I am building a 60×100 Metal building for a trucking company and need help with some foundation details. A Engineer has designed the foundation plan. The Engineer shows a 4″ Slab 3500psi with 6×6-W1.4xW1.4 wire mesh, I would like to use rebar in place of the wire mesh as in my experience wire mesh usually ends up in the bottom of the slab, I also do not think a 4″ slab is enough for a shop that will have 80,000lb trucks in it. If rebar is used what should the spacing and size be? Also what is everybodys opinions about the 4″ slab versus a 6″
Discussion Forum
Discussion Forum
Highlights
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Replies
you are second question an engineer, if you change the plans, He off the hook. No matter what happens to the building, he has no liability. If you feel its not a good design. Talk to the engineer. we cannot offer any advice here.
As a guy who worked construction in the past and now is an engineer I will talk just the liability piece. Go back and talk to the engineer, you're doing him a huge favor. Just b/c he has some letter after his name and college degree doesn't make him infallible. Unfortunately in todays hectic schedule you don't get to do "peer review" (have a another guy look at your design) a lot. You giving him a sanity check is very cool and hopefully he will take it that way.
If he says no change needed then build it exactly to those specs b/c when he put his stamp and signature on the drawings he said it was good. That stamp is a very big deal in every state in the US. You build it it to sped and it fails then it is all on him.
A little personal story. I stamped some drawings for some duct work in a commercial building and had the HVAC guys come talk to me saying that it did not make sense. Parked my pride for a minute and realized that they were 100% right. Although was technically correct the design was really inefficient and would have been noisy. They probably saved us $5000 in materials and labor. Took some convincing but I got my boss to push some of those saving to those guys b/c they earned it.
As an Engineer what would you normally spec. in a building such as I have described? Also what do you think about welded wire?
I am a mechanical engineer and me specing a slab would be "practicing out of my area of expertise" I have done some concrete work in the Army but that was a long time ago and was very primitive.
I'll add that if you do have questions, and you do build it to spec, ask for and pay for a field inspection at the time of the pour. The engineer will then come out and sign off on the slab prep. It wouldn't hurt to have a cylinder to crush as well. Might save you some 'proving it' after the fact if the slab does not work out.
welded wire mesh and rebar serve different purposes, and are not really interchangable. I think rebar in a 4" slab would be of little benefit anyway. For that matter, I think a 4" slab is not really up to the task at hand here anyway. But I, like yourself, am not an engineer, so what do I know.
You could bring your concern to the engineer. As others mentioned, if he's professional he should not take it personally.
Is it shown on a cross section, or is it just a note on the plan? Alot of those notes are cookie cutter, but the section drawing may show something that would contradict the note on the plan. It is possable that a standard note got used and it went under the radar.
I guess I will have a talk with the Engineer as 4" of concrete with wire mesh does not seem adequate to me.
4" slab with wire as a floor slab that will have 40-ton trucks on it.
Interesting.
Ask your engineer where he bought his calculator, because you do not want to buy one there. They don't work, obviously.
FWIW, I am an engineer, PE, BS and all that. But just on intuition alone, and seeing the results of poor design, I wouldn't put a slab that thin in a residential garage, never mind a truck shop.
Call the engineer and tell him you aren't happy with the slab design. If you can't get satisfaction, get a second opinion.
I don't like it either, but would you change your tune if the soil baser were say, compacted mineral to 2000PSI two feet deep? Just for sake of discussion?
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
For discussion's sake, no. I think a 4" slab, regardless of reinforcing, is asking for trouble. If you have one spot under that slab that is or becomes unsupported, due to poor compaction or settlement, you have a potential failure point. The slab needs to be designed under the assumption that such settlement will occur.
The rebar will help to make the slab structurally more capable of withstanding point wheel loading, but only if it is installed exactly according to the design criteria. Most isn't, so it's hard to rely on it.
I wasn't comfortable with a 4" slab in my basement, which currently has a 1600-lb planer sitting on it. That'll be the heaviest load it sees. I made it a 5" slab, with wire mesh AND fiber. The wire mesh doesn't really prevent failure due to bending or shear, it's more of a major crack prevention. Same with the fiber - it helps against minor cracking (which I've avoided so far, after ten months).
The difference between a 4" slab and a 6" slab, in terms of quantity, for that size building, is 37 yards give or take. Say 40. 40 yards placed, material only since the floor costs already include the screeding and finishing, is maybe another five grand. For a minimum half-million-dollar building, what's another five grand and a few boxes of fiber (and some actual rebar chairs and tie wire!) if it will prevent major structural failure during the life of the building? Simply put, the original poster is right to be concerned.
My Engineers plans are very Generic as he had no idea of what type of soil the building is going on and is 150 miles away from the site. The building is going to be setting on top of 2 ft of fill that was compacted in 6 in. lifts. I chose the Engineer based upon reconditions from a metal building contractor who uses him daily, after talking to the contractor again he said he just uses the Engineers plans to get the permits and then does the foundation however he wants it.
There have been a lot of threads here about slabs and concrete. Using the search function should turn up a lot of info. My understanding of slabs based on limited experience and a heck of a lot of reading here is:1. The compaction UNDER the slab is more important than the thickness of the slab. 2. Wire mesh is better then rebar, if it can be kept up off the bottom of the slab. 3. Rebar only serves to limit the width of cracks AFTER the concrete cracks. It doesn't make concrete stronger.4. Don't let anybody talk you out of a vapor barrier under the slab.
When you have a lot to do in a day, it's always best to get your nap out of the way first. [Ann Landers]
#3 is wrong boss, sorry to point out Rebar only function is in load factors. it does nothing till a load is applied and then it counter act the bending up the concrete. wire mesh controlled cracking in the curing process does nothing for load factors.wire mesh cannot replace rebar and rebar cannot replace wire mesh. If your slab calls for rebar, it better have wire mesh in it too. In fact all slab should have wire mesh.side note. my garage has five inch slab. only thing on it is a toyota pickup. Its not thick enough, For big trucks its more common to have 8 Inch to 1 foot. The local peterbilt dealer here, last year placed 1 foot slab.. 2+3=7
Every bit of advice you have received here is sound.
I will add to it just a little. You definitely need to talk with your engineer, something is not right with that design. As already suggested, it wouldn't hurt to take a few cylinders and have them broken for a CYA.
What I would add is it would be prudent to get some compaction tests on the subgrade. Concrete failures are often a result of subgrade failures, you want to make sure that doesn't happen here. You don't need to spend much; geotech labs often have Proctors on file for locally available manufactured materials, even if you have to have one done on the native material it's cheap insurance. That and a few nuclear densometer tests and everyone can sleep well. Except the engineer if he screwed up.
A king can stand people fighting but he can't last long if people start
thinking. -Will Rogers, humorist (1879-1935)
When I design something in AutoCAD, I almost never start with a blank sheet (or screen). I save a lot of work by using one of my old drawings as a starting template. Thus, if I already have the details of a wall or a foundation, I can change the dimensions and move some features in a matter of minutes. I also import notes, details, and schedules. These are found in my other drawings. I have already created the work and am just cutting and pasting it into the new drawing. If I import a note from a 4" slab and forget to change it to 6", and then fail to catch the oversight, I have a problem. Your engineer will probably appreciate your pointing out the discrepancy.
BB has a lot of experience with concrete, his observations are some apt.
Now, I'd have to agree, a 4" slab, even with 3500 mix with WWM does not sound like a "heavy truck shop floor" to me, either. But, I did not run the calculations, I've no idea what sorts of perimeter and intermediate beams are built into the slab. It could be that the beam spacing is close enough to only need a thin "membrane" of floor to span between the beams. I don't know, and can only guess.
The last shop floor for a truck center I detailed used #5 @ 12" OCEW in a 6" flyash admixtured 3200 psi with stirrups at 36" OCEW to insure bar location--but that was a slab with very little additional beam structure, too--a very different sort of engineering design.
Wire mesh is just smaller reinforcement than rebars. NYC DOT standards for a sidewalk is 4", and 7" at drop curbs (truck traffic), however both are sufficiently reinforced with wire mesh. The key is that the wire mesh is above the center of the slab and the slab is placed over 4" minimum stone base over compacted material.
For heavy use, and larger than normal trucks, #5 at 12" c/c rebar is preferable. A typical bridge approach slab (to transition from the roadway pavement to the deck) is a 12" slab on 12" of well graded, compacted stone with #5 @12" each way, top and bottom. NY State DOT has been using a high density mix for the last decade with good results. It contains fly ash which reduces the micro-cracking that leads to corrosion of the rebar and accellerates freeze thaw, and also has high-early strength characteristics.
And as always, the most important aspect of a slab is proper curing....that's not a mistake, it's rustic
I come from a family of loggers. We have lots of big equipment and lots of big buildings. I can tell you for sure that if you run a truck that size on a 4" slab, it will soon crumble.
He may have made an honest mistake. If he doesn't correct his specs, there is no way I would pour that slab, no matter what seal is on the specs. If you do it and it fails ( which it will) then you are involved one way or the other and will be involved in a finger pointing contest and possible future liability problems.
Our shop floors are 8" thick w/ rerod. Drive roads at paper mills are usually 12" thick.