This paper investigates the importance of including the effects of the
flexibility of the horizontal diaphragms when using the P-delta metho
d of analysis, especially when considering the loads applied to interm
ediate frames on trusses that are not part of the lateral force resist
ing system. Analyses were conducted for structural systems with a vari
able number of stories, number of bays and diaphragm stiffnesses and s
upported by rigid jointed plane frames or vertical trusses.