Pre-engineered metal building slab

[jonathanwilkins]

In my area (southeast), metal building foundations are typically monolithic pours with large footers at the columns and hairpins to handle the thrust. Control joints are typically placed at 10-12' o.c. to prevent cracking. This is typically not overly detailed and from a purely technical standpoint somewhat flawed. This is, however, how it's been done and how I was taught (buildings are typically small) so it's what we do.

In a recent building, the concrete was cut with joints 20' and 25' o.c. I have not looked at it but the contractor has said that some cracking has started.

What's the best way to address these cracks or should we? Thanks.

 

[dik]

Sawcutting has to be done at the right time. Once the microcracking has initiated, then there's little you can do, except route out the cracks and seal.

If you are looking at a 5" slab typically, then the 10' to 12' spacing may be in order... 20' to 25' would likely be used for an 8" slab.

Dik

 

[msquared48]

I try to restrict the area between cuts to 400 to 600 square feet, trying to keep the areas as "square" as possible. Seems to work well for me.

I also put the joints/sawcuts that are parallel to the mainframes, between the mainframes, away from any hairpins or cross ties.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ron]

What do you need to do? Well, unless the client is unhappy with the result, nothing.

Cracks occur in concrete, whether controlled or not. The sawcut joints are just for control, not prevention. Neither is detrimental to the physical performance of the slab. It's just whether you (or the owner) want to live with the aesthetic detriment.

If you want to be proactive, you can fill the cracks with an epoxy repair material. You can also fill the existing cracks with structural epoxy and re-cut the control joints where they should have been, cutting them to about 30 to 35 percent of the slab thickness rather than the 20 to 25 percent that should have been done on the day of placement, assuming the slabs less than 30 to 45 days old at this point. It might not work, but certainly after, it would be futile.

Here is an FAQ for concrete jointing....faq592-162

 

[hokie66]

Joints are just straight cracks, so as Ron said, it is probably just an aesthetic issue. But if the cracks are direct tension cracks due to thrust from the frames, I think the design is open to criticism.

 

[jonathanwilkins]

No structural problems. Cracks are at this point only unsitely. We'll let it be for the time being.

I'll take this as a lesson learned and put more info into the slab plan as to control joints.

 

[Bobber1]

Contractors don't like it but a good water cure for at least 7 days helps a lot.

 

[jonathanwilkins]

Thanks to everyone for the advice.

Got a related question...What is the technically correct way to deal with these joints?

They were never brought up as an area of concern while I was training and consequently I've never, until now, put much (any) thought to them. I've read through the ACI-318 about joints and don't get much from that. ACI-302 isn't referenced by the building code so I'm a little unsure about using that, not to mention I'm not sure that the hairpins present in these slabs don't render all of these references inapplicable anyway.

I'd like to come up with a way to detail the joints so the slab doesn't crack. I've also got my hands tied in that the way these buildings are going to be built is predetermined...4" slab w/ large footers at columns. While not the best way to do this, it is the least expensive so that's the way it is going to be done, at least here in the southeast. Consequently, these joints must take this into account.

Thanks again. JEW

 

[waytsh]

You may have looked at these already but in case you haven't these documents discuss joint details and may help you.

1) ACI 224.3R-95 Joints in Concrete Construction
2) ACI 369R-06 Design of Slabs-on-Ground
3) Designing Floor Slabs on Grade (Ringo and Anderson)

Since you are using your slab as a tension tie are you running your reinforcing through the joints or are you making sure that the area to which the hairpin is attached is large enough to withstand the force through friction with the soil? I think ACI 318 states that if your slab on grade is used to resist seismic loads (which it likely will) you need to consider it a structural diaphragm and be designed in accordance with section 21.9. Then section 21.9.9 discusses construction joints in those diaphragms. Unfortunately however, it does not discuss spacing.

If you are using the subgrade drag formula for determining your required reinforcing you can strike a balance between the required reinforcing and a workable joint spacing. Although you should probably add the required reinforcing for the tension from the frames on top of the required reinforcing from the subgrade drag formula since the effects could be cumulative across the width of the building. Another option might be to use the chart shown in Figure 5.6 of ACI 360R-06, determine your joint spacing from your slab thickness as if it were unreinforced and only use the amount of reinforcing required to resist the tension load from the frames. Once again if you need to meet the requirements of a structural diaphragm then you are going to need to meet the minimum reinforcing requirements of ACI 318 Section 7.12.

Hope my rambling made sense.