Hay Loft floor structural analysis

[ScandieSconnie]

Hello everyone,

Greetings from southern Wisconsin! This is my first forum post ever, so please excuse me if I don't get the etiquette quite right..

I'm trying to do some structural analysis on the hay loft floor of an old dairy barn. My uncle wants to store big square bales up there, so it needs to support his JD 318E skid loader (~ 7,000 lb) with an 800 lb square bale on the front. The beams are rough-sawn oak that were made by the amish many years ago. They are still very solid, no sign of rot (I think they are white oak) Beam cross section is 3" x 8", 12' span, 2' on center. I am currently in a mechanical engineering program with mechanics of materials under my belt so I have a general idea how to do the analysis, but I can't seem to find consistent data on the allowable stress of the wood.. It would be easy if it was steel, but obviously wood is more complicated. Would the tension on the bottom govern the design, or compression on the top of the flooring? I was going to check both, but again, not sure where to find allowable stress for tension or compression when it comes to wood. I thought about adding a 2x6 on the bottom of each beam to increase the moment of inertia, but I still wouldn't feel comfortable without having a solid figure...

Here's what I have come up with so far (analyzing beam only):

Area moment of inerita I = 128 in^4
For a point load "P" in the center of the beam, Mmax = 36P
compression stress at top extreme fiber, Sigma_c = 1.125P
tensile stress at bottom extreme fiber, Sigma_t = 1.125P

For the flooring, we put 2 layers of 3/4" plywood on top of the old 3/4" tongue and groove boards. Any help would be greatly appreciated, thanks!!

 

[JStephen]

Go find a local structural engineer to check it out for you.
The floor may be the issue rather than the beams.
Dealing with "mystery wood" may be a bigger problem.
I assume driving a loader around would put a lot of horizontal loading into the building that it wasn't designed for as well, especially a skid-steer loader.
Is there anything that would keep you from driving through a wall?

 

[racookpe1978]

It is not only the "beams" that need be analyzed for static loads (of the loader, the stacked hay as a mass, and the "single" carried hay bale on the loader plus the driver weight); but the METHODS of attaching all of the beams to the walls, the wall structure to the frame, and then the frame down all the way to the foundation around openings and cutouts and windows and the like. The loader, for example, is really 4 point loads, but the stacked hay is a spread-out load - that "moves" as the stack is set up and torn down over the year.

Dynamic loads have to be checked as well - as pointed out above - but they can only be analyzed (approximated really) only after the load-bearing path is know and validated as adequate.

 

[JAE]

JStephen is correct - mystery wood is what you have and very difficult to pin down an appropriate allowable stress. You could make some very conservative assumptions (perhaps Fb = 500 psi or something) and see if your floor is well stronger than needed. But other than a load test or somehow sampling and testing a wood beam in a lab I don't know what else you can do.



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[Lomarandil]

I agree that there are more facets to this problem. One (typically in your favor) is how the wheel loader will distribute load to the beams. I agree with JStephen's concerns about the flooring and putting equipment load on an elevated story of a structure. Also, remember that the beams have to carry loads in addition to the skid steer -- other bales, tributary dead loads, and potentially loads due to the behavior of the rest of the structure (for example, they may help keep the building stable during wind storms).

The best course of action is to find a local structural engineer who is willing to take on the work (or if you're unusually lucky, mentor you through the process).

If you insist on wanting to proceed yourself, here are a few resources to get started in the right direction.

Timber design is governed by a code called the NDS. You may be able to find a copy at your university library -- but you'll find that contemporary timber design can't just be boiled down to straight tensile and compressive stresses. (Once again, if you're unusually lucky you may be able to find an old [30 years +] reference that would have guidelines for simpler design at the cost of being more conservative). Connections are also very important.

As JStephen mentioned, not knowing the species of wood might also be tricky. The allowable strength of members varies by both the species and the grade of the timber (as a natural material, the quality of timber varies -- it's graded so the best timber can be used for more critical applications). Be careful with what assumptions you make here.

Plywood is a tricky beast in and of itself. You'll want to find some resources from the American Plywood Association, and either will want to verify the orientation of the plies or assume they're all running in the weak direction.

 

[Tmoose]

Are the beams resting on a sill?

Or is some joinery involved?

https://www.thebarnyardstore.com/post-and-beam-barns/joinery/?for=carriage-barn

 

[ScandieSconnie]

Thank you all for your input.

Some more information:
The lower level of the barn is partially underground, so the floor of the loft (I guess calling it a loft is a bit misleading, they call it the "haymow") is effectively the ground level, at least on the side with the entry door. So the oak joists are resting on top of the concrete foundation wall. There are two main beams running parallel to this wall (length of the barn) that have steel columns about every 8 feet. So I am fairly confident that as long as the joists and the floor can support the weight, the rest of the barn will be fine. My plan was to keep it simple, making conservative estimates wherever needed. That way I know the floor is as strong or stronger than it needs to be.

 

[ScandieSconnie]

As for the wood - I know it is oak. Just not exactly sure what kind of oak, so the weakest oak wood could be assumed maybe?

 

[racookpe1978]

No, you don't "see" our points.

It is not just the " simplified floor" and joists that have to be checked. It is the whole "system" of ALL of the loads involved -including those you don't see - AND how those loads are transmitted THROUGH the wood floor down to (what we assume to be) a stable foundation.

It IS about safety of the occupants (people and cows) of the entire system that we are concerned about. Get a PE involved. Don't guess about lives.

For your own savings, get your prof's to approve a PE-signed paper that you've assisted with and research and written as a term project for credit. The PE signing can use your research (sizes, joint analysis, basement wall digging and measurements, drawings and sketches of load areas, sizes and beam shapes, and beam coefficients and wall-and-beam connections (nails? Screws? Wood dowels? Bolts?) that will
(1) give you experience
(2) use your previous classes as experience
(3) save him/her time and cost getting basic measurements done - and thus saving you cost.
(4) save your family grief and time re-building the barn.

 

[Brad805]

Being an ex-farm boy and working for my brothers on many different projects over the years I would suggest you start with a clear plan of what they think will work. You have mentioned a few key flags (ungraded lumber, rough sawn..) that are going to make this assignment a challenge. What age is the barn?

Am I correct you are doing this as a favor? Do they require a permit where they are located? If they need a permit I would suggest they hire a structural engineer since they will need sealed construction drawings. If this is a favor in a non-permit zone my guess is they will go ahead and do whatever they think might work once they find out the fee to hire someone. Been there.

Have you looked at the practicality of using a skidsteer on the loft? Unless this is a very large barn it would not surprise me that the operational space (travel path, turning space..) required to load/unload bales will reduce the number of bales they can store to the point that this will not make that much sense once they consider the costs involved. I have derailed crazy ideas many times in the past by pointing out the practicality of the idea. If it is practical to use the skid steer on the loft and this is the normal farmer case where labor is free, I think the next step is to post a clear framing plan and a maybe a typical section. The group tends to get more active when they have sketches to look at.

 

[oldestguy]

Scandi:

It may be a disappointment to get the comments above. However, remember this site is for general guidance of engineers, not for any designing. To guide you there may be Ag Engineering sites on this subject, but no design sites.

 

[SlideRuleEra]

ScandieSconnie - I agree completely with our colleagues... this is not a (structurally) good idea. Instead of adding to their comments, I suggest you take a look at the other side of the problem: the load. To proceed, you have got to know a lot more about the load than 7000 pounds weight and will be carrying a 800 lb. bale. With that load, what is the weight distribution on each of the wheels? I looked up the specs for the John Deere 318E. There is enough information there to make a reasonable estimate... but it will take some statics calcs and engineering judgment. Suggest you give us your answer.

http://www.SlideRuleEra.net

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[msquared48]

Go to 80# bales!!!!!!!!!!!!!!!!

Mike McCann, PE, SE (WA)

 

[boo1]

visually assess the lumber
Assume Fb = 800 psi, lower if there is visible sagging or moisture exposure.

for older lumber see

http://www.awc.org/standards/nds/archives.php

 

[Wallache]

Old hay barns were designed to store loosely stacked hay moved by pitchfork rather than tightly compressed bails moved by heavy equipment.

 

[dhengr]

ScandieSconnie:
If it is a barn which has one large side door up at the hay mow floor level, served by a ramped drive way, then the area immediately around that large side door, might actually have been designed/built a little stronger to support a loaded hay wagon, maybe even some small implements. But remember, dry loose hay weighs quite a bit less per cubic foot than today’s stacked and compressed bails, and a skid loader. And remember, that at some locations you will have stacked hay bails, and the front wheels of the skid loader (that is, 800lb. bail, plus .6-.7 times 7000lbs. of loader) on only one or two fl. jsts. Is there a trolley track running down the center of the barn up at the ridge? That’s how they unloaded the hay wagon, with a large hay lift/hay carrier, and then on to either end of the barn. I’ve seen plenty of barns like I think you are talking about, but tell you Uncle that this is not as simple a problem as you and he think it is. It seems to me that I have heard of instances where bailed hay stacked to high was enough to start to causing structural problems. I think I’ve also seen them park small tractors up there. This is probably not a very good structural design project for an aspiring young Mech.E., who’s total structural design experience is having completed a first Strength of Materials course to be cutting his teeth on. And, that’s nothing to be ashamed of not being able to handle alone. Many of the points made above are right on the money. You might learn a lot by following a qualified Structural Engineer around and helping him as part of the deal. The whole structure should be reviewed given what your Uncle wants to do.