In a structural framework, a tension member is a component that carries a force in one direction throughout its length. A clear example of a tension member is a rope used to sustain a weight or a cable used in a suspension bridge. Yet, there are special circumstances in which a member, primarily a tension member, may also be subject to a bending moment, either because of the eccentricity of the longitudinal load or because of the action of transverse loads in addition to the primary longitudinal stress.
Tension member: Features
Structures’ tension members bear axial tensile loads- a tension member only experiences stretching from direct axial pressures. Many elements, such as connection length, fastener size and spacing, cross-sectional area, shear lag at the end connection, the eccentricity of the connection, and manufacturing method, all contribute to the tension components’ ultimate strength.
It is possible to attach the ends of a tension member using bolts or weld them. A tension member’s effective sectional area is less than its gross sectional area because of the bolt holes. Tension members are distinguished from others by their inability to buckle. Extending a tension part to its breaking point allows it to undergo elongation. The member fails when the tensile stress on a member reaches its ultimate value. Both excessive elongation and section rupture may lead to failure in a member under strain.
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Tension member: Types
Wires and cables
Derricks, hoists, hangers for suspension bridges, rigging slings and guy wires all use tension members of the wire kind. Wires are individually twisted around a central core to form a strand. Wire ropes have several strands that are coiled around a central body. Cables consist of several strands twisted in a spiral around a central conductor.
Bars and rods
Tiny tension members are often made from round or square bars. Instead of threads, circular bars with pin connections at either end are employed. When forging rectangular plates or bars, the ends are widened and drilled to create eye bars. The eye bars may be used as an alternative to pin connections. Bars and rods are large cross-sectional, straight members. The cross-section might be round, square, or rectangular. They are used alone as structural elements instead of in bundles like cables, wires and strands. They often have threaded ends to allow for easy bolting to other members.
Single structural plates and shapes
Tension members are formed from individual structural forms like angle and tee sections. If you compare the angle sections to the wires, cables, rods and bars, you’ll see they’re much stiffer. The single-angle sections can become flexible if the length of a tension member is excessive. Single-angle pieces may be prone to eccentricity in both planes when riveted together. Just one axis of the channel segment is eccentric. The flexural strength of the single channel sections is greatest in the web direction and lowest in the flange direction. Angles and other common structural steel sections are also utilised as tension members. The length and width of these are generally provided.
Built-up sections
Built-up members consist of two or more tension members. Built-up sections are utilised instead of singular structural steel sections when the former cannot provide enough surface area. Tension members in the roof trusses are double-angle sections with unequal legs. The gusset plate has angle portions attached to both of its sides. Tension and bending may be applied to the gusset plate when both angle sections are located on the same side of the plate. Similarly, built-up parts see extensive application in building. Many different types of standard sections and plates are used to create these.
Causes of tension member failures
A tension member may fail in several ways, and the section of the member should be built to avoid failure in any of these ways. There are three forms of tension member failure:
- Failure of a tension member may occur due to gross section yielding, which occurs when the member deforms too much and ultimately ruptures. Before breaking, a member of this class will undergo considerable longitudinal deformation. Such severe distortion renders a building useless. Hence, the yielding of the gross cross-section is a limiting parameter in the design strength.
- Tension members are often joined to the main or other members via bolts or welds, which may lead to a net section rupture if the connection fails. Tension members with bolt holes experience a rerouting of stress. These bolts experience stress concentration next to bolt holes during service loads because the bolt holes have decreased the cross-sectional area.
- A section of the block is detached from the member in this sort of tension member failure. It happens when the shear strength of the bolts is greater than the bearing strength of the material. The likelihood of a block shearing off due to this is raised. The failure mode known as block shear failure involves a route that simultaneously experiences stress in one plane and shear in another.
Tension member: Slenderness ratio
For tension members, the slenderness ratio is the ratio of their unsupported length to their smallest radius of gyration. Nevertheless, IS: 800 (2007) stipulates the maximum effective slenderness ratio as the ratio of the effective length of the member to the least radius of gyration. There is no theoretical constraint on the slenderness ratio of tension members due to the absence of buckling in this kind of member. Nonetheless, IS 800 has preserved a constraint over the maximum value of slenderness of tension members to prevent buckling during the reversal of loads.
Tension member: The role of shear delay
Loading the tension component at an angle away from the connection results in consistent stress. As we approach the junction, the forces on the outstanding leg gradually transmit to the linked leg. The result is that the strained leg is the one that is attached, while the unstressed leg is the one that is sticking out.
The shear lag effect causes the outstanding leg’s stress distribution to become uneven, leading to failure. Tension members’ power is diminished since their full potential is not exploited. Uneven angle sections, having a longer linked leg and a shorter outstanding leg, mitigate the shear lag effect.
FAQs
What are the factors that affect the strength of tension members?
Several things affect the strength of a tension member, such as the length of the connection, the size and spacing of the fasteners, the net area of the cross-section, the type of fabrication, the eccentricity of the connection, and the shear lag at the end connection.
What purposes do tension members serve?
Tension members can be used as parts of a truss (in roofs, bridges, and towers), as bracing in buildings, as hangers in suspension bridges, cables in cable-stayed bridges, and so on.
Which tension member is the best?
The best-rolled steel section is flat for a tension member with no shear lag.
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