Slab-on-Grade and Grade Beam Design 1

[DMWWEngr]

I'm trying to find some "good" procedures that can be used to design a slab-on-grade and a grade beam. (Note: By grade beams I'm talking about a foundation system that is used to support walls above...similar to a wall/strip footing)

If there is a section in the ACI 318/99 code please direct me to that section. This information is not needed for a specific problem but for my own knowledge in the future.

For a slab-on-grade (and "mat" foundations for VERY small structures), I'm looking for the procedure including the appropriate checks (punching shear, moment, etc..?) I'm also looking for the minimum steel generally accepected in slab on grade construction. Is the ACI 0.0018 min steel overkill??

Grade beams are especially troublesome for me to find design information on. Searching this forum I see some people suggesting they are designed as a wall/strip footing. My concern with this is that wall/strip footings are generally longitudinally reinforced along the bottom only while most grade beams, I've seen so far, are doubly reinforced (top and bottom). Am I missing something??

Background: I'm 3 months out of school and I work at a water utlilty. They hired me on as a sturctural engineer but the sturctures knowledge around the engineer department is VERY limited and, therefore, I get very little help internally. My problem so far is that the loads on the projects I've done so far are SO small. It would be EASY for those with experience, but for me it is very confusing.

Any good structures books you could recommend would also be helpful as I slowly build my library up. Thanks in advance.

 

[ishvaaag]

"Any" non deep foundation you can design from a modulus of subgrade reaction viewpoint or for an elastic modulus of the soil. Hence it is the structural analysis shich will determine what reinforcement to place. Even if old books draw very shallow strip foundations reinforced the cantilever way transversally, modern practice uses full cages. You can ascertain even the collaboration with the wall if available through FEM, but if not you may choose to make the grade beam as stiff as to prevent wall damage from differential settlement, this is reasonable for small structures, and can be guessed for assumptions of soil going from bad to good.

So, from my viewpoint and with the target of precluding problems, reinforce top and bottom and make the structures as stiff as economy permits.

 

[DMWWEngr]

Thank you for the response. That explains why most grade beams I've seen are doubly reinforced.

I should have also added that at my work I do not have the aid of any structural analysis software or any FE software. I get to work off of VERY CONSERVATIVE assumptions!! Luckily everything I have designed so far, and most of what I will design in the immediate future, consists of VERY light loads!!

Thanks again for your response!!

 

[ishvaaag]

DMWWEngr, you may use some free software out there standing to help you. This will avoid excessive sizes or reinforcement schemes when the structure is important enough. On the other hand, any structural designer with practice can reassure you that the things learnt for structural design really work, you will be seeing with your first examples built. You need not be overly conservative but when the situation demands, that cases there are.

 

[DMWWEngr]

Do you have any links/contacts for the free software??

Thanks for your help. I'll be "lurking" and learning for the next few years!!

 

[ishvaaag]

I have one 3D freeware design program (I don't use it, but seems functional)

download analys.ese from

http://www-gmap.mecanica.ufrgs.br/gmap_dop.html

All resumes in making searches in a search engine for freeware structural design. Myself have thousands of sheets posted for free download at the collaboratory site..yet it requires access to not free- Mathcad 2000 Pro.

Make the searches, it is fruitful.

 

[DMWWEngr]

Thank again!! Will try some out!!

 

[Gourile]

I forward you to the book " Foundation Analysis and Design" by Bowles, in which different methods to analyse such foundations are argued in detail. It will also reffere you to useful ACI code recommendations.
For you case, however, I doubt that using finite element technics would be helpful, as the loads are so small, so does the scales, that the foundation under their practical dimensions act almost rigidly. A rigid foundation approach for strip foundation would satisfactorily and conservatively solve your problem, and save a lot of time for you.