CLASSIFICATION OF STRUCTURES | Tension Structures | Compression Structures | Trusses Structures |Shear Structures | Bending Structures
CLASSIFICATION OF STRUCTURES | Tension Structures | Compression Structures | Trusses Structures |Shear Structures | Bending Structures
INTRODUCTION :
CLASSIFICATION OF STRUCTURES:
As AAs discussed in the preceding section, perhaps the most important deci- sion made by a structural engineer in implementing an engineering pro- ject is the selection of the type of structure to be used for supporting or transmitting loads. Commonly used structures can be classified into five basic categories, depending on the type of primary stresses that may develop in their members under major design loads. However, it should be realized that any two or more of the basic structural types described in the following may be combined in a single structure, such as a build- ing or a bridge, to meet the structure’s functional requirements.
1. Tension Structures :
The members of tension structures are subjected to pure tension under the action of external loads. Because the tensile stress is distributed uni- formly over the cross-sectional areas of members, the material of such a structure is utilized in the most e‰cient manner. Tension structures composed of flexible steel cables are frequently employed to support bridges and long-span roofs. Because of their flexibility, cables have negligible bending sti¤ness and can develop only tension. Thus, under external loads, a cable adopts a shape that enables it to support the load by tensile forces alone. In other words, the shape of a cable changes as the loads acting on it change. As an example, the shapes that a single cable may assume under two di¤erent loading conditions are shown in Fig. 1.3.
2.Compression Structures :
Compression structures develop mainly compressive stresses under the action of external loads. Two common examples of such structures are columns and arches. Columns are straight members subjected to axially compressive loads, as shown in Fig. 1.6. When a straight member is subjected to lateral loads and/or moments in addition to axial loads, it is called a beam-column.
An arch is a curved structure, with a shape similar to that of an in- verted cable, as shown in Fig. 1.7. Such structures are frequently used to support bridges and long-span roofs. Arches develop mainly compressive stresses when subjected to loads and are usually designed so that they will develop only compression under a major design loading. How- ever, because arches are rigid and cannot change their shapes as can cables, other loading conditions usually produce secondary bending and shear stresses in these structures, which, if significant, should be con- sidered in their designs.
Because compression structures are susceptible to buckling or in- stability, the possibility of such a failure should be considered in their designs; if necessary, adequate bracing must be provided to avoid such failures.
3.Trusses:
Trusses are composed of straight members connected at their ends by hinged connections to form a stable configuration (Fig. 1.8). When the loads are applied to a truss only at the joints, its members either elon- gate or shorten. Thus, the members of an ideal truss are always either in uniform tension or in uniform compression. Real trusses are usually constructed by connecting members to gusset plates by bolted or welded connections. Although the rigid joints thus formed cause some bending in the members of a truss when it is loaded, in most cases such secon- dary bending stresses are small, and the assumption of hinged joints yields satisfactory designs.
Trusses, because of their light weight and high strength, are among the most commonly used types of structures. Such structures are used in a variety of applications, ranging from supporting roofs of buildings to serving as support structures in space stations.
4.Shear Structures:
Shear structures, such as reinforced concrete shear walls (Fig. 1.9), are used in multistory buildings to reduce lateral movements due to wind loads and earthquake excitations. Shear structures develop mainly in- plane shear, with relatively small bending stresses under the action of external loads.
5.Bending Structures:
Bending structures develop mainly bending stresses under the action of external loads. In some structures, the shear stresses associated with the changes in bending moments may also be significant and should be con- sidered in their designs.
Some of the most commonly used structures, such as beams, rigid frames, slabs, and plates, can be classified as bending structures. A beam is a straight member that is loaded perpendicular to its longitudinal axis (Fig. 1.10). Recall from previous courses on statics and mechanics of materials that the bending (normal) stress varies linearly over the depth
of a beam from the maximum compressive stress at the fiber farthest from the neutral axis on the concave side of the bent beam to the max- imum tensile stress at the outermost fiber on the convex side. For ex- ample, in the case of a horizontal beam subjected to a vertically down- ward load, as shown in Fig. 1.10, the bending stress varies from the maximum compressive stress at the top edge to the maximum tensile stress at the bottom edge of the beam. To utilize the material of a beam cross section most e‰ciently under this varying stress distribution, the cross sections of beams are often I-shaped (see Fig. 1.10), with most of the material in the top and bottom flanges. The I-shaped cross sections are most e¤ective in resisting bending moments.
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