Is it a design defect that the space frame structure is not integrally modeled and calculated with the lower main structure?

Problem introduction

At 14:01 on Sep. 10, 2021, the dust-proof and noise-reduction construction shed at Jianshe North Road Station Phase II of Chengdu Metro Line 17 collapsed during construction (Figure 1), resulting in 4 deaths, 14 injuries, and direct economic losses of 6.5 million yuan.

The investigation report concluded that “there are defects in the design. There are defects in the design model of the space frame, and the substructure is not calculated as a whole with the space frame roof.” This is one of the reasons for the collapse.

Netizen discussion

1. Zhou

I don’t think it is a design defect. Even if there is no overall modeling analysis, it does not mean that the designer did not consider the horizontal stiffness of the column top. Besides, the construction drawings will be checked, proofread, approved, and the construction drawing reviewed. I personally think it is more Many places are collapsed due to irregular construction.

2. Chen X

Logically, this simple substructure should still be calculated as a whole.

3. Shenzhou

The collapse during the construction process, the main constant live load has not been applied, can not be considered a design defect, the space frame is a statically indeterminate structure, and even if the bearing capacity of individual rods is insufficient, it will not cause the overall collapse of the space frame.

4. Peng Baoning

This accident has little to do with whether the design unit has an overall modeling calculation, and the failure of the overall modeling calculation of the space frame and the lower main structure is not a design defect. When it collapsed, the load did not go up, it was piled up at that point, and the whole thing collapsed. The lack of overall modeling and insufficient bearing capacity of some members can only be regarded as hidden dangers, and the disengagement of the support is the direct cause. Looking at the investigation report, the steel column is large at the top and small at the bottom, and the mesh frame is connected with the upper and lower strings of the column. It should be designed according to the hinged column foot. Looking at the photos of the scene, the 23.9 m high column has no stabilization measures at all, and I am very lucky to be able to stand on my own. Irregular construction is the direct cause of the accident.

5. lich

Looking at the picture, the column in the space frame section is still made of a portal frame, that is, the bottom of the column is hinged and the top of the column is rigidly connected, and the top of the column transmits the bending moment. If the upper and lower chords are provided with hinge supports in the structural design calculation book, it is assumed that the lower structure is a rigid structure without deformation, and it is no problem to use the single model to calculate separately under the condition that this assumption is satisfied. If the substructure (the lower part is hinged) has a large deformation outside the plane, it is true that all the upper and lower chord supports should be welded in place during the construction stage, otherwise the system is a variable system.

6. Forward

According to the pictures after the collapse, the space frame is relatively complete as a whole, and it is basically damaged at the support. The space frame is hinged and connected with the column top and the column corbel support, and the column feet are also hinged. This structure forms a three-rigid hinged variable system. Large horizontal loads, or any factor that causes horizontal displacement of this variable system, will cause the structure to fail at the junction of the column top and the space frame (without waiting for the vertical load to reach the design load). Therefore, it is necessary for the overall modeling calculation to accurately simulate the structural stiffness.

7. Wanliyunfan

In this project, the column top and the upper and lower chords of the space frame are connected. The column top and the space frame should be considered as rigid connections, so there is indeed a problem with the separate calculation of the upper space frame and the lower structure. It’s not a big problem.

8.Mr_Roshan

The lower steel column is a cantilever structure. If the model is calculated separately, it will be extremely unsafe if the value of the bearing stiffness is incorrect. The overall model calculation will avoid this problem.

9.Du Diao than Jiang Xue

When only the single model of the upper space frame is used for calculation, it is difficult to give the horizontal stiffness of the support, because it is related to the horizontal stiffness of the lower support column, and the horizontal stiffness of the lower column is different in different positions, so it is impossible to accurately input each support for the calculation of the single space frame model If the horizontal stiffness is, the calculated internal force of the member must be different from the overall model considering the substructure.

Problem analysis

Article 10.2.7 of GB 50011-2010 Code for Seismic Design of Buildings states that the seismic effect of the roof structure itself is the result of the collaborative work with the substructure. Because the vertical stiffness of the substructure is generally large, in the past, it was usually used to only use the roof structure as the analytical model when calculating the vertical seismic action of the roof structure. However, studies have shown that if the cooperative work between the roof structure and the substructure is not considered, the seismic action of the roof structure, especially the calculation of the horizontal seismic action, will be significantly affected, and even wrong results will be obtained. Even in the calculation of vertical seismic action, when the vertical stiffness provided by the lower structure to the roof is weak or unevenly distributed, the results calculated only by the roof structure model will produce large errors. Therefore, considering the synergistic effect of the upper and lower structures is the basic principle for the calculation of the seismic action of the roof structure.

The most reasonable way to consider the synergy of the upper and lower structures is to perform the seismic action calculation according to the overall structural model. Therefore, for the irregular structure, the seismic calculation should use the overall structure model. When the substructure is relatively regular, some simplified methods (such as the equivalent elastic constraint of the support) can also be used to take into account the influence of the substructure. However, this simplification must be based on reliable and dynamic principles.

It can be seen from the above description that it is necessary to conduct an overall analysis of this space frame structure.

If the calculation method of separate modeling is adopted, the space frame is generally calculated and analyzed separately, and then the bearing force is added to the lower main structure in the form of concentrated force, so as to consider the influence of the space frame load on the lower structure. In this calculation method, for the space frame, if the horizontal stiffness of the support cannot be accurately simulated, it will directly affect the calculation results.

The most reasonable way to consider the synergy of the upper and lower structures is to perform the seismic action calculation according to the overall structural model. Therefore, for the irregular structure, the seismic calculation should use the overall structure model. When the substructure is relatively regular, some simplified methods (such as the equivalent elastic constraint of the support) can also be used to take into account the influence of the substructure. However, this simplification must be based on reliable and dynamic principles.

In addition, due to the space frame being very rigid, designers generally use some approximate methods to simulate the effect of the space frame stiffness on the main structure, such as: using virtual beams, equivalent beams, rigid members, rigid floor simulation, etc. However, these simulation methods are inaccurate, because no matter what method is used to simulate, the upper space frame is not truly built into the model, and the overall calculation and analysis of the upper space frame and the lower main structure cannot be realized, especially the space frame alone. Differences in dynamic characteristics between analysis and overall analysis. Therefore, these simplified processing methods cannot truly reflect the stiffness of the space frame, the real deformation, and vibration of the space frame, the influence of vertical earthquakes, the interaction between the upper and lower parts, and the large roof structure and the lower part cannot be accurately considered. The overall effect of the structure.

Therefore, the mesh frame is not modeled separately from the lower main structure, which is an approximate processing method, and there is a possibility of inaccuracy. The overall calculation of the mesh frame and the lower structure is essential. Of course, the separate calculations will not necessarily lead to the collapse of the space frame. As long as various assumptions are simulated accurately and a certain safety reserve is reserved, separate modeling calculations cannot be regarded as a design defect.