1. Design of Tension Members

A member that supports axial tension loads is defined as a tension member. 

Tension members are simple structural elements to design, with perhaps the simplest being concentrically loaded uniform tension members, as they are nominally in a state of uniform axial stress. However, a tension member is not always connected concentrically. In many cases the fabrication of tension members is simplified by making their end connections eccentric, but this will induce bending  moments which interact with the tensile loads leading to a reduction in the ultimate strength. 

The effect of the bending action caused by eccentric connections is dealt with in AS 4100.

by introducing a correction factor kt.

 

AS 4100 provides two criteria which a tension member must meet: 

(i) yield : 

The stress at which steel begins to deform plastically (permanent deformation starts).

Up to this point → deformation is elastic (returns to original shape when load is removed).

Beyond yield → steel continues to deform without much increase in load.

Failure in yield means the member has reached its design limit because it can no longer carry additional load without large permanent deformations.

(ii) ultimate strength: 

The maximum stress the steel can sustain before it starts to neck and eventually fracture.

This occurs after yielding and strain-hardening.

Failure in ultimate strength means the member fractures (breaks apart).

Yield strength (Fy): governs serviceability & safety → we don’t want structures to undergo large permanent deformations.

Ultimate strength (Fu): governs ductility & collapse → ensures structure won’t snap suddenly without warning.

The logic behind these criteria is explained below. 

 

A ductile steel member loaded in axial tension can be expected to yield at a load N * = fy Ag where fy is the yield stress and Ag is the gross cross-sectional area. Although it will not fracture at this load because of strain hardening, it is unlikely to serve its purpose in the structure if it elongates excessively. Hence the yield criterion Nt = Ag fy.

The ultimate strength criterion is a little more complex. A tension member with bolted end connections will tend to yield first at a cross section containing one or more bolt holes, but this limited local yielding does not constitute failure because the overall increase in length of the member is negligible. However, the member may fail by fracture through the bolt holes at a load smaller than that required to cause general yielding on the gross area along the member length. Hence the second criterion, Nt = 0.85 kt An fu, where An is the net cross sectional area, fu is the ultimate tensile strength, kt as explained above is a factor to account for eccentric loading and 0.85 is a further safety or capacity factor.

Example: Truss Member in Tension

1. An equal angle section of Grade 250/300/350 steel is to be used in a truss such as the one shown in Fig. below. It is to be connected at the ends by welding one leg of the angle to a plate.  Select a suitable section for a factored design axial tension force N* = 100 kN. 

Truss structures spanning a clarifier in a water purification plant

2. Fig. below shows a compound tension member made up of 2L150x100x12 UA in Grade 300 steel (fu = 440 MPa, fy = 300 MPa). Determine the maximum design force N* that can be transmitted by the angles. Assume bolt holes are 2 mm larger than bolt diameter to allow for misalignment. Do not consider shear on the bolts, bearing or tearing at the holes. All bolts are M30 

Grade 8.8/S.