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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: Wed, 28 Feb 2007 22:31:11 -0800 (PST)
Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu> wrote:
I agree with a lot of what Bob said. Moment frames in wood are
technically feasible. You can certainly create a connection that will
have strength and rigidity.
I, however, don't typically recommend the use wood moment connections or
use them myself (I primarily work in the world of timber framing). The
reason for this is that while you can certainly create a moment connection
with rigidity, the truth is that it is typically rather difficult to
sustain that rigidity in the connection, ESPECIALLY in a situation that
resists seismic (cyclic) loads. As a result, I personally will not do a
wood moment connection to resist seismic loads and would only
remotely consider using such a connection in a VERY limited situation
for wind (i.e. not a whole lot of wind load and there is a LOT of
redundacy in the structure).
For seismic loads, the problem becomes the "loosing" of the connectors.
It is rather difficult to keep a bolt, lag screw, nail, etc from
"widening" a hole under load. So, under repeated loads, it would not take
much to loosen up such connections enough that the connection would lose
enough rigidity to behave more like a pin connection. Add to that the
difficulty in minimizing possible wood shrinkage effects on the
connection's "rigidity", especially if you do multiple rows of bolts,
screws, etc running parallel to the grain (i.e. you have more than one
bolt in line when looking at a line drawn perpendicular to the grain and
use a steel connector plate...that is a typical recipe for checks/splits
to happen at some of the bolts).
Knee braces are better in my opinion, but not by much. The advantage of a
knee brace system is that is easier to maintain the needed rigidity at the
beam to column connection due to the "truss" effect (you are essentially
creating a moment connection by tossing in a triangular shape that then is
largely dealing with just axial loads). The connections would still
loosen in a cyclic loading event, but the loosening of the individual
connections of the members would not have as much an effect on the overall
rigidity of the overall end connection. Regardless, I still would not
rely on knee braces for most situations in seismic situations (i.e. maybe
for a self-supporting small porch cover or trellis but not a main
structure) and would also be very limited in my use for resisting wind
loads (non-hurricane wind areas with rather small wind loads and lots of
From a pure techinical point of view, the problem that you will face for
both wood moment frames and wood knee brace (or other braced) frames is
that the codes don't have R values for them in the seismic provisions.
Thus, techinically, you cannot use them to resist seismic loading (unless
you convince a code official). This takes me back to what Bob said...if
you are going to use a wood moment frame to resist seismic loads, then I
would DEFINITELY suggest designing it for elastic loads (i.e. R=1) and
then STILL be VERY conservative on the connection design. If you can keep
the force in the connectors (i.e. bolts, screws, nails, etc) VERY low,
then they may not "work loose".
As others have pointed out, your likely best option is to use a
cantilevered column lateral system.
On Wed, 28 Feb 2007, Robert Kazanjy wrote:
> On 2/28/07, Jim Chatterley PE
> > Last week a county plan check engineer (Southern California) refused
> > to consider a connection detail using custom steel "T" straps 3/16" thick on
> > both sides - connecting a 6 x 12 beam to a 6 x 6 lumber column ( 2 each 5/8"
> > dia machine bolts in each leg of the "T" Straps). The lateral force on the
> > simple shed roof 8 foot high patio cover was 2,240 foot- pounds.
> > The plan checker insisted that unless the client would be willing to
> > use bolted lumber "Knee Braces" they would not issue a building permit.
> > Stating that there is no code approved heavy timber moment frame
> > connection.
> > Your comments please.
> > Jim L. Chatterley PE
> > Composite Framing Systems, Inc.
> > 2723 Currier Ave., Simi Valley, California, 93065
> > 805-520-3666 Fax 805-583-1434
> > www.Compositeframingsystems.com
> That's pretty much par for the course. Just about everyone in the industry
> believes (& insists) that moment connections in timber "can't work",
> "don't work", etc
> Years ago when I when I was doing cyclic testing on shear walls (full size &
> narrow ones, code / site built & factory fab), some of the customers wanted
> to test portals.....specifically to address garage door openings.
> Of course, they're first attempt was some sort of narrow panel strapped to a
> beam using Simpson straps...all designed with "design values" out of the
> I was unsuccessful in convincing them that the "strapped connection" from
> the panel (shear wall) to beam had to be WAY overdesigned, such that it
> would be very lightly stressed compared to the shear walls. Even to the
> point that the connection should probably remain elastic.
> That is, the connection (panel to beam) has to have enough capacity (before
> yield) to develop the ultimate strength of the panel. Otherwise the panel
> to beam connection yields (nails bending, wood compressive yield, straps
> stretching) and you wind up with a two panels (on either side of the garage
> door) tied together by a rigid strut....but no moment connection. And the
> portal lateral capacity reverts to that of the shear panels alone.
> Taking a look at your custom steel straps, a couple of questions / comments
> come to mind.
> You mention a "lateral load" of 2240 ft-lbs, I assume you mean that is the
> moment demand generated by the lateral load....and is that total or per
> beam/column joint?
> Are they T's or L's ? ie end columns or intermediate columns?
> Since you mention two bolts "per leg", I kinda think they must be L's
> between the beam & an end post.
> You 3/16" material has more than enough stiffness to match the members (I
> assume they close in width to the "depth" each member) but I think the "weak
> link" is going to be those 5/8 bolts & the timber is cross grain compression
> (bearing) depending on the actual geometry of the straps & the bolt
> What are the loads on the bolts? and the subsequent loading into the timber
> thru those bolts? Do the numbers work & the connection remains elastic, thus
> preserving the moment connection....cuz once something in that connection
> yields, there goes your moment capacity, now you've got a linkage.
> Unfortunately, even if this connection calcs & "works", your lateral system
> (if I understand it) has no yielding element so you really should design it
> for "real loads" so that it truly remains elastic.
> But in reality, this is JUST a patio cover (albeit, a rather heavily framed
> one so we don't want to drop it on someone)
> yeah, the plan checkers are in love with those knee braces; but I doubt the
> knee brace design & capacity would stand up to the preceding analysis /
> logical argument.
> How strong really is a knee brace design?
> I would suggest, showing by calculations that your alternative is much
> stronger than the knee brace. Those knee braces can't be very strong with
> a both thru the ends at 45 deg.
> btw I think someone may have previously asked the list about moment
> connections in a patio cover.
> Good luck with the plan checker.
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