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本文对桁架次应力计算理论的发展作了历史性的回顾,并对近20年来国内外有关钢筋混凝土屋架次应力研究结果进行了总结和评述,其中亦包括了近年来我院对此问题所进行的试验研究结果。试验与分析证明,钢筋混凝土屋架的次弯矩不仅降低了屋架的强度和抗裂度,而且使上弦的设计控制截面也发生了转移。一般强度安全降低0—30%,抗裂度的降低为5—25%,因此在设计中对此有考虑的必要,并须进行计算。在计算时考虑到屋架在超过弹性阶段后,次弯矩的增长速度往往超过荷载的增长速度,故应按破坏阶段考虑,此时受弯与受压构件的刚度可取弹性刚度的80%。试验分析证明,目前工程上通用的次弯矩计算方法是可行的。即将弦杆视作刚性支承连续梁求出其主弯矩,再考虑屋架节点的位移求出次弯矩,屋架的节点位移可按饺接桁架算出。上述计算方法对于手算仍嫌过繁,因此仍需有一个比较简单但有足够准确度的计算方法。
This paper makes a historical review of the development of the secondary stress calculation theory for trusses and summarizes and reviews the research results of the secondary stress of reinforced concrete roof trusses both at home and abroad in the past 20 years. This also includes the research conducted by our institute in recent years. Experimental study results. Tests and analysis have shown that the secondary bending moment of the reinforced concrete truss not only reduces the strength and crack resistance of the truss, but also shifts the design control section of the upper chord. The general strength and safety are reduced by 0-30%, and the crack resistance is reduced by 5-25%. Therefore, it is necessary to consider this in the design and must be calculated. In calculation, it is considered that after the truss exceeds the elastic stage, the growth speed of the secondary moment tends to exceed the growth rate of the load, so it should be considered according to the failure stage. At this time, the rigidity of the bending and compression members can take 80% of the elastic stiffness. The experimental analysis proves that the common method of secondary moment calculation in engineering is feasible. The chord is considered to be a rigid supporting continuous beam to find its main bending moment, and then the displacement of the roof joint is taken into account to find the secondary bending moment. The nodal displacement of the roof truss can be calculated according to the dumpling truss. The above calculation method is too complicated for hand calculations, so there is still a relatively simple calculation method with sufficient accuracy.