Section 5: Stage Construction
Anchor: #i1002041Overview
Stage construction is required when traffic must be diverted onto a portion of an existing bridge while part of the new structure is built, then moved over for reconstruction of the first part. This section is provided to give the bridge planner/designer some guidelines that generally apply for all staged construction. Topics include existing structure removal, new substructure, new superstructure, and temporary railing.
Anchor: #i1002051Existing Structure Removal
Texas Standard Specifications, Item 496 “Removing Old Structures,” outlines requirements for the removal of existing structure.
The partial removal of the existing structure begins with the cutting and removal of the slab. The location of the cut is called the breakback. The approximate location of the breakback is determined through coordination with the traffic and highway engineer and is based on lane width requirements of both the new structure and the partial structure to remain in place. The exact breakback point should be determined by the bridge designer and is based on the structural capacity of the existing structure.
The breakback is generally located over a beam and must be supported by a stable substructure. After the slab is cut and removed, the beams are removed and the substructure, or a portion thereof, is demolished. If necessary, footings are removed and drilled shafts and piles are cut and removed to a distance a minimum of 2 ft., or as specified in the plans, below the proposed ground.
Anchor: #i1002071New Substructure
The following are guidelines for the design of the new substructure.
Foundations. Consideration must be given to the room required for drilled shaft and piling installation. Both drilled shafts and piling require a 1 ft. minimum horizontal clearance from edge of foundation to the obstruction. Ideally, there should be no vertical obstruction above either type of foundation. Special drilled shaft rigs are now available that can work with as little as 6 ft. of headroom. This equipment is quite expensive, and placement of reinforcing steel and concrete is very difficult. Contact the TxDOT Bridge Division - Geotechnical Branch for information on the practicality and cost of these types of shafts.
The only way to install piling in limited headroom is to drive and splice short sections of steel piling. This process is seldom practical or cost effective, and should be avoided.
If possible, avoid the location of the existing foundations that remain. For widenings, foundations should be of similar type as those remaining in use.
Abutments. At the stage construction joint, it is difficult to leave reinforcing steel projecting from the abutments for splicing because of the conflicts with the temporary shoring that must retain the fill. Instead, locate foundations (drilled shafts or piling) close to the stage construction joint and dowel the two sides of the cap together, or provide a sealed expansion joint.
Interior Bents. If possible, use independent bents. If a single structure is required, the reinforcing steel can be spliced together using a lap or mechanically coupled together. If splicing is used, adequate horizontal and vertical clearances must be provided to account for the projecting reinforcement. The exposed reinforcement must be protected. If available clearances are limited, use mechanical couplers or butt welds. Due to the complexity of couplers and welds, accurate details and proper structural detail notes are essential.
Anchor: #i1002106New Superstructure
The following are guidelines for the design of the new superstructure.
The location of the stage construction joint in the slab and the available clear distance for splicing the mat reinforcing are critical factors in the slab design. The stage construction joint can be placed over a supporting beam or in a bay between beams. However, placing the stage construction joint over a supporting beam is the preferred method. When placing the joint over a supporting prestressed beam, the joint must be located 2 in. beyond the centerline of the beam to grab the R-bars with the first pour. Prestressed concrete panels are typically not allowed in the second placement in the bay adjacent to the construction joint. When placing the joint between beams, locate the joint at the quarter point of the beam spacing.
Joints should be located so that space for minimum reinforcing steel laps and 1 in. of cover beyond the ends of the bars is provided. The available construction clear distance may limit the available length required for an adequate lap length. If the clear distance is inadequate, mechanical couplers can be utilized. However, there are concerns about the performance of a construction joint using couplers in both mats, particularly in salt areas. If couplers are used, be sure the appropriate specifications are supplied. Consideration of raising the grade a few inches to allow the top mat to be lapped should be given. Shorter laps might be justified based on the AASHTO provision (As required /As provided) in areas where the slab has excess capacity.