Full Snow + Live Load

[dmdblkeng]

We are having a debate whether to consider full live load + full snow load when designing a member carrying both the roof and the floor, that is 1.2D +1.6L + 1.6S. For example a beam on the second floor level supporting 2nd floor live load as well as the wall supporting the roof trusses. Any thoughts?

 

[KootK]

I vote yes. I think that the load combination already deals with the probability of simultaneous occurrence. You could apply live load reduction to the second floor based on it's tributary area of course.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[00Z]

I'm not sure what code your using but if memory serves the IBC has a load combination of 1.2D+1.6L+0.5S. I would go with the most demanding load combo found in the bldg code unless in your judgement circumstances exist that justify using a higher load.

 

[MacGruber22]

If we are talking American codes, definitely not. Ignoring dead load, transient effects are classified as either primary or not in each load combination. If the primary is floor live, then the load factor on the non-primary effect (snow, wind, eq.) is less than the maximum. The probability of 1.6L + 1.6S is well above the target reliability by ASCE and associated codes.

More simply, the basic load combination you show (with typical building load effects) doesn't exist in ASCE therefore it isn't warranted.

"It is imperative Cunth doesn't get his hands on those codes."

 

[Triangled]

I think of load combinations as relating to the likelihood of worst case scenarios. What is the likelihood of a full floor live load at a Christmas / New Year's party occurring simultaneous with a full snow load? ....shrugs....

 

[MacGruber22]

Shrugs

ASCE says there are no shrugs allowed!!

"It is imperative Cunth doesn't get his hands on those codes."

 

[KootK]

I'll refine my answer. I vote yes for designing for the code load combos. I vote no for making up your own combos unless there's a project specific rationale for doing so.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Canuck67]

For reference, the Canadian Code would look at the following load combinations: (there are other combinations to consider, but these are the most applicable to this situation)

1.25D + 1.5L + 0.5S
or
1.25D + 0.5L + 1.5S

whichever produces the most severe loading case. This would be considered the minimum allowed by code and sometimes judgement warrants additional loading be considered.

 

[PSEPK]

Following ASCE 7-10 Section 2.3.2 and in absence of actual load values, I would go for MOST CRITICAL of the following load combinations:

1.2D + 1.6L + 0.5Lr
1.2D + 1.6L + 0.5S
1.2D + 1.6Lr + L
1.2D + 1.6S + L
1.2D + L + 0.5Lr
1.2D + L + 0.5S

A good structural engineer is often a blessing for others.

 

[manstrom]

Interesting concept.

Basically - should we design for full snow on the roof and full live load on the lower floors? I would vote no. From a rational point of view, if you have 3 feet of snow on the roof, you won't have a party in the apartment too.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[mtu1972]

Because this member is supporting both floor and roof loads I would error on the conservative side. I agree with Canuck67 that the Code loads are the minimum. When you are looking at a roof member only, then the 1.6S + 0.5Lr might be more applicable. If SL is of any real serious concern, the alternative 1.6Lr + 0.5S is probably ridiculous. But the full Roof Snow Load and full Floor Live Load could certainly occur at the same time.

Having grown up in the MI's UP with lots of lake effect snow, that full Snow Load can be there for a long, long time.

Also, having worked with Allowable Stress Design for a good deal of my career we would have just added DL + LL + SL and designed for that total. That mindset has not been changed just because the Code's have been changed.

gjc

 

[MacGruber22]

When you are looking at a roof member only, then the 1.6S + 0.5Lr might be more applicable.

Me no understand - roof live load and snow?

I understand that the nominal loads are a minimum, but circumnavigating their combination doesn't compute for me. Local jurisdictions have always had a history of increasing nominal loads (particularly ground snow, minimum roof live, etc.), but when do you ever see them change their combination?







"It is imperative Cunth doesn't get his hands on those codes."

 

[steellion]

The OP is not talking about roof live load and snow load simultaneously, he is talking about floor live load and snow load simultaneously which absolutely could happen (the Christmas party was a good analogy). ASCE 7-10 load combination #3 has 1.2*D + 1.6*S + 1.0*L. I don't see a load combination which requires 1.6 x Snow and 1.6 x Floor Live simultaneously. Consider all ASCE load combinations and design for the greatest.

 

[MacGruber22]

Exactly, steellion.

There are certainly plenty of code provision which require a grain of salt when reading. Load combinations with well-defined and typical nominal loads is not one of them.

"It is imperative Cunth doesn't get his hands on those codes."

 

[PicoStruc]

I want to precise that in Canada, exterior area are designed only for the maximum effect between the snow and the live load. we don't combine them. We have a minimum of 1.0 kpa of live load for roof.

See clause 4.1.5.5.

So, the combination of 1.25D+1.5L+0.5S become 1.25D+max(1.5L, 0.5S) for roof.

 

[Stewie_]

Thank you, guys. I learnt sth.

 

[dmdblkeng]

Yes steellion, a christmas party scenario is what I'm referring to.

Thanks gents!

 

[canwesteng]

So an apartment full of Christmas parties at the same time as max snow load? Seems awfully unlikely to me. FYI max snow load is generally going to happen January/February as snow load is cumulative, so maybe a groundhog day party is more appropriate.