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Re: Construction method for Second Story Additions

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>At 09:32 12/3/97 -0800, Norb Volny wrote:
>> .... You'll also find that
>>the older grading rules and published model codes show that the allowable
>>stresses in many types of lumber available 40 to 60 years ago were higher
>>than they are now. The lumber available and used then was of a definitely
>>higher quality than that used now.
>[Bill Cain]  I agree that the lumber in older structures was generally of
>much better quality than now, but...
>Allowable stresses for a given quality of lumber have also been reduced
>over time recognizing failures that have taken place, particularly in
>members (such as collar ties and trusses) subject to tension loading or
>bending members subject to a large portion of their capacity (e.g., widely
>spaced rafters with many layers (too many to meet the code in some
>instances) of roofing material added).  It is important to use quality of
>the old lumber from observation and/or formal grading along with current
>knowledge and methods to evaluate what stress should be used rather than
>relying on the allowable stresses from old grading rules and codes.
        Thanks for the additional comments on lumber and its allowable
As this project of Kate's is obviously a residential structure that will be
rebuilt from the top plates up I didn't mention the decrease in allowable
tensile stresses due to relatively recent new knowledge. If the structure
had been at timber bridge or large industrial building it would have! Its
good to have second and third "inputs" on materials as complex as timber.
Even though I'm not a timber grader I've found the West Coast Lumber Assn's
grading rule books usfull adjuncts to my "engineer's judgement" when
visually inspecting highway bridges and industrial and residential timber
trusses. Not every 2x that is stamped with a grade mark is absolutely that
grade.! Its obvious to me as an engineer that the most important part of a
project involving a remodel or an existing structure is a thorough visual
inspection of said structure and all its material elements. If the visual
inspection reveals some questionable elements then the questions should be
answered before continuing design. Timber is a complex material and we've
all seen that steel is much more complex that most of us thought under
dynamic loading. Timber reacts to moisture and the ambient relative humidity
in climates and structures in ways that are well understood but forgotten by
some of us until we attempt to restrain its movements due to climate
changes. If trusses are to be used in the new roof structure that are timber
then an awareness of emc of the timber as manufactured versus as intended
for use is absolutely essential. 
        Norb Volny, SE
        Bend, Or