The Most Cost-Effective Option for Increasing Punching Capacity
Outside of the construction industry, “punching capacity” and “punching shear” would seem to be terms associated with boxing matches. Within the industry, these terms have a bit in common with their pugilistic counterparts. If punching capacity isn’t sufficient, punching shear failure can occur at the intersection of a concrete slab and a support column, and “knock out” a construction project.
The most common image of punching shear failure is that of a collapsed slab around a series of columns that appear to have “punched” through it. That image, combined with the commonly used terminology, has given rise to the erroneous conclusion that failures result from columns literally punching through a slab, much like a screwdriver punching through a piece of thin steel.
In fact, punching shear failures are much more complicated than that. As opposed to a slab having collapsed because a column “punched” through it, failure typically results from reactions between a column and slab causing the slab to fracture in areas well outside of the column footprint. Slab-to-column intersections are more heavily reinforced than the adjacent sections of a slab so failure occurs in those weaker areas as opposed to at the intersection. That can be seen in photos of punching failures which typically show sections of slab still in place at the top of the columns around which most of the slab has collapsed.
Hopefully, the potential for punching shear failures are addressed in the design phase, rather than after clean-up, following a failure. The most common options for assuring sufficient structural integrity to prevent punching shear failures are shear capitals, drop heads or drop panels, thicker slab depths, larger column size, closed rebar stirrup reinforcement, or ShearailsTM headed stud shear reinforcement.
All of these options are effective solutions to one degree or another. However, shear capitals, drop heads or drop panels compound the complexity of the formwork process, resulting in higher formwork expenditures when compared to a flat plate formed slab. Furthermore, the aesthetics of a drop capital on an exposed ceiling for condominiums, apartments, and hotels usually preclude their use in these applications. Thicker slabs increase structural capacity but require considerably more concrete which increases foundation requirements, the volume of concrete, amount of rebar, and column size. Each of these options increase costs as does the expense of forming a thicker slab. Larger column size is another potentially effective structural solution, but the additional size may not be practical as it would encroach into adjacent interior spaces.
Closed rebar stirrup reinforcement has traditionally been used to increase the punching strength of flat plates, but in slabs thinner than 10 inches it is difficult to achieve anchorage of the stirrups. From a field installation standpoint, rebar stirrup reinforcement results in a very congested reinforcement condition at the slab-column connection zone. In addition, the full capacity of a closed rebar stirrup cannot be achieved in thin slabs as a result of slippage at the stirrup bends. This limitation is addressed in the ACI-318-14 Building Code which prohibits the use of rebar stirrups in thin slabs, and specifies additional requirements in thicker slabs that make placement of rebar stirrups impractical. To decongest the slab-column reinforcement ShearailsTM are a proven solution to facilitate constructability.
The preferred, most cost-effective solution is ShearailTM Brand Shear Stud Reinforcement, which eliminates the problems and costs associated with other options. This is especially true for structures in areas of high seismic activity, moment reversals and high unbalanced joint moments, which are at higher risk of premature punching failure. ShearailsTM improve the lateral drift capacity of a column-slab joint by absorbing energy with their ductility and superior anchorage, thereby minimizing or eliminating the potential for punching failures.
ShearailsTM are manufactured and certified under strict quality control ICC manufacturing and testing certification program. With accurate field placement drawings prepared by Tendon System’s experienced engineering and drafting staff showing exact rail locations, the simplicity of SHEARAILTM installation, and the attendant cost savings are unsurpassed.