Commonly-Used, Code-Approved Structural Detail Proven Susceptible To Premature Failure

ENR’s Nadine Post reported recently on research findings that may carry broad implications. Representatives from American Concrete Institute (ACI) and Gustavo Parra-Montesinos, a researcher at University of Michigan, Ann Arbor, state that a popular structural design detail may not perform as intended, despite being compliant with existing building codes.

The detail in question: shear-studs-on-a-rail, used for reinforcing slab-column connections at flat-plate concrete frames. The current application detailed in ACI 318, was tested according to Section 11.11.5 of ACI 318-11. The result: the easier-to-install orthogonal configuration resulted in premature punching shear failure. What is causing the red flag to be raised by ACI and researchers is the fact that premature failure is possible without seismic loads. They are concerned because it is estimated that “tens of millions of sq ft of flat-plate systems reinforced using shear studs” exist in seismic zones alone.

Parra first discovered the premature failure problem by accident in 2007, during National Science Foundation-funded physical testing of fiber-reinforced concrete systems. “We wanted to show that our solution for fiber reinforcement would perform as well as accepted practice with shear studs so we tested a shear-stud specimen under seismic loads as a control,” says Parra. “We got surprised by a very early failure.”

The test was performed at the Minneapolis-based Multi-Axial Subassemblage Testing laboratory (MAST), under a grant from NSF’s Network for Earthquake Engineering Simulation (NEES). Carol K. Shield, director of MAST, says: “The specimen couldn’t hold the gravity load during reasonably small displacements of the column. The code would tell you it should have been able to hold the gravity load for a larger column displacement.”

ACI plans to publish articles throughout various technical journals in advance of changes to the planned 2014 update of ACI 318. In addition, a retrofit detail is being studied, possibly in conjunction with NEES.