A Study On The Design Method Of Foundation Beam

The reinforced concrete plate on foundation is commonly used in civil and hydraulic engineering, and it is usually simplified as a beam on foundation in design. The current design approach of beam on foundation is unreasonable in theory and is too conservative waste in practice, in which the bending moment M_e is calculated on assumption of linear elastic material while the area of reinforcement is determined by ultimate strength design method. It is because that in the ultimate strength state, the elastic modulus of soil will increase as soil is under high compression stress, and the stiffness of beam will decrease as the steel bars are yielded and concrete is crushed and cracked. These make the actual bending moment of beam on foundation M_n much smaller than M_e.In the paper, a lot of beams on foundation are computed by reinforced concrete finite element method in order to discuss the influence of reinforcement ratio on behavior of the beam, in which soil and concrete are taken as nonlinear materials. A new design method of beam on foundation is suggested according to the regulation of bending moment and displacement varying with reinforcement ratio. The detail is as follows:1. M-n is smaller than M_e after the beam on foundation is cracked. The more cracks of the beam increase, the more M_n / M_e decreases. Under same loads, as reinforcement ratio drops, the maximum of vertical displacement increases little, and M_n decreases, but the decreasing level is smaller than reinforcement ratio.2. Ultimate strength design is not required and the reinforcement can be determined only on basis of allowable crack width in design of beam on foundation. The bending moment under service loads can be taken M_n = 0.8M_e, in which M_e is obtained by Guo's forms. The crack width is calculated by the following formulas presented in the paper.ω_max=aσ_ss/E_s(c + 0.12d/ρ_te)or ω_max=aσ_sl/E_s(c + 0.12d/ρ_te)...