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Re: Lumber Moment Frame Connection

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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
 
 

"Jordan Truesdell, PE" <seaint1(--nospam--at)truesdellengineering.com> wrote:
Okay, I usually don't use moment frames in wood, nor do I count knee
braces in timber structures, and the easier answer is to embed the
poles. However, I'm going to take Jim's position here for the sake of
argument.

We all know how wood reacts with fasteners. We all know statics. In a
purely academic sense, we can determine the loads, forces, and reactions
on just about any simple structure by hand. Presuming that you use an
R=1, a phenomenally conservative value (as I will expand later), there
is no reason that the combination of steel design - including buckling
stability - and wood fastener design - using the 6 interaction
equations - can't be used to make a moment frame of defined capacity.
If this were not the case, then shear connections without bearing would
be forbidden! That 2x2 ledger supporting a joist would never last, as
the forces over time along with the shrinking of the wood would allow
the nails to deflect and the system to collapse. You couldn't bolt a
ledger into a band - the same thing would happen, but even worse since
the fastener isn't a displacement type. The band would flop around like
a wet noodle causing anyone walking on the adjacent surface to fear for
their lives. And hurricane ties and seismic hold-down anchors - wow,
talk about a recipe for disaster.

As for the performance in a seismic event, what could be better than a
moment connection in a non-linear material with mechanism that builds in
a significant amount of hysteresis in the dynamic performance. Owners
pay millions of dollars to implement such systems using dampers and
plastic hinge connections. Would you give a lower R value to this type
of system than you give to URM - possibly the worst conceivable lateral
force resisting system for seismic loads?

I say there's no reason that a properly designed and installed
steel-braced moment frame system shouldn't be considered as a legitimate
structural element. And don't bother trying to bring up the red herring
of cross-grain bending - cross-grain bending occurs in every connection
which uses more than one fastener.

Jordan


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Benjamin H. Maxwell, S.E.


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