Ch 12 pg 338-344 Flat Timber Trusses Overview

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Questions and Answers

What is a primary consequence of using gusset plates in trusses during a fire?

They allow for easier ventilation access.

They strengthen the connections between trusses.

They prevent fire from spreading inside wood.

They contribute to the truss collapsing. (correct)

Which of the following best describes the impact of drilling holes into steel tubes and wood-chord trusses?

It facilitates fire prevention measures.

It improves the structural integrity of the wood.

It allows fire to travel more easily through the chord. (correct)

It enhances the aesthetic appeal of the construction.

What is the primary risk associated with cutting through trusses during ventilation?

It can create openings for smoke to escape.

It improves the overall safety of the structure.

It might weaken the trusses and lead to failure. (correct)

It ensures that the roof does not collapse.

What responsibility is placed on individuals who cut through any part of the truss system?

They are accountable for the repair and engineering costs. Signup and view all the answers

What is a significant factor contributing to the vulnerability of trusses when sawing?

The shallow cut potential due to thin coverings. Signup and view all the answers

What is a primary characteristic that makes flat timber trusses more dangerous than bowstring trusses?

They have a flat roof that lacks visual cues for firefighters. Signup and view all the answers

What spacing is typically used for flat timber trusses?

20 ft apart Signup and view all the answers

Under what conditions should firefighters avoid working on or under flat timber trusses?

If there is heavy fire inside the structure. Signup and view all the answers

What can occur as a result of the failure of one flat timber truss?

It can cause a chain reaction leading to the failure of nearby trusses. Signup and view all the answers

What is critical for ensuring firefighter safety when dealing with flat timber trusses?

Preplanning before entering the structure. Signup and view all the answers

What are the primary materials used in the most common truss design?

Short pieces of 2x4s or 2x3s with stamped metal fasteners Signup and view all the answers

Which component is primarily associated with connecting web members in truss designs that use steel?

Tubing stamped flat at each end Signup and view all the answers

What is a significant fire hazard associated with common truss designs?

Sheet metal plates that act as heat sinks Signup and view all the answers

What leads to the potential failure of a truss in a fire situation?

Burning through a single piece of wood Signup and view all the answers

Which describes the web members in a truss design that uses gusset plates?

Thin pieces of sheet metal Signup and view all the answers

What term did Professor Frank Brannigan use to describe the void spaces created by open-web trusses?

Truss loft Signup and view all the answers

What is a major concern associated with the void spaces in parallel-chord trusses?

Uncontrollable fire spread Signup and view all the answers

Which type of trusses has been often used instead of solid joists for floor support?

Parallel-chord trusses Signup and view all the answers

What does the use of parallel-chord trusses instead of solid joists primarily address?

Increasing void space for utilities Signup and view all the answers

What characteristic of steel affects its performance in a fire?

Steel can become weakened quickly when heated. Signup and view all the answers

What is a common feature of steel bar joists used in metal deck roofs?

They typically span distances of up to 60 ft. Signup and view all the answers

What is a significant drawback of the design of steel bar joists?

They collapse after a few minutes in a structural fire. Signup and view all the answers

How are the top and bottom chords of the steel bar joists typically formed?

By welding thin angle iron sections to the steel bar. Signup and view all the answers

Which of the following statements about the behavior of steel under fire conditions is true?

Steel does not burn but can be significantly weakened. Signup and view all the answers

What is the typical time frame for firefighters to search and attack before roof or floor sagging becomes noticeable?

10 Signup and view all the answers

What type of beam is particularly dangerous due to its potential sudden failure without noticeable sag?

Plywood I-beam Signup and view all the answers

Which component composition primarily makes up a plywood I-beam?

1 1/2 × 2 in plywood top and bottom with a 3/8 in plywood web Signup and view all the answers

What is a key concern regarding new lightweight structural systems mentioned in the content?

They can fail within 10 minutes without prior sagging. Signup and view all the answers

What misleading assurance do marketers of plywood I-beams often provide to homeowners?

They will eliminate squeaky floorboards. Signup and view all the answers

What is the primary hazard for firefighters when cutting ventilation holes in roofs with metal decking?

Falling into the ventilation hole Signup and view all the answers

What is the thickness of the average steel decking used in roofing, as mentioned?

1/16 in. Signup and view all the answers

Which insulating material can be used beneath the tar layer in this roofing system?

Fiberglass or rubber sheeting Signup and view all the answers

What additional layer is laid over the insulation for waterproofing in this roofing system?

A layer of tar Signup and view all the answers

What is the width of the steel sheets used for roof decking?

3 ft Signup and view all the answers

What is a key difference in the behavior of steel joist-supported roofs compared to wooden trusses under fire conditions?

Steel joists sag rather than snap. Signup and view all the answers

How quickly do steel joist-supported roofs typically fail when exposed to heavy fire?

Less than 5 minutes. Signup and view all the answers

What advance warning do firefighters receive when a steel joist-supported roof is under fire?

The roof will sag before it drops. Signup and view all the answers

What is the primary reason steel joists do not experience sudden failure in fire situations?

They sag rather than snap. Signup and view all the answers

In terms of firefighter safety, what advantage do steel joist-supported roofs provide during a fire?

They provide ample warning before collapse. Signup and view all the answers

What is a major risk for a firefighter when cutting a hole in a roof?

The possibility of falling through unmarked joist spaces. Signup and view all the answers

What can happen if a firefighter inadvertently steps on unsupported metal decking?

They may plunge through the roof. Signup and view all the answers

Which factor increases the risk of a firefighter falling when making a cut in the roof?

A lack of visibility regarding the placement of metal sheets. Signup and view all the answers

What could lead to a firefighter being thrown off balance while cutting through a roof?

Cutting near the edge of a sheet that could buckle. Signup and view all the answers

Why is it critical for firefighters to know the positioning of joists before cutting?

To prevent falling through unsupported areas. Signup and view all the answers

What is the primary action that should be taken to ensure safety before a deck collapse occurs?

Evacuate the area Signup and view all the answers

Why is it important to cool steel during a fire?

To freeze the steel and keep it in place Signup and view all the answers

What technique should firefighters employ when operating a hoseline during a deck fire?

Use a straight or solid-stream Signup and view all the answers

What should firefighters do after knocking down the fire?

Use large hoselines for a straight stream Signup and view all the answers

What is a risk associated with firefighters not being able to see a collapsing roof?

Increased likelihood of structural collapse under them Signup and view all the answers

Study Notes

Flat Timber Trusses

Constructed similarly to bowstring trusses using 2x8s or 2x10s for chords and 2x4s or 2x6s for web members.
Spaced approximately 20 feet apart, can create a 40-foot wide hole upon collapse.
Flat roof design disguises danger, resembling older standard flat roofs, posing risks to firefighters.
Preplanning is crucial for safety; roofs should not be approached if heavy fire is present.
Collapse can occur from one truss failure triggering others, leading to total structural failure.
Both flat and bowstring trusses are high-risk; firefighters must adopt defensive positions immediately with heavy fires.

Truss Design Concerns

Common designs include gusset plates (joined with metal fasteners), tubular steel webs, and sheet metal gusset plates.
Lightweight materials and air spaces act as heat sinks, increasing fire vulnerability.
A single 2x4 can burn through, compromising entire truss integrity.
Gusset plates penetrate wood minimally, leading to truss collapse when fire damages surface.
Drilling holes into trusses weakens connections and allows fire spread inside chord members.
Power saws can accidentally penetrate top chords during operations, risking major structural issues.

Truss Loft and Fire Spread

Truss lofts create large void spaces, promoting uncontrolled fire spread especially in parallel-chord trusses used for floor joists.
Older structures with solid joists typically had the ceiling attached, reducing fire risk in comparison.

Firefighter Response Time

Firefighters have 10-20 minutes to perform search and rescue before structural sagging becomes noticeable.
Many modern truss systems can fail in under 10 minutes without visible signs of distress.

Lightweight Roof Hazards

Open-web steel bar joists supporting corrugated steel decks are common in commercial buildings.
Steel is noncombustible but weakens rapidly when heated, particularly in thinner forms.
Steel bar joists are formed into a Z pattern; can be up to 60 feet long and spaced 6 feet apart.
Lack of fireproofing and insulation means unprotected steel can fail quickly in a fire.

Cutting Risks for Firefighters

Cutting ventilation holes presents risks due to unknown locations of joists below the deck.
Firefighters may inadvertently step on unsupported metal deck, risking falls.

Structural Collapse Insights

Steel joists typically sag before collapsing rather than snapping like wooden trusses, providing some warning.
Experience indicates these roofs fail in under 5 minutes of heavy fire exposure.

Fire Safety During Deck Collapse

Evacuate area before collapse; visibility may be impaired due to smoke.
Use solid-stream hoselines to cool structures ahead of personnel.
Proper cooling can restore steel strength and stability; caution is essential when advancing into compromised areas.

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Description

Explore the unique characteristics of flat timber trusses and how they compare to bowstring trusses. This quiz covers their construction, spacing, and behavior during fire conditions, highlighting the implications of a potential collapse. Test your knowledge on this crucial aspect of structural engineering.