Assuming pin-ended members and loads applied at the nodes, the members of one-way systems such as trusses or two-way systems such as spatial structures will be primarily in tension or compression and not bending. It has to be noted that in reality the members of trusses or spatial structures are not pin-ended, however, this is assumed for the sake of simplicity in the structural analysis and design, as it generally results in a more conservative structural system. Due to the two way action and modular form of most spatial structural systems, a symmetrical layout is usually the best choice. The goal is to distribute loads over as large a distance as possible.
The compression members of spatial structures have to be designed to prevent buckling.
The load carrying capacities of spatial structures are limited due to the restricted capacities of the nodes. When the structure is highly loaded, welded connections should be used.
When the spatial structure is made of the same size member, it usually performs in a more ductile manner. This means that the structure does not suddenly fail when subjected to overloads. In contrast, if the spatial structure is optimally designed, i.e., the member sizes are varied based on their internal forces, the structure behaves in a more brittle manner and its failure occurs abruptly if subjected to overloads.
Member axial forces in group A have the same direction, i.e., the top layer members are all in compression and bottom layer in tension. For this group (with supports along all edges) the maximum forces on the top and bottom layers (bending moment, considering the slab analogy) are at the center of the grid and the maximum forces in the bracing members occur at the center of the edge (maximum shear force).
For groups B, C, and D supported along the edges there is a reversal of stresses in their corner members (the top layer members are in compression at the center of the bay but turn to tension towards the corners).
The square-on-diagonal configuration provides a more uniform stress distribution and less deflection than the diagonal-on-square.
The top layer of some spatial structures directly support the decking and therefore the members are subjected to axial and bending stresses. The nodes should allow easy fabrication, and provide needed tolerances for field assembly and maintenance.
Providing the joint rigidities (moment connections instead of pin connections) for flat double-layer grids changes the stress distribution by a small amount (about 10%-15%), however, supports' location, type and number have larger effects on member forces and maximum deflection.
If the supports are considered as pinned versus roller the following effects may be expected:
A reduction in the top layer forces with more even distribution, however, the change in the bottom layer forces is not significant.
Member Forces of Spatial Structure with Pin-Pin and Pin-Roller Supports (Red=Compression, Blue=Tension)
A reduction in the deflections. As compared to the use of roller supports the deflections can be reduced by up to 70 %.
Deflection of Spatial Structure with Pin-Pin and Pin-Roller Supports
