`a constantly debated topic’ (pole barn versus post frame)
Pole barn versus post frame … how to keep the structure from flopping over? Consider a barn, or perhaps a shop, made using timber and `posts’. There are two main ways to keep it from flopping over. One way is to embed the posts, or poles, deep in the ground, enabling them to act like vertical cantilevers. The other way is to keep all the wood above ground, using posts (columns), trusses, beams or girders, etc., and keep it from flopping over using the diaphragm and bracing wall action of the metal cladding. Yeah, there are other methods, such as X-bracing, knee bracing, etc., but this `post’ (blog post) will focus on the first two methods, acting singly, and in another `post’, in combination.
With embedded posts (or poles) the main issues are: 1) selecting poles or posts that will not (reasonably) rot, as they will be assumed to be in a constant or periodic damp environment, inviting decay; 2) making sure the posts are deep enough; and 3) making sure the posts have good, and enough, `side bearing’. (By `post’ I might also mean `pole’, and in some contexts, even `column’.) Perhaps the easiest way to get good side bearing is to auger a nice round (cylindrical) hole, clean out the loose dirt, drop the post in the hole, and cast concrete around it. Done. (In another post we’ll discuss how deep and wide the hole must be.) For good side bearing the concrete must be poured against undisturbed ground. If the hole is over-sized, or if concrete is not being used to fill the space between post and ground, then the material used to fill in and around the post must be compacted. Not just lightly `stepped on’ here and there, but indeed compacted. Preventing decay is accomplished using posts that are preservative treated or naturally resistant to decay.
In the `post frame’ method, the post or column is typically installed on a `post base’ bracket. These brackets are typically metal, have something that is embedded in some kind of a concrete foundation, and with `straps’ or sides into which the post fits, and is secured by bolts, screws, or nails. The idea is that the bracket and foundation keep the wood above ground. Such posts typically do not need to be preservative treated or naturally resistant to decay. A typical post base is not counted on to, alone, stay upright, and thus cannot keep the structure as a whole upright; the rest of the structure must do that. This is where the roof planes, the side walls, and the end walls, all act together to keep the whole structure stable … from flopping over, say, when a strong, or even not-so-strong wind comes along. Take away a roof plane, or a wall, and the structure is in trouble. We’ll talk more about this elsewhere. And how do the roof and walls do this? … with the metal cladding (properly specified, detailed, and constructed). And until all the roof and wall planes are clad, you better have some other means, even if only temporary, of keeping the structure stable.
So here’s the question, or at least some of the `debate’: won’t the post base (bracket) itself provide stability? … keep the post from flopping over? And thus the structure? Prove it!
Let’s look at some of the issues involved.
→ If the post alone, say a post in a long side wall, is acting like a cantilever to resist a lateral force effects, such as wind, or an earthquake happening when there’s a bunch of snow on the roof, it might be required to resist high hundreds, or maybe even a thousand or so pounds of lateral force, which ends up applying a shear force and bending moment to the base of the post. Consider a shear force of a thousand or so pounds. The post will typically be installed in the bracket with one or more through bolts. Maybe the wood section itself is strong enough, but suddenly the effective section of the post is reduced per the hole(s) for the bolt(s). I’ll post the calculation some time. Yikes.
→ The bending moment at the base of the posts will likely be in the `thousands’ of pound-feet. Check the bending stress in the post to make sure it’s not excessive. (I’ll post an example some time.)
→ The bending moment at the base of the post must be transferred to the bracket via at least one pair of bolts, or perhaps some bearing of post on metal. Certainly don’t count on both. And, in general, I’m leery of using bearing in this case, since I know that wood shrinks. There may not be very good contact between wood and metal, particularly on the sides of the bracket. Note that the loading on the bolts will be perpendicular to grain. Do the calculation. I believe you’ll find that the moment resistance of this pair of bolts is, unfortunately, quite low.
→ Let’s say your posts, bolts, and brackets are robust enough to resist the lateral load in question. Then comes `deflection’. A pure cantilever has a lot of deflection. By `pure’ I mean that the bottom of the post is help perfectly vertical (no rotation), and the top of the post deflects (move laterally) purely by the bending stresses in the post. Add some gap between bracket and wood, and some crushing and perhaps oversize in the bolt holes, allowing the post to move (rotate) before taking on any significant load, and that post may sway considerably! Presumably no gypsum, or other brittle material will be attached directly, or indirectly, to the post, or it will CRACK! I’m assuming that there is NO gypsum in the barn or shop in this conversation.
→ Now that you have got down past the post and bolts, and into the bracket, make sure the manufacturer of the bracket purports that it can be used to resist moments. Oops. Yikes. Yeah. There are very few on the market (that so purport). Game over. Try something else.
→ Say you ignore the fact that the manufacturer of the bracket does not publish a moment capacity of the bracket, or if it does, and you deem it sufficient to keep going. Now you need to transfer the moment (and shear) into the concrete. Presumably the bracket has rebars or bolts/studs welded to the bottom that are cast-in-place in the concrete. If, instead, you `wet-set’ them, that may be a different story, especially if the bracket manufacturer installation instructions forbid wet-setting. But because you’ve already ignored that the manufacturer didn’t provide a moment capacity, we’ll keep going anyway. Are the rebars or studs deep enough to prevent pullout due to the tension effect of the applied moment? Is there enough concrete bearing to prevent concrete crushing? Let’s say there is! Yay! On to the next question.
→ ls the concrete reinforced? If the bracket is part of an unreinforced concrete wall, or concrete pier, what will keep a chunk of concrete from breaking out of the wall, or breaking off the top of the pier?
→ If we get this far, let’s keep going. We need to look at the bearing of the concrete on the surrounding soil. Chapter 18 of the International Building Code (IBC) provides presumptive allowable soil side bearing pressures, and means (equations) to calculated the pressure applied laterally to the soil, based on either nonconstrained (at top) or constrained. You’ll find, in a hurry, that the necessary foundation or depth and size (pier diameter) are probably more than you expected.
→ Finally, how was the concrete cast against the soil? Was the formwork removed? If not, will it decay and be `crushable’ (or even gone) under lateral load, allowing the foundation, or pier, to move, before and during its task of keeping the building from flopping over?
These are tough questions. There are some post brackets (bases) that have published moment resistance capacities. Take a close look, choose carefully, and certainly don’t neglect the other questions above if you are going to rely on such devices to keep your structure or building stable.