ASD LOAD COMBINATION OR LRFD COMBINATION????

[keenkeem]

I am currently working on investigating a truss using AISC code. I am confused whether what type of load combination to use. Whether to use ASD load combinations or LRFD load combinations or even both. WHAT TYPE OF COMBINATION SHALL I USE ANYONE????

 

[structSU10]

Whichever design method you feel most comfortable with. You use the safety factors in the code that correspond to ASD or LRFD when you design the members, so it becomes a preference thing.

 

[SteelPE]

If you are asking this basic question, maybe you should asking your boss what method he prefers.

 

[mtu1972]

If you are investigating an old truss, it was probably designed with allowable stresses. Not quite the same as ASD, but more similar than LRFD. That would be more of a comparison of apples to apples. Then once you understand where the problem areas are, it would be OK to re-analyze it with LRFD as it may provide some benefit to you.

If you are looking at this truss, it must be because something is or has changed - more loads, etc. If it is a retrofit, then the current Code's loads, etc. should be used with either analysis method being acceptable.

gjc

 

[keenkeem]

Just to be clear... I am not designing, i'm investigating. Turns out you can use both??

 

[structSU10]

Yes, either design method can be used. As stated above, ASD was used for older stuff, so it may be a good idea to stick with that approach for the investigation. LRFD can sometimes lead to a smaller stress ratio when its all said and done, so it could be helpful.

Note also that todays ASD is very different than earlier ASD methods. Really ASD and LRFD with respect to the steel code only differ in the way load factors and safety factors are applied - they typically give results within a few percent of one another.

 

[steellion]

Regardless of which method was used to originally design the truss, if you are investigating an existing truss, you can check with either. If it's an older truss, you can probably squeeze a little extra capacity out of LRFD if you are trying to show a truss meets strength requirements. ASD should always be used for service limit state checks such as deflection.

 

[JedClampett]

For high ratios of live to dead load (>3.0), ASD will give a overall lower load factor. For lower ratios, LRFD will give you a smaller overall section (in your case a higher safety factor).

 

[TDIengineer]

Agree with Steelion. I would start with LRFD check as you typically can get a bit more capacity out of your element using that method. And has Jed has pointed out, the ratio of your dead load to live load will determine which method yields more capacity.

If you are checking a roof member, I am typically not shy about maxing out the member as I really feel that getting a full 20psf live load on a roof member is not likely unless someone has stacked a bunch of materials in one spot, or there is a party on the roof.