Bridge Engineering: Seismic Design (Principles and Applications in Engineering)

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The book also includes an overview of seismic design practices in Japan, including a study of the damage to highway bridges caused by the Hyogo-ken Nanbu earthquake and the changes in retrofit practices precipitated by that earthquake. Help Centre. My Wishlist Sign In Join. Chen Editor , Lian Duan Editor. Be the first to write a review. Add to Wishlist. Ships in 7 to 10 business days. Link Either by signing into your account or linking your membership details before your order is placed. Description Table of Contents Product Details Click on the cover image above to read some pages of this book!

Geotechnical Earthquake Considerations p. All Rights Reserved. In Stock. Basic Building and Construction Skills 5th Edition. Welding For Dummies For Dummies. Building Construction Illustrated. Principles of Structure 5th Edition. Check bearing compression and rotation Art. Check bearing stability Art. Check bearing steel reinforcement Art. Temperature Movement From Art. Bearing Maximum Rotation The bearing rotational capacity can be calculated as 7. Combined Bearing Compression and Rotation a. Bearing Stability Bearings shall be designed to prevent instability at the service limit state load combinations.

The average compressive stress on the bearing is limited to half the predicted buckling stress. Bearing Steel Reinforcement The bearing steel reinforcement must be designed to sustain the tensile stresses induced by compression of the bearing. The thickness of steel reinforcement, hs, should satisfy: a.

Elastomeric Bearings Details Five interior lays with 20 mm thickness each layer Two exterior lays with 10 mm thickness each layer Six steel reinforcements with 1. Stanton, J. Although piers are traditionally designed to resist vertical loads, it is becoming more and more common to design piers to resist high lateral loads caused by seismic events. Even in some low seismic areas, designers are paying more attention to the ductility aspect of the design.

Piers are predominantly constructed using reinforced concrete. Steel, to a lesser degree, is also used for piers. Steel tubes filled with concrete composite columns have gained more attention recently. This chapter deals only with piers or columns for conventional bridges, such as grade separations, overcrossings, overheads, underpasses, and simple river crossings. Reinforced concrete columns will be discussed in detail while steel and composite columns will be briefly discussed. Substructures for arch, suspension, segmental, cable-stayed, and movable bridges are excluded from this chapter.

Chapter 3 discusses the substructures for some of these special types of bridges. However, from time to time, it is also used particularly for a solid wall in order to distinguish it from columns or bents. From a structural point of view, a column is a member that resists the lateral force mainly by flexure action whereas a pier is a member that resists the lateral force mainly by a shear mechanism. A pier that consists of multiple columns is often called a bent. There are several ways of defining pier types.

One is by its structural connectivity to the superstructure: monolithic or cantilevered. Another is by its sectional shape: solid or hollow; round, octagonal, hexagonal, or rectangular. It can also be distinguished by its framing configuration: single or multiple column bent; hammerhead or pier wall. Typical cross-section shapes of piers for river and waterway crossings. Selection Criteria Selection of the type of piers for a bridge should be based on functional, structural, and geometric requirements.

Figure 2. Often, pier types are mandated by government agencies or owners. Many state departments of transportation in the United States have their own standard column shapes.

Solid wall piers, as shown in Figures 2. These features lend themselves well for providing minimal resistance to flood flows. Hammerhead piers, as shown in Figure 2.

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They are used to support steel girder or precast prestressed concrete superstructures. They are aesthetically appealing. They generally occupy less space, thereby providing more room for the traffic underneath. Standards for the use of hammerhead piers are often maintained by individual transportation departments. A column bent pier consists of a cap beam and supporting columns forming a frame. Column bent piers, as shown in Figure 2.

The columns can be either circular or rectangular in cross section. They are by far the most popular forms of piers in the modern highway system. A pile extension pier consists of a drilled shaft as the foundation and the circular column extended from the shaft to form the substructure. An obvious advantage of this type of pier is that it occupies a minimal amount of space. Widening an existing bridge in some instances may require pile extensions because limited space precludes the use of other types of foundations.

Selections of proper pier type depend upon many factors. First of all, it depends upon the type of superstructure. For example, steel girder superstructures are normally supported by cantilevered piers, whereas the cast-in-place concrete superstructures are normally supported by monolithic bents. Second, it depends upon whether the bridges are over a waterway or not.

Pier walls are preferred on river crossings, where debris is a concern and hydraulics dictates it. Multiple pile extension bents are commonly used on slab bridges. Last, the height of piers also dictates the type selection of piers. The taller piers often require hollow cross sections in order to reduce the weight of the substructure. This then reduces the load demands on the costly foundations. Table 2. Where expansion bearings are used, forces caused by temperature changes are limited to the frictional resistance of bearings. In the following, two load cases, live loads and thermal forces, will be discussed in detail because they are two of the most common loads on the piers, but are often applied incorrectly.

There are other special loading conditions peculiar to the type or location of the bridge structure which should be specified in the contracting documents. Live-load reactions obtained from the design of individual members of the superstructure should not be used directly for substructure design. These reactions are based upon maximum conditions for one beam and make no allowance for distribution of live loads across the roadway.

Use of these maximum loadings would result in a pier design with an unrealistically severe loading condition and uneconomical sections. For substructure design, a maximum design traffic lane reaction using either the standard truck load or standard lane load should be used. For the calculation of the actual beam reactions on the piers, the maximum lane reaction can be applied within the design traffic lanes as wheel loads, and then distributed to the beams assuming the slab between beams to be simply supported Figure 2. Wheel loads can be positioned anywhere within the design traffic lane with a minimum distance between lane boundary and wheel load of 0.

The design traffic lanes and the live load within the lanes should be arranged to produce beam reactions that result in maximum loads on the piers.

Principles and Applications in Engineering

Live-load reactions will be increased due to impact effect. Piers should be checked against these forces. Design codes or specifications normally specify the design temperature range. Some codes even specify temperature distribution along the depth of the superstructure member. The first step in determining the thermal forces on the substructures for a bridge with integral bents is to determine the point of no movement. After this point is determined, the relative displacement of any point along the superstructure to this point is simply equal to the distance to this point times the temperature range and times the coefficient of expansion.

With known displacement at the top and known boundary conditions at the top and bottom, the forces on the pier due to the temperature change can be calculated by using the displacement times the stiffness of the pier. The determination of the point of no movement is best demonstrated by the following example, which is adopted from Memo to Designers issued by California Department of Transportation [2]: Example 2. The size of the column is 1. Other assumptions are listed in the calculations. The calculation is done through a table. Please refer Figure 2. A pier should be designed to withstand the overturning, sliding forces applied from superstructure as well as the forces applied to substructures.

It also needs to be designed so that during an extreme event it will prevent the collapse of the structure but may sustain some damage. A pier as a structure component is subjected to combined forces of axial, bending, and shear. For a pier, the bending strength is dependent upon the axial force.

In the plastic hinge zone of a pier, the shear strength is also influenced by bending. In current design practice, the bridge designers are becoming increasingly aware of the adverse effects of earthquake. Therefore, ductility consideration has become a very important factor for bridge design. Failure due to scouring is also a common cause of failure of bridges.

In order to prevent this type of failure, the bridge designers need to work closely with the hydraulic engineers to determine adequate depths for the piers and provide proper protection measures. Small deflection theory is usually adequate for the analysis of beam-type members. For compression members, however, the second-order effect must be considered.

According to AASHTO LRFD [1], the second-order effect is defined as follows: The presence of compressive axial forces amplify both out-of-straightness of a component and the deformation due to non-tangential loads acting thereon, therefore increasing the eccentricity of the axial force with respect to the centerline of the component. The synergistic effect of this interaction is the apparent softening of the component, i.

To assess this effect accurately, a properly formulated large deflection nonlinear analysis can be performed. Discussions on this subject can be found in References [3,4]. However, it is impractical to expect practicing engineers to perform this type of sophisticated analysis on a regular basis. When the cross section dimensions of a compression member are small in comparison to its length, the member is said to be slender.

Whether or not a member can be considered slender is dependent on the magnitude of the slenderness ratio of the member. However, a lower value of K can be used if further analysis demonstrates that a lower value is applicable. Lu is defined as the clear distance between slabs, girders, or other members which is capable of providing lateral support for the compression member. For a detailed discussion of the K-factor, please refer to Chapter 8. Any detailed analysis should consider the influence of axial loads and variable moment of inertia on member stiffness and forces, and the effects of the duration of the loads.

It is an approximation of the effects of creep, so that when larger moments are induced by loads sustained over a long period of time, the creep deformation and associated curvature will also be increased. The following discussion provides an overview of some of the major criteria governing the design of compression members. Interaction Diagrams Flexural resistance of a concrete member is dependent upon the axial force acting on the member. Interaction diagrams are usually used as aids for the design of the compression members. Interaction diagrams for columns are usually created assuming a series of strain distributions, and computing the corresponding values of P and M.

Once enough points have been computed, the results are plotted to produce an interaction diagram. In an actual design, however, a few points on the diagrams can be easily obtained and can define the diagram rather closely. At this condition, the section has the highest moment capacity. However, in a seismic design, the shear is very important. In recent years, the research effort on shear strength evaluation for columns has been increased remarkably. The concrete shear capacity component and the angle of inclination of diagonal compressive stresses are functions of the shear stress on the concrete and the strain in the reinforcement on the flexural tension side of the member.

It is rather involved and hard to use. The recommendations are listed as follows. ATC [5] offers the following equations to address this interaction. With the end region of columns extending a distance from the critical section or sections not less than 1. The term ductility defines the ability of a structure and selected structural components to deform beyond elastic limits without excessive strength or stiffness degradation. This is a measure of the ability for a structure, or a component of a structure, to absorb energy.

The goal of seismic design is to limit the estimated maximum ductility demand to the ductility capacity of the structure during a seismic event. For concrete columns, the confinement of concrete must be provided to ensure a ductile column. Transverse Case 2 Max. Longitudinal — — Example 2. The typical section of the structure is shown in Figure 2. The concrete box girder is supported by a two-column bent and is subjected to HS20 loading.

The columns are pinned at the bottom of the columns. Therefore, only the loads at the top of columns are given here. Note that a load reduction factor of 5. Provide 9 30 longitudinal reinforcement. The reinforcement ratio is 1. The moment and axial force interaction diagram is generated and is shown in Figure 2. Following the procedure outlined in Section 2.

M Long M Comb. Pcy k Long Pcx k P k Critical Buckling Note: Column assumed to be unbraced against side sway. My Long Mx Comb. Column lateral reinforcement is calculated for two cases: 1 for applied shear and 2 for confinement. Typically, the confinement requirement governs. Apply Eq. For seismic analysis, the unreduced seismic shear forces should be compared with the shear forces due to plastic hinging of columns.

The smaller should be used. The plastic hinging analysis procedure is discussed elsewhere in this handbook and will not be repeated here. The lateral reinforcement for both columns are shown as follows. Steel and Composite Columns Steel columns are not as commonly used as concrete columns. Nevertheless, they are viable solutions for some special occasions, e.

Steel pipes or tubes filled with concrete known as composite columns Figure 2. Steel at the perimeter of the cross section provides stiffness and triaxial confinement, and the concrete core resists compression and prohibits local elastic buckling of the steel encasement. The toughness and ductility of composite columns makes them the preferred column type for earthquake-resistant structures in Japan.

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In China, the composite columns were first used in Beijing subway stations as early as Over the years, the composite columns have been used extensively in building structures as well as in bridges [6—9]. White, D. Galambos, T. Cai, S. Zhong, S. Louis, MO, December Lin International 3. They project above the superstructure and are seen from all directions by viewers and by users.

Towers give bridges their character and a unifying theme. They project a mnemonic image that people remember as a lasting impression of the bridge itself. As examples of the powerful imagery of towers, contrast the elegant art deco towers of the Golden Gate Bridge Figure 3. Or contrast the massive, rugged stone towers of the Brooklyn Bridge Figure 3. Towers can be defined as vertical steel or concrete structures projecting above the deck, supporting cables and carrying the forces to which the bridge is subjected to the ground.

By this definition, towers are used only for suspension bridges or for cable-stayed bridges, or hybrid suspension—cablestayed structures. The word pylon is sometimes used for the towers of cable—stayed bridges. Both pylon and tower have about the same meaning — a tall and narrow structure supporting itself and the roadway. In this chapter, the word tower will be used for both suspension and for cabled-stayed bridges, to avoid any confusion in terms.

Both suspension and cable-stayed bridges are supported by abutments or piers at the point where these structures transition to the approach roadway or the approach structure. Abutments are discussed in Chapter 4. Piers and columns that support the superstructure for other forms of bridge structures such as girders, trusses, or arches, usually do not project above the deck. Piers and columns are discussed in Chapter 2. The famous bridges noted above were opened in , , , and , respectively, and, if well maintained, could continue to serve for another years.

Bridge engineers will not design structures like these today because of changing technologies. Courtesy of Charles Seim. Robust designs, durable materials, provisions for access for inspection and maintenance, and a well-executed maintenance program will help ensure a long life. The towers must perform these functions in a reliable, serviceable, aesthetic, and economical manner for the life of the bridge, as towers, unlike other bridge components, cannot be replaced. Without reliability, towers may become unsafe and the life of the entire bridge could be shortened.

Without serviceability being designed into the structure, which means that it is designed for access and ease of maintenance, the bridge will not provide continuing long service to the user. The public demands that long-span bridges be attractive, aesthetic statements with long lives, so as not to be wasteful of public funds. The towers reveal the character or motif of the bridge. The bridges used as examples in the introduction are good illustrations of the image of the structure as revealed by the towers.

Indeed, perhaps they are famous because of their towers. Most people visualize the character of the Brooklyn Bridge by the gothic, arched, masonry towers alone. Seim [7] measured the ratios of the visible components of the towers of the latter two bridges and found important, but subtle, diminution of these ratios with height above the tower base. The proportions of the towers for any new long-span bridge should be carefully shaped and designed to give the entire bridge a strong — even robust — graceful, and soaring visual image. The aesthetics of the array of cables many times are of secondary importance to the aesthetics of the towers.

However, the array or form of the cables must be considered in the overall aesthetic and structural evaluation of the bridge. Main cables of suspension bridges always drape in a parabolic curve that most people instinctively enjoy. The large diameter of the cables makes them stand out as an important contribution to the overall visual impression as the supporting element of the roadway.

The cables of cable-stayed bridges are usually of small diameter and do not stand out visually as strongly as do the cables of suspension bridges. However, the array of the stays, such as harp, fan, radiating star, or others, should be considered in context with the tower form.

The separated, parallel cables of the harp form, for example, will not be as obtrusive to the towers as will other arrangements. However, the harp cable form may not be appropriate for very long spans or for certain tower shapes. The cables and the towers should be considered together as a visual system. Billington [2] presents an overview of the importance of the role of aesthetics in the history of the development of modern bridge design. Leonhardt [5] presents many examples of completed bridges showing various tower shapes and cable arrangements for both suspension and cablestayed bridges.

Conceptual Design Perhaps the most important step in the design of a new bridge is the design concept for the structure that ultimately will be developed into a final design and then constructed. The cost, appearance, and reliability and serviceability of the facility will all be determined, for good or for ill, by the conceptual design of the structure. The cost can be increased, sometimes significantly, by a concept that is very difficult to erect.

Once constructed, the structure will always be there for users to admire — or to criticize. The user ultimately pays for the cost of the facility and also usually pays for the cost of maintaining the structure. Gimsing [4] treats the concept design issues of both cablestayed and suspension bridges very extensively and presents examples to help guide designers. A proper bridge design that considers the four functions of reliability, serviceability, appearance, and cost together with an erectable scheme that requires low maintenance, is the ideal that the design concept should meet.

A recent trend is to employ an architect as part of the design team. Architects may view a structure in a manner different from engineers, and their roles in the project are not the same. The role of the engineer is to be involved in all four functions and, most importantly, to take responsibility for the structural adequacy of the bridge. The role of the architect generally only involves the function of aesthetics.

Their roles overlap in achieving aesthetics, which may also affect the economy of the structure. Since both engineers and architects have as a common objective an elegant and economical bridge, there should be cooperation and respect between them. Towers, as the most visible component of the bridge, seem to be a target for this type of conflict. Each professional must understand that these differences in viewpoints will occur and must be resolved for a successful and fruitful union between the two disciplines. Materials Until the s, steel was the predominant material used for the towers of both cable-stayed and suspension bridges.

The towers were often rectangular in elevation with a cross-sectional shape of rectangular, cruciform, tee, or a similar shape easily fabricated in steel.

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Since the soil bearing pressures are less than the allowable soil bearing pressure, the soil bearing stability is OK. The program will then focus on the design requirements and present several examples covering a range of conditions frequently encountered in this type of building. The very long span m Humber Bridge, England, , used uniformly spaced, multi-strut concrete towers Figure 3. Since the minimum soil bearing pressure under the footing is in compression, the tension at the footing top is not the case. The two shafts of cable-stayed bridges can be inclined inward toward each other to form a modified A-frame, similar to the Luling Bridge towers Figure 3. Emphasis will be placed on exploring the considerations that influence bridge configurations.

Examples of suspension bridge steel tower design are the plain, rectangular steel towers for the two Delaware Memorial Bridges; the first constructed in and the parallel one in Figure 3. An example of a cable-stayed bridge that is an exception to the rectangular tower form is the modified A-frame, weathering-steel towers of the Luling Bridge near New Orleans, Figure 3.

The cross sections of steel towers are usually designed as a series of adjoining cells formed by shop-welding steel plates together in units from 6 to 12 m long. The steel towers for a suspension bridge, and for cable-stayed bridges with stays passing over the top of the tower in saddles, must be designed for the concentrated load from the saddles.

The steel cellular towers for a cable-stayed bridge with cables framing in the towers must be designed for the local forces from the numerous anchorages of the cables. Since the s, reinforced concrete has been used in many forms with rectangular and other compact cross sections. Concrete towers are usually designed as hollow shafts to save weight and to reduce the amount of concrete and reinforcing bars required.

Courtesy of D. Luling Bridge, New Orleans, Louisiana. Steel towers will generally be more flexible and more ductile and can be erected in less time than concrete towers.

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Steel towers will require periodic maintenance painting, although weathering steel can be used for nonmarine environments. For pedestrian bridges, timber towers may be economical and aesthetically pleasing. During the conceptual design phase of the bridge, approximate construction costs of both materials need to be developed and compared.

If life-cycle cost is important, then maintenance operations and the frequencies of those operations need to be evaluated and compared, usually by a presentworth evaluation. Stay cables can also be arranged in a variety of forms. To this value must be added the structural depth of the girder and the clearance to the foundation for determining the approximate total tower height.

The final height of the towers will be determined during the final design phase. The simplest tower form is a single shaft, usually vertical Figure 3. Occasionally, the single tower is inclined longitudinally. Stay cables can be arranged in a single plane to align with the tower or be splayed outward to connect with longitudinal edge beams. This form is usually employed for bridges with two-way traffic, to avoid splitting a one-way traffic flow. For roadways on curves, the single tower may be offset to the outside of the convex curve of the roadway and inclined transversely to support the curving deck more effectively.

Two vertical shafts straddling the roadway with or without cross struts above the roadway form a simple tower and are used with two planes of cables Figure 3. Courtesy of T. Lin International. This allows the cables to be aligned in a vertical plane and to be attached to the girder, which can pass continuously through the towers as used for the Talmadge Bridge, Georgia Figure 3.

A horizontal strut is used between the tower shafts, offset to stabilize the towers. The two shafts of cable-stayed bridges can be inclined inward toward each other to form a modified A-frame, similar to the Luling Bridge towers Figure 3. The two planes of stay cables are inclined outward, producing a more desirable compression component across the deck support system. The form of the towers of cable-stayed bridge below the roadway is also important for both aesthetics and costs.

The shafts of the towers for a modified A-frame can be carried down to the foundations at the same slope as above the roadway, particularly for sites with low clearance. However, at high clearance locations, if the shafts of the towers for a full A-frame or for an inverted Y-frame are carried down to the foundations at the same slope as above the roadway, the foundations may become very wide and costly. The aesthetic proportions also may be affected adversely.

Projecting the A-frame shafts downward vertically can give an awkward appearance. Sometimes the lower shafts are inclined inward under the roadway producing a modified diamond Figure 3. For very high roadways, the inward inclination can form a full diamond or a double diamond as in the Baytown Bridge, Texas Figure 3.

For very long spans requiring tall towers, the A-frame can be extended with a single vertical shaft forming an inverted Y shape Figure 3. Yang Pu Bridge, China Figure 3. This form is very effective for very long spans where additional tower height is required and the inclined legs add stiffness and frame action for wind resistance. The number of shafts or columns within the towers of cable-stayed bridges can vary from one to four. Three-shaft towers generally are not used for cable-stayed bridges except for very wide decks.

Four-shaft towers can be used best to support two separate structures instead of a single wide deck. The towers could share a common foundation or each have its own foundation depending on the cost. Suspension bridges can have from one to four cables depending on structural or architectural needs.

Only a few single-cable suspension bridges have been designed with an A or inverted Y form of towers. Usually towers of suspension bridges follow a more traditional design using two vertical shafts and two planes of cables, as illustrated by the steel towers for the Delaware Memorial Bridges see Figure 3.

However, concrete towers have recently proved to be economical for some bridges. The very long span m Humber Bridge, England, , used uniformly spaced, multi-strut concrete towers Figure 3. The crossing of the Great Belt seaway in Denmark Figure 3. For conceptual designs, the height of suspension bridge towers above the deck depend on the sag-to-span ratio which can vary from about to A good preliminary value is about To this value must be added the structural depth of the deck and the clearance to the foundations to obtain the approximate total tower height.

Some form of strut is usually required for suspension bridges as the large cables carry lateral wind and seismic loads to the tops of the tower shafts, which then need to be braced against each other with cross struts to form a tower-frame action.

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The cost of unusual tower designs can be difficult to estimate and can add significant cost to the project. For important bridges and for long-span cable-supported bridge projects, special design criteria may have to be developed by the designer. The special design criteria may have to be also developed in cooperation with the owners of the facility to include their operations and maintenance requirements and their bridge-performance expectations after large natural events such as earthquakes.

Troitsky [8], Podolny and Salzi [6], and Walter [9] present detailed design theory for cable-stayed bridges. Design methodology for the towers should follow the same practice as the design methodology for the entire bridge. The towers should be part of a global analysis in which the entire structure is treated as a whole.

From the global analyses, the towers can be modeled as a substructure unit with forces and deformations imposed as boundary conditions. Detailed structural analyses form the basis for the final design of the tower and its components and connections. Both cabled-stayed and suspension bridges are highly indeterminate and require careful analysis in at least a geometric nonlinear program. The towers, as well as the entire structure, must be analyzed, designed, and checked for the controlling loading cases.

The weight of the superstructure, including the self-weight of the towers, is obtained in the design process utilizing the unit weights of the materials used in the superstructure and distributed to the tower in accordance with a structural analysis of the completed structure or by the erection equipment during the construction phases.

Loads from traffic using the bridge such as trains, transit, trucks, or pedestrians are usually prescribed in design codes and specifications or by the owners of the facility. Courtesy of Ben C. Gerwick, Inc. These are all gravity effects that act downward on the structure, but will induce both vertical and horizontal forces on the towers. A current trend for spanning wide widths of waterways is to design multispan bridges linked together to form a long, continuous structure.

With ordinary tower designs, the multispan cablestayed girders will deflect excessively under live loads as the towers will not be sufficiently stiffened by the cable stays anchored within the flexible adjacent spans. For multispan suspension bridges with ordinary tower designs, the same excessive live-load deflection can also occur. Towers for multispan cable-supported bridges must be designed to be sufficiently rigid to control live-load deflections. Towers are also subject to temperature-induced displacements, both from the superstructure and cable framing into the towers, and from the temperature-induced movement of the tower itself.

Towers can expand and contract differentially along the tower height from the sun shining on them from morning until sunset. These temperature effects can cause deflection and torsional twisting along the height of the tower. Wind blowing on the towers as a bluff shape induces forces and displacements in the tower.

Forces will be induced into the cables by the pressure of wind on the superstructure, as well as by the wind forces on the cables themselves. These additional forces will be carried to the towers. For long-span bridges and for locations with known high wind speeds, wind should be treated as a dynamic loading. Under certain wind flows, the wind can also excite the tower itself, particularly if the tower is designed with light steel components.

In the rare instances in which wind-induced excitation of the tower does occur, appropriate changes in the cross section of the tower can be made or a faring can be added to change the dynamic characteristics of the tower. The seismic excitation should be treated as dynamic inertia loadings inducing response within the structure by exciting the vibrational modes of the towers. Induced seismic forces and displacement can control the design of towers in locations with high seismic activity. For locations with lower seismic activity, the tower design should be checked at least for code-prescribed seismic loadings.

A full analysis of the structure will reveal all of the forces, displacements, and other design requirements for all loading cases for the final tower design. A cable produces a large vertical force and smaller, but important, transverse and longitudinal forces from temperature, wind, earthquake, or from the unbalanced cable forces between main and side spans.

These forces are transmitted through the cable saddle anchorage at each cable location to the top of the tower. The towers and the permanent saddle anchorages must be designed to resist these cable forces. The erection of a suspension bridge must be analyzed and the sequence shown on the construction plans. To induce the correct loading into the cables of the side span, the erection sequence usually requires that the saddles be displaced toward the side spans. This is usually accomplished for short spans by displacing the tops of the towers by pulling with heavy cables.

For long spans, the saddles can be displaced temporarily on rollers. As the stiffening deck elements are being erected into position and the cable begins to take loads, the towers or saddles are gradually brought into final vertical alignment. After the erection of the stiffening deck elements are completed, the saddles are permanently fastened into position to take the unbalanced cable loads from the center and the side spans.

At the deck level, other forces may be imposed on the tower from the box girder or stiffening truss carrying the roadway. These forces depend on the structural framing of the connection of the deck and tower. Traditional suspension bridge designs usually terminate the stiffening truss or box girder at the towers, which produces transverse, and longitudinal, forces on the tower at this point.

Contemporary suspension bridge designs usually provide for passing a box girder continuously through the tower opening which may produce transverse forces but not longitudinal forces. For this arrangement, the longitudinal forces must be carried by the girder to the abutments. The most critical area of the tower design is the tower-to-foundation connection. Both shear forces and moments are maximum at this point. Anchor bolts are generally used at the base of steel towers. The bolts must be proportioned to transfer the loads from the tower to the bolts. The bolts must be deeply embedded in the concrete footing block to transfer their loads to the footing reinforcement.

Providing good drainage for the rainwater running down the tower shafts will increase the life of the steel paint system at the tower base and provide some protection to the anchor bolts. Concrete towers must be joined to the foundations with full shear and moment connections. Lapped reinforcing bars splices are usually avoided as the lapping tends to congest the connections, the strength of the bars cannot be developed, and lapped splices cannot be used for high seismic areas.

Using compact mechanical or welded splices will result in less congestion with easier placement of concrete around the reinforcement and a more robust tower-to-footing connection. Careful coordination between the foundation designers and tower designers is required to achieve a stable, efficient, and reliable connection. The cable arrangements for cable-stayed bridges are many and varied. Cables terminating in the tower can pass completely through the tower cross section and then anchor on the far side of the tower.

This method of anchoring produces compression in the tower cross section at these anchorage points. Cables can also be terminated at anchors within the walls of the tower, producing tension in the tower cross section at the anchorage points. These tension forces require special designs to provide reliable, long-life support for the cables.

Just as for suspension bridges, the erection of cable-stayed bridges must be analyzed and the sequence shown on the construction plans. The girders, as they are erected outward from the towers, are very vulnerable. The critical erection sequence is just before closing the two arms of the girders at the center of the span. High winds can displace the arms and torque the towers, and heavy construction equipment can load the arms without benefit of girder continuity to distribute the loads. Cells must be large enough to allow welders and welding equipment, and if the steel is to be painted, painters and cleaning and painting equipment inside each cell.

The steel tower components are transported to the bridge site and then erected by cranes and bolted together with high-strength bolts. The contractor should use a method of tensioning the high-strength bolts to give constant results and achieve the required tension. Occasionally, field welding is used, but this presents difficulties in holding the component rigidly in position while the weld is completed.

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Field welding can be difficult to control in poor weather conditions to achieve ductile welds, particularly for vertical and overhead welds. Full-penetration welds require backup bars that must be removed carefully if the weld is subject to fatigue loading. Towers constructed of reinforced concrete are usually cast in forms that are removed and reused, or jumped to the next level.

Concrete placing heights are usually restricted to about 6 to 12 m to limit form pressure from the freshly placed concrete. Reinforcing bar cages are usually preassembled on the ground or on a work barge, and lifted into position by crane. This requires the main loadcarrying reinforcing bars to be spliced with each lift.

Lapped splices are the easiest to make, but are not allowed in seismic areas. Slip forming is an alternative method that uses forms that are pulled slowly upward, reinforcing bars positioned and the concrete placed in one continuous operation around the clock until the tower is completed. Slip forming can be economical, particularly for constant-cross-section towers.

Some changes in cross section geometry can be accommodated. For shorter spans, precast concrete segments can be stacked together and steel tendons tensioned to form the towers. Tower designers should consider the method of erection that contractors may use in constructing the towers.

Often the design can reduce construction costs by incorporating more easily fabricated and assembled steel components or assembled reinforcing bar cages and tower shapes that are easily formed. Of course, the tower design cannot be compromised just to lower erection costs. Some engineers and many architects design towers that are not vertical but are angled longitudinally toward or away from the main span. This can be done if such a design can be justified structurally and aesthetically, and the extra cost can be covered within the project budget. The difficulties of the design of longitudinally inclined towers must be carefully considered as well as the more expensive and slower erection, which will create additional costs.

Many towers of cable-stayed bridges have legs sloped toward each other to form an A, an inverted Y, a diamond, or similar shapes. These are not as difficult to construct as the longitudinally inclined tower design. The sloping concrete forms can be supported by vertical temporary supports and cross struts that tie the concrete forms together. This arrangement braces the partly cast concrete tower legs against each other for support. Some of the concrete form supports for the doublediamond towers of the Baytown Bridge are visible in Figure 3.

Both of these secondary effects must be adjusted by jacking the legs apart by a calculated amount of force or displacement to release the locked-in bending stresses. If the amount of secondary stress is small, then cambering the leg to compensate for the deflection and adding material to lower the induced stress can be used.

The jacking procedure adds cost but is an essential step in the tower erection. Tower construction usually requires special equipment to erect steel components or concrete forms to the extreme height of the tower. Suspension bridges and some cable-stayed bridges require cable saddles to be erected on the tower tops.

Floating cranes rarely have the capacity to reach to the heights of towers designed for long spans. Tower cranes, connected to the tower as it is erected, can be employed for most tower designs and are a good choice for handling steel forms for the erection of concrete towers.

Bridge Engineering: Seismic Design - Google Libros

A tower crane used to jump the forms and raise materials can be seen in Figure 3. Occasionally, vertical traveling cranes are used to erect steel towers by pulling themselves up the face of the tower following the erection of each new tower component. The erection sequence for a suspension bridge may require that the towers be pulled by cables from the vertical toward the sides spans or that the cable saddles be placed on rollers and displaced toward the side spans on temporary supports.

The tower restraints are gradually released or the rollers pushed toward their final position as the erection of the deck element nears completion. This operation is usually required to induce the design forces into the cables in the side spans. The cable saddles then are permanently anchored to the towers.

Because the tower erection must be done in stages, each stage must be checked for stability and for stresses and deflections. The specifications should require the tower erection to be checked by an engineer, employed by the contractor, for stability and safety at each erection stage. The construction specifications should also require the tower erection stages to be submitted to the design engineer for an evaluation. This evaluation should be thorough enough to determine if the proposed tower erection staging will meet the intent of the original design, or if it needs to be modified to bring the completed tower into compliance.

Being the most visible elements in a bridge, they give the bridge, for good or for ill, its character, its motif, and its identifying aesthetic impression. Towers usually form structural portals through which people pass as they travel from one point to another. Of themselves, towers form an aesthetic structural statement.

Towers are the most critical structural element in the bridge as their function is to carry the forces imposed on the bridge to the ground. Unlike most other bridge components, they cannot be replaced during the life of the bridge. Towers must fulfill their function in a reliable, serviceable, economical, and aesthetic manner for the entire life of the bridge. Towers must also be practicable to erect without extraordinary expense. Practicable tower shapes for cable-stayed bridges are many and varied. Towers can have one or several legs or shafts arrayed from vertical to inclined and forming A- or inverted Y-shaped frames.

Suspension bridge towers are usually vertical, with two shafts connected with one or several struts. The conceptual design is the most important phase in the design of a long-span bridge. This phase sets, among other items, the span length, type of deck system, and the materials and shape of the towers. It also determines the aesthetic, economics, and constructibility of the bridge. A conceptual erection scheme should be developed during this phase to ensure that the bridge can be economically constructed. A practical erection method should be developed during this phase and shown on the construction drawings.

If an unusual tower design is used, the tower erection should also be shown. The specifications should allow the contractor to employ an alternative method of erection, provided that the method is designed by an engineer and submitted to the design engineer for review. It is essential that the design engineer follow the project into the construction stages. The designer must understand each erection step that is submitted by the contractor in accordance with the specifications, to ensure the construction complies with the design documents. Only by this means are owners assured that the serviceability and reliability that they are paying for are actually achieved in construction.

The successful design of a cable-stayed or a suspension bridge involves many factors and decisions that must be made during the planning, design, and construction phases of the project. Towers play an important role in that successful execution. The final judgment of a successful project is made by the people who use the facility and pay for its construction, maintenance, and long-life service to society. References 1. Billington, D. Cerver, F. Gimsing, N. Leonhardt, F. Podolny, W. Seim, C. Troitsky, M.

Walter, R. Although there are numerous types of abutments and the abutments for the important bridges may be extremely complicated, the analysis principles and design methods are very similar. In this chapter the topics related to the design of conventional highway bridge abutments are discussed and a design example is illustrated. Unlike the bridge abutment, the earth-retaining structures are mainly designed for sustaining lateral earth pressures. Those structures have been widely used in highway construction. In this chapter several types of retaining structures are presented and a design example is also given.

For the open-end abutment, there are slopes between the bridge abutment face and the edge of the roadway or river canal that the bridge overcrosses. Those slopes provide a wide open area for the traffic flows or water flows under the bridge. Also, future widening of the roadway or water flow canal under the bridge by adjusting the slope ratios is easier. However, the existence of slopes usually requires longer bridge spans and some extra earthwork. This may result in an increase in the bridge construction cost.

The closed-end abutment is usually constructed close to the edge of the roadways or water canals. Because of the vertical clearance requirements and the restrictions of construction right of way, there are no slopes allowed to be constructed between the bridge abutment face and the edge of roadways or water canals, and high abutment walls must be constructed.

Since there is no room or only a little room between the abutment and the edge of traffic or water flow, it is very difficult to do the future widening to the roadways and water flow under the bridge. Also, the high abutment walls and larger backfill volume often result in higher abutment construction costs and more settlement of road approaches than for the open-end abutment.

Generally, the open-end abutments are more economical, adaptable, and attractive than the closed-end abutments. However, bridges with closed-end abutments have been widely constructed in urban areas and for rail transportation systems because of the right-of-way restriction and the large scale of the live load for trains, which usually results in shorter bridge spans.

The monolithic abutment is monolithically constructed with the bridge superstructure. There is no relative displacement allowed between the bridge superstructure and abutment. All the superstructure forces at the bridge ends are transferred to the abutment stem and then to the abutment backfill soil and footings. The advantages of this type of abutment are its initial lower construction cost and its immediate engagement of backfill soil that absorbs the energy when the bridge is subjected to transitional movement.

However, the passive soil pressure induced by the backfill soil could result in a difficult-to-design abutment stem, and higher maintenance cost might be expected. In practice, this type of abutment is mainly constructed for short bridges. The seat-type abutment is constructed separately from the bridge superstructure.

The bridge superstructure seats on the abutment stem through bearing pads, rock bearings, or other devices. This type of abutment allows the bridge designer to control the superstructure forces that are to be transferred to the abutment stem and backfill soil. By adjusting the devices between the bridge superstructure and abutment, the bridge displacement can be controlled.

This type of abutment may have a short stem or high stem, as shown in Figure 4. For a short-stem abutment, the abutment stiffness usually is much larger than the connection devices between the superstructure and the abutment. Therefore, those devices can be treated as boundary conditions in the bridge analysis. Comparatively, the high stem abutment may be subject to significant displacement under relatively less force. The stiffness of the high stem abutment and the response of the surrounding soil may have to be considered in the bridge analysis.

The availability of the displacement of connection devices, the allowance of the superstructure shrinkage, and concrete shortening make this type of abutment widely selected for the long bridge constructions, especially for prestressed concrete bridges and steel bridges. However, bridge design practice shows that the relative weak connection devices between the superstructure and the abutment usually require the adjacent columns to be specially designed.

Although the seat-type abutment has relatively higher initial construction cost than the monolithic abutment, its maintenance cost is relatively lower. Abutment Type Selection The selection of an abutment type needs to consider all available information and bridge design requirements. Those may include bridge geometry, roadway and riverbank requirements, geotechnical and right-of-way restrictions, aesthetic requirements, economic considerations, etc. Knowledge of the advantages and disadvantages for the different types of abutments will greatly benefit the bridge designer in choosing the right type of abutment for the bridge structure from the beginning stage of the bridge design.

An abutment should be designed so as to withstand damage from the Earth pressure, the gravity loads of the bridge superstructure and abutment, live load on the superstructure or the approach fill, wind loads, and the transitional loads transferred through the connections between the superstructure and the abutment. Any possible combinations of those forces, which produce the most severe condition of loading, should be investigated in abutment design. Nonseismic design loads at service level and their combinations are shown in Table 4.

It is easy to obtain the factored abutment design loads and load combinations by multiplying the load factors to the loads at service levels. Under seismic loading, the abutment may be designed at no support loss to the bridge superstructure while the abutment may suffer some damages during a major earthquake.

However, due to the uncertainties in evaluating the soil response to static, cycling, dynamic, and seismic loading, the service load design method is usually used for abutment stability checks and the load factor method is used for the design of abutment components. The load and load combinations listed in Table 4. For the abutment with spread footings under service load, the factor of safety to resist sliding should be greater than 1.

For the abutment with pile support, the piles have to be designed to resist the forces that cause abutment sliding, overturning, and bearing failure. The pile design may utilize either the service load design method or the load factor design method. The abutment deep shear failure also needs to be studied in abutment design. Usually, the potential of this kind of failure is pointed out in the geotechnical report to the bridge designers.

Deep pilings or relocating the abutment may be used to avoid this kind of failure. Abutment stability damage during an earthquake is mainly caused by foundation failure due to excessive ground deformation or the loss of bearing capacities of the foundation soil. Those foundation failures result in the abutment suffering tilting, sliding, settling, and overturning.

The foundation soil failure usually occurs because of poor soil conditions, such as soft soil, and the existence of a high water table. In order to avoid these kinds of soil failures during an earthquake, borrowing backfill soil, pile foundations, a high degree of soil compaction, pervious materials, and drainage systems may be considered in the design. Abutment component damage is generally caused by excessive soil pressure, which is mobilized by the large relative displacement between the abutment and its backfilled soil.

Those excessive pressures may cause severe damage to abutment components such as abutment back walls and abutment wingwalls.