A modified strut and tie model was presented to predict ultimate strength of cap beam of single column pier aiming at design of cap beam of single column pier in ultimate limit stage. The nonlinear analysis of a severe cracking cap beam- was conducted using smeared cracking model in ANSYS for reinforced concrete. Results show that the beam theory and deep beam theory are not appropriate for the analysis of such member with small depth-span ratio where the stress distribution is very complicated. Then the strut-and-tie model (STM) based on Mohr-Coulomb failure criterion was presented by considering the bent up steel bar, the stirrup and tensile strength of concrete contribution on restricting shear crack in the reinforced concrete cap beam. The model described the real behavior feature of cap beam in the failure stage. The effectiveness and applicability of the presented model used in different depth-span ratio was compared with other theories. The values by the presented model accorded well with the results obtained by finite element model.%针对独柱式桥墩帽梁在承载能力极限状态下的设计计算问题,提出改进的撑杆-系杆模型来预测该类结构的极限承载力.以某座独柱式桥墩发生严重开裂的桥墩帽梁为背景,利用ANSYS的有限元弥散裂缝模型对开裂的桥墩进行非线性分析.计算结果说明,对于钢筋混凝土独柱墩帽梁这类剪跨比较小的构件,帽梁的弯剪复合受力十分复杂,基于经典的欧拉伯努利梁理论和铁木辛柯梁理论的设计计算方法不合适.对比数值分析的结果,提出基于节点区域双向拉压破坏的改进撑杆-系杆体系模型.该模型进一步考虑了弯起钢筋、箍筋以及混凝土抗拉强度对斜裂缝开展的限制作用,更能反映独柱墩帽梁在破坏阶段的受力特征.对比独柱墩帽梁常用的剪跨比下各理论的适用性可知,该改进模型结果与有限元模型的计算结果吻合.
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