A roof bay is 120 ft long, and open-web joists are spaced at 5 ft on center along that length with a joist on both end lines. What joist count fits the bay?
Correct Answer
D) 25 joists
A 120 ft run at 5 ft spacing has 120/5 = 24 spaces. With a joist at both end lines, count spaces plus one: 25 joists. Other choices omit an end joist, add an extra bay, or use 4 ft spacing.
Why This Is the Correct Answer
A 120 ft run at 5 ft spacing has 120/5 = 24 spaces. With a joist at both end lines, count spaces plus one: 25 joists. Other choices omit an end joist, add an extra bay, or use 4 ft spacing.
Why the Other Options Are Wrong
Option A: 24 joists
joist count, choice a: the answer choice lifts 24 joists from the working line.
Option B: 26 joists
joist count, choice c: 26 joists is wrong because 26 joists counts a different spacing, panel, or repeated member quantity.
Option C: 30 joists
joist count, choice d: 30 joists counts a different spacing, panel, or repeated member quantity.
Memory Technique
Say the joist count rule aloud as a chain; the chain ends at 25 joists, not an intermediate stop.
Reference Hint
Open Technical Digest No. 9 - Steel Joists & Joist Girders, 3rd Ed., 2008; 29 CFR 1926 Subpart R; use the index entry or section for "Steel joist layout spacing takeoff" to confirm that A 120 ft run at 5 ft spacing has 120/5 = 24 spaces.
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