Multi-story wood structure considerations??
Multi-story wood structure considerations??
(OP)
I'm getting ready to design my first 3-story wood frame structure. I've done plenty of structures where my 2nd story was "balloon framed" but this one is going to be "platform" framed, and with two intermediate decks, so I'm a little concerned about shrinkage and such. I've seen all kinds of information about shearwalls and "take-up" devices that accounts for shrinkage, but my situation is a little unique in that my exterior corrugated metal siding is going to be my diaphragm and so I won't be counting on segmented shearwalls to carry my lateral load. In a way it seems that that would lessen my issues, but then I wonder that since my exterior cladding will be one-piece corrugated metal, what will happen if my structure "settles" a bit? Do I need to account for some "slip" at one or two points, perhaps at each story? I'd really rather not do that unless I really had to. Anybody have any red-flags to be thrown up here as things to watch out for and consider?
Thanks.
Thanks.






RE: Multi-story wood structure considerations??
I'd be leery about using exterior metal siding as teh diaphragm. What happens when the next building owner (or condo board) decides to do a complete overall of the exterior fascia. And then because it's just a residing job they don't get an engineer. There goes your diaphragm.
RE: Multi-story wood structure considerations??
Regarding the diaphragm, I understand where you're coming from. This is an ag-project that is matching other farm buildings, and in the post-frame construction world, people understand that when you remove the siding you've removed your strength. We're dealing with an operation who understands this concept well. On the flip-side, not to be reckless, but shame on the person who decides to sabotage a structure without consulting with an engineer or somebody with good knowledge of the building system. I'm in the ag-market most of the time so I'm not dealing with aesthetics as the rest of the world probably does, but I suppose it's more common than I realize that somebody just up and changes the look of a building with little regard for the structural ramifications. To what extent will you other engineers go to prevent the irresponsible actions of others in the future causing damage to the structure at the time of new construction? I don't have the luxury to build in multiple MWFRS's into a structure usually.
Thanks.
RE: Multi-story wood structure considerations??
The wood will shrink and well. Most of the shrinkage is in the flat members, not the studs, so the plates and joists will swell the most.
The steel won't like to move with the wood. Additionally, it will move with temperature and sun. This makes a tricky connection if you want to use it for lateral too.
If this is a residential building where people will notice shrinkage and "oil canning" of the corrugated, then you will want to use something else for the lateral resistance.
If this is a barn. Do whatever. Make sure no one cares about the looks. Also, check the detailing on similar structures
RE: Multi-story wood structure considerations??
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Multi-story wood structure considerations??
I would not use 2x joists between the walls, they might shrink to much. Rather frame the wall up to the floor sheathing and hang the 2x joist off the top of the wall. TJI or LVL joists could be placed between the walls. If you are using wood floor trusses I would hang them or have them be top chord bearing on the walls.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Multi-story wood structure considerations??
If the exterior metal cladding is one-piece corrugated steel from top to bottom, expansion and contraction cannot be ignored.
Your cladding needs room to breathe. At the very least, there should be a joint at each floor level with sheets overlapping to allow movement.
BA
RE: Multi-story wood structure considerations??
Differential shrinkage can be an issue. In the old days where architects kept these designs simple it was fairly easy to work around. Nowadays it seems architects are always wanting large open areas, column size restrictions, or some silly detail that leads to columns other than those built into the wall. Any of these details can be a problem if the wood starts at a relatively high moisture content. You do not have a lot of control over the wood moisture unless you specify engineered lumber, but if you start doing that the cost savings in the framing may become an issue for the client.
I am not a fan of wood framed buildings over two stories. Most of the time our clients select this material because it is what they can do or what they have seen others (we work for a lot of contractors). I am not convinced there is a huge savings if one considers current labor costs, course of construction insurance and all the other various factors that come to play. I am sure the savings is in the realm of 15-25%, but in the grand scheme of things I do not think it is always worth it. The easiest way to determine if your client is interested in pursuing an alternate option is to ask if they view their building as an expense or an investment. If they look at you with a puzzled look you know immediately that they consider it to be an expense and you are wasting your breath trying to convince them to consider anything else. Sorry for going off topic.
RE: Multi-story wood structure considerations??
RE: Multi-story wood structure considerations??
RE: Multi-story wood structure considerations??
Maybe dealing with 1/4" expansion and contraction is a bigger pain. Expansion will result in buckling of cladding. Contraction will result in nails popping or shifting in the wood.
BA
RE: Multi-story wood structure considerations??
RE: Multi-story wood structure considerations??
I’ve seen these conditions cause plywd. type sheet goods to buckle becuase they were improperly attached to account for this type movement. You must detail carefully to tolerate/accommodate this movement, and still allow the shear panels to function properly. The same type of considerations should be given to some mechanical systems which are often less tolerant of the vert. movement in multi-story construction. And, the elevator shaft and equip. fall in this category. You can have elastic shortening (compression, usually pretty small), shrinkage due to moisture change, and settlement due to poor fit-up and joinery; and they might all combine. A similar differential movement can be seen in old bldgs. with masonry exterior bearing walls, and interior wood beams and posts. Each floor seems to slope progressively to the middle of the bldg.
RE: Multi-story wood structure considerations??
Thanks all for your help.
RE: Multi-story wood structure considerations??
Not for me.
RE: Multi-story wood structure considerations??
If the building is to be platform framed, then it isn't really a post-frame building...or is it?
BA
RE: Multi-story wood structure considerations??
RE: Multi-story wood structure considerations??
RE: Multi-story wood structure considerations??
RE: Multi-story wood structure considerations??
BA, the pole bldg industry seems to rely upon the cladding to act as a diaphragm, so they avoid sheathing. The AB government (not familiar with US guidelines) has a paper out that illustrates the methods to create such a diaphragm or shear wall, but what I have seen in the industry in Canada is far from following those guidelines. The AB guidelines show the individual cladding sheets (as thin as 30g low profile tin) fastened all around the perimeter of the sheets similar to how we would specify a metal deck or plywood sheathing. In reality what I have seen is most nail the horizontal 2"x4" strapping (24" o/c) to each stud using 2 or 3 nails and there is nothing between them at the sheet joints to attach cladding even if the thought crossed their minds. The cladding is usually only screwed to the horizontal strapping. There is one building in a nearby municipality that is 60'x300'x20' with a 4/12 peaked roof that is done exactly this way without any interior shearwalls. The endwalls both have large openings for vehicle access. Most of these companies build the roofs exactly the same using the cladding as an adhoc diaphragm. Sure they put in some 2x4 bracing at some locations, but it is rare calculations ever suggest they are anywhere near adequate from a connection standpoint.
RE: Multi-story wood structure considerations??
RE: Multi-story wood structure considerations??
I have never designed a pole barn building because the practice in Alberta before I retired was to embed the poles about 4' to 6' into the soil with compacted gravel around them. I have always been in the habit of ignoring the upper five feet (the active frost zone) in the case of a concrete friction pile, hence I couldn't see how pole building practice was anywhere near equivalent to my normal practice or how it could be considered to support substantial roof areas. The minimum friction pile normally used is 12" diameter 12'-0" deep and most geotechnical engineers advise a minimum depth of 16' to avoid frost jacking.
It appears from what you are telling us, that the superstructure is falling far short of what most engineers would consider good practice. I am wondering why some engineers would place their professional seal on details such as you describe. I am also wondering why we have not heard of more failures of pole barn type of buildings.
BA
RE: Multi-story wood structure considerations??
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Multi-story wood structure considerations??
We have been seeing 26' - 28' minimums in the geotechnical reports over the last couple of years (5hr North of you). I think it is getting a bit much.
Koot, my business partner is a plane owner. They are a unique breed when it comes to their hangers. I have watched him contemplate everything under the sun to save a few dollars for a hanger. On their forum there is apparently a section dedicated to the topic of building cheap hangers. I suspect them going elsewhere was a blessing in disguise. Not only did it avoid the headache, but it gave you that much more time to contemplate shear friction :). I had tracked down the drawing set for the building in the picture. I recall the drawing set was four pages. The drawings were basically some generic plans, elevations and a some typical details.
RE: Multi-story wood structure considerations??
I suspect that there might be something to this. I don't have my wood manual with me but I'm pretty sure that there are diaphragm capacity tables for gapped 2X decking. You'd think that those provisions could be extended to 2X strapping somehow.
I wonder if a metal plate connected truss supplier could manufacture strapping panels that were internally braced, improving the capacity of a "strapping diaphragm". When I was an MPC truss wiz, we used to fabricate strapping "ladders" that would go on top of roof hip ends. They were't braced but that would be no big deal from a fabrication standpoint. Not much different from structural gable end trusses really.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Multi-story wood structure considerations??
I found the quote," There is a need to educate design and building regulatory professionals about lateral design of post-frame buildings," from the 2012 article below interesting.
http://nfba.org/uploads/Simplified_Lateral_Design_...