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RE: LRFD, ASD, and USD

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Very good points here. I agree with Ernie (isn't that a first:>))  I prefer
to design wood structures with Trus Joist products - this is not an
endorsement.  I like their performance series and have learned to use it
based upon the type of job I'm doing. On a scale of 20 to 70, TJ defines
what type of motion and feel the floor or roof will yield. Therefore, I can
be assured that if it is a high end custom home, I'm going for the 70 points
whereas I might be satisfied for a 50 in a commercial structure and a 30 in
an Industrial building.
Deflection criteria won't provide you with the experience of how the
occupant perceives the floor or roof to feel. After Northridge, I did a
repair to a Condo Complex in Simi Valley. Those in damaged units claimed the
floor was much bouncier than before the earthquake. There was no
justification, but there certainly was a greater perception by the occupants
after the earthquake. This became the psychological factor. The floor was
the same before and after, but the occupants believed it to be less safe and
bouncier after.  The numbers proved that it was a long span bouncy floor
before the quake.
You do need to consider performance in wood design - not simply run the
numbers to satisfy a code. I have also found it useful to meet the owners of
residential projects and hand hold them through the design - letting them
make some decisions as to how they want their homes to perform. My
experience is that the owner will tend to upgrade to a stricter code when
they have the options explained and they plan to live in the house. If it's
a spec job - expect to be down around 25 on the performance level.
Developers are only interested in the bottom line profit and not the comfort
of those who by the homes.

Dennis Wish PE

-----Original Message-----
From:	ErnieNSE(--nospam--at)aol.com [mailto:ErnieNSE(--nospam--at)aol.com]
Sent:	Thursday, July 30, 1998 10:01 PM
To:	seaint(--nospam--at)seaint.org
Subject:	Re: LRFD, ASD, and USD

I agree with Mel, I design mostly wood-framed residential and small
commercial
buildings, and if I need to use steel or concrete, the quantities are small.
I
use steel beams where wood beams are too deep to fit within the depth of the
joist system and for lateral resisting sytems, and usually, deflection
governs. I haven't used LRFD and I use USD only on big footings and heavy
grade beams.

I think serviceability(or deflection limits) should be personally
evaluated..
I designed a long span wood roof beam a long time ago and the framers called
me and said the beam is sagging excesively and bouncing like a diving board.
I
checked my calcualtions and everything works fine(understressed  based on
design loads and within the code deflection limits). I went out to the
jobsite
and checked it out and they're right, I would not want that kind of beam if
this was my building. And I learned my lesson. They ended up adding a post
at
the midspan of the beam. And this is only a roof where only the framers can
see and feel it. What more for a floor system?

My point is, don't just design for minimum code requirements, design for
what
you and the ultimate user of the building think and feel is a safe
structure.

Ernie Natividad