Structural Engineering Research Projects:

Pacific Stair Tests

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Fig. 1 - Overall loading frame and stair assembly.

Tests were performed on two full-size prefabricated steel stair assemblies to assess seismic interstory drift response combined with factored gravity loads. The stairs assemblies were production-run units of the standard stair system fabricated by Pacific Stair Company of Salem, OR and were designed for a typical steel frame building with 12 ft story heights and one intermediate landing between floor levels (Fig. 1). Two different stair assembly units were tested: one with checker plate and one with concrete filled pans. A testing protocol was developed to evaluate the seismic performance of the prefabricated stair assemblies (Fig. 2) and used American Institute of Steel Construction (AISC) factors for seismic and gravity load combinations. Lateral drifts were imposed in both orthogonal stair directions separately (movie). After completion of the lateral drift tests, full factored live and dead loads were applied to the stair assemblies (Fig. 3).

 

Fig. 2 - Lateral displacement history imposed at top landing in each orthogonal direction.

Experimental results demonstrated that both stair assemblies successfully completed the testing protocol by exhibiting full factored design gravity load carrying capability after undergoing lateral displacements in both orthogonal directions and there was no appreciable difference in the performance of the two stair assemblies. The stairs supported the full factored gravity loads while undergoing four repeated cycles of lateral displacement equal to half of the maximum seismic interstory drift after application of the lateral displacement history. Lateral drift imposed in the parallel to stair direction produced single curvature in the stair runs and the highest deformation demand in the bottom landing angle connection. Lateral drift imposed in the transverse to stair direction loading produced double curvature in the stair runs and the highest deformation demands in the top stair to landing connection. Stair performance is dependent on the landing connections and the vertical weld joining the connection plate to the stringer. Careful detailing, fabrication, and inspection of the welds joining the landing connection plates should be followed to ensure desired performance.

 

Fig. 3 -Full factored live and dead load applied to stair assembly after completion of lateral drift test (two sets of concrete blocks on treads and landing).

Research Significance

Stairs serve as a primary means of egress from a structure after an earthquake and thus their role in achieving life-safety performance for a building is critical. As a result of newly adopted performance-based design provisions for buildings, the lateral drift performance of prefabricated stair assemblies during seismic events has become of interest to some designers. No standardized testing methods or loading protocols were available to evaluate the seismic performance of prefabricated stair assemblies. Further, data on the structural performance of stair assemblies under lateral and combined lateral and gravity loading were lacking.

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