The basic function of the membrane structure is to carry the load. The size and effectiveness of its carrying capacity depends on the building materials and structural forms used. And the two are interrelated. At first, the only building materials that humans could use were natural stone and wood, and because the wood was susceptible to moisture and decay, in order to obtain structures with permanent use functions such as pyramids, stone was almost the only choice. When the stone is not easy to obtain or the cost of obtaining is too high, the sun l'adobe and fired brick become its substitute. However, the size of stone, brick and other blocks are relatively small. To get the basic components that can bear the load, such as walls, roofs, columns, piers, etc., they must be bonded together. The bonding materials often used are plaster, mortar, etc. The component made of two components of block and dry knot material is called masonry, which is the structural component of the site.
It is clearly desirable to increase the use of tension members because tension members do not break until they reach tensile strength, while slender compression members tend to buckle before they reach compressive strength. Another effective way to reduce the weight of the structure is to apply prestress, so as to convert the adverse compressive stress into tensile stress, which is widely used in building structures and bridge structures (Schlaich, 2001)0
In addition to improving the form of structural components, another effective way to reduce the weight of the structure is to adopt a reasonable structural system. If the external load of the structure is the same, the self-weight of the structure that can spread the load to all directions, so that the components of the whole structure work together to achieve equal strength (reliability) design must be the smallest. The structure with this force characteristic is the spatial structure.
The thin shell is the earliest form of modern space structure. Due to the small thickness of the thin shell structure, the out-of-plane bending stress and torsional stress of the shell are relatively small when subjected to external loads, and the main stress is the in-plane stress (also known as film stress). The flow direction of in-plane stress is set up in a body shape and the force transmission path is simple, which makes the concrete shell become the most important spatial structure form in the 1960 s and 1970 s.
As mentioned above, after fully understanding the force characteristics of the solid abdominal beam, people gradually hollowed out part of its abdominal material to form a plane analysis frame; in order to improve the stiffness and carrying capacity of the analysis frame, people also arranged the plane analysis frame in two directions to form a space analysis frame, which in turn developed into a (flat) grid frame. Similarly, in order to reduce the self-weight of the thin shell of reinforced concrete and give full play to the characteristics of steel, which is a light and high-strength material, part of the reinforced concrete material in the shell is hollowed out, and the remaining reinforced concrete is replaced by steel members to form a steel mesh shell. Generally, the grid and the reticulated shell are collectively referred to as the spatial grid structure.
Under the action of external load, some members in the space grid structure are in tension and some members are in compression. For tension members, the cross-sectional size is controlled by strength and stiffness. The section size of the compression member is controlled by the stable bearing capacity; at the same time, in order to facilitate factory processing and on-site installation, the types of components must be reduced, which results in only some components being full stress, while other components are in a "strength surplus" state, so that the high strength performance of the material can not be fully utilized.
Suspension structure may be another structural form to make up for this defect. It uses high-strength steel cables under tension as the main load-bearing components instead of rigid components of spatial grid structures to form flexible grid structures. At this time, there is basically no stability problem in the structure, and the high-strength performance of steel can be utilized to the maximum extent.
It should be noted that the separation of the stressed structure from the roof envelope occurred during the development from the continuous shell structure to the grid structure. For the continuous shell structure, the shell is both the force structure and the cover enclosure structure, while for the grid structure, the enclosure structure must be attached to the force grid in order to transfer various live loads to the force grid and meet the functional requirements of the building. The enclosure components of the grid structure generally use various forms of plates. At this time, although the span of the board is only the size of a single grid, which is much lower than the span of the entire structure, it no longer has the spatial force characteristics of the shell. Compared with the shell, its thickness does not decrease in equal proportion with the decrease of the span, therefore, the self-weight per unit area of the grid roof structure (including the covering layer) is not necessarily lower than the self-weight of the reinforced concrete shell, but the span of the grid structure can be improved only due to the use of high-strength materials. In this way, it can be seen that if a lightweight covering material with a certain strength and load transfer can be developed, it will make a great contribution to reducing the weight of the structure.
Engineering Guide