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.

 

[Brad805]

BA, I recalled I had a picture of a pole building just before the cladding was to be installed. Honestly, the cladding was installed immediately after this without adding anything like plywood or bracing using typical screws you can find at any hardware store. It started as a shop, and has been converted into an industrial sales building.

 

[BAretired]

Brad,

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

 

[KootK]

A few months ago, I was approached about designing a good sized aircraft hanger to be constructed much like Brad's photo. I proposed a fee of $9,500 CAN for construction documents and field review. They literally laughed at me and said that people that know what they're doing with these buildings can get it done for $2,500 tops. Once you've done a few of these and have details developed, they must go pretty quickly.

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

 

[Brad805]

BA, from everything I have read you say I could pretty much predict this would not have been the type of building you would have sealed. I am not a fan as you can tell. I grew up on a farm, both my brothers own them (one owns rather large versions) and eons ago I helped build a couple as well. In practice I know they can work, but the numbers never seem to justify these designs. I think they are fine for the farm. The farm code is much more lenient when it comes to lateral loads. The worst failures I have seen have been related to their lack of a proper foundation combined with water problems. I have also investigated a few that have leaned significantly (2" - 3"). I have not witnessed collapses, but have been told of cases. I suspect the reason we do not see many collapses is because of the redundancy we build into our wood structures. I am on the same page as you as far as the frost is concerned. The Canadian foundation manual also cautions against using the passive soil pressure, but I suspect many of the engineers sealing these overlook that. One of these engineers also suggested to me that the strapping attachment forms nominal resistance. He did not have a suitable response when i asked to see the testing or something to validate such a suggestion. I was randomly selected for a practice review a few years ago. I brought this building type to our association's attention, but they never did anything to investigate.

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.

 

[KootK]

Yes, too much time to contemplate shear friction makes KootK CRAAAAyyyyyZZZZZY.

One of these engineers also suggested to me that the strapping attachment forms nominal resistance.

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.

 

[Brad805]

Koot, I suppose we are getting off topic. I do think the strapping attachment will form some resistance, but how much and the reliability of this mechanism is a mystery to me. There was an interesting discussion at

http://www.eng-tips.com/viewthread.cfm?qid=338373.

The last post shows a number of publications that have been published since I was involved in these. I have not designed any of these in more than 10years, so maybe this research has improved the situation. Regardless, those clients were never the easiest to deal with since their business model revolves around being the cheapest option.

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_of_PF_Buildings.pdf