混凝土设计基本原理 第8章钢筋混凝土构件的变形裂缝及混凝土结构的耐久性.pptVIP

Chapter 8 变形、裂缝及延性、耐久性Deformation, crack, ductility and durability Durability serviceability Safety Serviceability limit state deformation Crack width Detailing requirements 结构的功能要求（Functional requirements of structure） Ultimate limit state Strength design of members 8.1 钢筋混凝土构件的变形（deformation-deflection) 弯曲刚度（Flexural stiffness）-（产生单位曲率所需要的弯矩）bending moment needed for unit curvature of section. 8.1.1 截面弯曲刚度的定义 Definition of flexural stiffness of section 均布荷载 材料为理想弹性材料，刚度为常数，截面曲率与弯矩呈线性关系 When the material is elastic, the stiffness of section is constant. Curvature is linear to the bending moment. M ? 刚度的特征 Feature of stiffness of section M y M u M cr E c I 0 M ? Stage Ⅰ StageⅡ StageⅢ Reinforced concrete is not ideal elastic, so the stiffness of section changes with the increase in bending moment. 刚度的特征 Feature of stiffness of section 1、截面曲率Curvature of section 8.1.2 受弯构件的短期刚度Bs Short-term stiffness of flexural member Bs 弹塑性材料 弹性材料 2、Strain of steel and concrete at the cracked section Compressive area of concrete Strain at cracked section Compressive stress of concrete 3、Average strain 4、Expression for short-time stiffness Stiffness formula 5. Expression of parameters（参数） (1).力臂系数Lever arm coefficient (3) 8.1.3 截面抗弯刚度 长期荷载作用下刚度下降Stiffness will decrease under sustained load 混凝土徐变Creep of concrete 混凝土与钢筋滑移Slip between concrete and steel Stiffness formula 裂缝处混凝土压应变增大Strain of concrete at cracked section increases 裂缝处钢筋拉应变增大 Strain of steel at cracked section increases New Code 8.1.4 最小刚度原则Principle of minimum stiffness and computation of deflection 1.最小刚度原则 Principle of minimum stiffness Along the length of span in simple beam, deflection is computed applying the stiffness at the maximum-moment section ( maximum stiffness), based on mechanical formula not considering the shear deformation (在简支梁全跨范围内，按弯矩最大处的截面弯曲刚度，即最小的截面弯曲刚度，用材料力学方法中不考虑剪切变形影响的公式来计算挠度。) 2. Computation of deflection 3. Discussion How to control the deflection New Code 8.2 裂缝控制 Crack control o