Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: Residential Design Discussions

[Subject Prev][Subject Next][Thread Prev][Thread Next]

You know my position as we have discussed privately, but I will share it
here to hopefully bring a fresh voice. It's long winded of course ...

Most of my career has been structural design of commercial buildings.
For the past year now I have been doing almost exclusively residential
design. The reason is two fold, One is that I find the projects
challenging/interesting and the second is that commercial construction
is dead in California.

Here is what I have come to realize...

The 97 UBC is in desperate need of revision as it pertains to
residential light frame construction. There are two key areas that need
to be addressed. One is the RHO factor calculation and the other is the
rigid diaphragm distribution. 

I'll start with RHO...Ron Hamburger is I believe the brains behind the
RHO factor. He used to be on this list up until about 2 years ago. When
he did post on this topic, he stated the theory behind the 10/lw clause
for shearwalls in the RHO calculation. The INTENT was to give the
engineer a break when he/she had a 40 foot long shearwall. The engineer
could count the wall as 4 separate elements when determining rmax (or
3.4 walls if you had a 34 foot shearwall). The reasoning was that this
big long wall is obviously a different animal than say a braced frame.
The 10 factor was based on an average story height of 10 feet for a
building and concrete cracks on a 45 degree plane. So it made sense that
each ten foot segment could be treated as and individual element in this
calculation for redundancy check.

At this point in my career, I was doing mostly tilt-ups. On the front of
a modern tilt up building, you are lucky to get 4 feet of solid panel
and even less when you consider the panel joints. When you do the 10/lw
calc you get a number greater than 1.0 for anything shorter than a 10
foot segment. So I had to treat say my 9.75 foot wall segment as less
than a single resisting element and this increased my RHO value. But
this was not the intent. The intent was to help when you had a long
wall, not punish when you had a short wall. There is no exemption or cap
that 10/lw need not be taken grater than 1.0 - this is a problem when
one tries to follow the letter of the code. The same thing applies to
wood/residential construction where shearwalls are not usually 10 feet
long. Gary Searer destroyed the whole concept of RHO. Everyone should
spend the 30 minutes and educate themselves on this joke code
requirement. Gary looked at more than wood frame construction and did a
fine job punching holes in it. I think it can be downloaded on Dennis's
structuralist web site. The seismology committee has reviewed it, but
doesn't seem to want to commit to anything concrete.

On the rigid diaphragm requirement, I have come to realize that it is
virtually impossible to prove a wood diaphragm is flexible - It just
doesn't occur in most house configurations. Now I will say that I feel
that the diaphragm will most likely distribute loads in a semi-rigid
manner, more rigid than flexible. The problem lies in calculating the
rigidities of walls and diaphragms. The equations used to determine
these values are largely based on tests in a controlled environment.
Wall rigidities take into account nailing, nail slip, shear thickness of
sheathing, hold-down deflections, and shrinkage. Diaphragm deflections
are based on similar equations for BLOCKED conditions. Due to the
limitations on shear capacity for walls and spans for wood floor
members, shearwalls are too close together to ever come out as having
the proper deflection ratios to diaphragms considered flexible. 

Now, your million dollar house is framed by some clown like me sitting
in front of Home Depot every morning sipping a 7 Eleven decaf waiting to
get pick up by you local California Licensed contractor. I can't read
blue prints so I proceed to make swiss cheese of the OSB left out in the
rain the night before as I shoot nails into the wall framings to the
beat of Metallica in my headphones, because more is always better.

So after all my sophisticated analysis, I have a shearwall in the field
that has twice as many nails as calculated, and the OSB has swelled up
and been shot to hell. Since I have built 2000 houses with the same set
of drawings, and I also sat there all day and watched the framer drive
each nail flush and I measured the nail spacing on each panel, is all my
sophisticated rigid analysis going to predict the behavior during the
next 7.4 earthquake? Of course it is, because like nailing, more
calculation is always better.

Now, what do those of us who do this type of engineering need to do? Do
we ask those who write the codes to listen to us. Yes we do. But will
they take these concerns seriously and try to re-analyze why these
things came into the code to begin with and the impact it has if
followed to the letter? Are code writers taking into account the
uncertainties of construction, code interpretation by building
officials, the rationale of equations based on controlled tests, and
past performance of buildings that may have had other causes to create a
concern (i.e. weak/soft stories, or concrete topping slab on plywood)?
What is the goal, save lives or save re-straightening of picture frames?
Some want to dumb the code down ... I don't think this is necessary,
just make it realistic. We have enough codes and I think much of the
1997 UBC is good. But the code cycle in California has made things too
risky as we sit in limbo.

Can we be legally protected when we refuse to follow irrational code
requirements ? Can we (small firms who do 90% of residential work) just
call it performance based design and call it a day? Can we be expected
to participate in committees that seem to be largely tools for
self-promotion of firms and building departments who have the resources
to pay someone to travel and spend time in this process. Are we at the
mercy of those who wish to complicate the code to keep business for
themselves by making it difficult for other to catch up with new
methods? E&O Insurance continues to rise for structural engineers, there
is big money in expert witness testimony. When I get sued, Is Ron
Hamburger going to cancel his Performance Based Design seminar to come
to my trial and say, "Gerard is correct in his interpretation of the
intent", or is the hired gun going to simply say - "Gerard has stated
publicly that he understood the code and knowingly ignored the
requirements. The code is clear on this and is not open for
interpretation. If Gerard is allowed to go unpunished, what's to stop
him from using a live load of 35 psf on a residential floor instead of
40 psf? Send a message to Gerard and other rogue engineers ignoring the
code as they see fit". 5 Years after the 97UBC was published, and maybe
another 5 years before another code is adopted in California, we are
left hanging out to dry by our code developed by our PEERS on the 21st
floor overlooking San Francisco Bay and spending the week at the SEAOC
convention sipping Margaritas.

I await your barrage. 

Santa Clara, CA

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at:
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*   Questions to seaint-ad(--nospam--at) Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: 
******* ****** ****** ****** ******* ****** ****** ********