Abstract: In order to thoroughly understand the calculation of punching shear capacity of pile caps, the impact of pile spacing on punching shear capacity of the rectangular pile cap have been discussed based on mathematical method and the literatures. The upper bound solution and lower bound solution are used in the explanation. The results indicate that the punching shear capacity decrease with the increase of pile spacing when the size of the pile cap meet a certain condition. In the calculation of code for design, punching shear capacity contains only the concrete shear of shearcompression zone of inclined section, and the calculation formula can understand by use of the concept of the nominal shear stress of the pile cap punching. The decline of punching shear capacity is due to the change of the destruction pattern of inclined section form shear compression failure to diagonal tension failure. Based on the spatial strutandtie theory on punching failure of pile caps, the decline of punching shear capacity is due to the increase of the load carried by the strut within the punching shear cone, which make the shearcompression zone at the top and bottom of the inclined cracks failure more easily.

Key words: Pile Caps, Punching Shear Capacity, Pile Spacing, Spatial Strut-and-Tie Mode