Multi-story wood structure considerations?? 1

[PostFrameSE]

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.

 

[jayrod12]

I've never seen shrinkage be an issue on a 3 storey wood structure before. Usually by the time you get around to putting the exterior finishes on the majority of the settlement and shrinkage has taken place.

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.

 

[PostFrameSE]

Thanks Jayrod12. That helps. Is that everybody else' experience?

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.

 

[manstrom]

I'm working on a 6 story wood framed now. 3 story is not a big deal with respect to shrinkage for most things. But the corrugated metal siding is a weird one.

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

 

[KootK]

I never would have thought of it before you brought it up but now I am concerned about the differential movement. If the wood shrinks and the three stories of corrugated expands in the sun, what does that mean for your fasteners near the roof? Do they just plow through the corrugated? I don't think that the shrinkage is a big deal on a three story but, if you don't like the numbers, you can switch to LSL plates which will eliminate much of the shrinkage. Breaking the corrugated up into single story spans sounds like a great idea to me too. I'd be curious to know what the shrinkage / numbers actually are. Perhaps the scale of the problem is simply too small to worry about.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[woodman88]

It depends on your floor to walls detail.
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.

 

[BAretired]

If you are using dimension lumber, shrinkage is a major concern. The amount of shrinkage depends on the moisture content of the lumber when installed. Shrinkage will occur in both joists and plates.

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

 

[Brad805]

We have had issues with the elevator shaft on wood framed buildings. I am not a fan of the wood frame elevator shaft that I have seen some attempt so we always use a concrete, steel or masonry for that component. The differential shrinkage between the different materials makes for a difficult situation. Count up the plates from top to bottom of a three storey building (could be up to 9 plates) and consider the worst case scenario where the wood starts at 19%+ and reduces to 6 - 12% after the mechanical system is functional. You will see it can be a rather large value if you are considering floor tolerances for an elevator. One elevator company we deal with has refused to install their elevators in some of these buildings because of the long term maintenance issue.

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.

 

[PostFrameSE]

We are going to be using engineered wood I-joists and an engineered rim joist so that will help some. From a constructability perspective, multiple-piece cladding is kind of a pain. Perhaps like KootK said, the scale may be too small to worry about. A 100 degree temperature differential yields a .257" expansion. Assuming this is installed in 30 degree weather, that 1/4" is definitely realistic.

 

[Brad805]

We all use engineered lumber for the floors. It is the top and bottom plates in each level of framing where the shrinkage occurs. Wood does not change length with moisture variations significantly, but perpendicular to grain it can change enough to cause problems. In the CSA guideline they indicates an average perp to grain dimensional change from wet to 12% humidity is approximately 3.0% for spruce (as high as 4.2%). If you are in a dry climate or the building has a low end mechanical system your humidity level can drop lower. With 9 plates (one bottom and two top plates per floor) there is a total of 13.5" of wood that can shrink perp to grain. This could yield a total shrinkage of .405". I have seen this.

 

[BAretired]

From a constructability perspective, multiple-piece cladding is kind of a pain.

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

 

[hokie66]

I have never used corrugated steel as a diaphragm, but would it work installed horizontally? That way, the vertical shrinkage of the wood just bends the corrugations a bit.

 

[dhengr]

PostFrameSE:
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.

 

[PostFrameSE]

Corrugated steel is the only way to make a post-frame structure stand unless you're counting on the frames to carry the load. I'm not talking post-and-beam construction, I'm talking what is commonly called "pole-barns." We use code-referenced standards for determining the strength and stiffness of metal-clad-wood-diaphragms. What we find is that the frames carry a small fraction of the total lateral load in a building like that, so that is why we use it every day hokie66. I like the idea of horizontal, which would work, but isn't practical for the construction practices and even the look that we're after. If you're interested, our trade association is located at

http://www.nfba.org.

Thanks all for your help.

 

[Brad805]

Pole building = Wood Tent. Their diaphragm testing is not up to the same standard as Hilti, Canam, or the SDI.

Not for me.

 

[BAretired]

It was not clear to me until the last post by the OP that the structure was to be a post-frame structure. I wasn't sure what that was exactly, so I looked at the nfba.org site briefly, but I'm still not sure how a post-frame structure differs from normal construction. I have seen pole barns where the columns extend from deep in the ground to the roof, i.e. the piles and columns are the same member. It is also not clear why "corrugated steel is the only way to make a post-frame structure stand".

If the building is to be platform framed, then it isn't really a post-frame building...or is it?

BA

 

[jayrod12]

I wouldn't consider it a post-frame building if there are interupptions in the posts. All post-frames I've see the post is embedded in the ground and is "continuous" from base of hole to u/s of roof. I put continuous in quotes because I've seen lately they've been getting metal plate laminated posts instead of trying to acquire full length members. This also allows them to be treated lumber for the portion of the post in the ground.

 

[PostFrameSE]

My apologies for the confusion. I'm mixing my construction types. I was responding to hokie66 about not using corrugated metal as a diaphragm and the fact that the post-frame industry uses this 95% of the time and got off topic. The building that the questions surround is a stick-built, platform constructed building which will be clad with corrugated metal siding. We WILL have interruptions at each floor level and we will have to strip the studs with girts in order for our vertically-oriented corrugated siding to be fastened. The more I think about this, the more I'm considering using a Parallam or something for a column that can reach to the roofline rather than these intermediate studs/floors. I could then balloon frame my intermediate stories and not worry about this whole settlement issue.

 

[mike20793]

Along the lines of what Brad mentioned, when we have a 4+ story wood building with masonry or concrete stair shafts and elevator shafts we will use engineered lumber for the top and bottom plates around the shafts to reduce the differential shrinkage. We then use the typical dimensional lumber for the rest of the building top and bottom plates. We also include a detail that calls out the expected shrinkage per floor and the contractor mechanism to deal with it. It's fairly easy to calculate but I believe Simpson also has a calculator on their website for shrinkage. Also another problem we have run into with large floor plans is that it is very difficult to frame the floor fast enough to prevent additional moisture from getting into the wood. One thing I always try to stay away from is shear walls near chase walls. That is just begging for the contractor to make excessively large holes in your shear wall studs/sheathing for MEP. We have had to deal with this countless times over the last few years. Fortunately, I believe we are starting to see a transition from wood bearing wall buildings above podium slab to more cold formed stud bearing walls (at least on our dorm projects, condos are a different story).

 

[Brad805]

Everything you have been describing suggests lowest cost. I would not bother wasting your time re-inventing the wheel. Your client is unlikely to be willing to pay the increased costs after his contractor whispers in his ear that you are selling him a story. Given the amount of 3 or more story real estate that is stick framed you will have a hard time arguing against the industry norms. Be aware, and protect yourself with appropriate notes so the risks remain with the parties that should accept the risks associated with a lower cost solution.

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.