Large Agricultural Structure

[mmmbmf]

I would greatly appreciate some opinions as to what would be the best possible options for lateral support for an enclosed agricultural structure. The structure will house chickens. Eave height of 20 feet. 140 mph Florida. Clear span of 70 feet. No interior walls. I have looked at using knee braces to the wood trusses at 4 feet oc like they used to use on these type of structures and anticipate providing the wood truss engineer with the compression and tension loading (approx 4k)at the points of connection of the knee braces. Another option is to use a vertical steel cantilever at some spacing oc. Another option would be to price out a steel building with moment frames. The major problem with the steel is the corrosion factor from the chickens. Any thoughts would be greatly appreciated.

 

[jayrod12]

What about cross bracing a couple of the bays between the columns and design the foundation portion to take the lateral load?

 

[mmmbmf]

I have looked at that option in the long direction (400 ft) and that is ok, it is the transverse clear span direction I cannot have cross bracing in. I have also looked at cantilever columns from the foundation and I can make that work but I am still searching for more economical options if they exist. Thanks.

 

[Archie264]

If you're interested in using cantilever columns then it sounds like you're not too far away from some of the principles that make a "pole barn", specifically, columns, i.e., "poles" embedded into the ground in such a way as to create a fixed base.

I don't know if it's kosher to point you to the site of another member here but Mr. Slideruleera has some guidance on that topic on his site. Look for the publication that mentions pole barns, see link:

http://www.slideruleera.net/miscellaneous.html

I will be watching to see if that proves to be applicable to your situation since I know of some structures with pinned bases and knee braces and the topic somewhat interests me. Good luck.

 

[msquared48]

With a 140 mph wind with 20 foot eaves you are really pushing the envelope with a 70 foot span for a pole structure, even with the poles at 4 feet. I would consider using Parallam or Glulam columns, or making the end walls plywood shear walls with the roof diaphragm of plywood spanning to the endwalls. The opposite direction could use the poles for lateral. You may want to mix the pole and standard plywood shear wall scenario with this one.

If you cannot use interior knee braces for the columns, perhaps you could use exterior ones.

Mike McCann
MMC Engineering

 

[XR250]

I would be surprised if the knee brace calcs actually worked out. Even if they did, it would be a detailing nightmare and then you have to account for the additional uplift in the trusses when designing their bracing. A pole structure is probably the easier of the two or maybe just a pre-engineered metal building.

 

[mmmbmf]

Thank you all for your input. After reading the pole barn article, I have done some preliminary calculations and determined that this may still be the best way to go on this structure. Some things just don't change very much from the engineering side, but certainly do from the code side!!! I am going to recommend them to price out the pole structure and the metal building structure for comparison and let them decide. It is my understanding that the steel does not do very well in that type of environment. Thanks again.

 

[JedClampett]

My question is why does the structure need to be clear span? The owner is going to pay a premium for this.

 

[ajh1]

Consider having the metal building frames hot-dip galvanized and use G90 galvanized secondary structurals. That should solve most of your environmental corrosion problems.

 

[civeng80]

I really think galvanized steel would be the way to go.

Propped in the middle it would be economical and strong.

 

[mmmbmf]

With using the allowable stresses recommended in the paper by Donald Patterson, I came up with 10 inch dia. poles at 8 feet oc. A truss manufacturer says that if he has enough height for the truss he can design it to meet the code. I still have to provide the truss manufacturer with the worst cast knee brace load for him to incorporate into the truss design. I am going to give the owner the option to price out the typical section that I came up with along with the metal building option. I have a question into the owner as to the clear span issue and see no reason as to why there can not be an intermediate support as they have shown me the rack layout for the interior. I am comfortable with the wood roof and wall design as we are planning to use the typical purlins and sheet metal roofing and siding. I have modeled the roof with additional wood cross bracing to add a "truss effect" in the transverse direction for distributing the lateral forces. I appreciate your comments and welcome any additional thoughts.

 

[Splitrings]

If you stay with the pole building I would consider steel posts, wye and knee braces. The headers and trusses could be wood. My experience with pole buildings is that the wood connections require more than just a few nails! The 40' and 50' pole buildings I have designed require numerous 3/4" bolts in each connection. It becomes difficult to get enough fasteners into each connection. They are designed for a 60 psf ground snow load that you won't have. Getting timbers large enough for posts is not an issue. We have installed 14" square SYP posts that were readily available and weren't as expensive as I would have thought. Use ASABE 486 for embedded post design.

 

[ornerynorsk]

Perhaps it's time to bring back flying buttresses as an architectural fad!

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[jayrod12]

Use [highlight #EF2929]ASABE 486[/highlight] for embedded post design.

Anyone know if there's a equivalent for this in Canadian Codes?

 

[Splitrings]

ASABE EP486 lists the shear and moment equations for embedded posts and some of the derivation. The allowable vertical and lateral soil pressure values come from the IBC. There are in Section 1807 of the 2012 IBC. Your Canadian code must have a similar section?

 

[mmmbmf]

Believe it or not, I designed buttresses for an older existing similar chicken house to take the new wind loads. They were large shear panels placed perpendicular to the exterior walls. No where near flying, but rather unattractive and effective. I don't think the chickens minded.

 

[dhengr]

There are laminated 2x6, 2x8, etc. posts, which have the bottom plys PT’ed. for embedment (bot. third or half), and the upper plys untreated. Then, why not two exterior posts and a middle post at each primary truss line. There would be two identical trusses on each line, running from the ext. wall to the center post. The trusses would be 4' deep at the eave and 8' deep at the center post; and side bolted to the posts, with split rings if you wish. No, not that guy, not that Splitrings! One outer ply on the ext. post is cut short to act as the bearing for these trusses, and both outer plys are cut short on the center post; thus a truss on each side of the center post, and through bolts. Be careful of all the truss and roof bracing required in this type of roof system.

 

[Archie264]

I see churches built with engineered lumber (i.e. laminated wood) that have flying buttresses. They're still in use occasionally. But then, as mentioned, a chicken coop might be more governed by aesthetics.

 

[Splitrings]

You want to see a contractor sputter? Specify a few splitrings! "What the @#$% are splitrings?? Where do I get them?? Did you realize it is going to take me three weeks to get them?? And did you know the dapping tool is $1200!!" I have heard it all before.

 

[charliealphabravo]

I did preliminary designs for a very similar chicken barn in that was going to be built in Florida (110MPH). I don't think my eaves were 20' high. Anyway I had all the same questions and geometry demands. I think I got knee braces to work for transverse loading. I have found that Simpsons self tapping wood screws are superior to bolted or nailed connections in terms of efficiency and design.

Looking back I would probably use a double post configuration, at the end of each truss, where either the inner or outer post was canted to serve as a brace for lateral loads. I am skeptical that a single cantilever post will work for 20' eaves and 140mph wind even if completely open. One consideration is that you will have no control over the amount of exterior walls/hording/screens that will be installed once the structure is completed. These things may end up being partially enclosed at some point in the future with no consideration for the additional loads on the transverse frames.