Residential Deck Engineering Seismic
Residential Deck Engineering Seismic
(OP)
I am designing a 10x36 covered residential deck attached to the end of a 36' wide house with 4x4 knee braces at the 8' tall posts in an area with high seismicity.
The plan reviewer here requested a seismic load calculation for the 36' long line and I'm curious what R factor is appropriate? From other posts/examples I've seen, it seems most have used the R factor for the overall structure the decks are attached to, and the ACSE 7-05 (code we are using) doesn't really seem to have an appropriate R factor choice for a knee brace situation...
The plan reviewer here requested a seismic load calculation for the 36' long line and I'm curious what R factor is appropriate? From other posts/examples I've seen, it seems most have used the R factor for the overall structure the decks are attached to, and the ACSE 7-05 (code we are using) doesn't really seem to have an appropriate R factor choice for a knee brace situation...






RE: Residential Deck Engineering Seismic
RE: Residential Deck Engineering Seismic
RE: Residential Deck Engineering Seismic
http://www.structuremag.org/?p=1353i
Our seismic forces are usually high enough that we end up designing for ~5 PSF lateral load. This is a bit lower than recommended in the structure magazine article.
I also saw a research paper which tested decks laterally. The conclusions are not great but I think the jist is that for a 1:1 or less aspect ratio that using lag bolts to attach the ledger provides a fair amount of lateral strength. They also recommend using screws for joists hangers. We haven't typically done this but do specify DTT2Z holdowns at 8'-0" oc typically.
http://www.forestprod.org/buy_publications/resourc...
RE: Residential Deck Engineering Seismic
Yeah, it's almost as though the timber frame industry forgot to show up to the whole seismic design discussion. Common systems that seem to have no representation:
- knee braced timber frames
- timber braced timber frames
- steel braced timber frames
I imagine that the nature of the connections would usually be such that R is always going to be pretty close to one. Still, it would be nice to see that in print someplace if that's the case.
It's tempting to just assume that all timber frame lateral systems are cantilever column systems just because that's the only place where the phrase "timber frame" shows up. On the other hand, "cantilever column" suggests a fixed base which is pretty rare in timber frame except when columns are embedded fence post style. One can argue that knee braced columns actually cantilever down from the fixed-ish connection at the top. It seems like a bit of a reach though, particularly given the flexibility of the beam forming the top of what is effectively a moment frame.
In Canada, it works like this:
R=3.00; Braced or moment frames in timber with ductile connections - moderate ductility
R=2.25; Braced or moment frames in timber with ductile connections - limited ductility
R=1.00; Other timber stuff not listed
In this context, I'd go R=1.00. To my knowledge, the Canadian codes don't elaborate on which connection systems are limited/moderately/not ductile but it's certainly difficult to see how a knee brace would generate much ductility. And, whatever thee would be, it would probably be limited to just the compression cycle and perhaps prone to ratcheting.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Residential Deck Engineering Seismic
Oddly enough timber braced frames used to be in the UBC before us western states changed to the UBC.
RE: Residential Deck Engineering Seismic
-know what the UBC R values were? I'm curious.
-I go 1.0 based on the canadian options. I'd be fine with 1.5 in your environment with building officials accepting etc. 1.0 and 1.5 are close enough to the same number here in my book.
-if one can get any reliable ductility out of the connections, it wouldntn't surprise me to find that the overall system is quite ductile. Probably closer to a moment frame. It doesn't take much local joint rotational ductility to translate into a whole lot of global displacement ductility in a moment frame.
- do you have any idea what kind of system the timber cantilever column system is meant to apply to? It just seems like such an improbable choice for the only timber frame option. Pole barns?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Residential Deck Engineering Seismic
Found this relevant FEMA paper looking around:
https://www.fema.gov/media-library-data/1393891642...
thanks for the input...
RE: Residential Deck Engineering Seismic
Thanks.
Slightly OT: In our area most knee braces are 4x4's lagged at each end. Except at the top, they have found a way to get capacity from a lag by installing it between the plys of a double. 2x10 beam. I won't even design a deck with kneebraces anymore. If they really want to use them, I call out, "Bracing per code". Most arch's want to use hidden bracing system anyway which I can design more easily with less push back on the details.
RE: Residential Deck Engineering Seismic
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Residential Deck Engineering Seismic
RE: Residential Deck Engineering Seismic
I have used this type of system many times.
Mike McCann, PE, SE (WA)
RE: Residential Deck Engineering Seismic
As for knee braces and taking a cantilever column approach, ASCE 7-05 section 12.2.5.2 and the 15% of axial design load max can really beef things up...
RE: Residential Deck Engineering Seismic
RE: Residential Deck Engineering Seismic
XR250 - how do you anchor the diagonal to the main house? It looked like just a couple of bolts to the rim joist?
RE: Residential Deck Engineering Seismic
I thru bolt a 6x6 pt block to the house band with (2) bolts. The house band is an LVL. The block is also typically end-nailed to the adjacent joists
RE: Residential Deck Engineering Seismic
RE: Residential Deck Engineering Seismic
RE: Residential Deck Engineering Seismic
RE: Residential Deck Engineering Seismic
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Residential Deck Engineering Seismic
RE: Residential Deck Engineering Seismic
RE: Residential Deck Engineering Seismic
RE: Residential Deck Engineering Seismic
RE: Residential Deck Engineering Seismic
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.