Floor loading and joist sizing

[rozett]

OK, I'm sure someone on this board can help me. I've spent an hour on the web looking for joist loading information and all I can find is span tables.

Background: We have a barn that is well over 150 years old. The post and beam construction is raised above ground level. The north wall sits just about ground level, with the south wall about 5 feet above ground level (35 foot span). The barn was rehabilitated by the previous owner. Besides reskinning it, they removed all of the original floor boards and replaced them with 2x10's @ 16" OC topped with 3/4 ply. This results in joists of 11' 2" span hung from 2x12 rim joists with standard stamped hangers.

I've built two 11x13 stalls on one side of the barn, where we house our two horses (1000 & 1200 lbs). The floor always had noticeable deflection with the horses moving around their stalls. Well, after about four years, the floor joists have started splitting or broken completely. Obviously, the 2x10's aren't enough to support the load. I've looked for tables that provide loading info for various joist sizes, but all I can find is span tables for the standard residential loads of 30, 40, and 50 psf.

My thoughts are to replace the single 2x10's with a pair of sistered 2x8's with 2x4 blocking nailed below the joist hangers and to use 3/4 underlayment instead of regular 3/4 ply. I'm trying to find the loading specs for 2x8's vs 2x10's. I want to make sure that's going to be enough. I could sister 2x10's but wouldn't be able to block them.

So, there's got to be a contractor in this group.

 

[bugstruck]

I don't have time at the moment to think thru this. I'll try and get back to this later, but reducing joist depth and doubling up is not the answer.

 

[CurlyDave]

I've spent an hour on the web looking for joist loading information and all I can find is span tables.

I am not understanding why span tables are not good for what you want.

If I look here: http://www.southernpine.com/spantables.shtml and say that you need to design for 125 to 150 psf, I get some good choices.

I would personally go with 2x12 No.2 or better at 12" centers. But then I am an overkill kind of guy.

Doubled 2x8s are just not going to make it.

 

[Egon]

Have you considered a center wall for less span?

Is the 3/4 in. plywood heavy enough for horses?

As per Dave 2x12.

 

[MikePA]

My thoughts are to replace the single 2x10's with a pair of sistered 2x8's with 2x4 blocking nailed below the joist hangers and to use 3/4 underlayment instead of regular 3/4 ply. I'm trying to find the loading specs for 2x8's vs 2x10's. I want to make sure that's going to be enough. I could sister 2x10's but wouldn't be able to block them.

I am with the others, nothing less than 2" x 12" on 12" centers.

By way of reference, our hay loft is built almost like your current stalls.

Our hay loft is above the center aisle (12' wide) of our barn. 3/4" OSB, on top of 2" x 10" on 16" centers with bridging down the center. When full of hay, there's approximately 15,000 pounds (300 bales at 50 lbs each).

However, this load is a dead load, while yours is a live load and depending upon where the horses are standing, their entire weight may only be supported by one floor joist.

 

[rozett]

Thanks guys! I knew I would get some good feedback....

I am not understanding why span tables are not good for what you want.

Good question... first, because all of the tables I found were for residential loading. Obvioulsy, not what I need. Secondly, I have a degree in engineering (although not civil or architectural) and I want to see the numbers. That's in my nature. Also, none of the span tables provide any info for sistered joists. I had no clue what the change in loading is if I doubled the joists. But the link you provided has much of the info I was looking for. Thanks!!

Have you considered a center wall for less span?

Yes, unfortunately the stalls are on the uphill side of the barn. So I can't do the work from under the barn, I would need to tear out the existing floor/joists in order to access the ground to dig footings. So, I'm looking at at least a week to dig/pour/cure footings, pour/cure posts, backfill, build a 26' beam, and then rebuild the floor. The horses would be very unhappy about spending a week out in the pasture overnight. I know, who's in charge, me or them.... already had that discussion with the CFO.

Is the 3/4 in. plywood heavy enough for horses?

While I'd love to remake the original barn floor (full 2" hemlock planks cut on site), that's not practical. I do have standard rubber stall mats atop the ply. I think thats sufficient.

So, THANKS very much for the input. Looks like it will be 2x12's on 12's. I can do one stall a day and get the boys (well, they used to be boys ) back in for the evening.

 

[vmarc]

I would use all pressure treated and at least 2x for the floor.I find it amazing that the horses didn't go through the plywood.Horse urine is potent and will rot anything in short order.I'm a wussy,so for horses I would double up the joists and sandwich plywood between.I think you can get oversized joist hangers to fit..Check the whole thing for rot very often.On second thought it might be better to have an engineer who knows horses spec it out.It's frightening to me to have horses suspended on a wood floor.They can generate tremendous forces hopping around and landing on 1 or 2 hooves.

 

[bugstruck]

The recommendations for 2 x 12 @ 12" centers should work fine. However, you need to distribute the point load across them better than 3/4" plywood can do. If those horses are pushing 2 x 10's at 16" to failure it wouldn't be impossible to have a 2 x 12 fail too, even at 12" centers. The problem is they are putting 600 lbs or more directly over (1) joist at times and probably point loading near 1,500 lbs with impulse. If the decking won't span that load to the adjacent two joists, you could have another failure. Frankly, I'd rather have sistered 2 x 10's or 2 x 12's (later preferred) at 16" on center than single 2 x 12's at 12 if you are married to the 3/4" ply for load distribution. Another important method of load distrubution is blocking or bridging between the joists. Not where I prefer the initial load take-up to occur (prefer that as a secondary take-up point) with animals that size but it's rather better than nothing transferring the loads. Additional rows also limit deformation of the joists that can promote failure. That span is pretty short but I'd still have (2) minimum and (3) preferred rows of bridging if you are doing the 3/4" ply with 12" centers. Then I'd start to worry about degradation of the plywood over time and a hoof dropping thru. No doubt the 12" centers would be better protection against that. The simple solution is to do 2 x 12's at 12" OC and deck this with 2 x 6 decking. You could ply over that and screw it down if so preferred clean-up purposes. That plywood could be replaced if needed. All this is assumed for PT # 2 yellow pine.

I'm aware that 3/4" plywood has some spanning capability. It just isn't designed for this environment.

 

[rozett]

I would certainly prefer not to have them on a wood floor at all. I've even considered buying the cement block used to build retaining walls and building that around the perimeter of the two stalls and filling the space with crushed rock topped by the stall mats. But as the costs go over a grand, I start backing off.

As for the horse urine, there is 2" of pine shavings sitting atop the stall mats and it absorbs the vast majority of it. When I installed the 4'x6' mats, I drilled small holes through the ply in order to drain any urine that seeped through the seams between the mats. That has been pretty effective. Although the ply has deteriorated because of the urine, the joist failures have been structural failures, without any sign of deterioration from urine. I suspect what happened was that a section of ply rotted and deflected significantly. The 3/4" rubber stall mat prevented the horse from stepping through, but put lateral pressure on one of the joists causing it to fail. The rest of the joists have just cascaded into failure from overload. The breaks in the joists are free of any urine stains.

Bridging is another great idea (Thanks, bugstruck). There is no bridging at all in the current construction. If my failure hypothesis is correct, I suspect that bridging would have prevented the joist failures. The bridging will also provide better support for the seams between sheets of ply. That's not a problem in residential applications, but definitely a problem at these loads.

Again, THANKS for the advice. I knew I'd get some good input from you guys.

 

[bugstruck]

Bruce,
You understand the failure and the impetus very well. Armed with that and the comments here, you can work the solution. You also know to keep an eye on the plywood from time to time. Have at it and hurry up.... gotta keep the CFO happy . Don't we all know