The Constructor

Types of Tension Members in Structural Steel Construction

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Types of Structural Steel Tension Members A tension member is designed as a structural member subjected to tensile force in a direction parallel to its longitudinal axis. A tension member is also called a tie member or simply a tie.

Types of Tension Members

The types of structure and method of end connections determine the type of a tension member in structural steel construction. Tension members used may be broadly grouped into four groups.
  1. Wires and cables,
  2. Rods and bars
  3. Single structural shapes and plates
  4. Built-up members

(i) Wires and Cables

The wire types are used for hoists, derricks, rigging slings, guy wires and hangers for suspension bridges.

(ii) Rods and Bars

The square and round bars are shown in figures are quite often used for small tension members. The round bars with threaded ends are used with pin-connections at the ends instead of threads. Figure: Square and circular rods and bars The ends of rectangular bars or plates are enlarged by forging and bored to form eye bars. The eye bars are used with pin connections. The rods and bars have the disadvantage of inadequate stiffness resulting in noticeable sag under the self weight.

(iii) Single Structural Shapes and Plates

The single structural shapes, i.e. angle sections and tee-sections as shown in figures are used as tension members. The angle sections are considerably more rigid than the wire ropes, rods and bars. When the length of tension member is too ling, then the single angle section also becomes flexible.

Figure: Single structural shapes for tension member

The single angle sections have the disadvantage of eccentricity in both planes in a riveted connection. The channel section has eccentricity in one axis only. Single channel sections have high rigidity in the direction of web and low rigidity in the direction of flange. Occasionally, I-sections are sued as tension members. The I-sections have more rigidity, and single I-sections are more economical than built up sections.

(iv) Built-up Sections

Two or more than two members are used to form built up members. When the single rolled steel section can not furnish the required area, then built-up sections are used. The double angle sections of unequal legs shown in the figure are extensively used as tension members in the roof trusses. The angle sections are placed back to back on two sides of a gusset plate. When both the angle sections are attached on the same side of the gusset, then built-up section has eccentricity in one plane and is subjected to tension and bending simultaneously. The two angle sections may be arranged in the star shape (i.e. the angles are placed diagonally opposite to each other with leg on outer sides). The star shape angle sections may be connected by batten plates. The batten plates are alternatively placed in two perpendicular directions. The star arrangement provides a symmetrical and concentric connection. Two angle sections as shown in the figure (a) are used in the two-plane trusses where two parallel gussets are used at each connection. Two angle sections as shown in figure (b) have the advantage that the distance between them could be adjusted to suit connecting members at their ends. Four angle sections as shown in figure (c) are also used in the two-plane trusses. The angles are connected to two parallel gussets. For angle sections connected by plates as shown in figure (d) are used as tension members in bridge girders.

Figure: Built up steel sections

A built-up section may be made of two channels placed back to back with a gusset in between them. Such sections are used for medium loads in a single plane-truss. In two-plane trusses, two channels are arranged at a distance with their flange turned inward. It simplifies the transverse connections and also minimizes lacing. The flanges of two channels are kept outwards, as in the case of chord members or long span girders, in order to have greater lateral rigidity. The heavy built-up tension members in the bridge girder trusses are made of angles and plates. Such members can resist compression in reversal of stress takes place.
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