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Old 09-14-2021, 10:52 AM   #1
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1984 32.5' Airstream 325
Gainesville , Florida
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Concrete thickness etc

My apologies if this has been done a 1000 times but I can't not find the relevant information.

We have a 325 MoHo, nice ol beauty but in dire need of work. In the process of building a 40'x60' pole barn (so excited to finally have a spot for a shop). We've now in the process of enclosing it and putting a concrete floor down.

The engineering drawings call for a sand base with 4" reinforced concrete.... is that going to be enough to prevent unwanted cracking from he 325?

I called the engineering company to clarify I'm putting a 16000lb RV on it and he asked what the axle weights are (a good question obviously... wish he'd asked months ago when I told them. T

here seems to be some data out there but 99% of what I've found is gross weight empty or loaded - not discrete axle weights. I've seen ~ 4500lb on the front (n=1) but nothing for a 2 axle rear (tag).


Does anyone have approximate front and (2x) rear axle weights I can give the Engineer to run the calculations?

Cheers and thanks -
Pete
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Old 09-14-2021, 12:43 PM   #2
FYS
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What's the strength of the concrete you're going to use? The concrete is for compression strength but it's the steel reinforcements that are going to take the sheer and bending moments. Too little rebars will cause crack failures, but too much reinforcement is the worst as it will pop/explode the concrete without warning. There are standard plans and specs for concrete pads that you can search for so to make your life easy. For mine, I used 4,000 psi strength concrete for a 4" pad, wire mesh reinforcement with rebars at certain areas and made sure there's proper concrete cover top and bottom before pouring the mix. I did plan the a 1/4" per foot slope for drainage, and joints at 5' intervals so I don't have to make a monolithic pour, and it function as controlled crack joint (so it'll crack there instead of somewhere else undesirable). As the concrete cured, I hand brushed it so the finished concrete pad isn't slippery.
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Old 09-14-2021, 02:13 PM   #3
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Quote:
Originally Posted by FYS View Post
What's the strength of the concrete you're going to use? The concrete is for compression strength but it's the steel reinforcements that are going to take the sheer and bending moments. pad isn't slippery.
Thanks - useful information - cheers.

With the local permitting codes here, we were forced to have engineering plans, with the engineer prescribing how it has to be done (to get permit approval and closure) -

I have to give him axle weights so he can modify the plan to suit and the inspector will pass it. So far the plan calls for 4" thick with WWF or fiber mesh.

He didn't notice in the emails I said it was for a 16000 lb RV. If yours (at 4") is working that gives me some confidence (also a lot cheaper). Tho' I have to consider I'll be lifting the whole thing up to do an engine swap so the load will be more concentrated.
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Old 09-14-2021, 04:34 PM   #4
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If you have to go 4” (I went with 5” @4000psi 4 sack mix). I’m surprised the engineer didn’t recommend a mix design for the batch plant to copy. Regardless of the thickness he recommends, the weakest points are the perimeter, especially where you drive in or out. So, a 12” beam with #4 rebar. As mentioned, have control joints to control where the cracks appear. You’ll probably have to have a couple of expansion joints, and I would place some sleeves dowels to allow for expansion/contraction while keeping the surface in the same plane. The engineer knows all this stuff. I know you’re not wanting to over engineer, but you do want it to last. Whether they machine trowel or bull float, I would still put a broom finish for foot traction. My barns here I park my tractor and/or truck, I did go with 5”, perimeter beam and #4 rebar tied on 16” centers and chaired off the base. But keep in mind there are two types of finished slabs - the kind that’s cracked and the kind that’s going to crack. We also kept our slabs wet for a day to minimize drying cracks. Good luck
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Old 09-14-2021, 05:29 PM   #5
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Hi

The concrete is one part of the equation. What the concrete is going on is the other part.

If you had solid granite bedrock in the back yard, you would be fine with 1/4" of concrete. If your garage is going into a swamp, 12" isn't going to be enough. Without knowledge of the "back yard" there is no way to know how much concrete is needed.

Bob
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Old 09-15-2021, 05:46 AM   #6
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Agreed, the foundation is important, but without knowing the expected load it's an educated guess by the engineer of how thick and what reinforcing approach used. Typically, here in Florida, a base of a 'yellow' coarse sand is used, which is compacted down and the standard reinforcing used. So, his calculations for thickness are based on that typical sand mix for an average use shed.

The question is still if anyone knows the individual axle weights for a (ideally) 325, but a 345, 350 would work. Obviously a 5000lb load will need more strength and a 1000lb load (for a given base foundation).

I can make the floor thicker and be okay with the permit, but it's as much as 30% more concrete and at $145-$150 a cube, that's an additional $3000 that maybe unnecessary. Of course if it is needed then it's fine, but I'd rather spend the $3K on the busses remodeling if it's not
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Old 09-15-2021, 08:18 AM   #7
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Quote:
Originally Posted by Shinytoaster View Post
Agreed, the foundation is important, but without knowing the expected load it's an educated guess by the engineer of how thick and what reinforcing approach used. Typically, here in Florida, a base of a 'yellow' coarse sand is used, which is compacted down and the standard reinforcing used. So, his calculations for thickness are based on that typical sand mix for an average use shed.

The question is still if anyone knows the individual axle weights for a (ideally) 325, but a 345, 350 would work. Obviously a 5000lb load will need more strength and a 1000lb load (for a given base foundation).

I can make the floor thicker and be okay with the permit, but it's as much as 30% more concrete and at $145-$150 a cube, that's an additional $3000 that maybe unnecessary. Of course if it is needed then it's fine, but I'd rather spend the $3K on the busses remodeling if it's not
Hi

Something in the $10 to $15 range at the local CAT scale will give you exact information on your vehicle. Takes less than 10 minutes once you get there.

Bob
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Old 09-15-2021, 08:47 AM   #8
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Here is the factory numbers for our 350.
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Old 09-15-2021, 08:50 AM   #9
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I use fiber glass mesh instead of rebar, have in run in barn between stalls [horses]12 ft wide 60 ft. long all done 1 pour, 4 in thick no expansion joints, some times will raise a little in severe freeze [below 0] settles down in spring no cracks in over 20 yrs. Only draw back kind of hard to get really smooth finish [[like sand finish]as it's strands of fiber glass that comes in bag. Redimix plant mixes in when mixing cement. a slight broom finish works well or if some kind of paint, epoxy etc. will make very smooth. Wen I used rebar for years always had cracks. the only drawback that I see with FG mesh is if have to tear out, very difficult as will not break like concrete w/rebar, every tear out I did when in bus. had to saw cut in sections to remove as will not break even w/jack hammer mounted on bobcat [only makes holes].
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Old 09-15-2021, 09:45 AM   #10
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We had very high clay soil where we last built and the contractor had to dig out th etop soil and recompact it, then apply the sand layer, then 4 inches of slab with rebar, so as said your soil type is important.

Interesting about the fiberglass. I see they even have fiberglass rebar too! https://www.americanfiberglassrebar.com/
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Old 09-15-2021, 02:23 PM   #11
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Trouble is the engine dies after 10 minutes driving... one more issue to solve
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Old 09-15-2021, 02:34 PM   #12
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Hi

Based on the sticker posted above 7,000 to 10,000 pounds max per axle would seem to be the range to target.

Bob
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Old 09-15-2021, 02:59 PM   #13
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Yeah - I guess I'm a little surprised the tag isn't carrying a little more weight. Good data tho' not as easy to find as I thought. Thank you!
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Old 09-15-2021, 03:22 PM   #14
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Quote:
Originally Posted by SYC2Vette View Post
We had very high clay soil where we last built and the contractor had to dig out th etop soil and recompact it, then apply the sand layer, then 4 inches of slab with rebar, so as said your soil type is important.

Interesting about the fiberglass. I see they even have fiberglass rebar too! https://www.americanfiberglassrebar.com/
They make good snake sticks too. The only jobs we ever used the FRP rebar was in Galveston around the salt water. More often than not, if steel rebar is burning the forms (too close to the concrete surface) it will rust, expand and spawl the surface. I like the FRP, but a little more cost. Thanks for jogging my memory.
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