Calculating lbs/sq ft.

[raykos]

Hi all,

If I wanted to build a floor out of 2x's, spaced 16" O.C., covered with 3/4" plywood, and park my tractor on it,(max 3000 lbs.), how do I figure out the lbs/sq. ft. the floor will need to support?

I'm thinking that I probably take the length & width of the wheel base as the sq. ft. "foot print", & then divide the weight by this "foot print"? But, I'm not positive?

Thanks,
Ray

 

[Jerry/MT]

Hi all,

If I wanted to build a floor out of 2x's, spaced 16" O.C., covered with 3/4" plywood, and park my tractor on it,(max 3000 lbs.), how do I figure out the lbs/sq. ft. the floor will need to support?

I'm thinking that I probably take the length & width of the wheel base as the sq. ft. "foot print", & then divide the weight by this "foot print"? But, I'm not positive?

Thanks,
Ray

You really need to get the tire foot print and the load on each wheel to do what you want to do. That will give much higher loadings than what you propose.

If it were me I sprinkle some powder or flour on some pieces of plywood and drive the tractor on them, mark the foreward and rear edge of the footpint , then drive off. I'd measure the length and width of each "footprint" (it's liable not to be a rectangle but it will be close enough).
Because the cg of the tractor is unknown, you have to weigh each wheel to get the load to calculate the loading in psf.

I'd guess that 2/3 of the weight on the rear tires so that 1000lb per rear wheel for a 3000lb tractor. Divide that by the rear foot print area and you 'll have the loading for the rear tire. Multiply that 1.25 for a safety factor and use that for the rear tire loading. repeat the process for the front wheel at 500lb per front tire. that should get you in the ballpark.

 

[CurlyDave]

Hi all,

If I wanted to build a floor out of 2x's, spaced 16" O.C., covered with 3/4" plywood, and park my tractor on it,(max 3000 lbs.), how do I figure out the lbs/sq. ft. the floor will need to support?

I'm thinking that I probably take the length & width of the wheel base as the sq. ft. "foot print", & then divide the weight by this "foot print"? But, I'm not positive?

Thanks,
Ray

That is the way I have always done it.

Sure, if you just count the tire contact area you will get a much higher #/sq. ft. number, but you are ignoring that all the space where the tires are not in contact has no load at all on it.

You just have to be certain that there is enough load transfer between the floor joists to equalize the loads on them, or you may overload individual joists.

 

[Oldchuck]

No way would I put a tractor, or any other vehicle for that matter, on 2X4s. I would think maybe 2X8s, 12" OC, and a 1" deck. Better yet, concrete on grade.

 

[/pine]

Just because a framed floor (joists and decking) is designed to support say a 40# live load (per square foot) does not mean it would support a tractor of which its entire weight is supported by the four points where the tires bear.

over time you would end up with a floor that has four big dips (birdbaths)....

it is all about distributing the weight over the entire structure

 

[WVBill]

Here's a reference that discusses floor joists and loads - this one is about residential loads so it dosen't directly answer your question but the basic concepts are applicable:
Understanding Loads and Using Span Tables - Publications - BM&WT - UMass Amherst

A real valuable source of documents I found through a post here on TN several years ago is the US Army' General Dennis J. Reimer Training and Doctrine Digital Library

Found here: https://rdl.train.army.mil/soldierPortal/soldier.portal

You can find all kinds of construction and tractor-related documents there (mostly search under "Engineer" for this kind of stuff)
- like this one:
https://rdl.train.army.mil/soldierPortal/atia/adlsc/view/altfmt/9736-1

That said - I don't think plain 2x lumber will support your tractor - for long...

Best of luck to you

WVBill

 

[RWolf]

Also keep in mind that wood however treated will over time abosorb moisture. I think concrete is a better choice. As a swag most likely reinforced at about 4 to 6 inches.

 

[Egon]

How is the floor supported?

 

[Jerry/MT]

That is the way I have always done it.

Sure, if you just count the tire contact area you will get a much higher #/sq. ft. number, but you are ignoring that all the space where the tires are not in contact has no load at all on it.

.

That's the point! you need to size floor structure for the maximum "point loads" which are where the the tire contact is made. To use the average load of the area under the tractor will lead to undersized structure. That will likely cause a failure unless you put some huge safety factor in to use for sizing.

 

[RobS]

Study the links that Bill provided, good stuff there. Yes, you have to worry about the point loading but there is also the overall loading and how the joists are supported. This gets into spans and such. Don't forget to figure all the scenarios, not just the one where your tractor is parked in the middle. There could be higher loads during approach and such. Wood will work, given enough of it. They've been making wooden covered bridges to hold much more than your tractor for a long time now.

 

[radioman]

you have gotten my curiosity. Is there a reason you need to use lumber for a floor for your tractor ?

 

[bncsauve]

Just estimating: I would say my tractor, roughly #3500 lbs without bucket or backhoe or bh subframe and estimated 200 sq in contact patch for each back tire and 50 sq in for each front tire seems to be 1750 lbs on the back divided by 400 = 4.4 lbs per sq in in the back x 144 = 634 lbs per sq ft in the back and 1750 divided by 100 = 17.5 lbs per sq in in the front x 144 =2520 lbs per square foot in the front. Pretty large point load. Your floor decking will need to accomodate the point load. Area loading is roughly 5 feet x 6 feet = 30 sq ft, divided into 3500 lbs = 117 lbs per sq ft area load. Your floor joist will need to accomodat that. Without looking up load ratings, I would guess you would need at least 2x10s 12 inches on center with 3 layers of 3/4 inch plywood, with spans between supports no longer than 6 ft for the 2x10s.

 

[texbaylea]

Do not use milled limber!! The strength is too variable. Look at engineered floor joists where you can get span/load tables. Also with 1" or better tongue and groove plywood screwed and glued to the joists, with glued joints.

When I built a second floor in part of our utility building I used 14" I-joists, 24' O.C. with 3/4" T&G screwed and glued. The span is 20'. No tractor but a lot of heavy household items in storage. When you walk on it it feels like a poured slab. I admit I found the I-joists at a bargain (surplus)

Vernon

 

[CrazyQ]

Just estimating: I would say my tractor, roughly #3500 lbs without bucket or backhoe or bh subframe and estimated 200 sq in contact patch for each back tire and 50 sq in for each front tire seems to be 1750 lbs on the back divided by 400 = 4.4 lbs per sq in in the back x 144 = 634 lbs per sq ft in the back and 1750 divided by 100 = 17.5 lbs per sq in in the front x 144 =2520 lbs per square foot in the front. Pretty large point load. Your floor decking will need to accomodate the point load. Area loading is roughly 5 feet x 6 feet = 30 sq ft, divided into 3500 lbs = 117 lbs per sq ft area load. Your floor joist will need to accomodat that. Without looking up load ratings, I would guess you would need at least 2x10s 12 inches on center with 3 layers of 3/4 inch plywood, with spans between supports no longer than 6 ft for the 2x10s.

This is great advise. Just make sure to be conservative in the calcs since there will be some dynamic loading.

 

[orezok]

Just estimating: I would say my tractor, roughly #3500 lbs without bucket or backhoe or bh subframe and estimated 200 sq in contact patch for each back tire and 50 sq in for each front tire seems to be 1750 lbs on the back divided by 400 = 4.4 lbs per sq in in the back x 144 = 634 lbs per sq ft in the back and 1750 divided by 100 = 17.5 lbs per sq in in the front x 144 =2520 lbs per square foot in the front. Pretty large point load. Your floor decking will need to accomodate the point load. Area loading is roughly 5 feet x 6 feet = 30 sq ft, divided into 3500 lbs = 117 lbs per sq ft area load. Your floor joist will need to accomodat that. Without looking up load ratings, I would guess you would need at least 2x10s 12 inches on center with 3 layers of 3/4 inch plywood, with spans between supports no longer than 6 ft for the 2x10s.

Actually. it's much simpler than that. If you have 30 PSI in a tire, it will exert 30 psi on the floor. Every action has an equal and opposite reaction. Raising or lowering the pressure will vary the footprint, but the load will equal pressure.

 

[carpenter383]

I would just figure out where the tractor will be in the building and strengthen that section of floor from the door to the tractors' resting place. Set posts under the floor with beams on top of them that will support the bottom of the floor joists.

 

[CurlyDave]

If you have 30 PSI in a tire, it will exert 30 psi on the floor.

This is a common misconception about tires and how they work.

A tire works by the air pressure inside stiffening the sidewalls, which are the primary load-bearing mechanism.

If you increase the pressure in a 30 psi tire to 60 psi, the area in contact with the ground will not be cut in half.

 

[wedge40]

you have gotten my curiosity. Is there a reason you need to use lumber for a floor for your tractor ?

I'm going to guess it's taxes. Most states/counties charge taxes on a permanent buildings but none on temporary building. Cement = permanent, wood = temporary. I bought one those little tin cans at the local home improvement chain. I think 10X14 and decided I wanted a raised wood floor, but I was going to put my cub cadet garden tractor in there. I believe I used 2x8 on 12" centers supported every 18". When I first got done I had put down 1/2" treated plywood and the floor sagged in spots when I was walking on it. No way was this thing going to support my garden tractor. So I went back and got another layer of 3/4" ply to put down. I dont think I trust driving a car on it but it don't budge at all when I drive the cub on it. I think the shed kit cost about $500 or $600 and I ended up with well over a grand in the deck. Dont get me wrong I'm still glad I did it because when I get the barn done it will be easy to tear the shed down.

Wedge

 

[Charlton John]

I have a wood floor in my pole barn. My tractor weighs roughly 3k w/ loaded rears,fel and the BB on the back so not quite as heavy as yours, but I do also have plenty of other toys in there as well. My floor is constructed of 2x6's but 6" OC w/ 1" ply screwed and glued. The floor is extremely strong I could drive a truck on it I beleive w/ no issues.

 

[WVBill]

My floor is constructed of 2x6's but 6" OC w/ 1" ply screwed and glued.

Good info - critical question, though - what's the unsupported span of the 2X6's?