| Nowadays the body of buildings is becoming larger and larger, therefore temperature movement joints are settled because of the outing of length while the size of foundations is increasing under the heavier load of superstructure. As a result, the collision and overlap of foundations usually appear. To solve the problem, several supercolumns are constituted on the same foundation which transfers load to soil and this means combined shallow foundation under group-column or combined pile foundation under group-column is settled. Related rules about calculation of bearing capactiy aren't included in the national code while instructions are only given in engineering design software PKPM comparing to just be referred in codes abroad without detailed calculation methods. When this problem appears in practical engineering, different treatments are adopted by designers based on collective drawings or individual experience and this leads to great difference in the design which brings difficulties to engineering execution due to shorting of unified, suitable designing theory.Owing to the above conditions, calculation methods about bearing capacity of combined shallow foundation under group-column are studied when engineering calculation methods for combined pile caps under group-column based on code of single-column combined caps are analyzed and the application of the space truss theory in the combined pile caps under group-column is discussed based on current code of foundation design. Research work is mainly on the following parts:(1) Calculation methods about bearing capacity of combined shallow foundation under group-column are studied. Two kinds of representative calculation methods are analyzed-one based on the national code and the other based on the ACI code. Bearing capacity, amounts of rebar etc. are calculated based on these methods before analyzing their load capactity and economy index. Methods based on the national code are suggested to calculate while the shortage of the ACI code is pointed which is needed to be revised on the basis of the national code.(2) Two sorts of different assumptions of punching failure cone are given by adopting two kinds of popularized based on the calculation methods of pile caps of the current national code-setting up a hidden beam or not. The flexural, shearing and punching bearing capacity about combined pile caps under group-column are calculated. The opinion that if 800≤h0≤2000 andλ<0.54 and when shearing force is equal to punching force, shearing bearing capacity is less than that of punching, which means shearing failure in control at the time are put forward(3) Costitutive relations, failure criterion and crack, broken theory are summaried. Different models of combined pile caps under two-column are constituted and nonlinear finite element analysis is adopted. Influence on bearing capacity of pile caps by different layout style are analyzed to draw the preliminary conclusions of the failure reasons of pile caps.(4) Finite element models of different combined pile caps under columns are studied, transfer mechanism is concluded and space truss model is established by comparing its pictures of stress and strain. Revising formulas suitable for engineering are analyzed by comparing the results of current formulas. It is verified by comparing the results of example calculation in according to traditional calculation methods. It is indicated that the methods suggested by this article are feasible and the results match for engineering.
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