Design of Tension Member, types | Civil Final Year students. |Steel Structures Design.

 INTRODUCTION

When a tension member is subjected to axial tensile force, then the distribution of stress over the cross-section is uniform. The complete net area of a member is effectively used at the maximum permissible uniform stress. Therefore, a tensile member subjected to axial tensile force is used to be efficient and economical member. The procedure of the design of a tension member is explained below with help of example problems.

Tension members :- Are structural elements that are subjected to axial tensile forces. Examples of tension members are bracing for buildings and bridges, truss members, and cables in suspended roof systems.

When things feel so tight they might snap, that's tension. The noun tension has its Latin roots in tendere, which means to stretch, and tension occurs when something is stretched either physically or emotionally. ... Strained relations between countries can cause political tensions to rise.

Types of Tension Members

i)Wires and cables,

ii)Rods and bars.

iii)Single structural shapes and plates.

iv)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.

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.

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.

STEPS TO(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.

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.

 BE FOLLOWED IN THE DESIGN OF A TENSION MEMBER

The following steps may be followed in the design of axially loaded tension members.

Corresponding to the loading on the structure of which the tension member is a part, the tensile force in the member is first computed.

The net area required for the member is determined by dividing the tensile force in the member by the permissible tensile stress.

Now, a suitable section having gross area about 20 per cent to 25 per cent greater than the estimated area is selected. For the member selected deductions are made for the area of rivet holes and the net effective area of the section is determined. If the net area of the section of the member so determined is greater than the net area requirement estimated in step i, the design is considered safe.

The slenderness ratio of a tension member shall not exceed 400. In the case of a tension member liable to reversal of stress due to the action of wind or earthquake, slenderness ratio shall not exceed 350. If the reversal of stress is due to loads others than wind or earthquake, the slenderness ratio shall not exceed 180.

Example 1: Determine the tensile strength of the 12 mm thick plate shown in Fig 9.1. Rivets used for the connection are 20 mm diameter. Allowable tensile stress is 150 N/mm2.

Solution

Diameter of the rivet hole       = 20 + 1.5 = 21.5 mm

The critical section to be considered is a section like ABCDE.

Effective width at critical section           = b – nd = 180 – (3 x 21.5) = 115.5 mm

Effective net area                                 = 115.5 x 12 mm2              = 1386 mm2

Strength of plate                                  = 1386 x 150                      = 207900 N     = 207.9 kN.