It is important for a structural engineer to recognize the various types of elements composing a structure and to be able to classify structures as their form and function.
Structural Elements.
Commonly used structures can be classified into several basic categories as follows.
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Figure 3. Frame Structure |
Frames. Frames are composed of beams and columns that are either rigid (moment-resisting) connection or by hinged connection to form stable configurations. Unlike trusses, which are subjected only to joint loads, the external loads on frames may be applied on the members as well as on the joints. The members of frame subjected to bending moment, shear, axial (compression or tension), torsion (for space trusses) under the action of external loads. Structural steel and reinforced concrete frames are commonly used in multistory buildings and industrial plants. Example shape of frame shown in Figure 3.
Arches. The use of trusses to support bridge deck becomes
impracticable for longer than moderate spans. In this situation arches
are commonly used. Figure 4 shows an arch in which the bridge deck is
carried by columns supported, in turn, by the arch. Alternatively the
bridge deck may be suspended from the arch by hangers as shown in Figure
5. Arches carry most of their loads by developing compressive stresses
within the arch itself.
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Figure 4. Arch Structure. Bridge Deck Supported by Column |
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Figure 5. Arch Structure. Bridge Deck Supported by Hanger |
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Cables. Other structures than arches to support long-span bridges are cables. Because of their flexibility, cables have negligible bending stiffness and can develop only tension. Thus, under external loads, a cables adopts a shape that enables it to support the load by tensile force alone. In other words, the shape of a cable changes as the loads acting on it change. One of the popular cable structure is
suspension bridge. In suspension bridge, the roadway suspended from two main cables by means of vertical hangers. The cables pass over saddles on the tops of tower and are fixed at each end within the ground by massive anchor blocks. The weakness of suspension bridge and other cable structure is lack of stiffness in lateral directions, they are susceptible to wind-induced oscillations. Bracing of stiffening system are therefore provided to reduce such oscillations
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