Truss Roofs

Question 1

Size-up indicators of a truss roof in a building

Answer 1

1. A large, open space without columns
2. For Bowstring trusses only - a mounded roof shape
3. Certain occupancies frequently use truss roofs - Supermarkets, Bowling Alleys, Garages, Theaters, Places of Worship, Auto Dealerships, Piers, and Armories

Question 2

Truss Members

Answer 2

• A Truss is composed of two major members: the TOP CHORD, and the BOTTOM CHORD
• Webs are shorter members, connecting the TOP and BOTTOM CHORDS

Question 3

Wooden Bowstring Truss

Answer 3

• Found in older commercial structures in NYC
• Have a characteristic hump-like roof profile
• The longer the span, the higher the bow

Question 4

Failure of Truss Roofs

Answer 4

• Failure can occur in the early stages of the fire
• Failure of one element (aka “member’) can cause the failure of the entire truss. In turn, this may pull down a number of trusses, in a domino effect. This may cause the entire roof, or a large portion, to collapse

Question 5

Truss Roof Characteristics

Answer 5

• Trusses may be spaced 10 or 20 feet apart w/ roof beams installed between the trusses to support the roof coverings
• Wood truss roofs appear to fail without warning. The roof does not sag or get “spongy”
• Steel trusses tend to “stretch” when losing their strength due to elevated temperatures, but wood tends to “snap”

Question 6

Unprotected Open Web Steel Joists

Answer 6

-Particularly vulnerable to elevated temperatures and may COLLAPSE AFTER ONLY 5 OR 10 MINUTES