At expansion bearings and at other points where necessary, expansion joints should be installed in the floor system to permit it to move when the span deflects or changes length. If apron plates are used, they should be designed to bridge the joint and prevent accumulation of dirt on the bridge seats. Preferably, the apron plates should be connected to the end floorbeam. For amount of movement to provide for, see Art. 11.21. However, jointless bridges have many advantages and should be considered where possible.
Short-span bridges usually have expansion joints at one or both abutments. Longer-span structures usually have such joints at pier or off-pier hinges. Although these joints may relieve some forces caused by restraint of thermal movements, the joints have been a major source of bridge deterioration and poor ridability. The LRFD specifications of the American Association of State Highway and Transportation Officials (AASHTO) acknowledges that Completely effective joint seals have yet to be developed for some situations. . . . To provide more durable bridges, the goal in design should be to minimize the number of joints.
One way to do this for multiple-span bridges is to use continuous beams or girders. Another, more general, alternative is to eliminate joints completely.
Some states permit jointless, or integral, steel-girder bridges with spans up to about 400 ft or longer. With this type of construction, restriction of the change in bridge length due to maximum temperature change induces longitudinal forces at fixed piers and abutments. This must be taken into account in design of substructures. Experience has shown, however, that the effect of these forces on superstructure design is negligible and that, with proper detailing, substructure design is relatively unaffected.
Tennessee is a major user of jointless steel-girder bridges for spans of 400 ft or more.
Through experience, they have developed details that are able to resist thermal forces and movements (Fig. 11.15), thus eliminating leaking bridge joints. Tennessee has successfully completed a two-span continuous bridge 473 ft long with integral abutments at each end.
The AASHTO Standard Specifications for Highway Bridges specifies that movement calculations for integral abutments take into account not only temperature changes but also creep of the concrete deck and pavements. The abutments should be designed to sustain the forces generated by restraint to thermal movements developed by the pressures of fills behind the abutments. (The specifications prohibit use of integral abutments constructed on spread footings keyed into rock.) Approach slabs should be connected directly to abutments and wingwalls, to prevent intrusion of water behind the abutments. Nevertheless, means should be provided for draining away water that may get entrapped.
The AASHTO specifications also require that details comply with recommendations in Technical Advisory T5140.13, Federal Highway Administration. These recommendations include the following:
Steel bridges with an overall length less than 300 ft should be constructed continuously and, if unrestained, have integral abutments. (An unrestrained abutment is one that is free to rotate, such as a stub abutment on one row of piles or an abutment hinged at the footing.˜ Structure Memorandum, State of Tennessee.) Greater lengths may be used when experience dictates such designs are satisfactory.
In the area immediately behind integral abutments, traffic will compact the fill where it is partly disturbed by abutment movement, if not prevented from doing so. For the purpose, approach slabs should be provided to span this area. The span length should be at least equal to a minimum of 4 ft for bearing on the soil plus the depth of the abutment (based on the assumption of a 1:1 slope from the bottom of the rear face of the abutment.) The Advisory suggests that a practical slab length is 14 ft.
The Advisory recommends that approach slabs be designed for live-load bending moments
as indicated for the case of main reinforcement parallel to traffic in Table 11.27, with
S slab length minus 2 ft.
The Advisory also recommends that the slabs be anchored by steel reinforcement to the superstructure. In addition, positive anchorage should be provided between integral abutments and the superstructure. Figure 11.15 is an example of such construction.
The Advisory calls attention to a detail used by North Dakota that it considers desirable.
To accommodate pavement growth and bridge movement, the state inserts a roadway expansion joint 50 ft away from the bridge.
Properly detailed and constructed, jointless bridges eliminate the maintenance that would
be required if expansion joints were used, especially corrosion and deterioration of substruc ture and superstructure because of leakage. Also, jointless bridges provide better ridability. As a bonus, the cost of joints is eliminated. The LRFD Specification encourages the use of jointless bridges to improve rideability of the roadway surface, but provides minimal design guidance. However, comprehensive design and detailing provisions for bridges with integral abutments are available from the American Iron and Steel Institute (AISI), as Integral Abutments for Steel Bridges. A design procedure for the piles supporting the integral abutment is included.
Where foundation conditions are not considered acceptable for integral abutment bridges, semi-integral abutments are acceptable, within the same length limitations. A semi-integral abutment is virtually identical to an integral abutment, except that there is a horizontal joint separating the backwall and beam from the pile footing. Thus, bridges with battered piles or rock foundations are candidates for semi-integral abutments. Semi-integral abutments are also used effectively in bridge rehabilitations to eliminate joints.