PDD Practice Test - Size of the Expansion Joint
An architect is designing a single expansion joint for a 200-foot-long building. The movement will be primarily controlled by the steel roof deck, which will experience a 120º F temperature differential.
What will the size of the expansion joint be?
Reference Formulas:
change in length = coefficient of expansion x original length x change in temperature
coefficient of expansion = .0000065 / º F
| A. | 0.156 inches |
| B. | 0.936 inches |
| C. | 1.872 inches |
CORRECT RESPONSE
1.872 inches
CALCULATIONS
1. 0.0000065 x 200-foot-long building x 120º F = .156 feet
2. 0.156 feet x 12 inches / 1 foot = 1.872 inches
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I am at a complete lost with this question. The question asks the size of expansion joint? The formula given solves for "change in length". Not sure how to interpret length - looking at all the options, I think the test is referring to the width of the joint? Plus "change" in length requires an initial length, and we are not given this information. What am I missing here?
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You are given an initial length: 200'
It's effectively telling you how much the material will expand at 120 degrees, so .156ft / 1.872 inches. That is the width of the joint.
It's not worded well, but that is the essence of the question. The roof can expand 1.872" and therefore the joint needs to be at least 1.872" wide to accommodate that movement.
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DC Thank you. I think I see what the question is asking now.
I was confused by the formula given and thought the formula was to solve for the change in length of the expansion joint material, not the change in length of the steel roof deck. Now that I read your answer and read the question again, it makes sense.
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I would argue the correct answer is actually B. (.936"). When sizing expansion joints, you don't calculate based on the entire length of the building (wall, roof, floor, whatever) you calculate based on the distance between the joints, so you would use 100' as the length, not 200'.
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The distance used should be based on the entire building area rather than just the distance between joists, as the expansion joints are meant to accommodate movement across the whole structure due to temperature fluctuations, and not just localized movement between individual joists.
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Hello Galchenko12. So we have one expansion joint that is 1.872 inches, based on a total building length of 200 feet. Now suppose we have 4 expansion joints instead of 1. If we calculate using total length, we now have 4 expansion joints that are 1.872 inches each.
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An expansion joint has to accommodate the maximum possible movement on each side of the joint if that expansion were to happen concurrently. If the first segment were to expand 0.936 inches (per the formula) and the second segment were also to expand by the same amount, the single expansion joint would have to accommodate movement in both segments at once, totaling 1.872 inches.
If you were to add more expansion joints the total movement to be accommodated would still be 1.872, but the joints could be sized differently depending on the length of the segments on each side of each joint, and whether expansion is permitted in just one direction or two (in other words, is there and expansion joint on both sides of a segment or just one).
This (bi-directional expansion) introduces a level of complexity that ncarb obviously wishes to avoid. In my experience these types of questions are not trick questions: they really just want you to take numbers directly from the question and plug them into the formula that they give you. Basic arithmetic plus maybe a single conversion from feet to inches or something simple like that.
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