Need a book? Engineering books recommendations...

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

RE: structures & ethics

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Guatam,
I must, with all respect, disagree with you. Deflection is much more than an
indicator - it is a function of bending stress and while you should consider
service loading as a means to calculate if creep is the cause of deflection
(or some other issue such as installing crown side down), it does tell me
that there is a bending problem or that deflection due to bending exceeds or
comes close to exceeding the code allowable limits for deflection of wood
products. If I recall my calculus (and please - it's been over twenty years
- deflection is a derivative of moment or bending, which is a derivative of
shear. Therefore the relationship of bending to deflection is a real issue.
Furthermore, in a flat roof or low slope roof, deflection can lead to
Ponding problems that can exceed allowable bending due to increased short
term live load.
Most panelized roofs are designed for economics. They are designed for the
lowest cost and the least weight of materials than other methods of design.
Creep occurs over time and in some cases, additional roofing without taking
cores to see if the maximum thickness of applied roofing has been exceeded
often takes place. This causes potential failure in bending stresses despite
the "indicators". If see excessive deflection in a wood member, I pay
attention to it. There are few indicators that give us warning of something
more serious - deflection happens to be one of them. 
I hope I did not misunderstand you, but this was the impression I got after
reading your post.
Respectfully,
Dennis S. Wish, PE

-----Original Message-----
From: Gautam MANANDHAR [mailto:GMANANDH(--nospam--at)ci.alameda.ca.us] 
Sent: Tuesday, November 23, 2004 1:59 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: structures & ethics

Bill-

If the building posed a life safety issue, then by all means, the
engineer has a moral duty to inform the client and the building
official.  You rightly indicated that "Deflections can be an indicator
of a problem but also can be just the result of the normal naterial
behavior of wood.  They are a warning sign, nothing more, nothing less."
 I am presuming that serviceability is not an issue as Steve did not
mention about it.  

The building currently is, and has been for the last 30 years,
overstressed based on code forces and code allowable stresses.  It has
been through two earthquakes and now has been hit by a car.  (I am
assuming - and I may be wrong - that except for the area that was hit by
the car, the building shows no distress sign.)  Unless the building is
in snow country, the roof may not see the full code live load.  As Steve
and others stated, the factor of safety is between 4 to 6; the
overstress for DL+LL was 70%.   With all these facts, I do not see why I
cannot use Chapter 34 of the code.   Of course, if there are other sign
of distress which Steve has not discussed, then the reasons mentioned
above may be meaningless.

Gautam

>>> BCainse(--nospam--at)aol.com 11/23/2004 12:40:14 PM >>>
Gautam-
The real issue raised by Steve is not really the serviceability issues
such as deflection which he noted in passing as supporting his concerns
but rather they were about basic structural safety and his professional
responsibilities relating thereto. Deflections can be an indicator of a
problem but also can be just the result of the normal naterial behavior
of wood.  They are a warning sign, nothing more, nothing less.

I applaud Steve's efforts to methodically and carefully look at the
structure and seek second opinions.  Based on some information he sent
me off-list, I now don't believe he has a life-safety concern requiring
immediate warnings or action involving the Building Official although
stresses are about as high as one would want to see in such a structure.
 Deflections may be creep related or material related.  As Dennis Wish
has pointed out, wood is not a uniform material, but its
non-uniformities do not necessarily require major upgrades.

Steve has provided great service to his client in following up on
concerns that developed in the course of his investigations and seeking
to resolve questions he had. He has recieved good advice from several on
the list and I believe will now make the decisions he needs to make
based on sound reasons.  He is basically a careful structural engineer
keeping his client's and the public's interests in the forefront.

Regards,
Bill Cain SE
Berkeley CA



 

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp 
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp 
*
*   Questions to seaint-ad(--nospam--at)seaint.org. 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: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ******** 

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. 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: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ******** 


******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. 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: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********