Understanding Structural Members and Forces

In structural engineering, different members are designed to carry specific types of forces. These forces are primarily either tension (pulling) or compression (pushing). Identifying the primary function of a structural member is crucial for understanding how a structure behaves under load.

What is a Compression Member?

A compression member is a structural element designed primarily to withstand compressive forces pushing along its axis. When a compressive force is applied, the member tends to shorten. Common examples include columns in buildings or struts in trusses.

Analyzing the Options

Let's examine each option to determine its primary function:

• Purlin: Purlins are horizontal beams that span between rafters or truss top chords in a roof structure. Their primary function is to support the roof covering (like sheeting) and transfer the load to the rafters or trusses. They are primarily subjected to bending, although they can experience axial forces depending on the overall structural system.
• Strut: A strut is a structural member specifically designed to resist compressive forces. It is often used in bracing systems, trusses, or frameworks to prevent members from buckling or to transfer compressive loads. Struts are typically relatively short and rigid to resist buckling effectively.
• Girt: Girts are horizontal members used in the walls of metal buildings. They span between columns and support wall cladding (like metal panels). Like purlins, girts are primarily subjected to bending forces due to wind pressure or other loads on the wall cladding.
• Tie: A tie is a structural member designed specifically to resist tensile forces. When a tensile force is applied, the member tends to lengthen. Examples include the bottom chord of a simple roof truss or tension rods used for bracing.

Identifying the Compression Member

Based on the analysis above, the member specifically defined and used primarily to resist compression is the Strut.

Here is a summary in a table:

Therefore, out of the given options, the Strut is the correct answer as it is classified as a compression member.

Revision Table: Structural Member Types

Revisiting the key terms from the question:

Additional Information: More on Compression Members

Compression members are critical components in many structures. Their design must account not only for the compressive strength of the material but also for stability against buckling. Buckling is a sudden lateral instability that can occur in slender compression members even if the material's yield strength is not reached.

• Examples of Compression Members: Columns, struts, top chords of trusses, vertical members in some bracing systems.
• Buckling: The tendency of a slender compression member to fail by bending or deflecting laterally under compressive load.
• Factors Affecting Buckling: Length of the member, cross-sectional shape and size, material properties (like modulus of elasticity), and end conditions (how it's supported).
• Design Considerations: Engineers use formulas (like Euler's buckling formula for ideal slender columns) and design codes (like AISC for steel or ACI for concrete) to ensure compression members are safe against both crushing and buckling failures.