×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Slab on Grade - Wall Loading

Slab on Grade - Wall Loading

Slab on Grade - Wall Loading

(OP)
See attached from the Army Manual. I am not sure of the origins of this formulation. Is it empirical? It does not make sense to me dimensionally.

RE: Slab on Grade - Wall Loading

ARMY Intelligience and you ask such a question

RE: Slab on Grade - Wall Loading

No, the formulas have a mathematical basis--they are similar to the ones I have seen in the PCA slab on grade design book.

DaveAtkins

RE: Slab on Grade - Wall Loading

(OP)
Could you look through the attachment. I don't see how they work out dimensionally. Thanks.

RE: Slab on Grade - Wall Loading

slickdeals,

I agree that they do not work out dimensionally.  Also, they don't seem to agree with values published in Table 3-2 of the same document.

BA

RE: Slab on Grade - Wall Loading

(OP)
Thanks for concurring. I thought I was missing something basic.

RE: Slab on Grade - Wall Loading

slickdeals,

Maybe I was too hasty.  The units of I are in^4/ft.  The expression for λ is (12k/4EI)1/4.  I think the factor 12 is converting feet to inches, so the units work out okay.

Using the final expression for P, with f'c = 4000 psi, te = 10 in. and k = 100 #/in^3, I find P = 1375 lb/lin ft.

This compares to Table 3-2 values of 1340 to 1605 for flexural strengths of 550 to 700 psi which seems reasonable, but I am not sure how flexural strength relates to f'c.

BA

RE: Slab on Grade - Wall Loading

According to the following link, the Modulus of Rupture is usually taken as 9√f'c.

http://www.cement.org/tech/faq_flexural.asp

Table 3-2 on page 3-5 gives a value of 1340#/ft when k = 100, t = 10 and flexural strength = 550.

The value of f'c corresponding to the above expression is:
f'c = (550/9)2 = 3734 psi which, when used in the final expression on page B-1, attached gives a value of 1340.

BA

RE: Slab on Grade - Wall Loading

Well, slickdeals, I was expecting some kind of reaction from you.  What do you think?

BA

RE: Slab on Grade - Wall Loading

(OP)
Sorry BA, I have not yet had a chance to look at it. I will look today and let you know.

RE: Slab on Grade - Wall Loading

(OP)
Agree with you. I had a brain fart and was ending up with (lb/in) instead of (lb-in) in the denominator. I guess you overlook really simple, stupid things and need someone else to point it out to you. Thanks.

RE: Slab on Grade - Wall Loading

slick,

I did something similar the first time, too.  It seemed right to consider I in units of in^4 whereas it should be in^4 per foot.  I also was not sure of the relationship between flexural strength and f'c, but it seems that 9√f'c is what is normally used.

I still feel more comfortable reinforcing the slab to spread the wall load over a width determined on the basis of allowable soil bearing than to rely on unreinforced concrete to do the job.  

Perhaps this would be a useful technique to use when adding a heavy wall load to an existing slab of known thickness and very little reinforcement.  Is that your situation?

BA

RE: Slab on Grade - Wall Loading

(OP)
Yes, my situation is the same. The client wants to add an electrical room with a roof slab inside an existing building and I was trying to check the slab on grade.

The drawings indicated a 5" slab with 6x6 WWR, but the cores came out to anywhere between 6.5" to 7".

I had realized the 12 came from converting in^4/ft to in^4/in, but I made a math error in the denominator and did not get it work dimensionally.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources