Punching Shear at the Perimeter of the Column
Column Base Design doesn’t check the punching shear at the perimeter of the column, this check can be done manually if desired.
Note: The following extracts are from South African Standards (SANS), see relevant Codes and Clauses.
The shear strength of the base in the vicinity of concentrated loads or reactions is governed by Clause 18.104.22.168.2 SANS 0100-1. See extract below:
22.214.171.124.1 The design shear force is the algebraic sum of all the ultimate vertical loads and reactions acting on one side or outside the periphery of the critical section.
126.96.36.199.2 The shear strength of bases in the vicinity of concentrated loads or reactions is governed by the more severe of the following two conditions:
a) shear along a vertical section that extends across the full width of the base (for pad footing, this section may be considered at 1.5 times the effective depth from the face of the loaded area and the provisions given in 188.8.131.52 will apply); and
b) punching shear around the loaded area, where the provisions given in 184.108.40.206 will apply.
Column Base Design is based on the Shear stresses in solid slabs under concentrated loads as per Clause 220.127.116.11 SANS 0100-1. See extract below:
18.104.22.168 Shear stresses in solid slabs under concentrated load
22.214.171.124.1 The following terms specific to perimeters are used in this subclause:
a) perimeter: a boundary of the smallest rectangle (or square) that can be drawn around a loaded area and that nowhere comes closer to the edges of the loaded area than some specified distance Lp (a multiple of 0.75d) (see figure11).
NOTE - See 126.96.36.199.8 for loading close to a free edge.
b) failure zone: an area of slab bounded by perimeters 1.5d apart (see figure 12);
Based on the theory in Clause 188.8.131.52 above. The shear perimeters are considered as a set distance away from the column faces. Various combinations of shear patterns are then compared against each other (See typical examples in the picture below) and the critical perimeter (Ucrit) will be used to determine the Area used for Shear. (See Punching Shear Stress/Reinforcement Calculations)
The required reinforcement for bending is used to calculate the shear resistance, Vc in the X and Y-directions. For punching shear, the value is based on the average required reinforcement in the two directions,
For punching shear, shear perimeters are considered a set distance away from the column faces. The distance is determined by the design code, and is typically 1.5 times the effective slab depth (e.g. BS 8110 - 1997) or 0.5 times the effective depth (e.g. ACI 318 - 2011). Various combinations as for internal, edge and corner columns are considered.