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Re: Lumber Moment Frame Connection

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Hi Jordan,
 
I don't think you're taking the discussion in a bad place.  I understand where you're coming from and having worked for Eric Elsesser for many years I am very familiar with pushing the envelope with regard to code provisions. I don't happen to agree with him all the time, but that is for another discussion.
 
First, let me state that I would have no problem installing a moment-resisting wood canopy in my own back yard. I'm not about splitting hairs. The heart of this discussion to me is diverging from the code in instances similar to this, except when greater consequences are at stake.
 
As an aside, as licensed professionals responsible for the safety of occupants, I believe it irresponsible for us engineers to dismiss the codes, which are really living documents of our collective engineering experiences and discoveries, based on our personal judgment.  We're all still responsible for our own engineering with or without the code and definitely not any less of engineers for staying with in their established limitations.  The codes are not there as  crutches. They are there to maintain rational constraint on the engineering community such that a reasonable and universal standard of care is provided to the public and building owner alike.
 
Now back to the matter at hand:
 
I agree with you that for given loads there is no magic in determining if the materials you are working with are adequate for the job.  But I don't agree that we are on the same page when it comes to knowing what the loads will be.
 
Yes, we all would know and would agree on what the 5% damped spectral loads would be for a given structure. But for an undefined lateral load-resisting system, choosing the appropriate pseudo-static load is a complicated process that cannot be guessed. 
 
The "R" values used in building codes to reduce elastic response for design purposes consider many factors from robustness of the connection type, anticipated damping, modal response assumptions, etc.  These numbers quantify the acceptable nonlinear response capacity of properly detailed systems, and are result of decades of research, observations, debate, consensus, etc. 
 
So when you decide to use an undefined lateral load-resisting seismic system, you are faced with the uncertainty of how that system will respond in the non-linear realm.  Start with untried assumptions and you could end up being surprised.  It is definitely not a simple matter.  If it were, earthquakes would not be problems anymore.
 
For a little back-up of the above, refer to ASCE 7-02, Section 9.5.2.2 bottom paragraph. This sums up what I'm trying to get at.
 
So, in conclusion, I did not intend to advise blindly following the plan checker regarding the original post.  My intent was to expand upon what I perceived to be the rational behind the plan checker's concern (be they unfounded or legitimate).
 
-Ben

"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:
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
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Benjamin H. Maxwell, S.E.


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