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.
Understanding Steel Truss Roof Construction
Let's analyze each statement regarding the construction of a steel truss roof to determine which ones are correct.
Analysis of Statement (i): Load Transmission
Statement (i) says: "Steel truss transmits self-weight and roof loads vertically on the walls."
Steel trusses are structural frameworks designed to efficiently carry loads over a span. They support the roof covering (like sheeting or tiles) and transfer the weight of the roof (dead load) and other loads like snow or wind (live loads) to their support points. These support points are typically walls or columns. The forces within the truss members are transferred as axial forces (tension or compression) to the joints, and ultimately, the reactions at the supports are predominantly vertical forces transmitted downwards onto the supporting walls or columns.
Therefore, this statement is correct. Steel trusses are indeed designed to transmit loads vertically onto the supporting structure, such as walls.
Analysis of Statement (ii): Spacing Between Trusses
Statement (ii) says: "Spacing between steel trusses are usually between 10 feet to 15 feet."
The spacing between adjacent roof trusses is a crucial design parameter. It affects the size of the purlins (the beams spanning between trusses) and the sheeting material requirements. While spacing can vary based on the design loads, span, and type of roof covering, a common and economical spacing range for steel roof trusses in many applications is indeed between 10 feet (approximately 3 meters) and 15 feet (approximately 4.5 meters). Wider spacing requires heavier purlins but fewer trusses, while closer spacing allows for lighter purlins but requires more trusses.
Therefore, this statement is correct as 10 to 15 feet is a typical range for steel truss spacing.
Analysis of Statement (iii): Dead Load and Stability
Statement (iii) says: "Steel trusses use reduced dead load of building making structure unstable."
Steel trusses are generally lighter in weight compared to some other roofing systems like reinforced concrete slabs or heavy timber constructions covering the same span. This means using steel trusses often results in a reduced dead load on the supporting structure and foundations. Reducing the dead load is usually advantageous in structural design. It means less weight needs to be supported, potentially leading to cost savings in foundations and supporting walls/columns. A reduced dead load does not inherently make a structure unstable. Stability is achieved through proper design considering all anticipated loads (dead load, live load, wind load, seismic load) and ensuring the structure can resist these forces safely.
Therefore, the claim that reduced dead load makes the structure unstable is incorrect. Reduced dead load is often a benefit in structural design.
Conclusion on Correct Statements
Based on the analysis:
- Statement (i) is correct.
- Statement (ii) is correct.
- Statement (iii) is incorrect.
Thus, the correct statements are only (i) and (ii).
Revision Table: Steel Truss Roof Statements
| Statement | Description | Correctness | Reasoning |
|---|---|---|---|
| (i) | Transmits loads vertically on walls. | Correct | Trusses transfer roof loads to supports (walls/columns) primarily as vertical reactions. |
| (ii) | Spacing usually 10-15 feet. | Correct | 10-15 feet is a common and typical spacing range for steel roof trusses. |
| (iii) | Reduced dead load makes structure unstable. | Incorrect | Reduced dead load is usually beneficial and does not inherently cause instability; stability depends on overall design for all loads. |
Additional Information: Steel Roof Truss Fundamentals
Steel roof trusses are widely used in construction due to their strength, efficiency, and ability to span large distances. Here are some key aspects:
- Components: A truss consists of straight members connected at their ends by joints. Members are typically steel angles, channels, or tubes. Joints are often made using gusset plates and bolts or welds.
- Load Transfer Mechanism: Trusses work by transferring external loads (applied at joints) into axial forces (tension or compression) within the members. This makes them very efficient for spanning distances without significant bending in the main members.
- Types of Loads: Structures must be designed for various loads:
- Dead Load: The weight of the structure itself (truss members, purlins, roof covering, insulation).
- Live Load: Temporary loads such as snow, wind pressure/suction, and maintenance personnel.
- Other Loads: Seismic loads, specific equipment loads, etc.
- Spacing Considerations: The optimal spacing between trusses depends on factors like the strength of the purlins, the capacity of the roof sheeting to span between purlins, overall building dimensions, and economic considerations. Common spacing ensures standard purlin sizes and connections can be used efficiently.
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 -
Select the incorrect statement from the following.
How does an increase in the pitch of the roof affects the amount of load that can be placed on it?
