Highway Bridge Structures, Use And Functionality
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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.
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