Select the incorrect statement from the following.
Purlin is designed as a tension member.
Understanding Purlins in Roof Structures
Purlins are essential structural elements in a roof system. They are beams that span between main supporting members, usually trusses or rafters, to support the roof covering such as sheeting or decking. The question asks us to identify the incorrect statement among the given options regarding purlins.
Evaluating Statements about Purlins
Let's carefully examine each statement provided:
Statement 1: Purlin is subjected to biaxial bending.
- This statement is generally correct. Purlins are typically installed on a sloping roof. The load they support (like roof sheeting, snow, or even self-weight) is vertical.
- Because the purlin's cross-section is usually oriented vertically or perpendicular to the roof slope, the vertical load components cause bending about both of the purlin's principal axes.
- One component acts perpendicular to the purlin's web, causing bending about the strong axis, and another component acts parallel to the roof slope (which is typically inclined relative to the purlin's web), causing bending about the weak axis. Hence, purlins often experience biaxial bending.
Statement 2: The span of purlin is center to center of truss; purlin is located at the panel point of the truss.
- This statement is generally correct regarding the span. Purlins transfer loads to the main supporting structures like trusses. The distance between the supports (trusses) is the span of the purlin, measured from center to center of the trusses.
- Placing purlins at the panel points (joints where truss members meet) is a common design practice. This ensures that the loads from the purlins are transferred directly to the joints of the truss, avoiding bending moments in the truss members themselves and simplifying the truss design as a pin-jointed structure.
Statement 3: Purlin runs perpendicular to truss.
- This statement is correct. Trusses typically run parallel to each other along the length of the building.
- Purlins are installed spanning across these parallel trusses to support the roof decking. Therefore, purlins run perpendicular to the plane of the trusses.
Statement 4: Purlin is designed as a tension member.
- This statement is incorrect. Purlins primarily support gravity loads such as the weight of the roof covering, snow, and maintenance loads.
- These gravity loads act downwards, causing the purlin to bend between its supports (the trusses). Therefore, purlins are primarily designed as flexural members or beams to resist bending moments and shear forces.
- While wind uplift could induce tensile forces in the connections or, in some scenarios, overall tension in the purlin depending on the overall structural system and bracing, the primary design consideration under typical loading is bending, not axial tension throughout the member's span like a pure tension member (e.g., a tie rod in a truss).
Based on the analysis, the statement that is incorrect is that the purlin is designed as a tension member.
Summary of Statements
| Statement | Assessment | Reason |
|---|---|---|
| Purlin is subjected to biaxial bending. | Correct | Vertical loads on a sloping purlin cause bending about two axes. |
| The span of purlin is center to center of truss; purlin is located at the panel point of the truss. | Correct | Purlins span between trusses and are often placed at truss panel points. |
| Purlin runs perpendicular to truss. | Correct | Purlins are laid across parallel trusses. |
| Purlin is designed as a tension member. | Incorrect | Purlins are primarily designed for bending under gravity loads. |
Conclusion on Purlin Design
Purlins are crucial structural beams in roof construction, designed primarily to resist bending loads from the roof covering and environmental factors like snow. They are not typically designed as members carrying only axial tension. Understanding the correct loading and design behavior of purlins is vital in structural engineering.
Revision Table: Key Purlin Concepts
| Concept | Description |
|---|---|
| Function | Support roof covering (sheeting, panels). |
| Supports | Span between main structural frames (trusses, rafters, beams). |
| Orientation | Run perpendicular to the main frames, along the roof slope. |
| Primary Loading | Gravity loads (roofing, snow), Wind loads. |
| Primary Design | As a beam (flexural member) for bending and shear. |
| Bending Type | Often biaxial bending due to sloping roof. |
| Typical Section | Rolled steel sections (like I or channel sections), Cold-formed sections (Z, C, hat), Timber sections. |
Additional Information on Structural Members
Structural members are designed based on the type of forces they are expected to carry:
- Tension Members: Designed to resist pulling forces that tend to elongate the member. Examples: Cables, tie rods, bottom chord of a simple truss under gravity load.
- Compression Members: Designed to resist pushing forces that tend to shorten or buckle the member. Examples: Columns, struts, top chord of a simple truss under gravity load.
- Flexural Members (Beams): Designed to resist bending moments and shear forces. They experience compression on one side and tension on the other. Examples: Beams, girders, rafters, and purlins.
- Torsion Members: Designed to resist twisting moments. Examples: Shafts.
Purlins, carrying vertical loads across a span, clearly fit the description of flexural members or beams, not tension members.
Important Questions from Roof Truss
The economical range of spacing of roof trusses is:
The bracing provided in the plane of end posts is called ____
In a trussed bridge, the maximum limit of span is -
During the construction of a steel truss roof, which of the following statements are correct?
(i) Steel truss transmits self-weight and roof loads vertically on the walls.
(ii) Spacing between steel trusses are usually between 10 feet to 15 feet.
(iii) Steel trusses use reduced dead load of building making structure unstable.
How does an increase in the pitch of the roof affects the amount of load that can be placed on it?
