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3.桁架桿件次应力影响问題分析鋼筋混凝土吊車桁架的上弦是勁性很大的連續杆件。由于各节点的剛性大,各杆件本身的刚度也比較大,所以其实际工作情况与假想为铰接桁架的工作情况不同。尤其是上弦杆的节点沉陷对上弦杆的弯矩变化有很大影响,因此是不容忽視的。下面就以試驗方案Ⅰ的加荷序号Ⅰ-(?)这一荷載阶段(圖8)为例,进行分析。由表8中看到,节点沉陷对上弦杆的受力有很大影响。如1—2跨的跨中正弯矩,由于节点沉陷而增大了79％(50.48-28.23/28.23·100=79％),节点2的負弯矩增大了51％(19.89-13.17/13.17·100=51％)。更不利的是节点1的負弯矩竟变成很大的正弯距,这样,鋼筋的位置就完全顛倒了,会严重地影响結構的安全。因此,設計时必须考虑吊車桁架上弦杆节点沉陷的影响。 (1)上弦杆 連續的勁性上弦杆,由于上弦节点的沉啗,使內力發生很大变化。茲以圖8中0—1—2梁段为例,如按各种不同的计算体系进行計算,可得很不相同的弯矩数值(如表8)。
3. Analysis of the influence of the secondary stress of truss members The winding of the reinforced concrete crane truss is a continuous member with high stiffness. Since the stiffness of each node is large and the stiffness of each member is relatively large, the actual working condition is different from that in the case of an articulated truss. Especially the node sinking of the upper chord has a great influence on the bending moment of the upper chord, so it cannot be ignored. The following analysis will be based on the loading phase I-(?) of the test plan I (Fig. 8) as an example. From Table 8, it can be seen that node subsidence has a great influence on the force of the upper chord. For example, the positive mid-span bending moment of 1-2 span increases by 79% (50.48-28.23/28.23.100=79%) due to node subsidence, and the negative moment of node 2 increases by 51% (19.89-13.17/13.17). 100=51%). What is even more unfavorable is that the negative bending moment of node 1 turns into a large positive bending distance. In this way, the position of the steel bar is completely reversed, which will seriously affect the safety of the structure. Therefore, the effect of the subsidence of the chord on the top chord of the crane truss must be considered when designing. (1) The top chord The continuous stiff top chord changes the internal force greatly due to the sinking of the top chord. Taking the 0-1-2 beam section in Fig. 8 as an example, if calculated according to various calculation systems, very different bending moment values can be obtained (see Table 8).