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RE: Lateral Load--Is it THE Design?

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Bill, Cliff;

I easily spend half my time designing and detailing the lateral load path
for the projects I design (mostly wood framed structures in UBC/CBC seismic
zone 4).

Long ago I figured out how to get around or through the questions about
"over engineering" when I started clarifying that I'm not designing to avoid
structural failure but I'm designing to comply to a prescribed code using
prescribed loads, allowable stresses and factors of safety. My design
documents compliance with the code, not to predict structural failure. After
explaining this, I ask rhetorically, "Will your structure actually
experience this load condition?" To which I respond, "Probably not, but
that's a good thing, right?". That ususally stops the argument.

Since Northridge, the questions/comments in the form of "I've never had to
do this before" are waning, but I usually respond "After today, you can't
say that anymore". In response to: "I've been building houses for 25
years...", I usually respond either with my first response or with something
like "The code has changed quite a bit since you first learned how to frame
a house." But, like I said, these comments have diminished. If the
contractor isn't so antagonistic, but just sharing his "tales of woe", I
commiserate with him and tell him that I'm having to do more calcs and
details than I had to do 25 years ago as well.

Amazingly, none of my clients or contractors have ever asked me what the
wind load design criteria I used for a particular project. If I told them
that I design for an unshielded, constant 70 mph wind with a gust factor of
1.3 on top of that, they would probably bust a gut. I know there are some
locations here in SoCal which experience some significant winds, but those
areas are requiring a design for 90 and 100 mph (sustained, not gusting).

Regards,

T. William (Bill) Allen, S.E. (CA #2607)	
ALLEN DESIGNS	
Consulting Structural Engineers	
http://www.AllenDesigns.com	
V (949) 248-8588	 .	 F (949) 209-2509	


-----Original Message-----
From: Cliff Schwinger [mailto:clifford234(--nospam--at)yahoo.com] 
Sent: Thursday, September 16, 2004 10:01 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Lateral Load--Is it THE Design?


Bill, 

If you are talking about wood framed residential
construction then I agree that investigation of wind
and seismic loads could easiliy consume half of your
design effort. Unfortunately a lot of this effort is
expended by structural engineers because they know
that whatever they design and detail, they will hear
complaints that "..I've never had to do this
before....my previous structural engineer never
required these hold down anchors,...I've been building
houses for 25 year and never saw anything like this
before...." 

My previous response to your post was for steel or
concrete framed construction.

Cliff Schwinger


--- Bill Polhemus <bill(--nospam--at)polhemus.cc> wrote:

> Folks:
> 
>  
> 
> Is it just me, or is everyone here pretty much
> resigned to the fact that
> lateral loads dicate maybe 80% of our efforts?
> 
>  
> 
> Of course I know those of you on the Left Coast who
> do commercial &
> facilities design know this because of seismic, but
> even those doing
> residential there admit that wind loads tend to
> govern their designs, and
> they still spend the greater portion of their
> efforts in this.
> 
>  
> 
> This comes to mind because in talking with typical
> clients, they just don't
> seem to "get it." They say "why are these roof
> purlins so big? There's not
> that much load on them!" and I have to tell them
> about design for wind. They
> say "but it doesn't get that windy here!"
> 
>  
> 
> Now, how silly a comment is that? Ask the folks in
> Gulf Shores, AL, just how
> "windy" it got last night! When I say to them that
> we have to design for
> hurricanes, they act like I'm just being
> outrageously conservative.
> 
>  
> 
> I have begun telling them that lateral loads account
> for the majority of
> building structural failures (I'm flying by the seat
> of my pants here; I
> have no way of knowing if this but every time I see
> photos of devastation
> it's hurricane or earthquake).
> 
>  
> 
> Anyway, just thought I'd solicit some comments on
> this topic.
> 
> 



		
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