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RE: allowable stresses for wood floor joists - Phila. PA

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It was not uncommon for the “old wood” of 75 to 100 years ago to have an

Allowable bending stress in excess of 1500 psi, regardless of species.


The younger “harvested” lumber of today simply does not get a chance to age enough

To develop the strengths the old growth timber did throughout the section…


Hence, the propensity of laminated and engineered wood products these days…




We’ve done a considerable amount of work with old structures (loft conversions of

Old mill buildings, etc…) to the point where the City of Chicago has made us in

certain instances, retain a testing entity to visually grade and then tensile test the specimens.



The tests typically returned values of 1500 psi or more.



David L. Fisher SE PE

Fisher + partners

372 West Ontario

Chicago 60610



312.573.1726 fax


312.622.0409 mobile

From: Ken Peoples [mailto:kspeoples(--nospam--at)]
Sent: Monday, March 28, 2005 11:17 AM
To: Seaint
Subject: allowable stresses for wood floor joists - Phila. PA


I have been asked to evaluate an existing wood-framed structure (with brick bearing walls) in center city Phila (Chestnut Street).  The existing wood joists are approximately 3" x 11" spaced at 16" to 22" on center and spanning about 21'.  I don't know the date of construction, but it would not surprise me if the floor framing is over 75 years old.  Since they would like to use the upper floors for offices, I will need a minimum live load of 50 psf with corridors being 80 psf and file rooms being higher.  From what I can see of the joists, they appear to be nice wood - free of knots - but I have no idea what type of wood it is.  Preliminary rough calcs indicate that the capacity is borderline in most areas with an assumed allowable bending stress of 1200 psi and sub-standard in other areas.  I have never gone through any wood testing to determine allowable stresses and wondered if any of you have and would recommend it or should I just assume conservative values and strengthen as required.   The building will be gutted, so strengthening will not be impossible or extremely disruptive.  There are three elevated floors plus a roof. 


Thanks for your help.


Best regards,




Kenneth S. Peoples, P. E.
Lehigh Valley Technical Associates, Inc.
1584 Weaversville Road
Northampton, PA 18067


Phone: 610-262-6345
Email: kpeoples(--nospam--at)