A steel tension member carries only direct axial forces that tend to stretch it. Bracing for structures and bridges, truss members, and cables in suspended roof systems are a few examples of tension members.
How to design steel member in tension?
Design of tension steel members in tension is governed by strength at two primary locations in the member: gross area and the net cross sectional area. Yielding of steel happens in gross area while tensile rupture or other types of failures appear in the locations of net area.
Design categories of Tension steel members as AISI 360
ANSI/AISC 360-16 Specification for Structural Steel Buildings gives the design guidelines for three categories of tension members:
- Tension members
- Pin connected tension members
- Eye bars in tension
Tension members are defined in general as any steel member experiencing forces in tension. A tension member could be also an eye member or pin connected member.
Pin connected tension members are special type of members under tension under tension, which are connected by pin joint. If the pin ended joint takes the form of an eye, it is called as an eye bar. Following picture illustrates the difference between them.
Slenderness limit for tension members
Tension member slenderness ration l/r must preferably be limited to 300
Failure modes/ Limit states of steel tension members
A steel member can fail in any of the following four failure mechanisms under tension
- Tensile Yielding
- Tensile rupture
- Shear Rupture
- Bearing failure
- Block shear
Tensile yielding and bearing failure are the ductile failure mechanisms and yielding strength of steel governs them. Shear and tensile ruptures and block shear are the brittle failure mechanisms, which are governed by the ultimate strength of steel.
Tensile yielding
It is the permanent elongation at the gross cross section of steel member due to yielding of steel.
Tensile rupture
It is the tensile rupture at the effective section of the tensile member. Affective section is defined as the product of shear lag factor and net area. Net area of the section is defined as the gross area minus the nominal area of holes. The nominal diameter of the hole (dh) is equal to the bolt diameter (db) + 1/16 in. The holes could be placed in staggered or non staggered manner. In case on non staggered holes, a term s2/4g should be also subtracted from gross area. If there are two rows of staggered holes, the term s2/4g should be subtracted once from gross area. If there are three rows of staggered holes, the term s2/4g should be subtracted twice from gross area and so on.
Shear lag occurs when the tension force is not delivered instantaneously to all cross-sectional parts. This happens when some cross-sectional parts are not linked. Shear lag factors is calculated using table D3.1 of ANSI/AISC 360-16 Specification for Structural Steel Buildings .
Shear rupture
It the internal shear localization of plastic strain due to tension forces
Bearing failure
It is the yielding of the steel around holes due to bearing pressure
Block shear
The tension member may fail owing to material ‘tear-out’ at the attached end. This is known as block shear. The ANSI/AISC 360-16 Specification for Structural Steel Buildings Chapter D on tension members does not expressly address block shear failure. However, it refers the engineer to Specification Section J4.3.
How to design a steel tension member
Here is the design flow chart.
