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There are several methods of connecting normal stringer bridges to the substructures. In most cases, these connections are designed to transmit the lateral seismic forces from the superstructure to the substructure. It is also possible to make the connection integral.

In most states, the connection of the superstructure to the substructure is detailed as a pinned connection. Integral connections are also specified, but are not as common.

There are other options for seismic restraint such as cable restrainers and seismic isolation devices. These methods are also acceptable; however they are not included in this document.

6.2.1 Keeper Blocks

Keeper blocks consist of concrete keys that are placed between two interior beams to transmit lateral forces from the superstructure to the substructure. Keeper blocks are often the most cost effective means of restraining a bridge for seismic events.

Bridges should be designed with only one keeper block per superstructure unit.

Keeper blocks are usually only used for lateral seismic forces. Longitudinal seismic forces can be resisted by the abutment backwall, or anchor rods.

Concrete keeper blocks have very low ductility. If two blocks were detailed, it is likely that one would carry all the seismic demand. Distribution of seismic forces to other keeper assemblies would most likely only occur after failure of the first keeper.

If keeper blocks are precast, the connection of keeper blocks can be made using several of the methods outlined in this document.

Consideration should be given for casting keeper blocks in place.

Keepers can be installed using grouted mechanical splices, or can be cast integral with the precast substructure component.

See Section 4.3.4.

6.2.2 Pilasters

Pilaster are used on adjacent box beam bridges near the fascia beam ends. They resist lateral seismic forces.

The design of pilasters is similar to keeper blocks. Each pilaster must be capable of resisting the entire lateral seismic force at each substructure unit, since only one pilaster will engage the superstructure at a time.

Pilasters are usually only used on adjacent box beam bridges since there is no gap between adjacent beams. On stringer bridges, keeper blocks are probably more cost effective, since only one keeper is required to resist lateral seismic forces in both directions.

The commentary from Section 6.2.1 is also applicable to this section. Also, see Section 4.3.4 for information on pilasters.

6.2.3 Abutment Backwall

Longitudinal seismic forces can be resisted by the abutment backwall. The design of abutment backwalls for seismic forces is similar to keeper blocks.

Designers should verify that the distance between the backwall and the beam/slab is sufficient for thermal movement, but small enough so that the beams do not slide off the substructures.

The design of seismic restraining systems is based on limited and repairable damage. Using backwalls for restraint will inevitably result in a structure that has shifted longitudinally during the seismic event. The structure may need to be jacked back into position after seismic events.

One of the most important aspects of seismic design is to prevent superstructures from sliding off foundations. This issue becomes more pronounced on multiple span bridges where all joints between spans are assumed to be closed in one direction.

6.2.4 Anchor Rods

The design of anchor rods for lateral load should take into account the bending capacity of the rod, edge distance to the concrete foundation, internal reinforcing around the embedded portion, strength of the concrete, and group action of the rods.

Anchor rods should be designed to be ductile. The use of high strength heat treated rods is discouraged due to low ductility.

The AASHTO specifications do not address the design of embedded anchors loaded in shear. Designing for the shear capacity of the rod is not acceptable. The rods tend to fail in combined bending and crushing of the concrete around the rod. The American Concrete Institute publication “Building Code Requirements for Structural Concrete (ACI 318-02) is recommended.

During a seismic event, it is inevitable that only a percentage of the rods will initially see load due to construction tolerances. Ductility in the rods will ensure that all rods will work

The embedded portion of the rod shall be properly reinforced in order to prevent brittle fractures of the surrounding concrete.

Material for anchor rods should be ASTM F1554, and should be either threaded (with nuts) or swaged on the embedded portion of the rod. The design yield strength of this material may be specified as36ksi (250MPa), 55ksi (380MPa), or 105ksi (725MPa), depending on the design. The yield strength should be given in the specifications or on the plans.

together to resist seismic forces.

The anchor rods should normally be surrounded by lateral reinforcing steel near the surface of the concrete. This will allow lateral forces to be resisted after the initial cracking of the concrete.

This material is specifically designed for anchor rod applications. Other materials have been used, but do not offer the economies of ASTM F1554. The designer should offer options of swaging or threading the anchor as different suppliers supply one or both of these options.

6.2.5 Integral Connections

Superstructure can be connected to substructures using integral moment connections. In most cases, this connection will be made with a cast-in-place closure pour or by using grouted mechanical splices.

The most common form of integral connection is between beams and abutments in stringer bridges. In these cases, the end diaphragm is usually cast in place between the beams due to the complexity of the shapes.

Integral connections have successfully been made between beams and abutments using grouted mechanical splices cast into the beam- ends.