Why does standard Welded Wire Mesh does not meet ACI criteria? 2

[seattlemike]

Hello fellow Structural Engineers,

This may be a dumb point, but 6x6 - W1.4xW1.4 WWM comes up woely short of the ACI requirement of 0.0018.... in a 4" slab, WWM = 0.28 sq in/ft, but ACI requires 0.0018*4*12 = 0.0864 sq in/ft.

what's that all about?
Why does standard Welded Wire Mesh that we always put in slabs not meet ACI shrinkage & temperature criteria (7.12) for slabs with welded wire mesh? Do the joints make up for it instead?

I guess 7.12 is for structural slabs only, and slab-on-grade is not meant to be included.

Mike

 

[WillisV]

Read R7.12.1.

 

[seattlemike]

WillisV,
So where are the minimum requirements for slab-on-grade?
Seems like it's just owner/geotech preference?
Mike

 

[DaveAtkins]

There is a subgrade drag formula (I can't remember it) that can be used to rationally determine area of steel required in a SOG. But I place CJ's at 12' oc, and then it doesn't matter what mesh is used.

DaveAtkins

 

[seattlemike]

Yes, I heard the rule of thumb was 3 times the slab thicknes... I wasn't sure if there was still a minimum steel requirement... I'd never heard of one... just the better-than-nothing W1.4xW1.4 - 6X6 WWM.

 

[vincentpa]

To answer your questions, there are no minimum steel ratios for slab on grade. You design it as if it were unreinforced. The wire reinforcement is nominal and does not affect the strength. It is only there to keep cracking to a minimum width or try to at least. It also helps aggregate interlock at the joints. There is a somewhat new philosophy since there have been many lawsuits regarding slab for industrial and warehouse floors. The wire reinforcement you are using is fine for a 4" slab. It will never get to the location in the slab it is supposed to anyway.

The new ACI 360-06 changes things AGAIN. They have even changed the name. It is now "welded wire reinforcement" (WWR)instead of welded wire fabric (WWF). It's more PC and marketable I guess. It's a chart for unreinforced but since the slab is only nominally reinforced, it is recommended to use these control joint spacings.

4" slab - about 8'
5" slab - about 12'
6" slab - about 14'
7" slab - about 16'
8" slab - about 17'

There are design methods in ACI, PCA, and WRI. The WRI are free to download. ACI 360-06 contains them all. PCA Slabs on Ground is a good book although dated a bit.

 

[DaveVikingPE]

WRI is the place to go. You'll "discover" that since WWR is usually of higher yield strength than regular grade 60 rebar and as such there's interesting conversions one can make and meet what the ACI needs.

Go here:

http://www.wirereinforcementinstitute.org/pages/pubs/structural.htm

and download all the Tech Facts. It was sort of a steep learning curve for me, but I figured out how things worked - and then, for my surprise, there's a chart in Seelye's "Design" from 1960 that has a similar example!

 

[271828]

8' CJ spacing in a 4" slab?!?!?

Does anybody actually do this?

I really think the contractor would try to have architect all over me if I tried to make them install CJ at 8'!

FWIW, my old firm probably puts in multiple million sq. ft. of 4" SOG per year with a 20-30' (depending on the engineer) CJ spacing and I've never heard of a problem.