Questions for the experts as I start the build

[mobilus]

I have read many threads and know that there is a lot of great info garnered by experience and shared through good will. But, and there's always a but, sometimes I just can't find the answer I'm looking for, and as I start this build, I thought that I'd create a thread solely for those questions.

First one: I hire a guy to put up a 30'D x 60W' x 12' H metal building, with 1 20' x 30' enclosed shop bay. He subs the concrete work. I read repeatedly on this forum that control joints are necessary, but the builder says that they usually don't cut pads as small as this one. www.concrete.org says that for a 4" monolithic slab, they should have cut it every 8'. I have a footing for a lift at the 15' mark, so I was thinking that I would have them cut one control joint down the middle (on the 30' direction) and two cuts at the 10' and 20' line. This would make it six 10'x10' zones.

My question is this: since the pad is about three weeks old, is there still a benefit to making the control cuts? I appreciate your input.

 

[cdsvt]

I'm not a concrete guy, but I'll parrot what concrete guys have told me. Concrete slabs WILL crack, control joints let you decide where they will crack. Again, I am not a professional. If it was me, I'd have the control joints cut.

 

[Zeke]

Cutting at 3 weeks is not a bad thing. One reason they cut right away is green concrete cuts like butter and is easy on the blade. In fact, I think there's a green blade. Your cuts will take a bit longer and use more blade. It just costs more, bottom line.

 

[readhead]

Absolutly should be cut.

 

[Garage Dog]

I would absolutely recommend you cut control joints - now would be best.

They are "necessary"? Not really, just depends on a a bunch of variables we don't know and what you want the slab to look like in 10 - 20 years.

Any configuration of cuts that will get you close to 10' x 10' sections and not affect your lift placement will significantly reduce the likely hood of random cracks.

GD

 

[BoostAddiction]

My 3+ car garage has a 4" monolithic pour, no cuts whatsoever, and has zero cracks after almost 14 years. It's even survived a minor earthquake or two will no apparent effect.

My guess is that control joints are a local thing, or a partial function of the foundation on which the slab is poured. My house is sited on a very large regional granite dome; perhaps that has something to do with the choice by my builder to not cut them.

I asked at the time, and was told they weren't needed for my house. I remember being skeptical, but trusted the builder. So far, he's been right!

No doubt a slab poured on a sandy substrate, or installed in an area with earthquake activity would be done differently.

 

[Zeke]

Well, Boost is right too. They don't cut the slab in the house part. But house slabs are usually more irregular with more footings. The base and site preparation are the deciding factors.

 

[nehog]

My 30x60 slab has cuts on 10' centers. No cracks (yet) after three (or is it four?) years.

 

[mobilus]

Thanks for all the input. Looks like control cuts are to be.

The structure is a weld-up. The concrete guys put 6" square weld plates at the same place I want to run the two cross-cuts.

Do you think the cuts in that location might somehow weaken the slab's hold on the plates and weaken the walls?

The I-beams are 8" tall, 3/8ths thick and 30' long. They are supported by 4x4" posts with 1/4" walls (or pretty close). So I know this is heavy structure, and they build 'em like this all the time...nothing to worry about, right? Wrong, this one is mine, and a lot of hard-earned money is exchanged.

 

[Zeke]

The concrete is thicker where the weld plates are? Any cracks that appear will go around the thicker parts, IMO. I'd just cut up to close to the plate and let the cut die.

 

[mobilus]

Thanks, Zeke...that was what I was hoping to hear. Yes, the anchor points are much thicker concrete. probably close to 1' cubed.

 

[mobilus]

Okay, not related to the last question (or project), here's another question:

I'm building a little 20Lx15W picnic shelter out near the pond. I built the trusses in place, and welded on the tabs for the purlin today. How do you guys think I should weld the purlin to the tabs, I+] or I+[?

I+] looks like



I+[ looks like



I can think of reasons why either would be better. I+[ has more welding area, but I+] would help prevent the upper lip from sagging.

Anyways, this is where I'm at...wanting to weld the purlins in tomorrow.



Any words of wisdom?

 

[Krodad]

I'd weld on the flat side and alternate stitch welds...looks like your tab is about 6", so 2" bead, then alternate to the other side of the tab another 2" bead, then back to the other side. And though it would be kind of a PITA, I'd tack everything on every tab before starting with the big heat. Those steel purlins can warp without a lot of effort.

 

[mobilus]

Thanks, Krodad. Especially for reminding me of the warp factor. My first trailer build showed that, and even though it has served me well for 15 years, it shows plenty of rookie mistakes.

I cut the tabs 1/2" too long and will have to grind them down once the purlins are welded on.