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用后张法生产预应力构件时,当混凝土强度达到设计要求后即可张拉钢筋。规范要求张拉时张拉力必须均匀、对称进行,以尽量避免出现偏心受力和应力损失。可是,在张拉叠层屋架时,除应满足以上要求外,还必须考虑预应力筋的超张拉问题。这种现象在后张法施工工艺中可以明显看到。采用后张法张拉预应力钢筋时,把屋架下弦端部混凝土作为反力支座。也就是说,钢筋达到设计张拉控制应力σ_k时,屋架下弦混凝土
When the post-tensioned method is used to produce pre-stressed components, the reinforcement can be stretched when the concrete strength reaches the design requirements. The specification requires that the tension must be uniform and symmetrical when tensioning to avoid the occurrence of eccentric stress and stress loss. However, in addition to meeting the above requirements, the problem of super tension of prestressed tendons must also be taken into account when tensioning laminated roof trusses. This phenomenon can be clearly seen in the post-tensioning construction process. When post tensioning prestressed steel is used, the concrete at the lower end of the truss is used as a counter bearing. In other words, when the reinforcement reaches the design tension control stress σ_k, the roof screed