ASD/LRFD Wood Design

[JMC3178]

Excuse me for being behind the ball a bit. I have just finished up all of my 2001 CBC work and now beginning to look at the new code requirements a little deeper. As I understand it, ASD wood design hasn't changed any (i,e, fa/Fa and fb/Fb), and LRFD is a similar fashion just added adjustment factors.

Now steel is a different animal where ASD is similar to LRFD in the sense that the CSI check is based on M/omega correct?

The reason for my questioning is I have started checking a couple of wood beams with a particular software, and it utilizes an ASD CSI check of Mult vs. M nominal /omega. I am unaware of an omega factor for bending or shear for ASD wood. Am I off base with my questioning? Can you check wood beams using a Moment over omega method, and where is the omega listed in the NDS?

 

[RARWOOD]

I am having a hard time following your question because of the abbreviations you have used. I am familair with ASD wood design and have a small amount of knowledge of LRFD wood design. I don't know what you mean when you say that your program uses an ASD CSI check of Mult verses M nominal/omega.

It appears to me that, the people who write codes and text books are going away from calculating stress and comparing it to an allowables stress even for ASD applications. Instead the trend seems to be the calculation of the member's capacity so that it can be compared to the load requirement.

I've also seen examples where the Demand Capacity Ratio is calculated dividing the load by the member capacity. Your program may be doing something like that.

 

[jdonville]

JMC,

There are no omega values, as far as I can tell, in the 2005 AF&PA NDS for ASD. The code (includes LRFD as well) is cheap enough ($100-$200, including supplemental references) to get a copy of it for little pain.

Jeff

 

[JMC3178]

Sorry for any confusion. CSI is the interaction between allowable and actual. I do own the NDS and I have gone through it quite well now. I am just questioning the use of the omega factor in ASD wood by the program manufacturer. I have questioned him and he claims it is an acceptable method of calculating the stress in the beam.

 

[RARWOOD]

I checked with Jeff Linville who is the AITC Technical Director he indicated that the only area he thought an omega factor might be used in wood design is for seismic design. He also forwarded my question to Phil line at AFPA.

The conclusion is that it would only apply to seismic design. Neither Jeff or I are Seismic experts, however.

My guess is that it has something to do with the way Seismic forces are calculated today. I think the rational goes something like this, if you calculate your seismic forces using ASCE 7-05 then you must provide, for timber frames, 1.5 times the capacity required to resist those seismic forces.

To me it makes no sense, if you want an increased safety factor then reduce the allowable stress. To me it appears more like LRFD advocates trying to drag every one into their methods.

 

[JMC3178]

Actually its another omega factor. This one is unrelated to seismic. As teh 13th AISC states the check for flexure is Ultimate Moment vs. the nominal moment over omega sub B. This omega value is always 1.67 Should I stop using the wood module of this program because it incorrectly evaluates the bending stress of an ASD wood beam?

 

[RARWOOD]

I would ask the program provider, what design standard they are using. When designing a wood beam subjected to bending, using ASD, you use unfactored loads to determine your moment and then compare that to the capacity provided by your member.

Regarding the 1.67 factor are you indicating that your program use that for wood design or only for steel.

 

[JMC3178]

I have asked him which standard they are using, and he claims 2005 NDS. He is using the 1.67 (i'm assuming) for both wood and steel. Its a fairly popular program that alot of people on here are using.

 

[RARWOOD]

I would tell the person that they need to contact AFPA and talk to some one who is an expert on the NDS. I would stop using the program for wood design.

I know very little about the new ASD steel design code. However one post I've read indicated that ASD steel is just another version of the LRFD approach.

I think before long, with the every changing codes, you won't be able to build anything out of wood.

ASD in wood design is still a true ASD method.

 

[jdonville]

As someone using both AISC and NDS, I've only seen the omega factors used for ASD for steel. This is likely due to the fact that steel is an engineered material and wood isn't.

Jeff

 

[JMC3178]

Well it seems that my assumptions are right then. Can someone else verify that Enercalc is doing wood beams incorrectly?