Tension member

A tension member is a structural element designed to carry loads primarily through tensile forces, meaning it is subjected to stretching rather than compression or bending. These members are integral components in engineering and architectural structures, such as trusses, bridges, towers, and suspension systems, where they provide stability, distribute loads, and resist deformation. Typically made from high-strength materials like steel, wire ropes, or composites, tension members are valued for their efficiency in transferring forces along their length while maintaining lightweight and durable construction. Their design and performance are crucial in ensuring the safety and functionality of structures subjected to dynamic and static loads.

Calculation

In an axially loaded tension member, the stress is given by:

F = P/A

where P is the magnitude of the load and A is the cross-sectional area.

The stress given by this equation is exact, knowing that the cross section is not adjacent to the point of application of the load nor having holes for bolts or other discontinuities. For example, given an 8 x 11.5 plate that is used as a tension member (section a-a) and is connected to a gusset plate with two 7/8-inch-diameter bolts (section b-b):

The area at section a - a (gross area of the member) is 8 x ½ = 4 in²

However, the area at section b - b (net area) is (8 – 2 x 7/8) x ½ = 3.12 ⁱⁿ²

knowing that the higher stress is located at section b - b due to its smaller area.

Design

Designers typically adhere to standardised design codes when specifying tension members, which are critical components of structural systems. In the United States, the Steel Construction Manual published by the American Institute of Steel Construction (AISC) is the primary reference for structural steel design, while in Europe, the design is guided by the Eurocodes published by the Comité Européen de Normalisation (CEN). These codes provide comprehensive guidelines to ensure the safety, reliability, and efficiency of tension member designs. Other similar design codes are: GB 50017 in China, IS 800 in India and AS 4100 in Australia.

The design of tension members requires careful analysis of potential failure modes, specifically yielding (excessive deformation) and fracture, which are referred to as limit states. The governing limit state is the one that results in the lowest design strength, as it dictates the member's capacity and prevents structural failure.

Under the AISC code, the ultimate load on a structure can be determined using specific load combinations, ensuring the tension members can withstand the applied forces while maintaining the integrity of the structure. The ultimate load on a structure can be calculated from one of the following combinations:

1.4 D

1.2 D + 1.6 L + 0.5 (L_r or S)

1.2 D + 1.6 (L_r or S) + (0.5 L or 0.8 W)

1.2 D + 1.6 W + 0.5 L + 0.5 (L_r or S)

0.9 D + 1.6 W

L= 14

• D… is the dead load or the weight of the structure itself
• L… is the live load which vary for different structures
• S… is the snow load
• W… is the wind load

the central problem of designing a member is to find a cross section for which the required strength doesn't exceed the available strength:

P_u < ¢ P_n where P_u is the sum of the factored loads.

to prevent yielding

0.90 F_y A_g > P_u

to avoid fracture,

0.75 F_u A_e > P_u

therefore, the design must consider the loads applied to this member, the design forces acting on this member (M_u, P_u, and V_u) and the point where this member would fail.

See also

• Compression member