Pole barn

[Chuck52]

After much soul searching (and wallet searching) I have decided to build a pole barn
rather than a more expensive all metal building to house my 30 year old Kubota and
the implements I have and will acquire. My plan is for a 24x32 structure with one long
side open....kind of like a loafing shed but using trusses. I went to Payless and got
a preliminary plan for the 16x32 building I was thinking of at the time, but Payless is now
going out of business. I think I've got a reasonable handle on how to proceed with this
project, but I am puzzled about one aspect of both the plan I have a drawing for and
pole barns in general. It seems to be common practice to use only a few roof trusses
for these buildings. These are identified as "Ag" trusses, and are rated for 8 foot spacings.
One then uses 2x4 purlins set on the edge to provide the nailing surface. Does anyone
know if the "Ag" truss is actually heavier duty than the standard truss. Also, wouldn't it
make the roof stronger to use joist hangers to connect the purlins to the trusses at the
same height as the truss top member? Is there any reason, like maybe ventillation, to have
the purlins toenailed on edge on top of the trusses?

Chuck

 

[cowboydoc]

Chuck,
No way for joist hangers!!! Not near the strength and stability of having the 2x4 on end and overlapped. That design is strictly because it is the strongest and stablest. That's another way you get away with 8' spacing, yes they are stronger trusses. Every 24" you place the purlins. The purlins then distribute the load. Not to mention it would take you probably ten to twenty times as long to build using hangers. It really is a good design and will hold up to just about anything.

 

[beenthere]

Cowboy is right (2x4's on edge and overlapping), and it has to do with bending moments. Advance warning: Don't make the mistake of walking on those 2x4 purlins, as they do not have that kind of strength (many accidents from construction workers who think they do). The 2x4's are not designed to take a point load, in flatwise bending, such as 200#'s on one foot using the purlins like ladder rungs. Just FYI.

 

[Chuck52]

Well, I did poorly in physics as an undergraduate, and my common sense is often way off,
but it just seemed to me that 2x4 purlins joined to the top members of the trusses via
hangers would in effect produce an extended truss system, with the 2x4's butted against the truss.
I guess I'm also prejudiced against toenailing, since every time I've done it I've walked the object of my attention several inches out of line. This time, however, I'll have a nail gun to shoot 'em in place,
instead of a poorly swung framing hammer, so I may do a better job. Clearly, running the 2x4 on top of the trusses will be faster and cheaper. By "overlapping", you just mean staggering the ends of the purlins so they don't all sit on the same truss? I was planning to to that anyway, though the design I was given shows all the 2x4x16's butted together on the same center truss of the 32 foot roof. I bet that was just a shortcut built into the design program. Hmmmm. With Payless going out of business, I wonder if they'll
be selling that software. Neat program. Click on the wall width and height and it spits out a framing
design along with a material list and necessary supplies. 3D views and all. I recently bought some home design software that works for normal house construction, but it doesn't do pole-type construction. I looked for something like that on the web, but I just found places that would do it for you for a fee. And the fee was more than I paid for the home design software that does quite a bit more.

 

[Bird]

Chuck, I'm sure these other guys know more about this than I do, but I've sure been happy with my wood framed, aluminum skinned 40 x 60 shop building and it's built with the hangers, but with 2 x 6 instead of 2 x 4s, and the trusses are 12' apart. Of course, it was here when I bought the place, and they'd have been closer together if I'd been building it myself, but I'd have still used the hangers.

Bird

 

[cowboydoc]

Chuck,
WHOA!! You don't toe nail those purlins on. You have to drive a 6" nail through them! Sorry but nail gun won't work on them.

 

[beenthere]

I suggest you look at some pole barns to see what the common practice is in your area. Snow loads in the North would probably dictate a different design. If me, I would put up 10' purlins, overlapping two feet with the next purlin (i.e. not butt jointed as you mentioned with the 16'rs). I would use brackets to fasten the purlins to each truss (important for wind resistance and work well with a nail gun). You didn't say if this is a metal roof or if it will be sheathed with panels. The panels may structurally tie the purlins and trusses together better than the metal, but that may be debatable. The grade of the lumber used will also be important too.

 

[radman]

Cowboydoc:
Some pole barn manufactures do use joist hangers for for the purlins (ie Astro buildings) but like you said, I have been told that purlins running on top of the trusses give better support and certainly quicker. Astro claims with joist hangers there are less places for birds to sit. I figure there are already plenty of other places for birds to sit as well. Purlins on joist hangers wont stop them. I will be building a 45X72 pole barn in a 10 days. I have the shed sitting on two trailers for the past month. Incredible pain to get necessary permits where I live. Had to have engineered drawings for 30# snow load and 80 wind load. Most of the footing pads are 20-22 inches. I put the building on 8 acres but in order to get the permit I also had to submit house plans, drawings etc and get a separate permit for a house. I got the final necessary permits today. Five separate trips to building and safety, calls to builders, structural engineers for both wood and metal specs etc. Finally got all the i's dotted and t's crossed.
I purchased my building from Menards. Menards even did the engineered drawings through Midwest Manufacturing. Seems very complete including door knobs, silicone caulk for windows, colored screws, nails, trim, entry doors, windows, wainscoating, 25 yr paint warrenty, sliding and overhead doors, skylights etc. Cost for building and erection about $18,000. This was about $10,000 less than Cleary and Astro and those buildings had considerably fewer options. Morton was still higher.

 

[cowboydoc]

Radman,
Same here I got my materials from Menards. I built mine myself so saved that cost. Even back in the west they recommend going the purlins overlapped route. I've talked to several engineers who have told me that this way is stronger. I actually wanted to build mine the hangar route this last time but was talked out of it by the engineer at Midwest Mfg. He said you just don't have the same strength. Also I'm glad I didn't after it took as long as it did anyway. I'm sure I would been a good week just putting purlins on that way. The other way we were done with that part in a day. I'm lucky here if you have over 40 acres you don't have to have any permits for house or barn.

 

[beenthere]

Chuck52; I found this site that has some free downloads of pole buildings and truss construction plans, that addresses the purlins and the overlapping being discussed. The site is: http://www.public.iastate.edu/~mwps_dis/mwps_web/tr_plans.html
It may or may not be of interest to you. There was a note that if trusses are on 8' centers, 2x4's on edge should be overlapped by using 18' and 16' lengths. Also, includes information on lumber stress grades that may be of interest.