On September 30, 1996 at approximately 8:10 a.m., a tractor trailer traversing the Tacony Palmyra Bridge toward Philadelphia changed its direction abruptly, crossed into the opposite lane and struck the downstream truss (truss B) of the bascule span near Pier F. More specifically, the truss diagonal 9-10 was impacted.

This diagonal, 9.5 m (31 feet) long from work point to work point, consisted of two C 380 x 50 channels, which are 380 mm deep with 50 kg/m (C 15 x 33.9, which are 15 inches deep with 33.9 lb/foot), with webs installed vertically and tied together by 63 mm by 10 mm (2 1/2 inch by 3/8 inch) lacing bars at the top and bottom. The point of impact was approximately 0.9 m (3 feet) above the riding surface.

Serving as an engineering consultant for the insurance company underwriting the Burlington County Bridge Commission, the owner of the bridge, we were asked to review, comment on and approve all analytical and repair procedures and to monitor their actual field implementation.

Damage Assessment

The original design drawings indicate that truss B, 9-10 diagonal experienced 592 kN (133 kip) compressive force due to dead load, although a more precise estimate showed 520 kN (117 kip) and 627 kN (141 kip) tensile force due to live load.

As a result of this accident:

• The diagonal was bent outward with an "inboard" channel bent of approximately 150 mm (6 inches) and an "outboard" channel bent of approximately 100 mm (4 inches).
• The lacing bars adjacent to the impact point buckled and one of the lacing bar rivets at the impact point was completely sheared off.
• The impact force propagated throughout the entire member and caused slippage of the lacing bars relative to the main channels, as indicated by cracks in paint around lacing bars.

The consensus was that the damaged diagonal had to be replaced "in kind" rather than repaired because of the extent and severity of the channels' deformation, buckling of the lacing bars and loss of the rivets' clamping force. Attention then shifted to devising an appropriate construction sequence and evaluating structural conditions at each step.

Construction Sequence and Structural Evaluation

The decision to replace the damaged member was helped greatly by the fact that dead load and live load produce axial member forces of approximately the same magnitude, but acting in opposite directions. It was expected, therefore, that by placing live load in strategic location(s), it would be possible to greatly reduce, if not totally eliminate, the axial compression in the member. This consideration also led to a decision to keep the bridge open to traffic.

Six strain gages were installed around joint #9 of the bottom chord members, side diagonals and vertical post in order to:

• Make sure that the axial compression in the member was indeed reduced or eliminated altogether
• Ascertain the magnitude of compressive force relieved by placement of live load
• Monitor force level during staged reconstruction.

The construction sequence for diagonal replacement was completed in two stages, shown in the accompanying box on the next page.

Stress Monitoring during Diagonal Replacement

The basic rationale for gauging and staged stress monitoring was as follows:

• Stresses induced by dead load are already present in the members and, therefore, must be screened out so as not to interfere with stress readings from special live load.
• Stresses induced by regular live load would be measured and taken into account during the screening out process.
• Stresses induced by the special live load would be recorded at each relevant stage of diagonal replacement and superimposed algebraically on the background stresses produced by dead load and regular live load.
• The residual axial forces acting in the members could be then calculated and compared with the capacities of members at each stage of diagonal replacement.

Table 1: Residual Axial Forces for Each Member

Stresses were read and recorded at five stages of diagonal replacement, when the:

• Bridge was subjected to dead load and regular live load (zero reading with traffic)
• Special live load was in place
• Inboard channel of the diagonal was cut completely after the outboard channel was reinforced
• Top and bottom lacing bars were cut completely
• Damaged inboard channel was removed and the reinforced outboard channel was carrying the load.

The strain gage readings (in psi) were recorded and residual forces acting in joint #9 members (in kips) were estimated (Table 1).

Successful Results

As can be seen in Table 1, the residual axial forces were reduced sufficiently to allow safe staged replacement of the Truss B diagonal. Subsequent trial lifting of the bascule span showed that the repairs corrected the damage and that the bascule span was ready for routine operations. All routine operations have continued safely.