In recent years, space structure systems have gained widespread application in large-scale buildings such as theaters, hotels, and conference venues due to their advantages of simple load transmission, lightweight construction, high stiffness, excellent seismic performance, easy installation, and flexible floor plan layout. However, in the construction of space frame ceilings, some construction personnel often apply the ceiling load and equipment loads such as air conditioning and fire protection pipes directly to the lower chord of the space frame by welding or overlapping. This practice is inconsistent with the stress characteristics of space frame structures and poses a significant safety hazard.
- Calculation model and stress characteristics of space structures are spatial stress systems composed of rods and nodes. Their structural calculation models are usually rod systems. In rod systems, the mass of the rods is generally combined at the nodes, assuming that the nodes are hinged, and the loads within the area under the jurisdiction of the nodes are concentrated on the nodes according to the principle of static equivalence. In this way, the rods only bear axial forces and do not bear shear forces and bending moments.
- Treatment of ceiling loads in space structure design When designing a space structure, based on its calculation model, the ceiling load is statically equivalent to the lower chord node. There are two specific methods:
(1) When the ceiling load is a uniformly distributed load, it is directly added to the lower chord plane as a surface load;
(2) When the ceiling load is not uniformly distributed or is a localized load, the surface load value is multiplied by the lower chord grid size, and the resulting node load value is directly applied to the lower chord node.
- Loading principles for ceiling loads during space structure construction During space structure construction, the loading method for ceiling loads should comply with the calculation model of the space structure and the handling method for ceiling loads during design, that is, it must be based on the principle of not generating shear force and bending moment in the members, but only generating axial force. According to this principle, the ceiling load should first act on the nodes of the space structure, then transmitted through the nodes to ensure that members only experience axial forces, rather than directly applied to members to induce shear forces and bending moments. Of course, in actual engineering projects, members may also experience shear forces and bending moments, such as due to their own weight or construction loads from workers walking on the chords. However, this should not be used as a basis to conclude that applying ceiling loads and equipment loads directly to the chords is acceptable; If the lateral loads from equipment and ceilings applied to the chord members are too large, under the action of shear forces and bending moments, the members may fail, causing the internal forces in the space frame to redistribute and potentially exceed the design control values, leading to the instability of the space frame, which is very dangerous. Therefore, designers often specify on the drawings that regardless of the load size, it must be applied to the space frame nodes. Whether loads can be applied to the members or how much load can be applied must be determined through calculations.
- Methods for connecting ceiling loads to the space frame Based on the above principles, the methods for connecting ceiling loads (including equipment loads such as air conditioning and fire protection pipes) to space frame nodes primarily include the following three approaches.
(1) When the suspended load is relatively large, suspension rods can be installed on the lower chord spheres using M20 process holes. The suspension rods can be constructed using the same method as truss members, and suspension brackets can be welded onto the rods. The type of steel used can be selected based on the load size. The load is then applied to the suspension brackets. This method is suitable for applications such as stage ceilings in theaters, audience hall ceilings, and suspended maintenance platforms.
(2) For complex ceilings with sprinklers, air conditioning ventilation, walkways, etc., a secondary truss made of square steel can be constructed in a grid pattern. The secondary truss is then connected to the spherical nodes and ceiling joists. The steel consumption for the secondary truss should generally be controlled at 8–5 kg/m.
(3) When the ceiling load is small and uniformly distributed, such as in general lightweight steel stud gypsum board ceilings (2–30 kg/m), suspension rods made of 10–1 rebar can be used. The upper end is connected to the spherical nodes of the space frame using clamps, while the lower end is connected to the main ceiling beams.
Regardless of which connection method is used, welding on the space structure should be avoided during the installation of the ceiling and equipment.
