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RE: structures & ethics

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Dennis:

I agree with you that one cannot neglect the effect of creep.  
However, the original post was regarding the overstress and not the
deflection.  I am assuming that except for the vehicular damage, the
building is in good shape.

Section 3403.2 of CBC allows repairs/addition to a existing structure
without requiring the rest of the structure to meet current code
provided the repair/addition does not overload the existing structure. 
In Steve's case, replacing the damaged purlin will not add additional
load to the structure.  Therefore, by code, the project
engineer/building owner is not obligated to upgrade the building in
spite of the existing calculated overstress. This makes sense -
otherwise, we would end up upgrading any building we touch -if not for
gravity loads, for seismic loads.  

Gautam Manandhar

>>> dennis.wish(--nospam--at)verizon.net 11/22/2004 11:58:33 PM >>>
Gautam,
You disregard 30 years over which creep has an effect on the original
deflection and framing. Not only may a camber have been considered in
the
original design, but wood is not perfect and a framer will most often
install the sub-purlins with the crown side up to accommodate the
weight of
the materials. Furthermore, water damage from roof leaks, drying of
lumber
depending on its location and original moisture content is an
important
consideration.
This is not a steel roof but one based on materials that are fibrous
and
originally a living thing. 30 years later the installation of roofing
materials can make a big different.
I am looking at deflection problems in the overhang of buildings that
are at
least 30 years old and which have been fixed a number of times to
compensate
for the cantilevered deflection. This is not uncommon - especially in
older
condominium (single or dual family units) where the roof was
cantilevered
out five feet or more and has, over the years, deflected to an
unsightly
dimension based on the weight of a tile roof originally calculated to
work.
I almost always try and get the HOA to allow me to cut back the
overhang
even if it is a good selling point. My argument is that it is unsightly
and
will continue to pose a problem when a layperson assumes it is a
structural
defect.
Dennis

-----Original Message-----
From: Gautam MANANDHAR [mailto:GMANANDH(--nospam--at)ci.alameda.ca.us] 
Sent: Monday, November 22, 2004 12:50 PM
To: seaint(--nospam--at)seaint.org 
Subject: Re: structures & ethics

Steve:

You mentioned about re-roofing. The live load was defiantly on the
structure when the roof was initially installed.  I am presuming
nothing
happened then; why should anything happen now?  If you feel that
adding
an extra layer of roofing could cause problems gravity wise, you could
instruct the owner to make sure he removes the current layer- although
I
have seen houses with more than one layer of roofing.

You stated:
"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?"

My question to you is - where do you draw the line.  What about
seismic
- does the building meet seismic requirements - collectors,
deflection,
hold down, etc.  I would presume not.  Do we limit ourselves to
gravity
design only or also include sesimc.

There are many buildings that do not meet code and , based on
engineering principles as we know it,  should not even be standing up. 

Consider a 1930-40 (??) era stick frame building.  The roof rafter
consists of 2x4, 2x6 spanning 14-16 feet; the ridge consists of a 1x
board; the hip beam are nailed to the 1x board; collar ties are
non-existent.   Based on wl^2/8 we use for wood frame design, there is
no way the roof should even be exsiting.  If one argues that the
moment
and deflection in the roof rafter are reduced due to a horizontal kick
at the sill plate level, one would expect to see damage in the plaster
at the sill plate level; you would be hard pressed to find one. 
Moreover, the vertical studs have hinges at the top and bottom -
another
no-no per code.  Now, if the owner comes to you to design a new
skylight
that fits in right between the rafters, would you recommend that the
owner replace the roof rafters because there is no way one can justify
the rafter design with the engineering principles we commonly use for
wood design.

I do understand your dilemma - especially considering the fact that
this is litigation loving nation.  However, you my find solace under
section 3403.2 of California Building Code.

Gautam

>>> mailbox(--nospam--at)sgeconsulting.com 11/22/2004 11:32:39 AM >>>
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 ----- 
  From: Gautam MANANDHAR 
  To: seaint(--nospam--at)seaint.org 
  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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