The punching shear dialog displays the actual and allowable punching shear or two-way shear values for each pier. If the punching shear under any of the piers is greater than the code allowable value, the check box is checked with the label “Exceeds Allowable” in red. The pier under which the punching shear exceeds allowable is also highlighted in red.
In determining the punching shear stress in the footing, Foundation3D considers the following for soil and pile supported foundations.
For square and rectangular foundations:
The punching perimeter for calculating punching shear under the pier considers the effective pier dimensions. The punching perimeter for piles considers either the circular pile shape or square pile shape as selected in the Options dialog.
For soil-supported foundations, the net applied ultimate load on the pier is the difference between the applied ultimate load on the pier including the load due to soil and footing within the punching perimeter and the soil reaction under the pier. The soil reaction used is the lowest average value determined separately in each orthogonal direction within the punching perimeter. This net ultimate load is then used in determining the punching shear stress in the footing.
For pile supported foundations, if the option to consider pile resistance is not selected on the "Piles" tab of the Options dialog, the net applied ultimate load on the pier is the applied ultimate load on the pier including the load due to soil and footing within the punching perimeter without considering any resistance from the pile reactions even if the piles fall directly under the pier. If the option to consider pile resistance is selected on the "Piles" tab of the Options dialog, piles with centers falling within punching perimeter are considered in resisting the applied compressive punching load if the pile reaction is compressive in nature. Partial or total pile reaction is considered to be contributing towards punching resistance depending on the location of the piles within the punching perimeter.
A similar approach is also used in determining pile punching shear for each individual pile. If the option to consider pile resistance is not selected on the "Piles" tab of the Options dialog, actual pile reaction is used in determining the pile punching shear for each load combination without considering the location of the pier in relation to the pile under consideration. In other words, even if the piles are located directly beneath the pier, pile punching shear does not consider the compressive pier load that would resist pile punching shear. However if consider pile resistance option is selected, the net pile punching load considers the compressive resistance from the pier if the pier is located within the punching perimeter of the pile.
For octagonal footings:
For soil-supported foundations, the net applied ultimate load on the pier is the difference between the applied ultimate load on the pier including the load due to soil and footing within the punching perimeter and the soil reaction under the pier. The soil reaction is conservatively estimated assuming the effective octagonal pier/footing dimensions. This net ultimate load is then used in determining the punching shear stress in the footing.
For pile supported foundations, if the option to consider pile resistance is not selected on the "Piles" tab of the Options dialog, the net applied ultimate load on the pier is the applied ultimate load on the pier including the load due to soil and footing within the punching perimeter. If the option to consider pile resistance is selected on the "Piles" tab of the Options dialog, piles with centers falling within punching perimeter are considered in resisting the applied punching load. Partial or total pile reaction is considered to be contributing towards punching resistance depending on the location of the piles within the punching perimeter. The net punching load is an approximate value based on the area of overlapping rectangles of the pile and the pier.
For Euro code, punching shear is determined at a distance of effective depth d and 2d from the face of the pedestal/pile. If punching shear exceeds allowable at both d and 2d, then a higher ratio of actual to allowable punching shear is considered as the governing condition.
Moment is considered in determining the net punching load.
The pile punching shear is determined for each individual pile and is listed in the Calculation Report. The pile punching perimeter listed in the Calculation Report assumes the perimeter around a single pile only. It may be possible to group perimeters based on pile spacing. However grouping is currently not considered in determining punching shear.
Note: For many concrete design codes, the allowable punching shear depends on the amount of reinforcement placed in the footing or pile cap. Therefore only after the "Footing Reinforcement" dialog is viewed, the correct values of the allowable punching shear are reflected on the "Punching Shear" dialog. If changes are made to the rebar size and spacing on the "Footing Reinforcement" dialog, the "Punching Shear" dialog must be revisited to view the change in allowable punching shear values.
Strap Footings:
Since strapped footings only resist axial load, the punching shear on the strapped footings do not include any contribution from the applied moments.