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Is it too conservative to include both snow and live load?

Is it too conservative to include both snow and live load?

Is it too conservative to include both snow and live load?

(OP)
Hi all,

I am using Forte to size my beams and I found that the program is considering dead load, live load and snow load at same time with the specific load factors (1.25D + 1.0L + 1.5S). Isn't it too conservative in that way?

CWW

RE: Is it too conservative to include both snow and live load?

Not if they can all exist at the same time.

Edit: They probably aren’t holding square dances up there at that time of year, but you will likely find some duffus shoveling snow out of a valley, or from a drift, into a 6’ high pile, 15’ in dia. before he gets close enough to the roof edge to throw it off. They just don’t very often walk every shovel full over to the edge of the roof and throw it off.



RE: Is it too conservative to include both snow and live load?

Guidance over this side of the pond suggests that it is too conservative and notes that snow+Roof imposed do not need to be combined. Nor does full wind+Roof imposed.

Wind+snow and wind+drift are however.

RE: Is it too conservative to include both snow and live load?

Yes.

Mike McCann, PE, SE (WA)


RE: Is it too conservative to include both snow and live load?

The engineer should decide that...and NOT the program

RE: Is it too conservative to include both snow and live load?

Let's start with more info. What code? What is the beam supporting?

RE: Is it too conservative to include both snow and live load?

In the IBC code, L is defined as live load except roof live load. Lr is defined as roof live load

The load combinations in the IBC code don't require Roof Live load and Snow load be applied at the same time.
However, you do have to apply live load and snow load per the load combination you noted.

For example, if you had a floor beam with a post supporting roof framing, you would need to consider live load and snow load.

RE: Is it too conservative to include both snow and live load?

In general ASCE7 does not require roof live load and snow loads to be considered simultaneously - BUT there are exceptions. If you have floor framing suspended from the underside of the roof, then snow and live loads must be considered simultaneously for design of the roof framing members. Similarly, we had a theater several years ago where there were catwalks and platforms suspended from the underside of the roof framing. We had to consider snow and live loads acting simultaneously.

RE: Is it too conservative to include both snow and live load?

Now I'm going to assume National Building code, based on the fact you appear to be out of BC.

I believe the 2015 code allows you to not account for roof live and snow acting simultaneously, however if you have a beam supporting second floor and roof loads, then you would need to account for them concurrently.

Forte doesn't know whether you have pt in roof live load, or floor live load, and therefore must use your inputs, and the code prescribed load combinations. If you only have roof live and snow, then you need to run the beam separately, one with the roof live, and the other with the snow.

RE: Is it too conservative to include both snow and live load?

In college I worked on a construction crew. One winter there was a ton of snow and a local manufacturing plant hired us to clear their roof on an emergency basis. There must have been 20 of us "doofuses" up there with shovels and snowblowers clearing snow grid by grid. I remember the owner nervously commenting that the roof joists were noticeably deflecting and our foreman telling us to just shovel faster! We ended up getting hired that summer to reinforce the joists and columns.

Even this roof didn't collapse, another crew from my company was shoveling a Walmart distribution center roof around the same time and were up on the roof shoveling when a few bays collapsed. I do recall that the main culprit was heavy snow followed by a soaking rain that increased the weight. Roofs were collapsing all over New England that year and we got a lot of reinforcing/retrofit work the following year.

All that being said, I still wouldn't necessarily combine roof live + snow for designing a roof beam. But in a certain climate that gets snow and rain events, with a flat roof that will likely require a big crew for shoveling, maybe I would make an exception there.

RE: Is it too conservative to include both snow and live load?

If all your extra roof capacity is used up in the hanging process piping,then you better consider both together.

Or maybe there's a big HVAC unit which is causing a heretofore ignored snow drift.

Or maybe it's a flat roof where your excess capacity is sapped by ponding loads.

The Engineer of Record gets paid the big bucks to figure these things out.

RE: Is it too conservative to include both snow and live load?

JLNJ:
When did “big bucks” become part of the Structural Engineer’s modus operandi? Mostly, they have become the project goat and insurer of last resort.

RE: Is it too conservative to include both snow and live load?

I have always designed in agreement with jayrod and IceNine’s approach.

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