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RE: factory-made roof trusses

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Shawn raises a couple of points that I would like to comment on.

It seems that much research nowadays is to justify something rather than to 
find out the truth.  It also seems that the results of research conducted at 
only one institution is used to change codes.  What ever happened to the 
requirement that the results of *any* research must be independently 
duplicated in order for the results to be valid?

I commend Shawn on recognizing that the research that he did raised questions 
about the validity of the DOL factor.  Research that shows what *doesn't* 
work is more important than research that shows what does work.

There is also another thing with regard to wood that *badly* needs research 
and that is the long term effect temperature on the capacity of wood and 
connections.  In the middle '80's I investigated the collapse of large 
parallel chord split ring connected wood trusses in which the split ring 
sheared the end off of the highly stressed end diagonal.  The trusses spanned 
60 feet, were about 7 ft deep spaced 10 ft on center, and had been in place 
for about 30 years.  Analysis of the trusses using both the code in effect 
when they were built and the code in effect when they failed showed that they 
were well within their capacity under dead and live load.  Dismantling a 
truss that did not fail showed that the end, edge, and spacing distances of 
the split rings were in conformance with the code.  However, at one joint, 
the core of wood within the split ring came apart with the split ring.

The trusses were installed in a well ventilated attic here in Tucson.  The 
only thing that I could attribute the substantial loss of strength to was the 
long term cumulative effect of elevated temperatures.  Codes and references 
state that the loss of strength from temperatures between 68 deg and 150 deg 
is "probably" reversible, but the loss of strength from temperatures above 
150 deg is "probably" permanent.  I think that more research into the effect 
of temperature in the 68 - 150 deg range is sorely needed.

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

Shawn wrote:

. > 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) <mailto:Shawn(--nospam--at)> 
. > (503) 315-4515
. >