Do Serviceability Loads Need to Match ULS Loads

KootK · Nov 12, 2017

For a long, long time, I designed stuff like this:

1) Get specified load from code.
2) Design strength based on specified load multiplied by appropriate factor.
3) Do serviceability checks based on specified load without load factor or, in some cases, based on a prescribed reduction factors.

So my question is, how appropriate is step #3? Does the the design serviceability load have to be the specified load?

A talented fellow that I used to work with called me out on this a while back for a stair that wasn't passing deflection/vibration checks. The ULS load was based on 100 psf so I based the deflection checks on that too. My colleague felt that 50 psf was more realistic and that, more importantly, we had the ability to decide that for our selves. Persuasive.

How about that? If you accept the premise, then the lid kinda comes off on a lot of things. Transfer slabs for example. They're usually governed by deflection concerns. Can I just design those for 50% LL when evaluating deflection? Obviously, I don't want to be the only guy in town not doing this if it's legit.

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.

IDS · Nov 12, 2017

I think this is a N America problem (or maybe just USA). Eurocodes and Aus/NZ codes define the basic loading and define load factors for different combinations at SLS and ULS, so there is no scope for reducing SLS loads based on guesswork. ACI 318 doesn't even use the term "Ultimate Limit State" and only uses the term "Limit State" once, in the commentary (2008 version).

In principle, the SLS loading should be the maximum anticipated loading, so I'm not sure why you would use a reduced loading for a deflection/vibration check anyway.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

rapt · Nov 12, 2017

As Doug suggested, ACI is very out of date on this.

Eurocode and Australian code base deflection design on a short term load condition which can vary from about .5LL (office/residential) to .9 - 1.0 LL (storage) depending on building/loading type.

In Eurocode crack control is based on various values of Permanent Live Load, Short Term Live Load and Characteristic Live load depending on building/loading type and building materials used.

But remember also that these are minimum values an intelligent designer should higher values if they consider that the real loading will be a higher value for their specific building and situation

RPMG · Nov 12, 2017

There is an IBC requirement that stairs used as a means of egress must meet a specific deflection requirement for 100 PSF. For everything else, service level combinations are per the engineer.

For code definition on SLS, I'm in the opposite boat. Codes are getting out of hand, and I see no reason to define SLS loads/combinations in the code. What happens if this stair has a perceived vibration? Nothing.

MrHershey · Nov 12, 2017

Right or wrong, IBC kind of locks you in by giving minimum requirements and no real outs. Live load minimums are table 1607.1 ('06 edition). Deflection requirements are 1604.3. Both are requirements, not recommendations or suggestions. Would probably require permission from AHJ to do something else and very few will stick their neck out to approve using less for serviceability.

Will throw it out there that for vibration you may want to actually look at a lower load despite what the IBC says. Having the load too high may be unconservative because it adds mass to the system, which helps reduce vibration. AISC Design Guide 11 has much lower live loads than the IBC for vibrations from both footfalls and rhythmic excitation, for instance.

If it's concrete, perhaps rather than playing with loads you play with stiffness instead? You can calculate out Ieff if you wish and for serviceability you can argue that Ieff should be based on unfactored loads.

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Do Serviceability Loads Need to Match ULS Loads

KootK

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IDS

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rapt

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MrHershey

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