LRFD vs. ASD

[palvarez83]

Hi guys and gals,

T minus 10 days for my exam. I'm pretty much done studying. I read the Hiner book twice and have done the problems several times. I grasp everything except LRFD/SD vs ASD.

Can someone provide me a layman's way of understanding this as they relate to the IBC? From my mechanical engineering days I remember that if you take the ultimate strength of a material and divide by a selected factor of safety you get your allowable stress. My current understanding of LRFD is that you take the actual loads and apply a factor to them and your ultimate strength has to be greater than that factored load.

However, the load combinations of the IBC don't seem that straight forward. What am I missing conceptually?

I've read various thing online such as this:

http://on.dot.wi.gov/dtid_bos/extranet/structures/LRFD/Training/LRFDvsASD_LFD-JerryD.pdf

Yet, that doesn't tell me how to relate it to the IBC. From working problems in the Hiner book, it appears that Strength Design is most common, for Earthquake loads it appears that in general ASD = 0.7*LRFD. Are most the equations (e.g. base shear, ect.) written to provide a LRFD/SD?

I think I can navigate my way through the problems enough to pass, but that's not the point. I'd like to have long term understanding....

Thanks in advance for your help.

 

[Lungshen]

Hi Palvarez,

Strength Design, or Load Factor Resistance Design is you mulitiply a load factor to your service load, thus called a "Factored Load". You then multiply your Nominal capacity by a reduction factor, then you compare the two. Your reduced nominal capacity needs to be greater than your factored load. Seisimic load is based on Strength design - mean that whatever load you came up by using the equations from IBC or ASCE7 are a "factored" load already built in from the equations.

Servie load design, or Allowable stress design is you divide your nominal capacity by a "safety factor" ( usually a safety factor of 2 or 3 or whatever per code), and then compare that to your actual service load. The beauty is that you don't have to adjust your service load so that it is one less step in calculation and a lot of times you can do the math in your head. Most structural engineers stick to ASD because it is quicker and easier to understand.

I learned steel design in LRFD in college. Learned ASD in 30 minutes first day of my job out of college and never go back to LRFD. because its quicker and easier.

Wind load, snow load, floor live loads are given in service load condition -

Concrete is in LRFD per ACI. Masonry, timber, steel are mostly design in ASD so if you are design masonry, timber or steel members for the Seismic load, you need to make sure you convert the strength level seismic down to service level so that you are compare apple to apple, orange to orange and not penalize your design by oversizing the members.

Look at the Load Combination under LRFD Section 2.3 and ASD section 2.4 in ASCE or IBC, you will see it.

For LRFD, floor and roof loads and wind loads are multiply by a load factor, because they are given in service load. But the load factor for Seismic is 1.0 meaning no factor applied to it, because it is given in strength level and already "built in" from the equations. Now tell me you can remember 1.4, 1.2, 1.6, 0.5, 0.9 all these different factors?

For ASD, all floor, roof, snow and wind loads are multiply by 1.0 - meaning no factors to the load combination, but there is a 0.7 factor to seismic - to reduce it back down to service level.

Hope that makes sense.

 

[palvarez83]

So does nominal capacity correspond with ultimately strength of the material?

 

[Jayman_PE]

That's where Allowable Stress (ASD) comes in - in my mind a more direct way of converting from nominal to allowable and visa versa. ASD is the best way to knowing just how far you are pushing the structure. I'm sure the LRFD fans are going to bombard me, but that's just based on my experience.