How to model Pile cap attached to batter and vertical piles in STAAD.Pro?
1) The pile cap can be modelled using either plate elements or solid elements. If the thickness of the cap is comparable to its plan dimensions, a solid element model is preferable. If the plan dimensions are much larger than its thickess, plate elements should be a better choice. One drawback of using solid elements is that, by their very nature, they lack rotational degrees of freedom. Consequently, for a monolithic structure such as a concrete pile cap with concrete piles, the rigid connection between piles and the pile cap cannot be properly accounted for, if the cap is modelled using solids.
2) The piles themselves can be modelled using beam members. The supports for the pile come in 2 varieties - skin friction and end bearing. Skin friction action can be accounted for by modelling each pile as several collinear members and specifying a spring support at each of those common nodes. End bearing action can be modelled using fixed or pinned supports. The support spring stiffness is obtained by multiplying the soil subgrade reaction by the influence area of the associated support node. A standard text book on pile analysis should be a great source of information on obtaining the spring constant of the supports.
Users also have the option to model the pile supports in STAAD.Pro using spring supports ( FIXED BUT supports). More details can be found in the STAAD.Pro Help section Technical Reference of STAAD Commands > TR.27 Support Specifications > TR.27.1 Global Support Specification. The pile stiffness needs to be defined through these springs. Also it may be useful to note that when piles are modeled as supports, one can run the analysis in STAAD.Pro and then take the supports to STAAD Foundation Advanced from within STAAD.Pro itself ( using the Foundation Design workflow) and subsequently one can design pile caps in STAAD Foundation Advanced.