Load Tests of Common Shoring Towers: Typical Detailing and Resulting Capacity Reduction

Following an in service collapse, three shoring towers were tested to failure at the University of Texas at Austin's Ferguson Structural Engineering Laboratory (FSEL) to better understand the cause of the collapse. Both numerical analyses and experimental results indicated a stability limit state governed the load capacity that may have not be adequately considered in the design of the shoring towers. The shoring system tested is widely used in construction and is constructed of modular aluminum components. A typical four-leg tower is constructed with paired frame segments, each containing two column legs. Frame segments are stacked and fitted with adjustable height extensions. Tower legs support a system of cribbage that includes beams and girders, which in turn support wooden formwork. The detail of placing a beam or girder directly over a column is known to increase the effective length of the supporting column, thereby reducing its buckling capacity. A number of structural collapses over many years have been attributed to this destabilizing detail. Consequently, the detail is typically either avoided or is modified to minimize its destabilizing effects. The shoring system tested includes the destabilizing beam-over-column detail without modification. To investigate the effects of the detail on the capacity of a shore tower, tests performed at FSEL included specimens with and without the beam-over-column detail. The first test, of three, applied load directly to the legs of a four-leg tower, and did not include the destabilizing beam-over-column detail. A buckling failure occurred at 96% of the manufacturers' provided ultimate load. The second and third tests included the beam-overcolumn detail and were loaded through wooden formwork and a cribbage system, to be representative of typical field conditions. The results of these tests, which included the detail discussed, showed an approximate reduction in ultimate load of 40% prior to failure.

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