Based on the structural feature of dumbbellshaped section of concretefilled steel tube (CFST)

a combined spatial beam element is presented. In this structure

two steel tubes

two core concretes

one web and the concrete in web cavity are firstly considered as general spatial beam elements

then they are assembled into a spatial combined beam element according to the plane hypothesis and transformation matrix formed from master and slave nodes. A FORTRAN program for CFST arch with dumbbellshaped section has been developed

and the numerical example shows that the steel tubes and core concretes bear mainly the axial forces with small moments

thus

a modified formula for lattice column method to calculate axial forces of the upper and lower limbs of CFST with dumbbellshaped section is deduced. Comparison of the results from the modified formula with those from an example indicates that the modified formula gives relatively accurate results. 