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Thursday, December 25, 2008

what are Cables, Arches, Trusses, Beams, Surface; Membranes, plates and shells

Cables are usually flexible and carry their loads in tension, cables are commonly used to support bridges and building roofs. When we use cables fro these purposes , the cable has an advantage over the beam and the truss, specially for spans that are greater than 46m, because they are always in tension, cables will not become unstable and suddenly collapse, as may happen with beams or trusses.

achieves its strength in compression, since it has a reverse curvature to that of the cable. The arch must be rigid, however, in order to maintain its shape, and this results in secondary loadings involving shear and moment, which must be considered in its design. Arches are frequently used in bridge structures, dome roofs and for opening in masonry walls.

Truss is a structure comprising one or more triangular units which are constructed with straight slender members whose ends are connected at joints. A plane truss is one where all the members and joints lie within a 2-dimensional plane, while a space truss has members and joints extending into 3 dimensions.

There are two basic types of trusses. The pitched truss or common truss is characterized by its triangular shape. It is most often used for roof construction.

Beams are primarily designed to resist bending moment; however, if they are short and carry large loads, the internal shear force may become quite large and this force may govern their design.

A beam is a structural element that carries load primarily in bending (flexure). Beams generally carry vertical gravitational forces but can also be used to carry horizontal loads (i.e. loads due to an earthquake). The loads carried by a beam are transferred to columns, walls, or girders, which then transfer the force to adjacent structural compression members.

In Light frame construction the joists rest on the beam.

Beams are characterized by their profile (the shape of their cross-section), their length, and their material. In contemporary construction, beams are typically made of steel, reinforced concrete, or wood. One of the most common types of steel beam is the I-beam or wide-flange beam (also known as a "universal beam" or, for stouter sections, a "universal column"). This is commonly used in steel-frame buildings and bridges. Other common beam profiles are the C-channel, the hollow structural section beam, the pipe, and the angle.

Surface structure; A surface structure is made from a material having a very small thickness compared to its other dimensions.

Membranes; The most commonly specified sheet materials are self-adhering rubberized asphalt membranes. These 60-mil-thick membranes are composed of rubberized asphalt laminated to a waterproof polyethylene film. The asphalt side is incredibly sticky but is covered by a release paper, which you remove during application.

Plates; A surface plate is a solid, flat plate used as the main horizontal reference plane (datum) for precision inspection, marking out (layout), and tooling setup.

Surface plates must be calibrated on a regular basis to ensure that chipping, warping or wear has not occurred. A common problem with surface plates are specific areas or a section that is frequently used by another tool (such as a height gage) that will cause wear to a specific point resulting in an uneven surface and reduced overall accuracy to the plate. Tools and workpieces may also cause damage when dropped on the surface plate or when material chips have not been removed. This will result in erroneous measurements and can only be fixed by resurfacing the plate.

Shells; Surface structure may also be made of rigid material such as reinforced concrete. As such they may be shaped as folded plates, cylinders, or hyperbolic paraboloids, and are referred to as thin plates or shells.


• Structural analysis / R.C Hibbeler / 6th edition

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