Post-Frame Building Design

[440MoPar]

What are the criteria to consider when designing a post-frame building (pole barn) that utilizes a scissor truss instead of a standard roof truss configuration? Thanks.

 

[msquared48]

Additional spreading of the poles due to the deflection of the scissors trusses.

is induces additional lateral force to the top of the poles, affecting the moment and shear seen by the pole, and the required embedment.

The amount of the relative effect depends on the lateral deflection allowed by the trusses.

Mike McCann
MMC Engineering

 

[jhoulette]

Consider tension ties to prevent the frame from spreading apart as McCann mentioned. You can do this with wood or metal rods.
Google image search "timber scissor truss" and you will see any different examples of wood and metal tie rods being used.

Jim Houlette PE
Web:

http://www.evstudio.us

Online Magazine:

http://www.evstudio.info

 

[440MoPar]

If 1/2 inch plywood is attached to the bottom of the bottom chords, would this reduce the lateral thrust from the scissor truss? and would that provide for an adequate diaphragm?

Also, the contractor is proposing to use a standing seam metal deck, similar to what he used on the roof, for attaching to the underside of the bottom chords. This would be attached directly to the bottom chords, without plywood. Would that provide an adequate diaphragm? Second, would the trusses alone provide an adequate diaphragm to transfer wind loads to the shear walls, to the foundation?

Wind load is 90 mph, exposure B, importance II, seismic loads not applicable.

Thanks again.

 

[msquared48]

If you have a true pole barn, unless it is very large, you don't need a plywood roof diaphragm.

Plywood on the underside of the trusswill not change the spread much. That is better done with larger top and bottom chord members or a greater truss depth at the ridge.

Do not use the underside of the trusses for a diaphragm location, only the top if needed, and not the metal deck if it is not rated. Usually the roofing is 28 otr 29 gage material - too light for a diaphragm.

Mike McCann
MMC Engineering

 

[msquared48]

Also, the trusses alone can transfer shear load to the poles, the usual primary lateral resisting element, but to transfer lateral forces to any shear walls, usually not needed in a small to medium pole structure, you need a plywood diaphragm or some form of bracing in the roof structure.

You really need to read up on how a pole structure functions to get a better feel for the animal.

Mike McCann
MMC Engineering

 

[440MoPar]

Mike,

Thanks for the info.

Can you suggest a good reference/document to read to better understand pole structures? Currently, I have the following documents on hand for review:

1. Post-Frame Building Design Manual
by National Frame Builders Association
December 2, 1999

2. Pole Building Design
by Donald Patterson
Sixth Edition, 1969

3. Post-Frame Construction Guide
by Southern Pine Council, APA
year 2000

4. Pole Building Construction - The Complete "How-To" Book
by unknown

5. Design for Code Acceptance - Post-Frame Buildings
by American Forest & Paper Association
year 2001

I am an E.I.T. and I understand timber design, standard roof truss configurations (i.e., fan, fink, howe), beginning to understand pole barn structures, but are unfamiliar with scissor truss design and their behavior.

Any further guidance would be greatly appreciated. Thanks again.

 

[msquared48]

#2 for the pole building. There are many others though.

Sliderule may have some good info in his website too.

The scissors truss is an animal separate in itself as to how it performs. Depending on the configuration, it can have significant lateral deflections at the ends, not the case with "normal" trusses with flat bottom chords. This does not mean that they cannot be used, just more things need to be considered in the design of the building.

I suggest you set up a scissors truss and a normal flat bottom chord truss on the computer and look at the lateral deflections at the supports if you let one end be pinned and the other a roller. It should open up your eyes here. And for Pole buildings, as many others, you should model the truss that way.

Experience is a great teacher.

Mike McCann
MMC Engineering

 

[Bobber1]

Standing seam roofs cannot provide diaphragm action unless their sidelap seams are crimped or somehow prevented from sliding at the sidelap seam.

 

[Splitrings]

440Mopar,
Are you building a true pole barn without side or end walls (walls that could act as shear walls)? If it is a true pole barn, will the connection between the truss and post be a fixed or a pinned connection? It is usually assumed to be a fixed connection when knee braces are used between the post and bottom chord of the truss. I doubt such a connection could be made between a scissor truss and a post. Otherwise this will be a pinned connection. The advantage of fixing this connection means you can design your post as a fixed-fixed column with a Ke = 1.2, instead of a fixed-pinned connection with a Ke = 2.1. I have designed wood pole buildings with both fixed and pinned connections at the truss. I am beginning to realize it may be more economical to increase the post size than to constructed this fixed connection. As far as trusses go, I have several wood truss designs from local manufacturers; gambrels, scissor, king post, etc. The horizontal deflections of a 40' wide x 12' high scissor, with 60 psf ground snow load, 90 mph base wind, category II, exposure B is only about 1" total. Probably not significant in a pole building.

 

[barret504]

Is there anywhere on line I can find information on post-frame buildings (pole barns) utilizing metal panels for roof diaphragm and shearwalls? The rest of the structure is typical wood post-frame construction.

 

[IceNine]

If the roof slope is steep enough, the plywood can act as a folded plate, spanning between end walls. This could significantly reduce the movement at the top of the side walls.

 

[PostFrameSE]

Post frame buildings are interesting creatures. If you do an analysis of the interaction between the columns/truss (frame) and the diaphragm, you'll find that the columns do about 10% of the work in resisting lateral loads. Dr. Bohnhoff has created a program called DAFI (Diaphragm and frame interaction) that is posted on the website

http://www.nfba.org.

Given some diaphragm stiffnesses and column stiffnesses, you can determine the forces that will be acting on the columns and any shearwalls (generally endwalls) that you may have.

I would disagree with the comments that msquared48 shared about 28 or 29 ga steel being too light for a diaphragm. There has been LOTS of research done on light gauge steel on wood diaphragms. I've done some myself. It is VERY easy to get diaphragm strenths of greater than 100plf.

Post-frame buildings are very resilient structures. What I have found is that the lateral deflection of trusses due to the scissor is insignificant, provided that the bottom pitch is no more than one half of the top chord pitch.

I certainly agree with Bobber1 on the standing seam. Use a wood deck for a diaphragm if you're going the standing seam route.

The PFDM published by the National Frame Builders Association is pretty old and is need of some update and revision. We are hopefully going to be doing that in the next year or two.