Tied Arch Roofs are tensioned with a bottom metal cord and can fail in as little as how many minutes of direct flame impingement?

Tied Arch Roofs are tensioned with a bottom metal cord and can fail in as little as how many minutes of direct flame impingement?

• A. 5 minutes
• B. 10 minutes
• C. 15 minutes
• D. 20 minutes

Answer: (B)

Direct flame impingement on tied-arch roofs is especially dangerous because the bottom metal cord acts as the tension tie that keeps the arch from spreading. When heat from direct flame hits that bottom cord, the steel weakens quickly and loses its ability to carry tension. As the cord loses strength, the arch thrust cannot be restrained, the joints begin to fail, and the roof can collapse rapidly. Because this failure can happen so quickly once heat exposure begins, the most accurate choice is the one reflecting a short duration of exposure before failure. In practice, this type of roof can go from intact to unsafe in a relatively brief period of flame contact, so the answer points to a very short time to failure under direct flame.

Direct flame impingement on tied-arch roofs is especially dangerous because the bottom metal cord acts as the tension tie that keeps the arch from spreading. When heat from direct flame hits that bottom cord, the steel weakens quickly and loses its ability to carry tension. As the cord loses strength, the arch thrust cannot be restrained, the joints begin to fail, and the roof can collapse rapidly. Because this failure can happen so quickly once heat exposure begins, the most accurate choice is the one reflecting a short duration of exposure before failure. In practice, this type of roof can go from intact to unsafe in a relatively brief period of flame contact, so the answer points to a very short time to failure under direct flame.