pole barn question

[patrick_g]

I recently moved a pole barn which is 35x70 with 18 ft side walls and 14x14 roll up doors centered in both ends. It was originally built with a slab floor and the 6 inch diameter steel pipe columns (poles) embedded in the foundation 2 feet.

I cut the columns off flush with the floor and moved the bld intact a quarter mile across one of my pastures to its new location. I now have a dirt floor and the columns are terminated atop 18 inch diameter 12-15 foot deep columns with embedded weld plates on top of the columns.

As the column to weld plate connection makes a good hinge and offers little resistance to the whole structure folding like a parallelogram I had to do some engineering. The metal skin offers considerable sheer strength and helps stiffen the walls against deformation but not nearly enough for me to be comfortable. I welded on diagonal bracing (approx 3 in dia pipe) to stiffen the structure to replace the rigidity lost by not having the columns buried.

The piers will easily carry the weight and will not move laterally but contribute essentially nothing to the prevention of the poles leaning, hence the diagonal bracing.

I also have a rotating jib crane with traveling winch on it. The vertical for it is a 12x14 inch I beam welded to a vertical column. It bends very little when the crane is loaded (2 ton capacity on traveler.) There was a problem though. if you swing the crane so it is oriented cross wise of the bld (the boom is a about 17 feet long) and lift a heavy weight then the force of the lift is translated to a lateral force trying to push the collocated truss laterally, deforming the structure. I added a large triangularizing brace on the opposite side to stiffen the structure against this lateral load at the height of the eaves.

What might I have done differently if I had it to do again?

Embed steel pipes vertically, deeply in the piers with a few feet sticking above the top. These pipes OD would be a decent fit (slightly larger than) the ID of the 6 inch columns. Split the part of the pipes above the piers longways with my plasma cutter. When sitting the building down at the new location maneuver to bring the bottom of the columns into the cut out vertical pipes, force them into intimate contact and tack weld them. After the moving crew has their stuff out of the way go back and weld the cut off vertical halves back in place capturing the columns.

This would have obviated the need for all the additional bracing and made the bld even stronger than original.

Now then, regarding the comparison of losing strength in a wood member by drilling it to receive a bolt vs nailing it with "less invasive" nails. I get the notion of using large bolts to fasten toothpicks together and it is something to discuss around the table between beers but not a very practical concern. If the "timbers" are so small as to be seriously weakened by bolts large enough to be safe for the loads and forces involved then you have serious issues with timber size/strength.

Pat

 

[tommu56]

Rodmo1

I had one put up last fall and they nailed the headers to the posts the header is a double 2 x 12 one inside one outside the posts
The trusses are on 4 ft center the posts are on 8' centers.
Every other truss is directly on the post the intermediat trusses have a 2x6 between the headers up along side the truss the posts are notched above the header and attached to the side of the truss too
it has 6/12 pitch on roof it is only 20 ft wide and survived this winter with all the snow and ice.

tom

 

[Rodmo1]

Thanks guys for all the responses. To elaborate on the design, it has 2x6 3 ply laminated posts spaced 8' on center except where the garage doors are it will have 9' spacing. Trusses 2' on center with asphalt roof. With the design it does not call for the posts to be notched out to support the trusses so all the weight will be upon the 2x12 header. Reading the reponses I will definately carriage bolt the header in place (probably 2 bolts per post or 4 where the header terminates). Glad to hear other people did it this way as well. As far as lateral strength due to shorter pole embedment, I am not as concerned as I was initially since looking further on the plans every post is Y braced and should be very strong. This morning I am going to set my first post and continue digging holes...Yes, I am digging my holes by hand. Takes me about 1/2 hour per hole 40" deep by 12" across. Unfortunately I am only good for about 2 holes per day. I live on slope of mountain so ground is too rocky for power auger. May have neighbor come with his Kubota with FEL and try digging some for me. As far as concrete depth in holes I am simply dumping an 80# bag of 4000 PSI (mixed) seems to give me about 6" depth. So in reality I am exceeding the plans by a couple inches however cutting back on the concrete depth by a few inches. I cant imagine this would negatively affect the plans. I should finally add there are no building inspections where I live just an inspection to ensure setback compliance.

 

[Rodmo1]

I also want to add that I am impressed that a lot of the previous posters have their trusses spaced 8' or more. Seems to me snow load would be a problem at least up my way. Those trusses used must not be the normal variety. Since this is a tractor forum I also want to add that I got my dads old cub cadet model 71 running yesterday after close to 10 years and will trailer it to my house this week so I can move some stone with the attached wagon. So the building already has its first inhabitant.

 

[tommu56]

Rodmo1

The truss design the truss manufacture you tell him the span and how far apart you want them and roof loading and the pitch you want.

A friend of mine has a morton building that the trusses art over 8 ft apart,no they aren't 2x4's they are like 2x10 syp and the purlins are 2x6's the truss do sit on post.

Mine is 2x4's though but engineered by the truss co for the span and pitch.

I'm not sure were you are in cpa but there are at least 2 truss manufactures between williamsport and mackeyville that I looked at (I was going to put building up my self) but didn't have the time so I got a contractor to do it.

As for the auger on a tractor they used a skid loader to drill mine and I only have about 18" to to the shale at my place.
with a skid loader you have the down pressure that a tractor doesn't have on its 3 point to help with the stone.

tom

 

[MarkV]

I also want to add that I am impressed that a lot of the previous posters have their trusses spaced 8' or more. Seems to me snow load would be a problem at least up my way. Those trusses used must not be the normal variety.

Much of the spacing difference has to do with the fact you are running asphalt shingles and there for plywood or OSB sheathing. Two foot on center is common for that combination where 8' centers is common for metal roofing.

MarkV

 

[yooperdave]

My last shed had the trusses and columns spaced at 10' on center.

We have a 40 psf roof snow load as well.

The further the spacing, the more economical overall - USUALLY (less pieces, less labor).

Yooper Dave

 

[orangebluegreen]

I found a great book at Tractor Supply several years ago when I had similar questions. I spent most of my life in home building and most of it is second nature to me, but this book is a great reference. It was titled Barns Sheds & Outbuildings by John D. Wagner and Clayton Dekorne.

I had the same question as you. On page 72-73 they say using 8" ring nails four per connection will carry approx 2500 pounds, and nails can loosen. Using two 4-6" lag screws can carry approx 5500 lbs and wont loosen. Machine or carriage bolts with nuts and washers can carry approx 8000 lbs. They weren't mentioning bolt diameters for those numbers. I had found a website at one time that gave load carrying specs for different bolt combinations but can't seem to find it now. They also show a specially designed (spike grid) to place between a girt\header and pole that looked like overkill to me. I calculated my desired design load and spacing and decided on two ½" lag bolts per connection for my building. I predrilled with a ¼" (from memory here, been a while, might have been smaller, you'll know by how hard it is to screw into) to make it easier to screw the lags in without splitting the wood and still maintain a tight fit. The smaller the predrill that works the better in my opinion. Some claim to use no pre-drilling but in my experience the risk of splitting the wood is too great and also risks breaking the screw while driving. I used an automotive style air wrench to drive the lags being careful to not over drive them and snap them off. I had used 7/16 on a smaller project and found them way to easy to break, ½" I didn't break any.

 

[wv1720]

I built a 24 x 24 a few years ago. I bolted the girts to the poles with pieces of all-thread rod. It was a little cheaper than carriage bolts. The building inspector told me a good rule of thumb for pole construction was for every three feet of pole above grade, it should be one foot in the ground. My poles were 12 ft high, so I put them four ft in the ground.

 

[patrick_g]

...to make it easier to screw the lags in without splitting the wood and still maintain a tight fit. The smaller the predrill that works the better in my opinion.

Not really.The body of the lag screw/bolt is tapered from tip to shank. When turning into wood the threads pull the screw/bolt into the wood and the tapered shaft acts like a splitting wedge. Pre-drilling, of course, makes screwing in the fastener easier but more importantly reduces the "wedge" effect. Different wood conditions (species, hardness, grain, moisture content etc) have an effect on the likelihood of splitting, now or later when the wood may change moisture content and be loaded by wind, snow, and so forth.

Lags are best installed similarly to most screws in wood. Pre-drill sufficiently such that the wedge effect does not split the wood now or later. The threads are what gives the holding strength. Never pre-drill so large a hole as to be greater in diameter than the shank of the lag (base of the threads.)

Lags have strength in two ways of interest. One is their sheer strength determined by cross section and material strength. The other is the clamping force generated by the threads pulling along the length of the lag. If the pre-drill hole size is "just under" the diameter of the base of the threads (shank diameter) you get full thread engagement and strength with minimum wedge effect to cause splitting now or later.

Drilling the holes significantly undersized (enough to make putting the fasteners in much harder) will not increase the holding power (clamping force) of the threads but will increase the chances of splitting.

It may seem counter intuitive that lags that are so hard to screw in are likely to provide poorer service than those with proper predrilled holes (easier to install) but that is the way it works.

Depending on lag size and wood conditions pre-drilling may not be required (soft wood or wood very resistant to splitting) but will never make for a weaker joint.

Pat