Residential Deck Loading - Snow + Live?
Residential Deck Loading - Snow + Live?
(OP)
I've been having an ongoing disagreement with a fellow engineer. Allowable stress load combinations in the IBC (require Dead + Live + Snow be considered. My associate says that combination does not occur, so does not design for it. I say he is violating Code.
Do you typically combine snow and live loads when designing exterior residential decks?
Thanks
Do you typically combine snow and live loads when designing exterior residential decks?
Thanks





RE: Residential Deck Loading - Snow + Live?
Yes, per the code - the D+L+S combinations REQUIRE the addition of snow plus live loads....they DO occur simultaneously...its just that the snow gets crunched down when you walk or drive over it.
You just don't add Roof Live Load (Lr) and Snow (S).
RE: Residential Deck Loading - Snow + Live?
RE: Residential Deck Loading - Snow + Live?
1.6S + 0.5L
1.6L + 0.5S
0.5L + 0.2S
For ASD they still have the straight up L+S in the combinations so the LRFD appears to be more logical (and consistent with RockEngineer) than the ASD combos.
RE: Residential Deck Loading - Snow + Live?
RE: Residential Deck Loading - Snow + Live?
I agree with both of you. We're in a 30 psf snow load area (Central Virginia). So while it may be unlikely to see both full snow and live loads, I'll design for both until the building code is changed.
I like RockEngineer's cut-off including live load above a certain snow load.
In fact, I'll send off an inquiry to the International Code Council for some feedback and let you all know what I find out.
RE: Residential Deck Loading - Snow + Live?
RE: Residential Deck Loading - Snow + Live?
Now does IBC want us to design for 42 psf snow and another 20 psf live load for the guy on his roof shoveling off the snow.
This last winter the heaviest snow fall at my house was 18".
I weighed a 12' x 12' x 18" section. The total weight of the snow was about 15 psf. After weighing the snow I didn't worry about shoveling off my deck or roof. That was a good thing because I know my roof wasn't design for snow + live load.
RE: Residential Deck Loading - Snow + Live?
RE: Residential Deck Loading - Snow + Live?
Here's my response:
Hello,
Thanks to all.
Regarding answer A1 – Table 1607.1 item 6 provides that deck live loads should be the same as the occupancy served – in the case of a residence, that would be a 40 psf live load. The load combinations to be considered according to 1605.3.1 don’t differentiate deck live loads from any other live load, so it’s a reasonable interpretation (in my view) to consider that the equation 16-9 (D+L+S) be applied, using the 25% load reduction per 1605.1.1.
The presence of 1607.11.2.2 (Special Purpose Roofs) suggest that if snow loads for the deck are developed using the code roof snow load approach, the total loading or load combinations would have to be directed or approved by the code official. I can tell you from personal experience, code officials in smaller jurisdictions are generally note willing or not able to provide “direction” and are hesitant to provide approval of anything not spelled out in the code.
The remaining option (per the code) for deck loads and load combinations would then be to apply the ground snow load to the deck, and per 16-9 to combine full D+L+S loads.
Given that deck collapses are a relatively frequent (and very public) type of structural failure, it seems appropriate to me that the code prescribed loading be quite clear (and reality-based of course), and subject to as little interpretation as possible. I encourage the ICC to prioritize this issue.
By the way, I floated this idea at a structural engineering on-line discussion group (Eng-Tips.com) and found that engineers take very different approaches to this situation. I will share this correspondence with this group of practicing engineers, who may have their own input and/or suggestions.
Thanks again for your response, and all that you do to help us provide safe and serviceable structures.
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Here's the Response from ICC:
Sir,
Your paraphrased question and our reply are as follows:
Q1. The 2003 IBC requires that the load combination Dead + Live + Snow be considered [e.g. Equation 16-9]. Some engineers neglect this combination for exterior decks, saying it is not reasonable to combine snow and live load on a deck, particularly in areas with heavy snow loads. Is this combination required for exterior decks?
A1. The combination is required, but the more appropriate question might be whether there are any special considerations when applying this load combination to the design of an exterior deck. The code offers no specific guidance on applying the load combinations to decks. This means that where a design proposes any relaxation of this load combination [other than the 0.25 reduction for multiple variable loads permitted in Section 1605.3.1.1], it should only be done with the approval of the Building Official in accordance with Section 104.11. In considering an alternative to the load combination required by the code the following items could be considered.
First of all note that Section 1608.1 requires that the design roof load shall not be less than that determined by Section 1607. Typically this is referring to the roof live load of Section 1607.11. That is also the reflected by Equations 16-9 and 16-10 which indicate the use of rain load, R, or snow load, S, or roof live load, Lr, to determine the maximum load effects for the design of structural members. Again these provisions are written for the typical situation where it is the roof that is designed for the snow load, but they consistently state that it is the roof live load OR the snow load that applies rather than applying them concurrently.
Now consider special purpose roofs in Section 1607.11.2.2 which must be designed for the appropriate live load. The statement in Section 1608.1 does not distinguish between special purpose roofs and any other roof. So the literal application of the provision to the special purpose roof is no different. That is that the roof design considers the snow load but the design roof load is not less than the live load determined by Section 1607. There is no indication here that the live load applicable to the special purpose roof and the snow load should be applied concurrently. There could be some parallels drawn between treatment of a deck live load and the applicable special purpose roof live load.
Q2. Would the answer be any different for residential uses?
A2. No.
Code opinions issued by ICC staff are based on published codes and do not include local, state or federal codes, policies or amendments. This opinion is based on the information which you have provided. We have made no independent effort to verify the accuracy of this information nor have we conducted a review beyond the scope of your question. As this opinion is only advisory, the final decision is the responsibility of the designated authority charged with the administration and enforcement of this code.
Alan Carr, S.E.
Code and Standards
International Code Council
Washington Field Office
2122 112th Ave NE
Suite C
Bellevue, WA 98004
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Here's my initial email
Sent: Thursday, May 03, 2007 1:21 PM
To: Alan Carr
Subject: Load Combination Question
Hello Alan,
I’ve been having an ongoing debate with some fellow structural engineers regarding exterior deck loading.
The Code (IBC 2003) requires that the load combination Dead + Live + Snow be considered. Has there been any discussion in committees that you know of about the ‘reasonableness’ of this combination for exterior decks, particularly for residential use? I’ve talked with several engineers who ignore this combination for exterior decks, saying its not reasonable, particularly in areas with heavy snow loads.
I say we (engineers) can’t just ignore the load combination, but also question how reasonable the requirement is.
It would seem appropriate to have some cut off for requiring live plus snow load combinations when ground snow loads exceed some amount.
Thanks for your response, or for forwarding this to the appropriate person for a response.
RE: Residential Deck Loading - Snow + Live?
But to include "roof" live loads in this discussion I think is off the mark. Decks receive loads from humans and other furniture as floors do - and do not represent the sort of live loads that are prescribed for roofs.
That is why I think that the L+S combination should be used, as opposed to the Lr vs. S which are never combined.
RE: Residential Deck Loading - Snow + Live?
I the ROOF load the ICC folks are pointing to is under "Special Purpose Roofs" 1607.11.2.2. That section requires appropriate live loads be considered on a roof used for assembly, roof gardens, etc.
I guess those loads would be considered ROOF live loads, by the code (though I would consider them as floor live loads myself).
You said you use the "appropriate snow reduction factor" when designing decks? I only see that reduction used for Roof snow loads - in fact, I don't see that the Code specifically addresses snow loads on anything BUT roofs.
That being said, I've designing decks for snow loads, reduced as if a flat roof (just as you've done) and treat the 40psf live load as a roof live load, allowing the 25% reduction when considering D+L+S. It seems reasonable to me, but techncally, I may be in violation of the Code (unless I get approval from the building official per 1607.11.2.2).
I think the code needs to be more clear.
RE: Residential Deck Loading - Snow + Live?
I would look at the deck and see whether it is up high where this might occur, or down low where it wouldn't and also include drifting on it as appropriate.
RE: Residential Deck Loading - Snow + Live?
it is permitted to multiply the combined effect of two or more variable loads by 0.75 and add to the effect of dead load. The combined load used in design shall not be less than the sum of the effects of dead load and any one of the variable loads.
So, if your snow load were high enough to exceed 0.75(L + S) then it seems just D+S would be appropriate. If the snow load were low, say 20psf and your LL is 40psf, then the design load of variable loads is 45 psf.
The variable load reduction seems similar to the ASCE 7 comment made by icenine.
RE: Residential Deck Loading - Snow + Live?
I don't think we ever came to a conclusion on it.
RE: Residential Deck Loading - Snow + Live?
RE: Residential Deck Loading - Snow + Live?
RE: Residential Deck Loading - Snow + Live?
This is the common practice in my area where the base snow load is 60 psf, and goes up from there. I have never seen calculations that use both S and L added together for design of roofs or decks.
The design snow load is based on the 2% probability of occuring in any given year. To add 40 psf live load to that value and assume that the deck will be fully loaded with people at the same time as the 50-year snowfall has occurred is quite unreasonable. I side firmly with MCurry's associate.
RE: Residential Deck Loading - Snow + Live?
It wouldn't hurt if the code explicitly stated to consider decks as special purpose roofs, with the live load considered as a Roof live load for load combination purposes.
RE: Residential Deck Loading - Snow + Live?
The second experience had to do with a precast deck for a porch of a high rise residential building. The deck had failed and fallen on the deck below, which thankfully held. When inspecting 100 or so of the decks in this building, it became apparent that many of these decks were carrying permanent live load, i.e., refrigerators, freezers...This load was there when it snowed.
Based on the second experience, I'd have to say that a deck is not a roof.
RE: Residential Deck Loading - Snow + Live?
What happens if the client has large pot plants (for example) out on the deck and then it snows the design snow load, you will have excess live load even though no-one has accessed it.
I would think that allowing 10psf live load on top of the snow load may be prudent in this case.
The Australian code is much better in this case as it gives you percentages (depending on the use) of the total live load that should be considered 'permanent' load and you use this in combination with the transient loads such as snow (it does snow in some areas over there).
csd