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RE: factory-made roof trusses

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Roger,

It is true that very little research in the area of the Load Duration Factor
exists, but what has been done suggests that 1.6 is a conservative
assumption for the DOL factor.  My graduate research was on the cyclic
performance of MPC trusses and specifically the DOL factor.  When I
researched the basis for the 1.6 DOL factor in the code, I discovered that
there was very little backing up the claim.  1.6 DOL for wind and seismic
was based on the testing of sawn lumber (rather than connections).   To make
a long story short, my research raised the question of DOL for MPC joints
and showed that the use of 1.6 is conservatively correct (from a strength
standpoint).  The joints performed very well when exposed to cyclic loading
(from a strength standpoint).  On the other hand, their deflections and loss
of stiffness due to the cyclic loading was significant.  So, I would say
from a survivability standpoint using 1.6 is adequate, but I would also
guess that the structure would have some significant cracking in the roof
and walls and should be inspected following an event.  

Just my 2 cents on MPC joints.

Shawn

Shawn Wicks Freilinger
Valmont/Microflect
Shawn(--nospam--at)valmont.com <mailto:Shawn(--nospam--at)valmont.com> 
(503) 315-4515


	-----Original Message-----
	From:	Roger Turk [SMTP:73527.1356(--nospam--at)compuserve.com]
	Sent:	Saturday, August 01, 1998 12:09 PM
	To:	seaint(--nospam--at)seaint.org
	Subject:	RE: factory-made roof trusses

	John Jones brought up a couple of items that I didn't touch upon.

	. > there is usually a little box labeled something like allowed
stress 
	. > increase and in my area (Central Alabama) the box is usually
labeled 
	. > 1.15. The code does allow a repetitive stress increase of 15%
for bending 
	. > members provided you meet a few requirements. This is not
clearly spelled 
	. > out the way the little box is described on the output.

	This could also be a load duration factor (2 months cumulative
maximum 
	loading over the life of the structure), however, you're right, it's
not 
	spelled out clearly, like many things on their shop drawings.
Another thing 
	with respect to the load duration factor is the recent code
authorized value 
	of 1.6 for wind and earthquake.  I am still hesitant to accept a
value that 
	high, so I specify that the maximum load duration factor to be used
is 1.33 
	for wind and earthquake.

	. > On our drawings and specs we always require that the shop
drawings and
	. > calcs be stamped by a licensed (registered ->whichever it is
this week)
	. > engineer in the state that the project is taking place in.

	At a Tucson Chapter SEAOA meeting a couple of years ago, a person
who worked 
	or had worked for a fabricator in Arizona stated that the way the 
	engineering was done was the fabricator (who may have no engineering

	knowledge or experience) input the load information and truss
information via 
	a local terminal and a computer in California spit out the results.
There 
	was an engineer in California who was a registered Civil in Arizona
that 
	stamped and signed the output, but he was so busy, that he never had
time to 
	review the output; besides, the computer program was *so* good, he
didn't 
	need to.  The output was then faxed to the fabricator who then
submitted it 
	to whoever for review and approval.

	A. Roger Turk, P.E.(Structural)
	Tucson, Arizona