Deflection Load Combinations

[TK406]

I have a question regarding Structural Deflection Limits. The way I see it, there are three ways of going about it - Follow the IBC verbatim, or use ASD load combinations, or load that aren't factored at all.

For example: What are the appropriate combinations to used if I have both a floor beam with Live load, Snow load, and Dead load.

For Total Load do i use:
TL = D+L+S (Actual Total Load)
-or-
TL = D+.75L+.75S (ASD Combo)
-or-
TL = D+L (all I care about per IBC)

For Live Load do I use:
LL = L+S (Actual Transient Loads)
-or-
LL = .75L+.75S (ASD Combo w/o Dead Load)
-or-
LL = L (separately from Snow per IBC)

I appreciate any thoughts, advice, and opinions.

 

[JAE]

What deflection limits are you talking about?

What member are you looking at?

 

[woodman88]

I fail to see how a full live load can occur during or directly after a full code snow load condition. So I would use the greater of the ASD combo or full live or full snow condition.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[msquared48]

And half live load plus half snow load, which is very reasonable.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Lion06]

Why is D + L all you care about per IBC? I've always used the ASD combinations to check all serviceability requirements - I sure hope I haven't been dropping the ball for the last 5 years.

 

[JAE]

Maybe my questions were a bit obtuse.

What I'm getting at is the primary live load (serviceability) deflection criteria is usually stated separate from load combinations. Live load is usually restricted to L/360 in many codes when you have brittle coverings (such as plaster type ceilings). This is seen in the ACI 318 spec. You use full L for that check - and again, it is just a check on live load by itself.

Other criteria suggest a TL limit perhaps of L/240 or some fixed amount. This again would be a TL (any load combination that occurs on the SPECIFIC MEMBER.

Another example - Seismic, uses ONLY the lateral seismic loads when checking against lateral sway limits such as 0.02h. Again, not a load combination, but a load case check independent of the combinations.

Masonry (ACI 530) suggests a vertical deflection underneath any masonry as L/600 or 0.3" for Total Loads. Not a specific load combination...just TL which suggests ANY load on the element being considered.

I've not seen any code put a deflection limit or criteria on the numerous load combinations found in ASCE 7.

 

[DaveAtkins]

Maybe this is a dumb question, but how would a floor beam get exposed to snow load? Is this a transfer girder carrying a column which supports roof framing?

DaveAtkins

 

[TK406]

Thank you for all of your responses.

@JAE: I guess I've always used the IBC to dictate what non-lateral deflection limits I use on rafters, beam, joists, etc.

But was thinking the other day about the difference btwn. the code and what people are doing in practice. (Obviously IBC is a minimum.)

In addition, i was thinking about when using a piece of software to design a beam, they often give you a LL and TL deflection result - What are they actually giving you. (A little investigation can make this determination, and it appears to me that there is not a set standard btwn. different pieces of software.) This is the root of my question as there doesn't seem to be much guidance in this practical matter what the appropriate way to check deflections are. My point is that IBC only has you check separate load cases (other than D+L), but in practice, it seems that we should be looking at deflections from ASD (service) load combinations. Any thoughts?


@ Dave Atkins: I can think of many examples of when this would occur (I live in snow country), but you are essentially correct. Typically I encounter this when, in the second floor of a two story structure, I can't continue a post to the foundation for arch. reasons, or a bearing walls don't align with one below and I need a beam under a wall to support both roof and floor...

 

[msquared48]

DaveA:

I have seen cantilever floor beams used to support a decks with no roof. In that situation is your answer.

Another would be if a roof structure was designed to support an assembly load, for whatever unknown architectural reason...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[TLHS]

Under my reading of the National Building Code of Canada you don't even need to apply snow and occupancy loading concurrently to the same floor area for strength design, let alone serviceability, just take the larger of the two unless there's some reason you expect that the area can be used to full capacity while there's snow all over the place. It also has a catch all that it still needs to be designed to its intended capacity, so you'd want to account for things like someone shoveling the area and putting the snow in a big pile somewhere if you expect someone will do that.

As far as steel in Canada, there are some drift limits in the NBCC which are hard numbers, but the beam deflection limits are a non-mandatory appendix to the steel code. The only hard requirement is that they be checked for serviceability in deflection and vibration, whatever that may be in the case in question.

Personally, I would likely assume that the worst of snow or live load should be used for deflection unless I expect that they can happen at the same time. I wouldn't include dead load in the deflection calculations unless there's something unusual (possibly a dead load that is applied after some or all of the architectural attachments have been made) because it's not a deflection that would be noticed by an occupant and any attachments to the structure will have been made with the structure already deflected by the dead load. That's pretty much what the non-mandatory Appendix D of CAN/CSA-S16-01 advises as well.

 

[csd72]

For strength you need to allow for worst possible load combination according to the code.

For serviceability I would focus more on worst probable case.

In an extreme event you may get away with a few cracks in the cieling but not with a collapsed building.

This philosophy is recognised in earthquake codes and in disproportionate collapse requirements where the focus is on strength rather than deflection due to the fact that these are unlikely extreme events.

I would also look at the consequence of serviceability failure, there are extreme situations where serviceability failure is just as bad as strength failure.

What is the reason for the limit? If just visual then a temporary infringement during extreme events is probably acceptable, if it is for very fragile cladding materials then maybe not.

 

[paddingtongreen]

The deflection limits are LL limits. Most of the DL deflection is in the beams when the plaster, etc. is applied.

Are they no longer called, "Live Load deflections"? They used to be.

Oops, I see JAE already posted this. Take this as support for his position.

Michael.
Timing has a lot to do with the outcome of a rain dance.