RCC stairs waist slab thickness

[Tstruct]

I have seen many experienced engineers in my surrounding using L/35 to calculate thickness of a one end continuous stair waist slab. Whereas, textbooks write L/24 or L/28. What is your practice? And which one is compliant to ACI-318.

 

[JedClampett]

I've never really thought about it, but the maximum rise is 12 ft, and the standard run is 11/7 times that, so that comes out to 18'-10". I typically use a minimum of 8", so that's L/28. But I'd run the numbers and see what kind of reinforcing that requires. If there's anything you don't want shaking and are exposed to impact loads, it's stairs. A couple of things:
[ol 1]
[li]I never knew how prevalent this was, but a lot of engineers use delegated designs for stairs. We don't, but I was at a meeting of structural engineers, and everyone else seemed to do that. Add to that a plan reviewer from the City was there and he said they were going to prohibit that (delegated stair design). [/li]
[li]We use typical details for our stairs and our recommended thickness is L/20.[/li]
[/ol]

 

[dik]

Most concrete stairs in these environs are used in apartment bldgs. The rise is typically about 4' and the span is about 4'x11/7 = 6.3' +7' (landings) = 13.3'. The throat (waist?) is typically 6"... or L/26. Most stairs are generally steel with pan and longer stairs with a rise of 12' would nearly always be steel with pan.

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-Dik

 

[JoelTXCive]

We don't know your specific project's geometry, but don't forget that you might have fall protection rails that will need to attach to the concrete. The railing posts can be cored into the concrete or bolted on the side.

I haven't done a lot of stairs, but I have done a lot of concrete walkways at wastewater treatment plants. Often, the requirements of the railing anchorage drives the thickness of these walkways.

 

[Agent666]

Usually, you're going to be governed by deflection and possibly vibration. Strength limit states are usually a secondary thing that just works for simply supported stairs for the main reinforcement, though sometimes the detailing at any change in direction with the flight/landings requires some consideration (we like to have links at the change in direction that go around the main reinforcement to deal with the tension across the joint considering strut and tie model of opening joint, this dictates a larger cover top and bottom to achieve cover to these bars). We almost exclusively use precast stair flight in New Zealand, that consist of both landings and the flight usually poured in several stages by the precaster, but delivered to site as one complete flight that's just lifted in on site.

Increases in thickness for concrete cover/durability is also a consideration depending on the environment, and may warrant an increase in thickness purely from that perspective over and above what might be more suited to an internal typical office or apartment loading if in a situation where more durability is required (you cannot say one size fits all really, as it 'depends' on loading and location).

I've seen a few people in my time just design stairs for strength or design for the gross section forgetting about consideration of the effects of cracking and creep. The corresponding increase in deflections can be significant, especially if you have thin stairs with only enough reinforcement for the strength.

I've seen a few of these designs go far enough that they were actually built, resulting in very larger deflections and expensive rectification. So stick to the bounds of whatever has worked in the past as a solid starting point is highly recommended (within your own design experience, or your offices, or local industry).

Had a look at our offices standard details and they hover between L/22 to L/23. L/35 seems quite excessive to me, but depends on loading and the cover required I guess.

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