The inventors and original supplier of a popular shear-stud reinforcement for two-way flat slabs in concrete frames are up in arms over charges, based on recent research, that the American Concrete Institute's model code governing use of the studs is flawed. The researchers claim that, consequently, there is potential for premature failure due to punching shear at slab-column connections, especially under earthquake loads.
A. Ghali, professor emeritus of civil engineering, University of Calgary, Alberta, Canada, and the lead researcher for original studies on shear studs dating back to the 1970s, disagrees. Extensive testing shows that code-specified "equations and detailing for the use of headed-stud shear reinforcement are conservative," he says.
"The shear stud reinforcement is proven ... as the most effective, practical type" of reinforcement, adds Ghali.
Chapter 11 of the model code, "ACI 318-11 Building Code Requirements for Structural Concrete," addresses shear reinforcement embedded in two-way flat slabs. Section 11.11.5 states, "Headed shear-stud reinforcement, placed perpendicular to the plane of a slab or a footing, shall be permitted in slabs and footings."
The code's main problem is the "orthogonal shear-stud layout" in the slab around the column, says Gustavo J. Parra-Montesinos, a professor of civil engineering at the University of Michigan, Ann Arbor, and the researcher raising the red flag about the potential for poor performance of the slab-column connection.
The orthogonal layout leaves slabareas adjacent to the column corners completely unreinforced for shear, says Parra.
Headed shear-stud reinforcement was introduced into the code in 2008. Ghali and his former colleague, Walter H. Dilger, invented a studs-on-a-rail product, which places studs on a base to keep them properly spaced and to provide an anchor until the concrete is cast. Stud rails, which look like combs with short tines, have become the standard way to install the vertical studs. Ghali and Dilger held the original patents on the stud rails; the patents have since expired.
Though structural engineers register concern and support further study of shear-stud assemblies, most do not think the situation presents an immediate threat to public safety. "The fact that we don't have failures is good news," says David Fields, a senior project manager with structural engineer Magnusson Klemencic Associates (MKA), Seattle. "However, this detail is intended for a very large seismic event, and it hasn't necessarily been fully tested by design-level events."
Cary Kopczynski, founder of the Bellevue, Wash., firm that bears his name, adds: "For buildings where loading is primarily gravity and seismic drift is minimal, which includes the majority of structures, I'm confident that shear studs perform as intended by the code. I remain concerned about the performance of shear studs in buildings with high gravity-shear ratios and high seismic drift."
Parra and University of Minnesota professor of civil engineering Carol K. Shield are co-principal investigators for the ongoing shear-stud research, which is funded by $940,000 from the National Science Foundation, $162,000 from the Charles Pankow Foundation and $30,000 from ACI's Concrete Research Council.
In seismic zones, structural engineers design a framing system of slabs and column to handle gravity loads and a shear- wall system to resist lateral loads, including seismic and wind forces, explains Shield, who is also director of the Multi-Axial Subassemblage Testing Laboratory (MAST) at the University of Minnesota, where most of the tests are performed.