The long term performance and constructibility of any bridge is highly dependent upon the quality of the design details. Unfortunately, given the unusual nature of segmental bridges, improper details can be overlooked by inexperienced designers and reviewers.
The number of different types of detailing pitfalls is rather large. Most experienced designers and contractors know the common problems for these type structures and they have known solutions. Some project specific conflicts, however, can only be revealed with accurate sketches integrating all of the various parts of the bridge. It is essential that the engineer make these sketches even if they are not part of the final plans.
Reinforcing fit-up. Conflicts can occur at anchorages, blisters, deviation beams and bearing diaphragms. The three-dimensional space requirements of tendon anchors and spirals need to be accounted for. Reinforcing should be arranged to accommodate horizontal and vertical tendon profiles. All reinforcing bends with the actual curve radius should be drawn to scale.
Nodes. All tensile reinforcing entering a structural node (Strut & Tie design) should be adequately developed. Post-tensioning bars require some distance from the free, perpendicular edge to become developed; cover requirements, bar extension, nut and plate thickness can combine to shift a node off of the center of the neutral axis of an entering thin wall element (e.g. web or slab entering a diaphragm).
Appurtenances. Details for access openings, drains, lighting fixtures etc. are often presented as separate details at the end of a plan set. These too should be drawn to scale with the reinforcing from typical plan sheets prior to final plan preparation. In complex projects, fully-integrated drawings may be necessary. Access hatches should swing free of external tendons, plumbing etc. Plumbing should be positioned so that it creates the least interference with passageways in the box. Additional reinforcing should be added if the element creates a disturbance in the flow of forces.
Short tendons. Post-tensioning bars should be used if the losses in short tendons are too great.
Tolerances. Details need to be buildable by conventional practices. A large, circumferential stirrup that engages both webs and flanges may need to be spliced into two telescoping U-shaped bars. Cast-in-place closures joining two soon-to-be prestressed elements should be wide enough to accommodate precast duct misalignments.
Jack Fit-Up. Hydraulic ram dimensions and stroke need to be accounted for when locating tendon anchors within the box.
Water Control. Methods to control water from broken plumbing and expansion joint seals should be integrated into the design. Holes cast into the bottom slab adjacent to bearing diaphragms and deviation beams will both remove the water from the box and serve as a tattletale for maintenance inspectors. These holes can easily be clogged with construction debris or grout and should be cleared before the contractor finishes the project. Transverse drip beads on the web and underside of the wings located one inch from the expansion joint should contain the longitudinal spread of stains resulting from a broken seal. Unsightly column staining can be controlled with either creative column detailing to channel water or an umbrella (drip pan) perched on top.
Temporary Drain Caps. Plans should specify a temporary plumbing line cap at the top of the column to discourage solid waste disposal by construction personnel.
Critter Guards. Roosting birds and varmints need to be kept out of the girder. Expanded metal mesh mounted to a metal frame on the inside circumference of the box (i.e. oriented in the planes of the webs and flanges) at expansion joints has worked well. Transverse screens impede movement of maintenance personnel through the box and should be avoided.
Future Post-Tensioning. During the design of the very first segmental bridges, long-term moment redistribution was an unknown phenomenon. To counteract unexpected deflections, additional post-tensioning was added after the bridges were in service. This was accomplished with some difficulty. Today the more likely cause of future additions of post-tensioning (although this is very rare in a well-detailed bridge) is from tendon corrosion. Bridge designs of today should have provisions for easily adding supplemental post-tensioning. Casting additional, empty post-tensioning anchorages and deviation pipes into diaphragms is the most common method. Allowances need to be made for transporting and positioning hydraulic stressing rams inside the bridge.
