Engineering Mechanics & Design of Steel Structures: Tension Members

Tension Member

Tension members are linear members in which axial forces act so as to elongate (stretch) the member. A rope, for example, is a tension member. Tension members carry loads most efficiently since the entire cross section is subjected to uniform stress. Unlike compression members, they do not fail by buckling. Ties of trusses, suspenders of cable stayed and suspension bridges, suspenders of buildings systems hung from a central core (such buildings are used in earthquake prone zones as a way of minimizing inertia forces on the structure), and sag rods of roof purlins are other examples of tension members.

Tesnsion Members in Structures

Cross Sections of Tension Members

Introduction

1. Tension member has no stability problem.
2. In tension, member net section will be effective whereas in compression member gross section is effective.

Net Sectional Area

(i) For plate

Net area = (b x t) – nd't 

where,

s₁ = Distance between two consecutive rivets in the direction of load, also called pitch.

g₁ = Distance between two consecutive rivets perpendicular to the direction of load also called gauge.

b = Width of the plate

n = Number rivets at the section

T = Thickness of the plate

d' = Gross diameter of the rivet

(ii) Single angle connected by one leg only.

(a) 

where, A₁ = Net cross-section of area of the connected leg.

A₂ = Gross cross-sectional area of unconnected leg. (out stand)

(b) 

(c) 

(d) 

(e) 

(iii) For pair of angle placed back to back (or a signal tee) connected by only one leg of each angle (or by the flange of a tee) to the same side of a gusset plate: or it the two angles are tagged along a-a.

where, A₁ = Area of connected leg

A₂ = Area of outstand (unconnected leg)

(c) The area of a web of tee = Thickness of web x (depth – thickness of flange)

(d) The outstand legs of the pair of angles should be tacked by rivets of a pitch not exceeding 1 m.

(iv) If two angles are places back to back and connected to both sides of the gusset plate. Then

 when tack riveted.

If not tack riveted then both will be considered separately and case (ii) will be followed 

Permissible Stress in Design

• The direct stress in axial tension on the effective net area should not exceed σ_at
  where σ_at = 0.6f_y
  and f_y = minimum yield stress of steel in MPa

Lug Angle

The lug angle is a short length of an angle section used at a joint to connect the outstanding leg of a member, thereby reducing the length of the joint. When lug angle is used k = 1

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