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RE: Historic Wood Structure; How to Determine Allowable Stresses

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Your comment about the addition of 1.5" concrete is perhaps more of an
overall problem. This would add significant DL which would tax the existing
lateral system (URM?). If that is the case, you might want to explore other
options with less DL impact.

I don't know the age of your building but as you may know, existing lumber
was generally of a higher quality that what is used today. The "exposure"
factor certainly adds to the uncertainty.

The girder concerns are valid however remember allowable horizontal shear
was significantly higher in lumber earlier. (For instance the 1956 WCLIB DF
Dense select structural Fv=120 psi) See if you can get historical values for
the species.



Barry H. Welliver
barrywelliver2(--nospam--at)earthlink.net
 

-----Original Message-----
From: Robert Rogers [mailto:RRogers(--nospam--at)lorwil.com] 
Sent: Friday, April 25, 2003 9:45 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Historic Wood Structure; How to Determine Allowable Stresses

Barry,

I guess the tough part here is two-fold ....

Load history

who knows ?...although identified as light manufacturing I have no
design basis information on what the allowable loading was at the time
of construction or anytime thereafter, the building is currently vacant
....3x14 Nom. Joists are at 14" o.c.,  are spanning roughly 18 feet, a
concrete topping (1.5" thick) is being added which increases the dead
load and although live loads are going down in some areas (manf. to
residential / office) it stays up in corridors (100 psf) and lobbies,
etc...additionally, shoring has been installed in the basement at
midspan which tells me they had trouble with deflection (because I
believe equipment used to be "wheeled" around on the first floor)... 

I'm not so concerned about the joists because I have redundancy
(repetitive members)...the girders & columns are the scary part.....when
I look at the most heavily loaded girder, spanning ~ 14 feet, if I
assume a mixed oak timber, select structural, I'm at about 80% of its
allowable stress.....if I assume a No. 2 I'm 40% over allowable....

Minimum species & grade

I'm not sure how you can assume a minimum grade when a building has been
subjected to the elements and left unmaintained....if the members were
not subject to water infiltration, insects, etc. I think this would be
okay.....and even the lumber that doesn't show signs of obvious
deterioration (fungi, wet spots, etc), I would think you have to
consider checks, splits, etc. which only a "grader" or someone familiar
with "grading" would be able to do.....

Am I off base here...?

Robert


-----Original Message-----
From: Barry H. Welliver [mailto:barrywelliver2(--nospam--at)earthlink.net] 
Sent: Friday, April 25, 2003 11:02 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Historic Wood Structure; How to Determin


I'm entering into this discussion late and admit to not having read all
the posts but here are my comments anyway............

The ASCE publication "Evaluation. Maintenance and Upgrading of Wood
Structures" 1982, is a good document if you do this kind of thing. The
general advice there is to use perhaps 90 % of the normal design
stresses when the loading history is changed. The recommendation is to
use the original allowable design values under the code in effect at the
time of construction. I realize this may not help in determining the
grade, but I'd go along with others regarding involving a testing agency
if needed. 

There are however, a couple of other ways to look at this. The 1997 UCBC
seems to support the assumption that all existing lumber will at least
stand up to an allowable stress level of DF No.1 (per table A-1-D
Allowable Values for Existing Lumber). This continued support also
occurs in Table 1-D of the GSREB. All this was stated by Roger below and
I agree with his experience and logic. I'd be inclined to do the member
stress checks first to determine if there is a need to pursue additional
testing.

My experience with framing systems such as you describe is that they
have a good deal of capacity when the live load drops significantly. 

Barry H. Welliver
barrywelliver2(--nospam--at)earthlink.net
 

-----Original Message-----
From: richard lewis [mailto:rlewistx(--nospam--at)juno.com] 
Sent: Friday, April 25, 2003 8:30 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Historic Wood Structure; How to Determin

Robert,

In the past I have sent samples of wood off to be analyzed.  You only
need a very small sample.  I believe it was to the Forest Product Lab. 
I'm not sure about that.  I haven't done it in several years and the
files for the projects I did do it for are at another engineering
business.  I recall that the FPL would do up to 5 samples a year free of
charge.  They just give you the species of wood, not the allowable
stresses, etc.  You have to come up with those based on your engineering
judgement.  You have to wait on them though, it is not a speedy process.

I would say that you also need to use your judgement about the damage to
the wood verses reduction in stress.  Hey, that's why your getting the
big bucks!  I recall a book by ASCE written at least 20 years ago about
restoring old wood structures.  You may look for a copy of it.  Again,
it is at the former engineering firm so I don't know the exact title.
Is your project big enough to warrant calling in a professional grader
to give you guidance on the grading of the wood in place?  

Rich



On Fri, 25 Apr 2003 08:53:05 -0400 "Robert Rogers" <RRogers(--nospam--at)lorwil.com>
writes:
> 
> Did I mention the building was constructed around 1918 ?....would
> you
> still use the same approach ?
> 
> Additionally, how would you take into account possible weathering / 
> exposure damage ?
> 
> Robert
> 
> -----Original Message-----
> From: Roger Turk [mailto:73527.1356(--nospam--at)compuserve.com]
> Sent: Friday, April 25, 2003 8:37 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Historic Wood Structure; How to Determin
> 
> 
> Robert,
> 
> Prior to 1970's, the lowest grade of structural wood was
> "Construction 
> Grade."  Comparing the visual grading rules of 1960's "Construction
> Grade" to 
> today's grading rules, these are equivalent to today's No. 1 grade,
> therefor, 
> I would use the allowable stresses for today's No. 1 Grade to 
> evaluate
> the 
> structural capacities of the existing members.  (Also, I believe 
> that
> the 
> UCBC states that No. 1 grade stresses be used.)
> 
> HTH
> 
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona
> 
> Robert Rogers wrote:
> 
> . > A question for all of my colleagues.......looking at an
> existing
> brick . > masonry shell & interior wood joist, timber girder, 
> timber
> column . > 4-story structure......roof was off top of structure for 
> 3~4
> year time . > frame and some water damage has occurred.  Joists are 
> 3x14
> @ 14" o.c. . > and timber girders are range from 10x12 to 14 x16 and 
> the
> columns range . > from 10x10 to 14x14.  Floor dead loads are 
> increasing
> but live load is . > decreasing (used to be an old light-industrial
> building; being converted . > to "lofts"; loads added to roof).  
> Joists
> span ~19 feet, girders span . > ~14 ft. and column height ~12-13 
> feet.
>  
> . > Material testing company performed visual / sounding of wood
> members
> and . > provided recommendation on replacement of specific members 
> but
> refused . > to provide any allowable stresses for the wood members.
> Material . > testing company provided species type but would not
> identify grade . > (thereby "locking-in" allowable stresses per NDS
> supplement for use in . > structural analysis).
>  
> . > How would any of you approach this situation to try and get a
> handle
> on . > the amount of strength of the existing wood joists, wood 
> girders,
> and . > wood columns (i.e, pin-down the allowable wood stresses) ?  
> I've
> spoke . > to a few "experts" and have their 
> recommendation......what
> would the . > everyday structural practitioner do ?
>  
> . > Robert Rogers, PE
> 
> 
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