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RE: UBC Section 1633.2.6 Collector Elements

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Hi David, 

I agree that the list seems to discuss only a few subjects which frequently
don't really tackle technical issues.  Of course the people who are the ones
to answer this question are probably getting the list in digest form so
there might be some more reasoned answers next week.  I believe that
interpreting (and designing) the UBC is something that we are not all
comfortable discussing.  It means that we, as peers, must expose ourselves
and our relative ignorances to each other.  Hopefully we are right in our
interpretations but it is important to be corrected.

Here is my take on collector elements:

Basically, all drag reinforcing and shear dowels (concrete) must be designed
for a force equal to 2.8Eh.  If you are designing a concrete shearwall
building, this means (or can mean) that your drag reinforcing can become
quite substantial, somtimes impractically so.  This is an additional way of
ensuring that the engineer provides additional redundancy in the system,
i.e. more or longer walls or move to a S.M.R.F. system.  Longer walls or
walls that are not well placed usually means (more) undesirable cracking in
the slabs.  It seems that for practicality and ease of design, the S.M.R.F.
system is the way to go, you reduce foundation issues and usually your
collectors aren't a problem. Maybe this is the intent since more walls are
usually not practical from an architectural and a durability viewpoint.  Of
course, it has been my experience that contractors make more mistakes
assembling the reinforcing in moment frames than in shearwalls and there is
less forgiveness with frames so it is a two-edged sword.

For concrete design, omega naught only comes in to play for the design of
the collectors and their connections to the lateral resisting system.
Something we should all be relieved about.  Steel design has load cases that
include the omega factor. I am not sure since I don't do steel design if
these are ASD or LRFD cases. I just checked to see where omega was applied
since the special seismic load combinations section says "as specifically
required by chapter...". Perhaps someone with a working knowledge of steel
can help out with what they or their company is doing.  It seems that for
light residential uses you can escape the application of omega to the drag
design.  ASD designed masonry will get hammered when designing the drag
system since you have to develop these heavily magnified forces at the joint
between the floor system and the wall system.  This is a good thing since
this is usually where problems develop.

Unfortunately there is usually a clash between what is needed structurally
and what is architecturally acceptable.  The changes introduced in the 1997
UBC Seismic section have yet to filter down to architects in a definitive
way, i.e. in order to comply with the redundancy requirements you need more
wall or more columns and the drag system may somewhat the dictate same.

I agree that it seems odd to design an ASD structure and then apply
(heavily) factored loads to it.  Maybe this is an additional way to
encourage the move to strength design for all materials. 

I hope this helps,

Nick


-----Original Message-----
From: David Hall [mailto:structbear(--nospam--at)earthlink.net]
Sent: Sunday, October 15, 2000 5:00 PM
To: seaint(--nospam--at)seaint.org
Subject: UBC Section 1633.2.6 Collector Elements


Can someone here give a quick explanation on this section pertaining to the
use of the Omega Factor in Ultimate Design and the increase of 1.7 in loads
for Allowable.  Thanks