Log home seismic design

[GVCivilGuy]

Switching over to the new code (CBC2007 VS UBC 1997). What seismic response modification coefficient (R) should I use for a bearing wall system that is designed out of logs? I can't find anything in the ASCE7-05.

 

[msquared48]

Most log cabins are not a true bearing wall system in that the loads are transmitted through the logs in bending to the intersecting corners. Many have gaps between the logs, but some do not. That being said, with the rods and spikes to transfer the lateral shear between the logs, and the rods to serve as holddowns, I just design them as a shearwall system.

In reality, if you model them on the computer, you could rationalize that they are really log moment frame systems with the rods and load bearing points serving as couples to develop the moment.

To keep things simple, I just use 6 or 6.5 for R (ASCE& or IBC). Never had my analysis questioned in ten years of doing them. Maybe because plans checkers really don't understand these animals very well?

Mike McCann
MMC Engineering

 

[WillisV]

See discussion of R factors to use for various configurations here:

http://www.structuremag.org/archives/2006/March-06/C-CS-Log-Structures-Standard-March-06.pdf

 

[DonPhillips]

Mike, if I review a plan that is sealed by a design professional, I seldom question the design unless it is way off base.

I once called an engineer who sealed a plan for a load bearing exterior wall being removed for an addition. The opening was something like 16' and the new beam was to carry the second floor bedroom floor loads and the roof truss loads. The design called for 2-2x12's.

When I called the engineer, he thought it was an 8' span for an interior load bearing wall. I faxed him the drawing and in a couple of days, received a revised drawing calling for something like 2-1.75x14" LVL's or seomthing like that. It was like 3 years ago but 2-2x12's scared the hell out of me since this was an engineer working for a large firm who does a lot of these smaller projects in central Ohio. Needless to say the engineer thanked me for catching that.

However, on another project, an engineer sealed something like 2x6 or 2x8 joists for a deck cantilevering out like 6' because that is what the owner wanted. I called the engineer and advised him the negative moment in the joist at the beam was probably very large and likely exceeds the allowable stress permitted by NDS (I normally do not do calculations to check another design professional). Now get this, he tells me the owner is only going to have 5 or 6 people on the deck and it should be fine. I told him fine, it was his license, and I was just advising him of my concerns should the owner have a party on the and 20 people are along the rail, dancing to the music while full of liquid energy. The certificate of occupancy would limit the deck to 6 people.

So the owner picks up the building permit and like 2 months later, the contractor comes to me asking why we issued a permit for such a bad design. He just could not beleive it. I advised him the engineer who sealed the drawing took responsibility for the design and the occupant load for the deck shall be limited to 6 people. He said he would talk to the owner and either modify the design or not accept the job.

About 2 or 3 weeks later, revised drawings were submitted and now, we have a new beam along the far edge and the cantilevered joists are only spanning about 2' feet. Contractor felt more comfortable, I lifted the reduced occupant load limitation, and life went on.

Oh, by the way, the engineer was a friend of the owner doing him a "favor".

Don Phillips

http://worthingtonengineering.com

 

[msquared48]

Don:

Thanks for the post, and I didn't mean to target you or criticize plans checkers in general. I, too, have had my bacon saved by more than one plans checker. We have all been there.

That being said though, I do feel that, particularly in smaller outlying jurisdictions where these structures are found a lot, at least where I practice, the level of structural expertise of the local plans checkers is not commeasurate with the complexity of these structures. Please correct me if I am wrong here, but I do not believe that the ICBO certification that many plans checkers get deals with much more than code compliance issues, not structural design philosophy or theory. That is where I was coming from in my previous post.


Mike McCann
MMC Engineering

 

[DonPhillips]

No apologies necessary, Mike. I did not feel targeted and I agree with you - it depends on the locality. Some are more geared towards residential and/or the building officials tend to come from the construction community and not the design community. Most codes and state laws do not require residential plans be sealed and the residential industry is used to the "cookbook" of the CABO/IRC to design buildings. If you grow up "designing" that way, then the more complicated plans need to go to more knowledgable reviewers. One of my clients sends me his larger and custom homes for that reason (very little money to be made there) with an occaisional commercial to keep me profitable.

I have had the good fortune of being involved with communities where to they want a CBO and commericial plans examiner in one package. They get better judgement during reviews (since administration of a code is key to keeping the public from getting pissed at the building department) and for the extra pay, get more thorough residential reviews.

The last ISO (the same insurance industry service that rate fire departments, also rate building departments) report I read, it looks like they are giving extra points for design professionals reviewing both residential and commercial, and now CBO's with masters degrees can earn more points.

So perhaps one day, even the smaller communities will have better plan reviews through increased knowledge and professionalism of the enforcement community. I know I am seeing the fruits of those labors in Ohio. In central Ohio, the really small communities (villages with less than 5000 people) contract through larger or more experienced departments or outsource it to firms like myself.

Okay, stepping down off my soapbox.

Don Phillips

http://worthingtonengineering.com