Study on Model Selection of Flat-plate space frame Structure in Toll Station

1 Classification of flat space frame structure (classified by the number of layers and support conditions)

1) Classified by the number of layers

The space frame structure can be divided into a double-layer space frame and a three (multi-) layer space frame according to the number of chord layers. The double-layer space frame is a spatial structure composed of an upper chord layer, a lower chord layer, and a web layer, and is the most commonly used space frame structure. The three (multi-) layer space frame is a spatial structure composed of an upper chord layer, a middle chord layer, a lower chord layer, an upper web bar layer, and a lower web bar layer. The three (multi-) layer space frames increase the height of the space frames and reduce the space frame size, therefore, the chord pressure and the length of the webs are reduced, which is convenient for fabrication and installation. The disadvantage of the three-layer space frame is that due to the increase in the number of nodes and rods, the rods connected to the nodes in the middle layer are denser. The calculation shows that when the span of the space frame frame is greater than 50m, the steel consumption of the three-layer space frame is less than that of the double-layer space frame.

2)Classification according to supporting conditions

According to the different support forms, the space frame can be divided into peripheral support, point support, and a combination of peripheral support and point support. Peripheral support means that all the nodes on the boundary of the space frame are support nodes. The support nodes can be supported on the top of the column or on the beam. This support method transmits force directly and uniformly. Point support means that the support of the space frame is supported on 4 or more supporting samples. In the point-supported space frame, in order to reduce the positive bending moment and deflection in the mid-span of the space frame, and to make the internal force distribution more uniform, the space frame should be The racks are arranged with a space frame of cantilevers of a certain length. For single-span multi-point support space frames, the cantilever length should be 1/3 of the middle span, and for continuous-span multi-point support space frames, it should be 1/4 of the middle span. The double-layer space frame is widely used, and the structure selection of the point-supported double-layer space frame is discussed in detail below.

2.Structure form of double layer space frame

The structure form and application scope of the double-layer space frame are shown in Table 1.

The 3 types of different 13 kinds double layers space frames as shown in the table1, two-way orthogonal space frame, rectangular square pyramid space frame and Evacuate square pyramid space frame can be made in both rectangular plane type and adopt the point support mode, therefore, these three types of space frame are more suitable for use in toll stations. It can be seen from the calculation and analysis of the three flat space frame structures suitable for toll stations. The stress characteristics of two-way orthogonal space frames are closer to those of space truss structures, while the stress characteristics of rectangular square pyramid space truss and evacuate square pyramid space frame belong to the standard space frame structures. The internal force distribution of the three structures is different. Under the same load conditions, the deflection of the evacuate square pyramid space frame is the smallest, and the smallest amount of steel used is the evacuate square pyramid space frame. Therefore, in order to meet the bearing capacity and use requirements, we should preferentially choose the evacuate square pyramid space frame, its economic performance is better than the other two types of the space frame.