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RE: Field Grading of Existing Wood Framing

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Jeff,

Generally speaking, what I understand is that the old lumber values were
arrived at by testing samples of clear, top-grade lumber and then
extrapolating that data to derive the allowable stress values for other
grades. After detailed testing of real lumber that had been graded they
found the actual stress ratings should be lower. Size factors may have
been adopted for the reasons you sited.

I think you're right. The grading rules for lumber have not changed that
much in the past 30 or 40 years and the lumber grades today are not that
different from what was available in the past. I have seen a lot of very
old lumber had very tight grain and was often milled as vertical grain
or free-of-heart centers. You don't get that kind of lumber from the
yard today unless you special order it.

I have been repairing older buildings for a few years now and we have
always used current allowable stresses for old lumber grades, not
allowable stress values from the code the structure was originally
designed under. You have to find a lumber grade stamp to verify the
grade before you use any allowable stress values. Otherwise you make
some assumptions based on the available lumber grading standards and a
visual inspection of the size and location of knots and other
imperfections. But that requires more expertise with lumber. The general
notes on original drawings may give you the lumber grades, but try to
verify they used that lumber on the building by inspecting the lumber
yourself.

Good luck,
Dave Gaines
(626) 794-4117 home
(626) 410-3631 cell
gainesengr(--nospam--at)earthlink.net

Photo Gallery at:
www.whaleshark.smugmug.com

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-----Original Message-----
From: Jeff Smith [mailto:jeffsmith7(--nospam--at)comcast.net] 
Sent: Monday, April 21, 2008 10:01 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Field Grading of Existing Wood Framing

David,

Your reply is consistant with some other sources I have reviewed. I have
heard of engineers using the allowables from the code they were
approverd under if the lumber was stamped, but that was a while ago. Old
lumber is not necessarily stronger than new lumber of the same grade. As
I understand it, the old grading procedures did not take into account
the degree of imperfections in lumber used in real conditions, they
tested smaller less representative sticks. It makes me wonder how many
buildings there are out there that may be subjected to failed members. I
have been looking for a code section or document that states that
current NDS values need to be used for old lumber, do you know where
this is?

Thanks for your reply,

Jeff


-----Original Message-----
From: Gaines, David [mailto:David.Gaines(--nospam--at)hdrinc.com]
Sent: Thursday, April 17, 2008 3:53 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Field Grading of Existing Wood Framing

Jeff,

If the joists have split at the knots, repair them by sistering on new
LVL or PSL beams on both sides. Shore up the damaged purlin before you
attach the new wood. If there are other purlins with  weak points that
are susceptible to cracking or failure, recommend that they be repaired
by the same means as well. It's a lot cheaper to repair them before they
crack. The new lumber can be bolted on to the 4x112 purlins below the
2x4 or 2x6 subpurlins.

Recommend to inspect as much of the roof as you can get to and itemize
the areas you can not inspect. You only need to upgrade the purlins and
beams you are adding loads to or remodeling in. Write a careful Scope of
work outlining what you're offering to do and what you're not covering.

By re-grading the existing lumber you are not likely to improve on a
Select Structural grade (nothing better than SS) and the allowable
lumber stress values for any grade will be lower today than what they
were in 1977.

If they're adding TI loads to the original, 1977 beams you need to
evaluate them by the current code, not the 1977 code. Current lumber
grading values have been revised to lower values and so most of the
purlins you check will not work with the original loads, much less
additional loading. You'll need to reinforce the structure anywhere they
modify, remodel or add loads. Check your beams and girders in areas that
may be affected by added loads too.


Dave Gaines, P.E.

Structural Project Engineer
HDR ONE COMPANY | Many Solutions
251 S. Lake Ave, Suite 1000
Pasadena, CA 91101
T: 626.584.4960
F: 626.584.1750
email: david.gaines(--nospam--at)hdrinc.com 


-----Original Message-----
From: Jeff Smith [mailto:jeffsmith7(--nospam--at)comcast.net]
Sent: Thursday, April 17, 2008 3:10 PM
To: seaint(--nospam--at)seaint.org
Subject: Field Grading of Existing Wood Framing

I am looking at a 70's office building that has 2 cracked 4x roof
purlins out of maybe 300. There is a knot in the tension zone and
looking around the building there are several other beams that also have
knots in the tension zone. It is stamped Sel Str. The tenant
improvements will add some additional load to the beams. Based on an
analysis using allowables from the
1977 UBC these beams are adequate. Do you think hiring a grader would be
a good idea and once graded would it be appropriate to use the design
values from the 1977 UBC, can a grader make that decision?

Thanks,

Jeff

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