Discussion on the Selection of Space Frame Support (Part 1)

Compared with the traditional concrete structure, the space frame structure has the advantages of large span, low dead weight, small amount of steel, flexible modeling and so on. The space frame structure is widely used in various public buildings. At the present stage, the design software of space frame support is becoming more and more mature, but there is no clear instruction on the software and specification of support selection. The design of space frame support has become the focus of the design of space frame structure and the difficulty of the difficulty. This paper introduces the selection of space frame support under several common support system for discussion.

1、Independent column support system

The common type of independent column support space frame is gas station, large transportation public buildings. This kind of structure has clear force, there is no beam knot between the columns, each column can be independent deformation without interference, can use independent calculation, can also use integral modeling calculation. The horizontal stiffness of independent columns can be calculated according to 3EI/l3. The horizontal stiffness is controlled by the column height. For 1m× 1m concrete square columns, when the column is taller than 5m, the column stiffness is less than 6kN/mm. For the grid support seat, the stiffness is small enough to ignore the effect of temperature. Instead, fixed hinged supports are used to transmit horizontal seismic forces and tensioned independent columns. For the lower supporting column is large and the plane ultra-long space frame structure, if all fixed hinged supports are set, the side column will have a large horizontal displacement, and the surrounding column space frame support can be changed into elastic support to deal with it.

2、Embedded support system around

This type is usually the roof lighting top, the supporting column is surrounded by beam plate constraints, the deformation of each column coregulation, and the horizontal stiffness is large. This type of space frame can not be generalized all the fixed hinged supports, so the design will make the horizontal reaction of the support under the effect of temperature is great, the space frame design is not economic, embedded parts design difficulties and other problems, also can not all use two-way sliding support to make the space frame floating on the main body under the earthquake. This type of structure can be integrated with fixed hinged supports, one-way sliding supports and two-way sliding supports so that the space  frame can not only extend freely under the action of temperature but also transmit force reliably under the action of earthquake. The specific operation is to assume a support as a fixed hinge, after fixing this point to do magnification or reduction of the space frame, compared with the movement path of the remaining support, make a judgment of the sliding direction required by the support, and finally according to the sliding square direction of the assumed trial calculation, the trial can be adjusted according to whether the natural vibration frequency of the space frame is the whole body translation, then the sliding stiffness. If the whole frame drifts, the elastic stiffness of the sliding side can be appropriately increased to provide constraints.

FIG. 1 Schematic diagram of the fixed embedded support layout of the space frame

Figure 1 assumes that point A is fixed hinge. Under the action of temperature rise, the space frame expands to the dotted line position. The vertical support of point A, namely point B and point C, only has unidirectional horizontal displacement, while the remaining points, namely point D, have bidirectional horizontal displacement. In accordance with this principle, fixed hinge at point A, unidirectional sliding hinge at point B and point C, and double-directional sliding hinge at point D can be arranged to release the temperature stress. Under the action of earthquake, point A and point B bear the X-direction horizontal force, and point A and point C bear the Y-direction horizontal force. The sliding direction stiffness generally takes 0~3kN/mm to release the displacement well. If the main mode is horizontal slip, the elastic stiffness in the sliding direction can also be appropriately increased, not more than 10kN/mm, so that the main mode is vertical vibration. For this kind of structure, if the above bearing selection is adopted, an independent calculation model can be used to simplify the design work. Considering that the horizontal stiffness and vertical stiffness of the substructure tend to be infinite, it is only necessary to input the bearing stiffness, which is little different from the overall calculation results.