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Punching shear reinforcement is often required in slabs around column heads. Traditional links are difficult to fix, particularly when there is fairly heavy top and bottom reinforcement. An alternative is the use of headed shear studs. The studs are lengths of high tensile steel with large heads forged on one or both ends. Generally the diameter of the heads is three-times that of the shaft, which will provide full anchorage; if the head size is smaller, the stud may not be fully anchored, reducing the capacity of the stud.
For ease of installation, a number of studs are welded to a thin strip of metal or otherwise connected. The required spacing of the studs along the strip will depend on the spacing of the main reinforcement. The lines of studs may be either placed on the soffit formwork (with suitable spacers to ensure the correct cover) prior to the main bottom and top reinforcement being fixed or else dropped in from the top once the top mat has been fixed. Traditional links hook around the top and bottom mats, forming the vertical legs of the classic strut and tie approach to shear. There can be no such mechanical connection when using studs; care must be taken to ensure that one end of the stud is located in the compression zone of the slab and the other in the tension zone.
While traditional links are generally arranged in an orthogonal grid, it may be more convenient and economic to arrange studs in a radial pattern.
(Traditional links may also be formed into assemblies to ease installation, see separate entry Reinforcement/Types/Prefabricated reinforcement.)
Acknowledgement: The Concrete Society
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