evolution.His plan is neither a culmination nor a conclusion but the starting point for contemporary architecture.The architecture of modern Italy is written upon this already dense palimpsest.
Structural Materials - Steel
The use of steel as a primary structural material dates from the late nineteenth century when cheap methods for manufacturing it on a large scale were developed. It is a material that has good structural properties. It has high strength and equal strength in tension and compression and is therefore suitable for the full range of structural elements and will resist axial tension, axial compression and bending-type load with almost equal facility. Its density is high, but the ratio of strength to weight is also high so that steel components are not excessively heavy in relation to their load carrying capacity, so long as structural forms are used which ensure that the material is used efficiently. Therefore, where bending loads are carried it is essential that ‘improved’ cross-sections and longitudinal profiles are adopted.
Pic. 1. Hopkins House, London, UK; Michael Hopkins, architect ;
Anthony Hunt Associates, structural engineers. The floor structure
here consists of profiled steel sheeting which will support a timber deck.
A more common configuration is for the profiled steel deck to act compositely
with an in situ concrete slab for which it serves as permanent formwork.
The high strength and high density of steel favours its use in skeleton frame type structures in which the volume of the structure is low in relation to the total volume of the building which is supported, but a limited range of slab-type formats is also used. An example of a structural slab-type element is the profiled floor deck in which a profiled steel deck is used in conjunction with concrete, or exceptionally timber (Pic.1), to form a composite structure. These have ‘improved’ corrugated cross-sections to ensure that adequate levels of efficiency are achieved. Deck units consisting of flat steel plate are uncommon.
The shapes of steel elements are greatly influenced by the process which is used to form them. Most are shaped either by hot-rolling or by cold-forming. Hot-rolling is a primary shaping process in which massive red-hot billets of steel are rolled between several sets of profiled rollers. The cross-section of the original billet, which is normally cast from freshly manufactured steel and is usually around 0.5m x 0.5m square, is reduced by the rolling process to much smaller dimensions and to a particular precise shape (Pic. 2). The range of cross-section shapes which are produced is very large and each requires its own set of finishing rollers. Elements that are intended for structural use have shapes in which the second moment of area is high in relation to the total area (Pic. 3). I- and H- shapes of cross-section are common for the large elements which form the beams and columns of structural frameworks. Channel and angle shapes are suitable for smaller elements such as secondary cladding supports and sub-elements in triangulated frameworks. Square,circular and rectangular hollow sections are produced in a wide range of sizes as are flat plates and solid bars of various thicknesses. Details of the dimensions and geometric properties of all the standard sections are listed in tables of section properties produced by steelwork manufacturers.
Pic. 2 The heaviest steel sections are produced by a
hot-rolling process in which billets of steel are shaped by
profiled rollers. This results in elements which are straight,
parallel sided and of constant cross-section. These features
must be taken into account by the designer when steel is
used in building and the resulting restrictions in form accepted
Pic. 3 Hot-rolled steel elements
The other method by which large quantities of steel components are manufactured is cold-forming. In this process thin, flat sheets of steel, which have been produced by the hot-rolling process, are folded or bent in the cold state to form structural cross-sections (Pic. 4). The elements which result have similar characteristics to hot-rolled sections, in that they are parallel sided with constant cross-sections, but the thickness of the metal is much less so that they are both much lighter and, of course, have lower load carrying capacities. The process allows more complicated shapes of cross-section to be achieved, however. Another difference from hot-rolling is that the manufacturing equipment for cold-forming is much simpler and can be used to produce tailor-made cross-sections for specific applications. Due to their lower carrying capacities cold-formed sections are used principally for secondary elements in roof structures, such as purlins, and for cladding support systems. Their potential for future development is enormous.
Pic. 4 Cold formed sections are
formed from thin steel sheet.
A greater variety of cross section
shapes is possible than with the
hot-rolling process
Structural steel components can also be produced by casting, in which case very complex tailor-made shapes are possible. The technique is problematic when used for structural components, however, due to the difficulty of ensuring that the castings are sound and of consistent quality throughout. In the early years of ferrous metal structures in the nineteenth century, when casting was widely used, many structural failures occurred– most notably that of the Tay Railway Bridge in Scotland in 1879. The technique was rarely used for most of the twentieth century but technical advances made possible its re-introduction. Prominent recent examples are the ‘gerberettes’ at the Centre Pompidou, Paris (Pic 5) and the joints in the steel work of the train shed at Waterloo Station, London.
Most of the structural steelwork used in building consists of elements of the hot-rolled type and this has important consequences for the layout and overall form of the structures.An obvious consequence of the rolling process is that the constituent elements are prismatic:they are parallel-sided with constant cross-sections and they are straight – this tends to impose a regular, straight-sided format on the structure. In recent years, however, methods have been developed for bending hot-rolled structural steel elements into curved profiles and this has extended the range of forms for which steel can be used. The manufacturing process does,however, still impose quite severe restrictions on the overall shape of structure for which steel can be used.
The manufacturing process also affects the level of efficiency which can be achieved in steel structures, for several reasons. Firstly, it is not normally possible to produce specific tailor-made cross-sections for particular applications because special rolling equipment would be required to produce them and the capital cost of this would normally be well beyond the budget of an individual project. Standard sections must normally be adopted in the interests of economy, and efficiency is compromised as a result. An alternative is the use of tailor-made elements built up by welding together standard components, such as I-sections built up from flat plate. This involves higher manufacturing costs than the use of standard rolled sections.
Pic. 4 The so-called ‘gerberettes’ at the Centre Pompidou in Paris,
France, are cast steel components. No other process could have
produced elements of this size and shape in steel
A second disadvantage of using an ‘off-the-peg’ item is that the standard section has a constant cross-section and therefore constant strength along its length. Most structural elements are subjected to internal forces which vary from cross-section to cross-section and therefore have a requirement for varying strength along their length. It is, of course,possible to vary the size of cross-section which is provided to a limited extent. The depth of an I-section element, for example, can be varied by cutting one or both flanges from the web,cutting the web to a tapered profile and then welding the flanges back on again. The same type of tapered I-beam can also be produced by welding together three separate flat plates to form an I-shaped cross-section, as described above.
Because steel structures are pre-fabricated, the design of the joints between the elements is an important aspect of the overall design which affects both the structural performance and the appearance of the frame. Joints are made either by bolting or by welding (Pic. 5). Bolted joints are less effective for the transmission of load because bolt holes reduce the effective sizes of element cross-sections and give rise to stress concentrations. Bolted connections can also be unsightly unless carefully detailed. Welded joints are neater and transmit load more effectively, but the welding process is a highly skilled operation and requires that the components concerned be very carefully prepared and precisely aligned prior to the joint being made. For these reasons welding on building sites is normally avoided and steel structures are normally pre-fabricated by welding and bolted together on site. The need to transport elements to the site restricts both the size and shape of individual components.
Pic. 5. Joints in steelwork are normally made by a
combination of bolting and welding. The welding is usually
carried out in the fabricating workshop and the site joint is
made by bolting.
Steel is manufactured in conditions of very high quality control and therefore has dependable properties which allow the use of low factors of safety in structural design. This,together with its high strength, results in slender elements of lightweight appearance.The basic shapes of both hot- and cold-formed components are controlled within small tolerances and the metal lends itself to very fine machining and welding with the result that joints of neat appearance can be made. The overall visual effect is of a structure which has been made with great precision (Pic. 5).
Pic. 6. Renault Sales Headquarters, Swindon, UK, 1983;
Foster Associates, architects; Ove Arup & Partners, structural
engineers. Joints in steelwork can be detailed to look very neat and
to convey a feeling of great precision
Two problems associated with steel are its poor performance in fire, due to the loss of mechanical properties at relatively low temperatures, and its high chemical instability, which makes it susceptible to corrosion. Both of these have been overcome to some extent by the development of fireproof and corrosion protection materials, especially paints, but the exposure of steel structures, either internally,where fire must be considered, or externally,where durability is an issue, is always problematic.
To sum up, steel is a very strong material with dependable properties. It is used principally in skeleton frame types of structure in which the components are hot-rolled. It allows the production of structures of a light, slender appearance and a feeling of neatness and high precision. It is also capable of producing very long span structures, and structures of great height. The manufacturing process imposes certain restrictions on the forms of steel frames. Regular overall shapes produced from straight, parallel-sided elements are the most favoured.
Highway Bridge Structures, Use And Functionality
When the average individual is asked to think of a bridge,some pretty impressive images usually come to mind.The Golden Gate and Brooklyn bridges might strike you if you are an American.Perhaps one would think of the Firth of Forth Bridge if you hailed from the United Kingdom. For the historically minded, Pont du Gard is almost always a favorite choice.
Without a doubt, these are magnificent structures and volumes have been written on their history and the engineering behind them; but what of the common highway bridge structure? Although you probably feel a bump every time your automobile hits an expansion joint, most people and even many engineers take these average highway bridges for granted. The common highway bridge structure, however, is one of the most integral components in any transportation network. It is also one of the most exciting design projects a civil engineer can be engaged in.
By common highway bridges, we imply structures which typically consist of a slab-on-stringer configuration crossing relatively short span lengths. The deck is usually a concrete slab which rests on a set of girders composed of one of the following types :
❏ Steel rolled sections or plate girders
❏ Prestressed concrete beams
❏ Timber beams
won’t be talking about in this text
There are a wide variety of other forms of bridge structures in use (suspension, cable-stayed, arch, truss, concrete, or steel box girder, etc.),however, the backbone of the modern transportation network is the slab-on-stringer type structure. The Golden Gate, and other major bridges like it, also carry traffic, and can quite rightly be called highway bridge structures.
However, the design and construction of the slab-on-stringer bridge is the focus of this text, not only because of its continued popularity as a structure in new design projects, but also due to the pressing issues of maintaining and rehabilitating existing slab-on-stringer bridges in an aging infrastructure. With regard to rehabilitation, today’s civil engineers are presented with a situation that their forerunners were, for the most part, unfamiliar with. Throughout the text we will see that rehabilitation design offers its own set of unique challenges. As young engineers, when we think of bridge design, we all dream of a magnificent project like the George Washington or Sydney Harbor bridges; but these are few and far between. In the trenches, so to speak, we are faced with the slab-on-stringer bridge which, while maybe not as glamorous,can prove every bit as challenging as its larger cousins.
and its representative components
LEGEND :
1: DECK AND OVERPASS
2: STRINGER
3: BEARING
4: PEDESTAL
5: FOOTING
6: PILES
7: UNDERPASS
8: EMBANKMENT
9: LIVE LOADING
Quite obviously, any integrated transportation network requires bridge structures to carry traffic over a variety of crossings. A crossing, which we will call an underpass, could be man-made (other highways, rail lines, canals) or natural (water courses, ravines). As facile as this point may seem, it should bring home the magnitude of the number of bridges currently in use and being maintained by various agencies throughout the world. It is very rare, indeed, when a highway or road of sizeable length can proceed from start to finish without encountering some obstacle which must be bridged. In the United States alone there are over 590,000 structures currently in service and that number grows every year as new highway projects come off the boards and into construction.
A HIGHWAY BRIDGE SITE is a complicated place and a point where a suite of civil engineering disciplines converge to form one of the most exciting challenges in the profession. A scan of the associated figure shows that a bridge designer must be concerned with :
Highway Design for the overpass and underpass alignment and geometry.
Structural Design for the super-structure and substructure elements.
Geotechnical Engineering for the pier and abutment foundations.
Hydraulic Engineering for proper bridge span length and drainage of bridge site.
Surveying and Mapping for the layout and grading of a proposed site.
Yet even with such a breadth of engineering topics to concern ourselves with, the modern highway bridge remains an intriguing project because of the elegant simplicity of its design and the ease with which its system can be grasped. For the new or experienced bridge designer one of the most helpful aids is continual observation. Bridge engineers have constant exposure to highway bridges as they travel the expanses of our transportation networks. By looking for different forms of elements, the reader will gain a better understanding of the variety of components in use in bridge design and possess a more well defined physical appreciation of the structure and design process.
The 1950’s through early 1970’s saw an explosion in the number of highway bridges being designed and built in this country. According to U.S. Department of Transportation, by 2003 over 27% of U.S. bridges were deemed structurally deficient and functionally obsolete [Ref. 1.1]. This situation means that in this century we will see a major push toward the repair and eventual replacement of many of these structures. It is with this in mind that we must identify the basic use and functionality of highway bridge structures.
1.1 Terminology and Nomenclature
As is the case with any profession, bridge engineering possesses its own unique language which must first be understood by the designer in order to create a uniform basis for discussion. Figure 1.2 shows a typical, slab-on-stringer structure which carries an overpass roadway over another road. This particular structure, shown in the figure, consists of a single span. A span is defined as a segment of bridge from support to support. The following offers a brief overview of some of the major bridge terms we will be using throughout the text.At the end of this section, the reader is provided with a comprehensive Bridge Engineering Lexicon which acts as a dictionary for the bridge designer. The lexicon contains many of the most common bridge engineering terms and expressions used on a day-to-day basis by bridge design professionals.
1. Superstructure. The superstructure comprises all the components of abridge above the supports. Picture 3 shows a typical superstructure. The basic superstructure components consist of the following :
■ Wearing Surface. The wearing surface (course) is that portion of the deck cross section which resists traffic wear. In some instances this is a separate layer made of bituminous material, while in some other cases it is a integral part of concrete deck. The integral wearing surface is typically 1/2 to 2 in(13 to 51 mm). The bituminous wearing course usually varies in thickness from 2 to 4 in (51 to 102 mm). The thickness, however, can sometimes be larger due to resurfacing of the overpass roadway, which occurs throughout the life cycle of a bridge.
■ Deck. The deck is the physical extension of the roadway across the obstruction to be bridged. In this example, the deck is a reinforced concrete slab. In an orthotropic bridge, the deck is a stiffened steel plate. The main function of the deck is to distribute loads transversely along the bridge cross section. The deck either rests on or is integrated with a frame or other structural system designed to distribute loads longitudinally along the length of the bridge.
■ Primary Members. Primary members distribute loads longitudinally and are usually designed principally to resist flexure and shear. In Picture 3, the primary members consist of rolled, wide flange beams. In some instances,the outside or fascia primary members possess a larger depth and may have a cover plate welded to the bottom of them to carry heavier loads.Beam type primary members such as this are also called stringers or girders. These stringers could be steel wide flange stringers, steel plate girders(i.e., steel plates welded together to form an I section), pre stressed concrete,glued laminated timber, or some other type of beam. Rather than have the slab rest directly on the primary member, a small fillet or haunch can be placed between the deck slab and the top flange of the stringer. The primary function for the haunch is to adjust the geometry between the stringer and the finished deck. It is also possible for the bridge superstructure to be formed in the shape of a box (either rectangular or trapezoidal). Box girder bridges can be constructed out of steel or pre stressed concrete and are used in situations where large span lengths are required and for horizontally curved bridges.
■ Secondary Members. Secondary members are bracing between primary members designed to resist cross-sectional deformation of the superstructure frame and help distribute part of the vertical load between stringers.They are also used for the stability of the structure during construction. In Picture 3 a detailed view of a bridge superstructure shows channel-type diaphragms used between rolled section stringers. The channels are bolted to steel connection plates, which are in turn welded to the wide flange stringers shown. Other types of diaphragms are short depth, wide flange beams or crossed steel angles. Secondary members, composed of crossed frames at the top or bottom flange of a stringer, are used to resist lateral deformation.This type of secondary member is called lateral bracing.
2. Substructure. The substructure consists of all elements required to support the superstructure and overpass roadway. In Picture 2 this would be Items 3 to 6. The basic substructure components consist of the following :
■ Abutments. Abutments are earth-retaining structures which support the superstructure and overpass roadway at the beginning and end of a bridge.Like a retaining wall, the abutments resist the longitudinal forces of the earth underneath the overpass roadway. In Figure 1.2 the abutments are cantilever-type retaining walls. Abutments come in many sizes and shapes, which will,like all elements described in this section, be discussed in detail later.
■ Piers. Piers are structures which support the superstructure at intermediate points between the end supports (abutments). Since the structure shown in Picture 2 consists of only one span, it logically does not require a pier. Like abutments, piers come in a variety of forms, some of which are illustrated in the sidebar. From an aesthetic standpoint, piers are one of the most visible components of a highway bridge and can make the difference between a visually pleasing structure and an unattractive one. Picture 5 shows a hammerhead-type pier.
PIERS, like abutments, come in a variety of shapes and sizes which depend on the specific application. The schematic figures below show some of the more basic types of piers which are popular in highway bridges. The physical conditions of the bridge site play an important role in deciding which type of pier to use. For example,to provide a large clearance makes a hammerhead attractive, while pile bents are well suited for shallow water crossings.
an important role in deciding which type of pier to use
■ Bearings. Bearings are mechanical systems which transmit the vertical and horizontal loads of the superstructure to the substructure, and accommodate movements between the superstructure and the substructure. Examples of bearings are mechanical systems made of steel rollers acting on large steel plates or rectangular pads made of neoprene. The use and functionality of bearings vary greatly depending on the size and configuration of the bridge. Bearings allowing both rotation and longitudinal translation are called expansion bearings, and those which allow rotation only are called fixed bearings.
■ Pedestals. A pedestal is a short column on an abutment or pier under a bearing which directly supports a superstructure primary member. As can be seen in Picture 2 at the left abutment cutaway, the wide flange stringer is attached to the bearing which in turn is attached to the pedestal. The term bridge seat is also used to refer to the elevation at the top surface of the pedestal. Normally pedestals are designed with different heights to obtain the required bearing elevations.
■ Back wall. A back wall, sometimes called the stem, is the primary component of the abutment acting as a retaining structure at each approach. Picture 7 shows a back wall integrated with a wing wall in a concrete abutment.
■ Wing wall. A wing wall is a side wall to the abutment back wall or stem designed to assist in confining earth behind the abutment. On many structures,wing walls are designed quite conservatively, which leads to a rather large wall on many bridges [Picture 7].
■ Footing. As bearings transfer the superstructure loads to the substructure, so in turn do the abutment and pier footings transfer loads from the substructure to the subsoil or piles. A footing supported by soil without piles is called a spread footing. A footing supported by piles, like the one in Picture 2, is known as a pile cap.
■ Piles. When the soil under a footing cannot provide adequate support for the substructure (in terms of bearing capacity, overall stability, or settlement), support is obtained through the use of piles, which extend down from the footing to a stronger soil layer or to bedrock. There are a variety of types of piles ranging from concrete, which is cast in place (also called drilled shafts or caissons) or precast, to steel H-sections driven to sound rock. Picture 8 shows piles being driven for the replacement of an abutment during a bridge rehabilitation project.
■ Sheeting. In cofferdams or shallow excavation, the vertical planks which are driven into the ground to act as temporary retaining walls permitting excavation are known as sheeting. Steel sheet piles are one of the most common forms of sheeting in use and can even be used as abutments for smaller structures. In Picture 9. a two-lane, single-span bridge is supported at each end by arch web sheet piling abutments providing an attractive and economical solution for this small structure.
3. Appurtenances and Site-Related Features. An appurtenance, in the context of this discussion, is any part of the bridge or bridge site which is not a major structural component yet serves some purpose in the overall functionality of the structure (e.g., guardrail). The bridge site, as an entity, possesses many different components which, in one way or another, integrates with the structure. Do not make the mistake of underrating these appurtenances and site features, for, as we shall see throughout the course of this text, a bridge is a detail-intensive project and,in defining its complexity, a highway bridge is truly the sum of its parts. The major appurtenances and site-related features are as follows :
■ Embankment and Slope Protection. The slope that tapers from the abutment to the underpass (embankment) is covered with a material called slope protection, which should be both aesthetically pleasing and provide for proper drainage and erosion control. Slope protection could be made of dry stone or even block pavement material. Picture 10. shows an abutment embankment being prepared with select granular fill. This type of slope protection consists of broken rocks which vary in size and shape. The form of slope protection varies greatly from region to region and is mostly dependent on specific environmental concerns and the types of material readily available. For water way crossings, large stones (rip rap) are usually used for foundation scour protection.
■ Under drain. In order to provide for proper drainage of a major substructure element, such as an abutment, it is often necessary to install an under drain, which is a drainage system made of perforated pipe or other suitable conduit that transports runoff away from the structure and into appropriate drainage channels (either natural or man-made).
■ Approach. The section of overpass roadway which leads up to and away from the bridge abutments is called the approach or approach roadway. In cross section the approach roadway is defined by the American Association of State Highway and Transportation Officials (AASHTO) as the “traveled way plus shoulders”. The approach roadway typically maintains a similar cross section to that of the standard roadway. To compensate for potential differential settlement at the approaches, a reinforced concrete slab or approach slab is sometimes used for a given distance back from the abutment.The approach slab helps to evenly distribute traffic loads on the soil behind the abutment, and minimizes impact to the abutment which can result from differential settlement between the abutment and the approach. An approach slab is typically supported by the abutment at one end, and supported by the soil along its length.
■ Traffic Barriers. A traffic barrier is a protective device “used to shield motorists from obstacles or slope located along either side of roadway”. Traffic barriers can range from a guard rail made of corrugated steel to reinforced concrete parapets. On bridges, they are usually called bridge railings.
4. Miscellaneous Terms. Some of the more basic expressions and terms that we will use throughout the course of the text are as follows :
■ Vertical Clearance. Vertical clearance is the minimum distance between the structure and the underpass. AASHTO specifies an absolute minimum of 14 ft (4.27 m) and a design clearance of 16 ft (4.88 m). The location of the structure (i.e., urbanized area vs. expressway) has a great deal to do with how this is enforced by the governing agency.
■ Load Rating. An analysis of a structure to compute the maximum allowable loads that can be carried across a bridge is called a load rating. The guidelines for load ratings are set forth in AASHTO’s Manual for Condition Evaluation of Bridges. Two ratings are usually prepared: the inventory rating corresponds to the customary design level of capacity,while operating rating describes the maximum permissible live load to which the structure may be subjected. Therefore, operating rating always yields a higher load rating than inventory rating.
■ Dead Loads. Permanent loads placed on a structure before the concrete slab hardens are called dead loads. For example, in a slab-on-stringer bridge the stringers, diaphragms, connection plates, and concrete slab itself (including stay-in-place forms) would be considered as dead loads.
■ Superimposed Dead Loads. Superimposed dead loads are permanent loads placed on the structure after the concrete has hardened (e.g., bridge railing, sidewalks, wearing surface, etc.). Superimposed dead loads are generally considered part of total dead loads.
■ Live Loads. Temporary loads placed on the structure, such as vehicles,wind, pedestrians, etc., are called live loads. In Picture 2. the truck traveling over the structure (Item 9) represents live load on the bridge. The vehicles used to compute live loads are not duplicate models of a tractor trailer seen on the highway but rather hypothetical design vehicles developed by AASHTO in the 1940’s and 1990’s.
■ Sheeted Pit. A temporary box structure with only four sides (i.e., no top or bottom) which can be used as an earth support system in excavation for substructure foundations is called a sheeted pit. The bracing elements used inside a sheeted pit to keep all four sides rigid are called wales (which run along the inside walls of the sheet piling) and struts (which run between the walls). When this type of structure is used where the ground level is below water, the sheeted pit is designed to be watertight (as much as possible) and is called a cofferdam. In Picture 11. a sheeted pit used for excavation at the center pier can be seen.
■ Staged Construction. Construction that occurs in phases, usually to permit the flow of traffic through a construction site, is called staged construction. An example would be a bridge replacement project where half of the structure is removed and replaced while traffic continues over the remaining portion of the structure. Once the first half has been removed and reconstructed, traffic is then diverted over to the new side while work begins on the rest of the structure. This is an aspect of rehabilitation design which offers some interesting challenges to engineers. A bridge rehabilitation under staged construction is shown in Picture 11.
1.2. Structure Types and Applications
As has been previously mentioned, the majority of bridges present in our infrastructure are of the slab-on-stringer configuration. There are, however, a wide variety of structures in use for a variety of different physical applications.By physical applications we imply man-made, natural, or climatological conditions which dictate the type of structure to be used at a given crossing. These could be in the form of
❏ Length to be bridged from the start to the end of the structure
❏ Depth of channel or ravine to be crossed
❏ Underpass clearance required
❏ Extreme temperature conditions
❏ Precipitation or snowfall
❏ Curvature of overpass alignment
❏ Aesthetics of the surrounding environment
Any or all of these criteria could play a critical role in the ultimate decision reached as to what type of structure is to be used in general, and what type of components in particular (i.e., wide-flange pre stressed concrete girders vs. steel stringers). While it is not within the scope of this text to present a detailed investigation into all different forms of structures, it is important for the reader to have an understanding of some of the major structure types in use and the conditions which make them more attractive than competitive solutions.
1. Slab-on-Stringer. In Pictures 2 and 3 the bridge superstructure consists of a concrete slab resting on a set of stringers, which are connected together by diaphragms to form a frame. The stringers could be steel beams, precast – pre stressed concrete girders, or of other suitable materials. Traffic passes over the top of the slab, which can be covered with a wearing surface,although sometimes the slab itself is made thicker to create an integrated wearing surface (i.e., using a portion of the slab rather than a separate layer to resist the wear of traffic). The principal advantages of this system are :
❏ Simplicity of design. It should be understood that simplicity is a relative term. From an engineering perspective, slab-on-stringer structures don’t break much new ground theoretically, but the complexity they offer from a total project perspective presents a challenge for any designer. Indeed, because of all the aspects involved in any highway bridge project, the need of providing a straightforward design is essential toward ensuring that costs be kept at a reasonable level for the engineering services portion of a bridge contract.
❏ The slab-on-stringer bridge lends itself well to a uniform design which can be standardized easily. This is an advantage because standardization and uniformity are critical for maintaining bridges in large transportation networks. Standardization minimizes the need for creating a plethora of codes and specifications for designers to follow,especially when many owners of bridges rely on private consultants to assist in the design of new bridges and rehabilitation of existing bridges. Uniformity means that consistent, and therefore economical,methods can be employed in repairing deteriorated structures. Imagine if a highway network had hundreds of unique designs with customized components for each structure!
❏ Construction is relatively straightforward and makes use of readily available materials. Prefabricated primary members like steel wide-flange stringers or pre stressed concrete beams allow for quick erection and a clean appearance while at the same time provide for an economy of materials that is a benefit to the contractor as well as the owner.
Slab-on-stringer structures, however, are primarily for short span lengths and average clearance requirements (we will quantify short and average a little bit later). When span lengths become excessive and the geometry and physical constraints of a site become excessive, other forms of structures must be investigated.
2. One-Way Slab. For a very short span [less than 30 ft (9 m)] a one-way concrete slab supported on either end by small abutments is an economical structure. Such a short span structure often gains the tag of puddle crosser because of the diminutive size of the structure. For short to median spans,[30 to 80 ft (9 to 24 m)] pre stressing steel is typically used. Circular voids in the slab are sometimes used to reduce the dead load.
3. Steel and Concrete Box Girder. When bending and torsion are major concerns, a box girder type structure offers an aesthetically pleasing, albeit expensive, solution. Since these types of structures do not make use of standardized or prefabricated components, their role is usually restricted to major highway bridges that can take advantage of their ability to meet relatively long span requirements. Picture 12 shows the KCRC West Rail Viaducts in Hong Kong.
4. Cable-Stayed. Although box girder bridges with span lengths of 760 feet(232 m) have been built, a significant number of modern bridges with span lengths from 500 feet to 2800 feet (153 to 853 m) have been constructed as cable-stayed bridges. These types of bridges have begun to be built in the United States only 40 years ago, but the response has been overwhelming.Low cost, ease of construction, and aesthetics are the major reasons why this type of structure is now a popular choice for medium and long span bridges. Picture 13 shows the William Dargan Bridge in Dublin, Ireland.
5. Suspension. Everyone immediately recognizes the suspension bridge as one of the consummate marvels of civil engineering. When presented with spans of significant length over impressive physical obstacles (e.g. the Mississippi River), the suspension bridge offers an elegant answer to a monumental engineering task. For the majority of structures in use,however, their application is relatively limited and their design relegated to the domain of a small group of engineers. Oddly enough, despite this limited role, numerous quality texts are available on the subject and the reader is referred to them for further discussion on these types of structures.
6. Steel and Concrete Arch. Like the cable stayed and suspension bridges described above, the arch is most often used for major crossings like the Hell Gate and Sydney Harbor bridges. Picture 14 shows a picture of the twin Thaddeus Kosciuzko bridges crossing the Mohawk River in upstate New York. In this particular site, the steel arches provide for an attractive-looking structure while also eliminating the need for a pier in the river. When the deck, as is the case with the structures in Figure 1.13, is suspended from the steel arch, the structure is called a through arch. When the deck is supported on top of the arch, this is called a deck arch. An arch bridge generates large reaction forces at its end supports. The horizontal component of these reaction forces is either resisted by abutment foundations, or in the case of a tied arch, resisted by a tie between arch supports. Other elements of an arch bridge are described in the sidebar accompanying Picture 14.
in upstate New York.
7. Truss. The truss bridge is encountered most often in historical engineering projects that require preservation or rehabilitation of an existing structure. For the most part, the day of the truss as a new bridge structure in and of itself is over, because truss members are typically fracture critical members (i.e., there is no redundancy in the load path, so should one member fail, the whole structure would collapse). Another major reason it becomes unpopular is that the construction and maintenance costs of truss bridges are very high. However, the use of trusses as bridge components in large structures is still prevalent. Trusses are also used as temporary bridges. Picture 15 shows a picture of American River Bridge near Sacramento, California.
