Types of Structures

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.

Beams. Beams are usually straight horizontal member and primarily designed to resist bending moment; however, if they are short and carry large loads, the internal shear force may become quite large and may govern their design. Beams are classified according to the way they are supported as indicated in Figure 1.

• Simply supported beam is a beam that supported at both ends which are free to rotate and have no moment resistance.
• Fixed supported beam is a beam that supported at both ends and restrained from rotation.
• Overhanging beam is a simple supported beam extending beyond its support at one end.
• Double overhanging beam is a simple beam with both ends extending beyond its supports.
• Continuous beam is a beam extending over more than two supports.
• Cantilever beam is a beam that supported at only one end and restrained from rotation.

a. Simply Supported Beam	b. Fixed Supported Beam	c. Overhanging Beam
d. Double Overhanging Beam	e. Continuous Beam	f. Cantilever Beam

Figure 1. Beam Classification Based on Supporting Type

Columns. Columns are generally vertical and resit axial compressive loads. Metal buildings usually use wide-flange cross sections as columns, while concrete buildings usually use square and circular section with reinforcing bar. Occasionally, columns are subjected to both an axial load and bending moment. These members are referred to as beam columns.

Types of Structures.

Trusses. As spans increase the use of beams becomes uneconomical. For moderately long span trusses are commonly used. Trusses consist of slender elements connected at their ends (joints) by hinged connections arranged in triangular fashion to form a stable configuration. When the loads are applied at the joints, ideal trusses are always either in uniform tension cause member elongate or in uniform compression cause member shorten. Real trusses, although some bending formed in the members when it is loaded, in most cases such secondary stresses are small, and the assumption of pure tension and compression are satisfactory. Trusses are commonly used in bridge and roof structure. Based on their shape, trusses are classified into several types shown in Figure 2.

a. Pratt Truss

b. Howee Truss

c. Warren Truss

d. Parker Truss

e. Baltimore Truss

f. Fink Truss

g. K Truss

Figure 2. Types of Plane Trusses

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.

Figure 4. Arch Structure. Bridge Deck Supported by Column

Figure 5. Arch Structure. Bridge Deck Supported by Hanger

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.