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Re: structures & ethics

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Gautam,
 
I hope I did not mentioned anywhere that the building is, or was supposed to, "come tumbling down."  I do not believe in that happening.  The building survived at least two good-size earthquakes (1971 and 1994), and is not going anywhere any time soon.  After all, the safety factors in wood design are in the order of 4-to-6, which, obvioulsy, quite safely covers the noted overstress.   
 
My problem is - can we allow the building to stand without strengthening, even in the "repaired to preexisting" condition, while knowing that it is not code-compliant?
 
Example: based upon my observation, I would assume that the building had not been re-roofed.   What will happen when the reroofing start?  Probably, nothing.  But at a 12' height, something may happen.  Then what?
 
I do not really care what my client would THINK about my degree.  I do care about what he will be willing to DO about my license in case of any problem - no matter how much he wants to save money now.        
 
Along with my professional and personal ethical considerations, this is my main concern. 
 
Thanks,
 
V. Steve Gordin, PhD
Registered Structural Engineer
Irvine CA
 
 
 
----- Original Message -----
Sent: Monday, November 22, 2004 10:58 AM
Subject: Re: structures & ethics

Steve:

Since the building has been standing up for the last thirty years in
spite of the 70-100% overstress for DL +LL and 15%  for DL,  I would
guess it will stand another 30 years without any problem (you did not
say that the members were sagging & assumming no major earthquak hits
it).   Either the loading is being overestimated or the strength
underestimated.

Even though the code requires a Live Load on the roof members, the
members  very seldom see that load - that could explain the high
overstress for DL+LL combinations without any detrimental effects to the
building integrity. (Some time ago, there was a discussion in this forum
about metal connectors in modern day pre-manufactured wood trusses that
failed when subjected to full design dead + live load.)

The factor of safety probably explains the 15% overstress for DL
without any detrimental effect.

If you tell your client that he has to replace the undamaged roof
framing because your calc shows that they are highly overstressed and
should have come tumbling down 30 years long ago, he will wonder if you
did earn your PhD the old fashioned way.  I think you should tell your
client what you found based on your calc and let him make the decision.
You should clarlify the scope of work  in your contract and limited to
the new work with the assumption that the existing framing is adequate
based on the fact that it is still standing.  After all, gravity is very
"unforgiving".

Gautam 


>>> mailbox(--nospam--at)sgeconsulting.com 11/22/2004 9:40:06 AM >>>
Good morning.

I was asked to provide repair plans and specifications for an existing
roof damaged by a vehicle (sic).  The problem now became not only
structural, but ethical, too.

THE LAYOUT

The building is apparently a former post office (1970s?), with 5 1/8"x
24-3/8"glulam beams @26' o.c. cantilevering 16' off brick wall along one
side.   Further into the building, the glulams also bear on wood
columns, and then cantilever and support other glulams spanning toward
the opposite brick wall.

Sawn purlins span about 26' between the glulams at 8-to-9 feet
on-center, supporting 2x4 @24", some insulation, lots of electrical
conduits and ducts, plywood diaphragm, and the composition roofing.

The "fascia" (still 6x12, nominal/typical) purlin of the overhang was
apparently hit and broken by the u-turning big rig, with subsequent
damage (delamination with lateral offset) to one of the cantilevering
glulams. 

THE PROBLEM

I ran the analysis based upon the current codes as well as upon the
1960s and 1970s codes (UBC).  The analysis shows that the undamaged
subpurlins, purlins, glulams, and columns are inadequate against DL+LL
(70-to-100% overstressed), and even against DL only (with 0.9 factor,
overstressed 15%). 

For the sawn lumber, I tried even select structural (dense select
structural) - did not help...

Some of the purlins are visibly sagging.  The glulams look OK.

I can just replace the damaged purlin (these are the only ones that are
adequate
due to 50% tributary area) and to repair only the damaged glulam to the
"preexisting condition" (I can do it with lag screws etc.).  After all,
the building may be OK solely by the fact of it existing like that for
about 30 years, right?

THE QUESTIONS

If I am right in my assumptions - how that could happen (don't answer
that)?

If I am wrong in my assumptions - what am I missing?

What should I do within the limitations of common sense, structural
analysis, and professional ethics?
  
Thank you.

V. Steve Gordin, PhD
Registered Structural Engineer
Irvine CA





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