Railway bridge resistance to earthquake ground motion: Properly-anchored bridge decks can provide significant resistance to ground motion, which may eliminate the need for seismic retrofits on many bridges - Ttci R&D - Transportation Technology Center Inc. research

Railway Track and Structures, June, 2001 by Duane Otter

Information is needed to quantify the resistance of existing bridges to horizontal ground motions to assess the ability of railroad bridges to withstand earthquakes With this information, decisions can be made about the need to retrofit a bridge, and, if so, what sort of retrofit might be appropriate.

Recent testing at the Transportation Technology Center. Inc., in Pueblo, Colo., shows that properly anchored rail and bridge decks can provide significant resistance to ground motion. This may be enough to eliminate the need for seismic retrofits of many bridges.

TTCI performed tests in 2002 to assess the ability of railroad bridges to withstand earthquakes and to make decisions about retrofitting bridges to withstand longitudinal movement. The objective of these tests was to determine the resistance to longitudinal movement provided by the track structure and approaches. The tests were conducted on an out-of-service, multi-span, open-deck bridge on an industrial lead of the Union Pacific.

The purpose of the tests on intermediate Span 5, show in Figure 1, was to quantify the resistance between rail and bridge deck, and the resistance between bridge deck and the span. The test was to determine how much resistance to longitudinal movement was contributed by friction, the hock bolts, and box anchoring of the ties. End Span 1, shown in Figure 2, was used to test the resistance provided by the full system, including the approach embankment.

Test results showed:

* Frictional resistance of a 53-foot intermediate span: Coefficient of frictional resistance for longitudinal movement with the bearings greased and rails disconnected was 0.21.

* Longitudinal resistance between rail and bridge deck: Total resistance was 31 kips for a 53-foot intermediate span on tangent, and 45 kips for a 53-foot end span in a four-degree curve with rails not anchored.

* Longitudinal resistance between bridge deck and span: Total resistance was 47 kips for the intermediate span, and 44 kips for the end span, with rails anchored and hook bolts loose.

* The longitudinal resistance of the whole system: Total resistance for the intermediate span with rails attached to both ends was 62 kips, with the rails box anchored at every other tie and hook bolts on every other tie tightened to specification. For the end span with rails attached only to the approach, the frictional resistance was 58 kips.

* Longitudinal resistance provided by the approach: The rail and/or deck moved before the approach could be pulled loose.

Anchor bolts on both spans were cut and the bearing plates greased. The walkway timbers were cut between spans and the guardrails were removed from the bridge. Each span was subjected to a series of longitudinal pushes by hydraulic jacks.

Displacement measurements were taken at the interfaces of the rail to tie, tie to beam, and beam to pier. Strain gauges were used to measure the forces in the rails. Load cells were used to measure the force needed to move the span.

Six tests were performed on intermediate Span 5, and four tests were performed on end Span 1. Each test consisted of a push to the west and then a push to the east. After each push, the span was inspected and any changes were documented.

Longitudinal resistance test results

The first three tests on the intermediate span were with:

* Rails disconnected

* Rails connected, hook bolts loose and no rail anchors.

* Rails connected, hook bolts tight, and no rail anchors.

These tests indicated that the frictional resistances for the girders were approximately the same; i.e., 26 kips. Figure 3 illustrates the movement of the deck and girder in reference to the rails. The ties moved with the span as can be seen by the mark on the girder next to the middle tie.

The next test had all the hook bolts loose on the intermediate span, every other tie box anchored, and all spikes driven down. Figure 4 shows the amount of resistance provided by the box anchoring of every other tie. The average resistance for this test was about 47 kips, which is 20 kips additional resistance compared to the base case of span friction only.

For the last test on the intermediate span, all of the hook bolts were tightened, every other tie on the span was box anchored, and all spikes were driven down. This situation created a stick-slip action in the span that yielded an average resistance of 62 kips, as Figure 5 shows.

The tests conducted on the end span were similar to tests on the intermediate span but with two differences in rail configuration; namely, the rails were cut between end span and the adjacent span (i.e., No. 2) and the approach rails were left connected. With the hook bolts loose and the rails not anchored, as Figure 6 shows, the average resistance was 44 kips. Box anchoring every other tie yielded an average resistance of 44 kips. The whole system combined, with all components tight, produced a peak resistance of 100 kips as Figure 7 shows and an average resistance of 62 kips. Failure to pull the east approach loose happened when the girder slid through the hook bolts. Figure 8 shows the movement of the girder through the hook bolts with all components of the rail to deck, and deck to beams, tight on end span.