The work in this thesis mainly includes following parts : 1 . as for box girder cross section , the three - bar stimulation method under typical loads and supports has been introduced . the force and stress formulae for flanges have been derived from the three - bar method . by reasonable simplification for multi - cell box girder , the three - bar method has been applied in multi - cell box girder ' s shear lag analysis 對于箱形截面主梁,本文介紹了三桿比擬法上、下翼板中比擬桿的計算公式,以及在典型荷載和支承的情況下翼板內各桿的應力計算公式,并對多箱式主梁截面進行合理簡化,使三桿比擬法應用于多室箱梁的剪力滯分析。