Minimum Area of steel using wire reinforcement

[BunnyRosecoe]

I have no experience in concrete design using wire reinforcement so I thought I would ask. I am reviewing shop drawings for a 12” thick machine slab and the engineer called for 6x6 (W2.9/W2.9) welded wire reinforcement. I think the 6x6 means the spacing of the longitudinal and transverse reinforcement respectively and the 2.9/2.9 is the area of the reinforcement. If that is correct, then the ratio of area of steel is (2x0.029in^2)/6”x12” = 0.0008. This means that they are not using the minimum for shrinkage control using 60ksi steel which is 0.0018.

Am I calculating this correctly? All of my experience is in heavy industry and I never used anything less that a #4 bar. Can someone with experience using welded wire reinforcement help me. Thanks

 

[EZBuilding]

6x6 W2.9 has an area of steel = 0.058 in^2/ft of width (from PCI Design Handbook) which gets you to 0.0004 ration in a 12 inch slab. 6x6 W2.9 is the typical wire mesh reinforcement for 4 inch slab on grades that I have seen out in practice. It is possible that the engineer had a note for a different slab thickness. Looking at it now, looks like this mesh only gets you up to 0.0012 for a 4 inch slab. You may also want to reference ACI's design guides specific to slab on grade. There are different criteria for slabs on grade - I have seen some arguments made for not having any mesh at all...

 

[BunnyRosecoe]

Thanks EZBuilding for the response. I forgot to state that they are showing 6x6 W2.9/W2.9 top and bottom. That is why I multiplied 0.029in^2 by 2.

 

[hokie66]

That amount of reinforcement doesn't do anything. What are the plan dimensions of the machine slab? Perhaps it doesn't need to be reinforced?

 

[BunnyRosecoe]

hokie66, I don’t understand how the amount of reinforcement wouldn’t matter. It’s a 12” thick slab and I calculated the steel ratio based on a 6” wide strip. The plan dimensions are 9’-0” x 6’-0”.

 

[BridgeSmith]

We use 4x4 W2.9 in our 4" thick slope paving, just to limit the width of the cracks. Perhaps flexural reinforcement in the machine slab is not required, just a little steel limit the width of shrinkage cracks.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[hokie66]

9' x 6'. In plain concrete, that won't have drying shrinkage cracking, provided all good concreting and curing practices are observed. The reinforcement would be a waste. Now if this is a footing, with flexural stresses, the story is different, and you should have at least the minimum flexural reinforcement, not shrinkage.

 

[BunnyRosecoe]

Thanks hokie66 for the response but I don’t think I agree with you considering they are specifying a 12” thick slab. Anyways, my question is did I calculate the rho correctly? Is the 6x6 the spacing and is W2.9 the area of steel? Is W2.9 = 0.029in^2?

 

[hokie66]

Perhaps you should read the section in your code about plain concrete.

 

[BunnyRosecoe]

Thanks hokie66 for the response, but the design criteria my client specified calls for reinforced concrete NOT plain concrete...so I don’t care to read the section in my code about plain concrete because it is not relevant in this case. No worries, I’ll make the comment. I just want to make sure my client gets what they paid for.

 

[BridgeSmith]

Bunnyrosecoe, I don't believe that amount of steel in a 12" slab is enough to consider the slab to be reinforced, per the codes I'm familiar with. You really should check your applicable code to see if it is or isn't for this slab.

Rod Smith, P.E., The artist formerly known as HotRod10