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Re: Lumber Moment Frame Connection

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On 2/28/07, Jim Chatterley PE <cfs2000(--nospam--at)pacbell.net> wrote:
    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



Jim-

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 catalog.

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 placement.

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.

cheers
Bob