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Re: Shear in NDS

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You may be applying the 2.5 Ch adjustment factor which I believe was eliminated in the new code and may have never applied to glulams anyway.

If you have big cantilevers, usually the issue is unbraced length of the bottom of beam.

Also, I think the allowable shears were bumped up several years ago for several GLB combinations, so maybe RISA is using an old one.

Last, if you are using an old version of RISA, it may be a unit problem when using PLF instead of KLF.... and PSI instead of KSI etc...

-gm

On Thu, Feb 26, 2009 at 12:42 PM, Timothy Hilton <thilton(--nospam--at)lhcengineers.com> wrote:

Hello All,

 

I’m new on the list and I’ve been enjoying reading it.  Here is my question.

 

I’m working on an outdoor pavilion/shelter that covers three basketball courts.  We are framing the whole thing with Glulams.  We have a few places with long cantilevers and I’m getting shear to control my designs.

 

The question is this: I’ve analyzed the structure in RISA software and it has a feature where you input design values and it will perform design calcs.  As my members were failing in shear so much I started poking around. 

 

My controlling V is 19.256k for one beam.  My shape is 7”x27” (the program can’t handle fractional dimensions).  RISA’s reports seem to be pretty lame, so I can’t get to all the numbers, but it gives me a shear check of 1.906.  This is interesting because by hand I get a fv of 19,256/(7*27)*(3/2) = 152.83 psi.  I gave the program Fv of 200 and I’ve neglected the load duration factor for the time being, so I thought the check should be 152.83/200 = .764.  This caught my attention because it’s off by a factor of exactly 2.5. 

 

So all this to say, is there some 2.5 safety factor somewhere in the NDS that I’ve never heard of?  It seems extremely conservative to me.  Or have I done something stupid?

 

Thanks a ton,

 

 

Timothy K. Hilton, E.I.

Engineering Intern

 

LHC STRUCTURAL ENGINEERS

P.O. Box 10567

1015 Wade Avenue

Raleigh, NC 27605-0567

 

919.832.5587 x 20

thilton(--nospam--at)lhcengineers.com