wood moment frame

[AlpineEngineer]

I did a search on this forum and I see that there has been some discussion on wood moment frames (2x4s, headers, OSB). In general I read on this forum and in Breyer's book that wood moment frames are a bad idea. I have seen the APA portal detail which if I'm not mistaken is basically a wood moment frame. From the research/testing done it looks like the APA portal frame works well. I can't find any resources with design guidelines and examples for a wood moment frame. The APA just gives the details on it, no design guide for it. Do you guys know of any design aid resources for wood moment frames? Why is a wood moment frame such a bad idea? Is a timber frame (post and beam) capable of becoming a moment frame with steel connections, if so do you know of any design aids for that?

Thanks in advance.

 

[csd72]

I believe it is due to the difficulty in achieving true rigid connections in the corners.

csd

 

[MiketheEngineer]

CSD72 is correct. Almost imoossible to achieve a truly moment resistant connection - the wood at the fasteners moves. Truss plates are fairly moment resistnat - but they too allow some differential rotation

I beleive APA came up with the idea and then just tested it. While not a perfect connection - it works well enough for the purpose intended.

 

[csd72]

There is also the tendency of timber to split if you put too many fasteners in it.

csd

 

[Lion06]

I think you can usually get the connection to provide a reliable moment capacity, it is the rotational stiffness that isn't reliable and can be an issue.

 

[woodengineer]

The APA detail seemed to appear as the IRC prescriptive code requirements for braced wall panels began to be enforced by local officials. For years houses have been built without this detail. I have had to answer many times the age old question, "I have built houses this way for years without structural failure; why the new detail?" The bottom line is that there are many redundant systems in wood structures which transfer load around these openings, specifically, via diaphragms. The APA detail (without hold-downs) shouldn't work.

woodengineer

 

[apsix]

Many timber portal frames have need built and used successfully, if not in the US certainly in other countries. Typically manufactured products such as LVL and plywood would be used.

 

[msquared48]

With older, better lumber before the LVL, etc, engineered wood, and cheaper labor, timber frames were more viable. With the decrease in the allowable stresses in the solid members over the past 20 years, plus the other issues raised, they have become far less feasible and reliable than steel frames. Even though I do not like to mix wood and steel in residences, I would never use a wood frame. The performance of the wood frames depends too much on the quality of workmanship for me. Plus, the member sizes are driven by lateral deflection in most cases, not stresses. Bolt holes leave room for lateral slippage for them to engage in bearing, resulting in more deflection. It would seem that on-site inspections of any timber frames would be mandatory if they must be used.

 

[WBUW]

Personally I wouldn't quantify the APA Lateral Restraint Panel as a wood frame. It is the same thing as a simpson strong wall. Built to spec and tested for capacity. The LRP like conventional shear walls relies on sheathing to transfer the load, IMO a moment frame relies on members, ergo glulams, steel columns, collectors, etc.

I think wood frames are generally looked down upon because it is difficult to provide a true moment resisting connection at the joints. They probably do not hold up to much load when compared to steel either.

If one was willing to use lots of steel knife plates, strong glulam/psl members I'm sure it can be done. I've seen them, just never designed them myself...

 

[hgallart]

Given the comments I read, as well as those I've heard in the industry about engineers trying to use a wood moment frame, what about a prefabricated, preengineered steel moment frame like the one Hardy Frame has (

http://www.hardyframe.com)?

They are ICCES and COLA prequalified and should be able to help you achieve the lateral load resistance you need without sacrificing technical performance and cost.