Residential Snow Load (USA)

[CrabbyT]

I recently branched out and started doing some freelance engineering. The majority of my projects have been residential and have involved wood framing.

With respect to snow loads, are we supposed to use the ground snow load as the design load? Or should I calculate it based on ASCE 7-10? Maybe I'm missing something, but the 2015 International Residental Code seems kind of unclear on this.

In steel and concrete design, I'm familiar with calculating the snow load in accordance with ASCE 7-10 and designing members based on their limit states. Residential design seems more like "determine span, determine ground snow load, look up member in a table".

 

[phamENG]

CrabbyT- first you have to understand the purpose of the IRC. It's to let non-professionals (i.e. anyone who can read) 'design' a house and build it. So yes, it's all based on tables and prescriptive rules. They are generally a combination of worst case scenarios, rules of thumb, and empirical design practices (it's worked for 70 years, so why change it? sort of stuff). So if you're doing a residential structure, you can certainly use all of that - it's easy and pretty fast once you know your way around. And you're right, for the roof you just need to figure out your ground snow load (ideally the municipality has filled out the table from R301 and all of your environmental design parameters are laid out on a nice little platter for you), your span, and pick the right rafter. But that doesn't mean it's using the ground snow load as the load - it means it's doing the calculation for ground snow load to sloped roof snow load and analyzing the rafter with that load for you.

But what if something doesn't quite work? The architect wants everything to align a certain way, but the tables are giving you sizes that are unworkable. Or the braced walls aren't quite cutting it. Or the wall is just a bit too tall and there's no entry for it in the table. Then you have to look back to Chapter 3 where it gives us an out - you can do whatever you want as long as it is in keeping with accepted engineering practice and designed in accordance with the International Building Code. So then you get to do it like normal - determine all of your loads and apply them, analyze the structure, size the members, etc.