Rules of thumb for 2 way slab

[struct_eeyore]

Does anyone have some rules-of-thumb to share for 2 way slab design? I currently use ADAPT for steel requirements, but have heard stories about old-timers calculating steel based on some quick proportion checks - wondering if anyone knows what those are. ( I don't particular want or care to do the full 2-way slab analysis by hand)

 

[dik]

It's been decades since I've done one... two way slabs were slabs with drop panels, as opposed to two way plate. I don't know if that's universal... as a rough guide, drop panel depths were determined by the depth of dimensioned lumber without cutting it... I seem to recall that the slab depth was approx L/35 or so, rounding downwards, and that drops were about the same thickness as the slab. We used to use fairly thick drop panels. Deflection calculations were always undertaken. Top mat bars were always staggered. It's been a while, and it's a little fuzzy.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[struct_eeyore]

Dik - can you elaborate on the staggered bars?

 

[jayrod12]

I see staggered top bars for top mats more and more again. Essentially not all of the steel is needed once you get to the outside edge of the negative moment region, so they shorten all of the top bars a bit, and then offset them from being perfectly centred over the support to half to the left of the support slightly, and the other half to the right. Therefore directly over the support you have all the negative bars fully developed you need, but at the outer edges of the top mat, you only have half of the bars fully developed.

It may only save a little bit of reinforcing when you look at an individual top mat. But on a multi-storey project with lots of mats, the savings can be substantial.

 

[dik]

Not only saving a little steel, but with the bars 'staggered' there is a more gradual termination that may reduce cracking. This is a common occurrence with FRP reinforcing where the bond strength is limited a tad. I haven't done a reinforced concrete multistorey for a couple of decades... but it was common practice with the firms I worked with.

The slab thickness is slightly reduced, but the drop panels are much stiffer than minimum... This tends to concentrate more of the negative moment over the supports. Deflection is not normally an issue.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[manstrom]

Staggering top bars is also very easy to mess up. I see this detail omitted a lot when I'm walking rebar.

It's more detailing, but idiot proof to have long bars and short bars.

 

[driftLimiter]

The direct 2way slab design method from ACI 318-14 is pretty easy. I don't really think that some back of the envelope calcs would be reliable for design for something like this. Are you just wondering about some quick method to give you sanity checks or preliminary gut feels?

The direct method takes just about as long as it takes to design a few concrete beam.

 

[struct_eeyore]

drift - yes basically looking for some easy hand calcs to check my software output. I'm all for the direct design method, but my slab geometry doesn't fall within the limitations.

 

[dik]

Bars are the same length... you just stagger them by a foot or so...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[RobertHale]

@structee. If you're outside the limitations of direct design, I'm not sure how much rules of thumb can help you when you get outside those bounds. Are you using Adapt PT-RC and equivalent frames to determine the As or a different Adapt product? And are you looking for validation of the span moments or just the As? I generally make sure that my moments are bounded by wL^2/12 and wL^2/8. I also just like to make sure the deflection diagrams pass the smell test.