Questions and Answers
What is a key design feature of flat roofs?
What does deflection under the weight of firefighters indicate?
Why should firefighters be cautious around lightweight roof constructions?
What might cause a flat roof to vibrate?
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What is a common misconception about deflection under weight?
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What should firefighters keep in mind about the weight a flat roof can support?
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What is an important safety factor for firefighters on roofs?
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What general advice is given regarding firefighter safety on roofs?
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How might firefighters interpret minor vibrations in roofs?
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In the context of roof safety, what is the significance of construction type?
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Study Notes
Roof Support Systems
- Firefighters must identify various roof support types and construction materials.
- Different roof support systems behave uniquely under fire and collapse conditions.
Flat Roof Support
- Flat roofs can be supported by open-web steel joists or steel beams.
- The simplest flat roof support utilizes wood joists, resting on load-bearing walls.
- Wood joists function as beams, supporting roofing materials similar to floor systems.
- Solid or laminated beams and columns may also support wood roof joists.
- Traditional wood-joisted roofs utilize solid wood joists, which lose strength gradually when exposed to fire.
- Gradual loss of strength causes roofs to become "spongy" prior to failure.
- Signs of softening or sagging in the roof are key indicators of potential structural failure.
- Misinterpretation of these signs may lead to overlooking imminent collapse.
- Thin plywood or oriented strand board sheathing can fail rapidly without warning.
- Advanced or heavy fire conditions indicate roof weakening; firefighters should be alert.
- Roof support systems can collapse unexpectedly under fire conditions.
- Box beams and I-beams, often made from plywood, may be used for flat roofs.
- Wood truss joints typically support flat roofs but have vulnerabilities due to their thin web sections.
Truss Joist Failure
- Slender components of truss joists are prone to quick failure during fire incidents.
- Open web design allows fire to spread rapidly perpendicular to the truss joist, rather than only along its length.
Fire Resistance of Open Web Joists
- Unprotected lightweight open-web joists can fail quickly under fire, depending on the fuel load.
- Any unprotected material, including lightweight open-web structures, is at risk of failure during a fire.
Building Codes
- In Type I construction, building codes may permit the omission of structural fireproofing for roof supports under specific conditions.
- Unprotected steel roof supports may be present in buildings with fire-resistive main structural supports.
Roof Design Considerations
- Roofs are engineered for lighter live loads compared to floors.
- Modern flat roofs often experience deflection or vibration due to weight.
- Flat roofs are primarily designed to bear the weight of maintenance personnel.
- Under the load of firefighters, roof deflection may not indicate imminent failure but warrants cautious assessment.
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Description
Learn to identify different types of roof support systems, their materials, and reaction to fire and collapse conditions. Important knowledge for firefighters!