Agricultural building - loads

[CEmonkee]

Hi - I have a question about the live load for a pole barn hay loft.

I can't find anywhere in the IBC that specifies what the live load should be for a loft of this type... I was going to use 40 psf, but I was wondering if anyone had some input as to what they would typically use.

Thanks for your help!

 

[msquared48]

An 80# bale of hay is roughly 1.5' X 2.5' or about 4 square feet for a load of about 20 psf. You are normally looking at 4 to 5 bales high, depending on how high you want to lift them, so 100 psf would not be unreasonable, depending on how many bales high you can stack.

Hope this helps.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Splitrings]

MWPS #1 lists baled hay at 6-10 pcf. A Penn. State Document lists baled alfalfa at 8-14 pcf.

 

[CEmonkee]

Hi Guys

So using Mike's dimensions (H x W x L = 1.5 x 1.5 x 2.5), the bale would be 5.625 CF. If it weighed 80#, the PCF would be 14.2. That would match the upper end of the Penn State Document that Splitrings listed (14 PCF).

I did some google searches and it seems like the size of a bale is not uniform, but I could just determine the volume of my loft storage area and then use 14 PCF to get the total weight the loft would hold at any given time.

Thanks!

 

[Splitrings]

CEmonkee,
It has been a long time since I have been around a baler, but I know density and or size are adjustable on most balers. On a recent project I did, I simply used the volume of storage times the bale density.

 

[CEmonkee]

Splitrings - thanks for your reply. If I use 14 pcf and my storage volume, I'm coming up with about 93 psf, which I would probably round up to 100 psf.

One variable is how high the hay would be stacked... I assumed a maximum height based on the available head room, but maybe that's too high. Does around 5 ft sound reasonable? I've never worked with hay bales, but I would think a guy of average strength could lift an 80# bale that high.

 

[Splitrings]

Certainly calculate the volume right to the ceiling. Bales are typically stacked in a staggered tier fashion, so there are "steps" to walk up with the bale in hand. If there is the smallest hole near the ceiling, you can bet there will be a bale stuff into it.

 

[CEmonkee]

Splitrings - thanks (you can probably tell I've never worked around a barn!).

 

[hokie66]

Splitrings is right. When I was a kid, I was at the top of the stack, right under the roof, finding another hole to fit a bale in. Now that I think of it, I'm sure that loft was not designed for the amount of hay we squeezed in.

 

[MJR2]

The orders in my barn is stack it so it doesn't touch the roof. We are definitly stacking ten layers high. I have seen it much higher in some barns. The farmer and engineer in me conflict on this subject. I've only been doing this the last fifteen years of this barn's 110 year life. So it is not an experiment. I've seen 100-120 PSF for new construction.

Mike

 

[CEmonkee]

Hi - in addition to the loft, I'm going to be doing the engineering for the entire barn. My plan was to consider diaphragm action when analyzing the structure, instead of just conservatively assuming the barn as a system of independent 2-D frames.

Specifically I was looking at the analysis procedure outlined in Chapter 5 of the National Frame Builders Association (NFBA) design manual. I have also downloaded the DAFI program from the NFBA website (

http://www.nfba.org/i4a/pages/index.cfm?pageid=3385).

My question is about calculating the horizontal shear stiffness of the roof diaphragm. The roof of the barn will be sheathed with 1/2" CDX plywood and finished with composition shingles. The NFBA design manual outlines how to calculate the stiffness of a metal clad diaphragm using tabulated diaphragm assembly test data. I have not been able to find any similar data for plywood diaphragms.

Has anyone done an analysis on a post-frame building with a plywood roof diaphragm? If so, did you account for the diaphragm action, and what did you assume for the diaphragm stiffness?

Thanks!

CEmonkee

 

[Splitrings]

All the designs I have been involved with were analyzed as frames without consideration for diaphragm action. Mostly because the structures were open type agricultural facitlites. Most had pulins and metal roof. I know both NDS and IBC list design values for diaphragms.

 

[CEmonkee]

Hi Splitrings - thanks for your reply. The barn will have openings at the top of the long sidewalls, so I've been thinking that analyzing the structure as independent frames is the best way to go.

 

[msquared48]

Most of the barns I have done were pole type structures with no structural diaphragms or shear walls. I think the last one did have knee braces and I did have to model it on the computer to get the relative stiffnesses of the columns and overall deflections.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[CEmonkee]

Hi Mike - thanks for your reply, I appreciate it. I've attached a PDF file of a plan I found that is similar to the barn I will be analyzing. The dimensions are different (length, roof slope, etc.), the roofing will not be metal, and the loading info on the sheet doesn't apply to my area. But the overall framing is close - 4 posts in each frame, loft in the middle, etc. The rafters (or trusses) frame into 2x girders. There is no knee-bracing. If you have any further comments or pointers, I would appreciate it.

Thanks!

CEmonkee

 

[msquared48]

Coupled of comments:

At the loft section:

1. Add horizontal 2X12's from the loft columns to the exterior posts at each stall to better transmit the seismic load from the hay loft to all the posts. You can add them just above the hay loft 2X12's.

2. Since this is intended to be a pole structure, I would recommend the exterior posts be embedded in 24" round, concrete filled, augered holes. As for the loft area poles, the spread footings are a nono if you intend these poles to carry lateral too. The soil has to be undisturbed. If you need more bearing for the vertical load, either go deeper with the hole, or increase the diameter, or both. The deeper you drill, the greater the allowable bearing value at the emd of the pier.

The truss sections seem fine, but I would still use the 24" concrete filled augered holes.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[CEmonkee]

Thanks for your comments Mike

Good idea on adding the 2x12's for better seismic load transfer.

The foundations shown on the interior posts are actually 8" thick 18" diameter pads. I think I probably will need to increase the diameter for bearing considerations (the loft load will be fairly large).

For the outside poles, what about using concrete pads with collars (see attached .jpg)?

 

[msquared48]

I wouldn't use that detail.

Just fill the bottom of the hole with 6" of gravel for a water break, and put the treated post directly on the gravel, filling the rest of the hole with concrete. Do not backfill with earth. The depth of the hole for lateral will start at the top of the gravel, so you will need to dig the hole 6" deeper than shown by the calculations.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[CEmonkee]

Thanks for you suggestion Mike. Quick question - why wouldn't you use the collar detail? Is it just simpler for the builder to use the method you described and achieve the same end result?

Thanks again.

 

[msquared48]

The collar is only to hold the bottom of the post in place over the footing. The earth fill above the collar is not useful for lateral bearing, but concrete would be. So forget the collar idea (which the concrete infil serves as anyway), and to achive full lateral capacity, replace the earth and collar with concrete, and the footing with gravel.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask