practice question need some help

Structural Layout Vignette and Multiple Choice

practice question need some help

Postby max8176 » Wed Mar 28, 2018 7:53 am

I have a question here from a friend who went through one of the practice exams. What should the spacing be for 4x18 and 15 feet long wood joists be with a 60 psf load and a Fb value of 1800? Anyone know how to resolve this?
Last edited by max8176 on Sat Mar 31, 2018 4:16 pm, edited 1 time in total.
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Re: practice question need some help

Postby glassfork » Wed Mar 28, 2018 3:27 pm

There are tables put out by AWC (American Wood Council). You just look it up:

http://www.awc.org/pdf/codes-standards/ ... 5-0606.pdf

Hope this helps
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Re: practice question need some help

Postby max8176 » Wed Mar 28, 2018 4:40 pm

I looked at the tables and they don't have 4x12 listed.
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Re: practice question need some help

Postby Coach » Wed Mar 28, 2018 5:04 pm

You need to know allowable deflection and use the formulas.
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Re: practice question need some help

Postby max8176 » Thu Mar 29, 2018 9:50 am

That's the only information given in his practice question.
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Re: practice question need some help

Postby Coach » Thu Mar 29, 2018 12:42 pm

max8176 wrote:That's the only information given in his practice question.

Does it say roof or floor?
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Re: practice question need some help

Postby Archer40 » Thu Mar 29, 2018 1:06 pm

Don’t want to mislead anyone, but I tried to resolve this question based on my understanding, the answer may not make sense, hope Coach or anyone else can review and verify, this is just for discussion:

Step 1, find out the maximum allowable moment:
S= M/Fb
S(section modulus) = 3.5” x 11.25” x 11.25” / 6 = 73.83 in3
so M=Fb x S =1200 x 73.83 = 88, 596 # in = 7, 383 #ft

Step 2, Find out the linear load on the beam:
M = w x L x L/8
So, w = M x 8 / 10’ x10’ = 590.64 #/ft

Step 3, based on the 80psf load, find out the spacing of joist:
Spacing = 590.64/80= 7.383 ft center to center

I really hope that I won't have this kind of question in the exam, too complicated!!
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Re: practice question need some help

Postby Coach » Thu Mar 29, 2018 1:49 pm

I didn't check your math but the process is correct.
What's important to remember though is that's the answer based only on max bending stress, and if that's all the information given, that's all you can do.
However, if designing for an actual joist/rafter, there is code maximum deflection to consider which could result in tighter spacing. Of course, you can't calc deflection without a given E.
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