Ice Load Combinations variance between ASD and LRFD

[MNach]

Has anyone ever ran ice load combinations in both ASD and LRFD? I am analyzing an existing structure that had a pipe rack added to the roof. The original design had 30psf Lr where as now code only requires 20psf. The current loading has 11psf more DL due to the pipe and over 13psf more of ice load. When comparing the original loading to the current (see attached PDF), the ASD combination spits out a way different max loading than LRFD. ASD, as expected, says the loading has increased by about 10%, whereas the LRFD combinations are saying the original design without any of the pipe or ice loads is higher. Does anyone know why the ice loads are applied so differently between the two? To me ASD makes more sense because load combination 3 adds most of the ice load with the snow load. However, LRFD load combination 3 applies a 1.6 factor to Lr or S but only applies the ice load to combinations 2 & 4 which then have .5 factors on Lr and S not the 1.6 factor. It's almost like they forgot to add the ice load to that combination. Do we think this is an intentional omission? It is quite the difference as one combination says the structure is overloaded and not to code, whereas the other says I can still add the 5% to the existing.

Thanks for your thoughts.

-MJ

 

[jimstructures]

What Standard/Code are you using to come up with the Load Combinations you are using?

How old is the existing building and what Code or Standard was used to come up with the existing loads?

It was common in some areas to include the Roof Snow Load and call it the Roof Live Load, Lr.

Jim

 

[MNach]

The current code I am using for the load combos is the ASCE 7-10. I gathered the existing loads from some hand calcs the precastors originally did for the extra pipe loading. Regardless of the existing loads though, ice is applied very differently between ASD and LRFD. With LRFD how the ice and snow loads are factored when combined, they essentially never control unless they are very large, whereas with ASD the ice and snow combined control very easily.

 

[dauwerda]

The commentary gives a little more insight and is consistent with what is in the body of the code so I assume it is intentional.
My thought is that the likelihood of full design atmospheric ice and full design snow happening at the same time is pretty slim as these conditions are typically caused by different storm cycles which would be in line with the LRFD load cases. But, if this is the case I'm not sure why the ASD load cases would consider full snow and ice together.

The IBC defaults to ASCE 7 for ice loads, so no help there either.

 

[r13]

There is no straight line comparison exists, you have to evaluate each case using the specified combinations.