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Re: Lumber Moment Frame Connection[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Lumber Moment Frame Connection
- From: Benjamin Maxwell <enginerd666(--nospam--at)yahoo.com>
- Date: Thu, 1 Mar 2007 15:54:28 -0800 (PST)
"Jordan Truesdell, PE" <seaint1(--nospam--at)truesdellengineering.com> wrote:
If the calculations were performed per the NDS and AISC spec and all show positive margins of safety using rational engineering analysis what grounds does the plan checker have - other than their gut feel - to deny the permit? What magic is occurring in the moment connection which does not result in tension, compression, and shear forces of a known quantity? Given the dimensional change and deterioration of lumber in the natural environment, why is it that bending moment, shear, and defined fastener-steel-wood interactions are allowed for nearly every other case? Is the lumber somehow aware that there is a moment connection at the free end? Does a post embedded 3' into concrete or soil figure there's no use in failing because there's a special formula in chapter 18 that allows such a use?
From experience we've all seen "moment connections" in poorly built lumber/timber that just don't have much rigidity left, but is it a good idea to avoid such designs because of anecdotal evidence of poor craftsmanship? I know of gambrel lumber roofs built with plywood gusset plates which have withstood the test of time, yet they function thanks to two moment connections using just plywood and nails.
While it's always fun to argue whether or not we can reasonably predict the forces imposed on structures from earthquakes, we nevertheless have all settled on a set of values. We also have a reasonable idea of where how a structure will respond, and can use conservative values where there is less certainty. Currently, there is no data in the IBC2003 which addresses the lateral shear performance of log homes. Should we outlaw their construction without steel moment frames, or perhaps require sections of SST panels with log siding interspersed among the 20-30" diameter logs? I'm not aware of any exhaustive research which would characterize such a system to the point that an effective R value has been determined, yet they appear to get built.
I don't necessarily mean to take the discussion in this direction, but have we, as engineers, fully given up engineering as a science in favor of checking off boxes and filling numbers into equations in a book? I suppose that depends on whether the plan checker asked for calculations or simply dismissed the plan as "not in the code". I believe it was the latter, in which case we're just checking boxes.Jordan
Benjamin Maxwell wrote:******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********Jordan,I thought I'd respond to your post to keep the conversation (gentleman's debate?) going...(Yes, it's a slow day at work, but I'm not moving to Palm Springs...yet)I don't think anyone on the List argued that a wood moment frame connection would not physically work (in terms of preventing collapse), just that it would not work within the defined limitations required by our building codes.We all would probably agree that for a small canopy, a metal strap connection between the beams and posts would probably hold the structure together for most earthquakes. The question is that would the system do so with a degree of reliability that would assure a plan checker to allow its use. I think this is the aspect of the question most engineers on the List responded to (me included).Without testing the proposed system, one could not decisively conclude that there is adequate ductility capacity and hysteretic energy dissipation to limit the performance of the structure to acceptable levels (stresses, strains, displacements, etc.). Even with testing, there is still a considerable amount of uncertainty (acknowledged or ignored) in how a system will perform under actual earthquake loads (Need I mention steel moment frames?).The wood - bolted steel plate moment connection in question could undergo brittle fracture of the steel strap at re-entrant corner in the first cycle of ground motion. Or, the wood could shrink and crack over the course of time, substantially reducing the capacity of the connection as intended. I won't bring up cross grain bending. The point is that you can't assume that a given connection possesses ductility and will exhibit robust hysteretic energy dissipation.With that established, I hold that the building official in this case was performing his job in asking the engineer of record to substantiate his design, since the connection is not a recognized lateral load-resisting system.-Ben
Benjamin H. Maxwell, S.E.
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- Re: Lumber Moment Frame Connection
- From: Jordan Truesdell, PE
- Re: Lumber Moment Frame Connection
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