Using LRFD with combined ASD loads

[DTS419]

What do you do when you must design concrete per ACI 318 using LRFD but your given loads have been determined in ASD with the live load and dead load combined? For example, you are a supplier engaged to design a concrete foundation but the EOR who is providing you the loads has designed the building in ASD and did not separate dead and live. It's not in your scope and wouldn't be feasible to determine all of the loads throughout the building yourself, and ACI no longer has Working Stress Design provisions. You could conservatively factor the whole load as a live load and end up with an inefficient design, or...

 

[phamENG]

Or submit an RFI to the engineer explaining that the ASD combined loads cannot be used in a code compliant manner to produce an efficient foundation.

I have to say, though...your situation is backwards from everything I've ever seen. As the EOR, I'm usually stuck checking a footing from the supplier's engineer's ASD combination...

 

[271828]

Ask the EOR for separate D and L loads or LRFD required strengths.

Or conservatively multiply their number by 1.6, as you mentioned.

 

[Eng16080]

Well, I would first ask the EOR for the separate dead and live loads. It should not be a tremendous effort for them to provide that!

In the absence of that, I know some engineers will use a factor of 1.5 to convert from the D+L ASD combo to the 1.2D+1.6L LRFD combo. Personally, I would conservatively assume the whole load is live and potentially "end up with an inefficient design." After asking the EOR nicely a few times for this information, if that fails, I would then make it clear to the necessary parties that the design will likely be inefficient due to the EOR not providing the actual loads.

 

[DTS419]

Evidently my theoretical example wasn't a good one... try this... You are pre-engineering a concrete member per ACI 381 to determine the allowable load through a range of variable dimensions for your client who wants a load table showing the allowable load for each span and depth, much like we see for K joists. How do you determine an allowable total load that includes both dead and live?

 

[DTS419]

...or put another way, you are going to determine w-sub-u by analysis- how do you present that as a practical allowable load?

 

[milkshakelake]

For the thing you're designing, you should have a rough idea of the dead and live loading it will be subjected to. For example, if it's a slab for residential, you know that the live load will be between 40-100 psf and the superimposed dead load will be between 5-25 psf. Knowing these low end and high end bounds will help you calibrate your factored load for the worst case scenario.

The extreme end of your liability will be assuming minimum live load and maximum dead load, resulting in an unconservative design. The other end of minimal liability is to assume 100% live load. Your comfort level and engineering judgment would be able to determine where you land on the spectrum. It also depends on how often this thing will be used, which affects exposure to liability and possible ethical ramifications of protecting the public. Is this thing used in one place, in about 20 members in one structure, or being used nationally?

In any case, I'd document the assumptions you'd made so that the user understands how you made the ASD conversion. Even if they don't understand, and use the table blindly, at least you protected yourself.

 

[phamENG]

much like we see for K joists. How do you determine an allowable total load that includes both dead and live?

That's just a capacity check, which is easy. Load combinations don't matter. You have a load table with spans and maximum factored load. Boom. Done. Whoever uses that table then has to combine the factored loads and compare it.

The tricky part, since it's concrete, is accounting for long term deflection. That would be a judgement thing, I think - either go crazy conservative (all of it is long term loading) or come up with a reasonable balance and state the limitation on the table, or come up with a simplified expression that can be run quickly and easily from a footnote on the table based on the short term/long term split in the loading.

 

[Once20036]

If you look at any joist design table, you`ll see that the ratio between ASD and LRFD is always equal to 1.5.
I believe that this is because, in short, LRFD reduced capacity by 0.9 and ASD reduces by 1/1.67.
1.67 * 0.9 = 1.5.

After that, the challenge with joists is how to split into DL & LL, but they leave that entirely to the user.
ACI doesn't provide that 1.67 ASD factor, but a similar concept could be applied.