max recommended height pole barn ?

[RWTschoeke]

I agree with others who suggest getting engineer/architect input; perhaps you will be lucky to find someone here on the forum, but whoever you get the trusses from should have someone on staff to answer many of these questions. In my experience, going up is less costly than one would think. Longer poles and additional siding are needed. Corner bracing as shown below helps with wind loads:

This building is 48x72x14'6" with 14x14 doors. The inside height will allow for mezzanine and the ability for 6'2" individual to stand both below and on mezzanine.

 

[quicksandfarmer]

The reason no one is answering your question is that this is actually a more complicated question than it seems.

There are three failure modes for a 6x6 post that I can think of in this situation -- there may be more that I'm not thinking of:

1. It buckles due to the weight that it's carrying and snaps.
2. The ends get crushed by the weight they're carrying.
3. The whole thing gets blown over by the wind.

Number 2 doesn't really depend on the height of the building, the weight carried by the post doesn't really change as you add height, the additional sidewall weight is nominal compared to the roof loads. The crushing point of wood is determined by the area and the type of wood and doesn't change as the post gets longer.

In short posts, crushing (#2) is the limiting factor, but as a post gets longer, its tendency to buckle (#1) increases. Doubling the length of a post quarters the buckling capacity. So if a 10' post can hold 10,000 lbs without buckling, a 20' post of the same material can only hold 2500 lbs. Every column has a characteristic length where buckling overtakes crushing as the concern. It's a handy rule-of-thumb thing to know. For instance, I know that you don't have to worry about bucking in 2x4's less than 8', or 2x6's less than 12' -- the ends will crush before it will buckle. I don't know what the number is for a 6x6, but it's going to be a lot more than a 2x6. Staying under that length saves you a lot of math, as you'll see below.

Numbers 1 and 3 are things that you don't just look up in a table. That's why people are saying talk to an engineer or architect. They depend a lot on local conditions: expected snow and wind loads. Also, design makes a big difference. Cross bracing helps with both buckling and wind loads.

Anyway, here's an overview of what an engineer would do. First he would calculate the weight carried by the post under the worst possible conditions. That would be the weight of the roof, plus the weight of the heaviest conceivable snowfall, all divided by the number of posts. Then he would calculate how long a 6x6 could be and still handle that load. To get an idea of how those calculations are done, see this wikipedia article: Buckling - Wikipedia, the free encyclopedia.

For wind load, he would consult a wind map to get maximum wind speeds for your area. Then he would calculate horizontal wind loads and see if your wall design can handle it. He would also calculate vertical wind loads -- wind tends to lift a building -- to see if your roof is firmly attached enough to keep it from flying off, and if your building is firmly attached enough to the ground to keep the whole thing from lifting. The wind loads on a building increase exponentially as building height increases.

Usually it's not that expensive to beef up a building using bracing to avoid buckling and wind collapse. Unless you can find pre-engineered plans it's really worth consulting an expert.

 

[puckett]

Thanks for that, it seems it is an open ended question with no definite answer since I don't have all the info needed myself. Thanks to everyone who helped out though, alot of good stuff from you guys here!

Jon

 

[MarkV]

Jon here are my thoughts. If you have to get a permit in your area you will most likely need an engineers stamp for the building department. The engineer can design what is required......BUT......you are going to have a heck of a time finding 6x6 pressure treated post longer than 20'. That is why 16' high is about as much as is common.

I think you will also have a time finding floor joist, without going to floor trusses, that will span 24'. The only problem with floor trusses is they eat up better than a foot of your head room. They do make roof trusses that are designed to have attic storage. You don't get the same amount of sq footage as the main floor though. You do get the floor framing for the second story though. A gamble roof, like old time barns, can offer more head room on a second floor and trusses are made for them.

In your case you might even consider stick framing the building as a house would be. No problem going 10' on the first floor, 2' for floor trusses and an 8' second floor. Not much different than a 2 story house.

With either building method don't be surprised if you are required to sheath the walls with ply or osb. Metal siding does not offer much lateral support and the taller the building the more that is needed.

MarkV

 

[schmism]

until the weight of the wall(s) becomes more than than the posts/beams can resist...

as part of my old job providing engineering services to the truss industry, we also did post designs.

the weight on the posts from the roof is a very small fraction of the load the post is designed for. by far the limiting factor to a post design is how high the side wall is. this is from the force of the wind on the side. most min wind loads are 60mph. ( a moderate downdraft as seen in a thunderstorm) which in the midwest area is not uncommon at least once a year.

16" OC is not uncommon for storage trusses especially for barns because of the high storage load from hay.

as for a 6x6 post (5.5x5.5) i would say 12' is toward the upper limit for 8'OC

 

[QRTRHRS]

IMHO, attic trusses are too expensive while limiting the floor space available for storage. I would put my money into a structural floor and either stick build the gambrel roof or build one with small trusses.

Small trusses can be had for relatively cheap. Picture two set at an angle meeting the horizontal truss on top that bridges them. Tie the three together with plywood gussets on both sides. Simple as that.

If you are fortunate enough to live near a truss builder, speak with them. The one's that I have dealt with have engineering deptments. They can look at your ideas and generally can give you a price and delivery cheaper than your local lumber yard.

That still won't help with your wall loading of course.