2 Way flat slab design resources - Papers or recommended textbooks?

[structuralCADspecialist]

I've read more than a few texts on 2-way slabs. Most of them just closely follow code and therefore focus on the direct design method as well as the equivalent frame method (if I recall correctly?). What most texts tend to lack is any info about how to detail the slabs themselves, aside from recommended bar cut-offs. And they are especially lacking in methods of detailing more eccentric slabs. Very few authors want to touch it. I'm sure there's been a grad student (or two) who's a bit braver? Is what I'm seeking actually covered in ACI 318? And if so what's the oldest (and therefore least expensive ;-) ) edition of 318 that would have everything pertaining to 2-way slabs?

Back (way back!!) when I actually did rebar detailing, we were better at it. And by we I mean the engineers and the detailers. Back then they had lots of knowledge and experience going into each design. We used noticeable less steel and the slabs, in place, were generally far less sloppy (way less cover and congestion issues).

A big concern with many of the slabs I'm seeing today is that they often don't follow code fundamentals, nor do they follow the precedence of "good field performance" which was a key driver of past designs. I think a lot of newer slabs are not failing simple due to using extra bottom steel. What I'm seeing today is slabs that use more steel (sometimes way more) and perform worse, if not in the short term definitely in the long term (poured one or two concerningly 'bouncy' slabs over the last few years).

I'm very much an armchair engineer (obviously not an actual engineer) and love reading about anything and everything relating to reinforced concrete. In my current line of work I'm not really responsible for inefficient designs per say, but I'd like to change that (for many different reasons). Verbosely explaining how things 'used to be' doesn't cut it. So looking to improve on my knowledge base in a way that allows me to actually reference a solid source. Any help is greatly appreciated!!

 

[WARose]

I haven't seen that many texts that take on the subject either. (At least beyond what ACI says on the subject.) It's not anything magic though....just a matter of a good model. In that regard, the slab not being sufficiently meshed in the FEA model is a frequent problem I see. I also see the "bouncy" issue as well.

 

[Settingsun]

Less steel and less bouncy means thicker slabs. No mystery. Maybe seismic design requirements are what changed. Going back far enough, dead weight only acted vertically so was easier to deal with.

What detailing info do you think is missing? If it's bar layout at odd-shape edges, no textbook or code will cover that. There are too many possible shapes for general rules to apply.

What do you mean by slabs not failing due to bottom steel? That's the point of reinforcement. Thinner slabs need more reo.

 

[Sheer Force Engineer]

Hi structuralCADspecialist, I've found that lesser experienced Structural Engineers will tend to throw more reinforcement at a slab through lack of confidence, especially if the slab is on the complex side of things.

By complex I'm referring to a combination of steps, folds, set-downs and columns which don't line up in straight lines in both directions.

As mentioned previously, thinner slabs means more reinforcement (all other things being equal). In this scenario, the thickness game usually wins out. If you are building a 75+ storey tower and can take of 50mm from your typical slab you can fit an extra full floor on top while keeping the same building volume envelope! SO you may be fighting an up-hill battle if it comes to thickness vs. congestion of reinforcement (although thickness vs performance is another story, if the slab is not performing in a satisfactory manner this is obviously a big no-no).

If you want to keep a Structural Engineer honest and make sure there is not too much reinforcement in your slab, you can do a quick volume take off. Its surprising how a well detailed and designed slab fits within general guidelines of reinforcement tonnage. As a general rule of thumb...

- Two-way flat slabs have reinforcement tonnage in the order of 80-100 kg/m3
- Two-way flat plate slabs have reinforcement tonnage in the order of 120-140 kg/m3

If your slab is 10-20% above the upper bound of these limits, something may be over-designed.

For the latest insights into the art of Structural Engineering

http://www.sheerforceeng.com

 

[dik]

With the Imperial Broadway Bldg in Winnipeg, the developer actually 'mocked' out the mechanical and electrical system for a floor to determine the minimum floor to floor height.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

I no longer remember the pounds per yard, but there were values for slabs, flat slabs, beams, columns and walls.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[structuralCADspecialist]

Hi Sheer Force Engineer and thanks for sharing all that useful information. Also enjoying some of the articles on your website (even though I don't have the time haha). Those tonnages will be super useful! - I'm actually developing ways to take-off reinforcing quickly and will be able to incorporate that in.

I've found that lesser experienced Structural Engineers will tend to throw more reinforcement at a slab through lack of confidence

That makes sense. I've also heard that it's usually the 'juniors' that are actually tasked with designing and detailing the slabs.

I think I tried to slip a little bit of a rant into my original post, which ended up consuming most that post ;-) . I do notice lots of actual errors in the slabs themselves, along with detailing that I personally don't consider efficient. The later is much harder to communicate. Some of the bigger, more substantial problems include:

[ul]
[li]Rebar congestion so profound that it likely exceeds (at least locally) the maximum reinforcement ratio per code,[/li]
[li]Top steel simply 'missing'.[/li]
[li]development lengths / bar cut-offs being too short,[/li]
[li]and hooked bar ends where they aren't needed (I might be wrong here... in some situations at least)[/li]
[/ul]

And... in general lots of little things that I think could use improvement, but would have me veering way off topic.

I've seen really good design examples, just not in my area (well not for a very long time). And the really good designs I've seen haven't been from more than two or three different engineers. I suppose I'm also curious to see what structural engineers in say, New York, Los Angeles, Toronto are doing compared to where I live.

As for the information I've found so far, the text I've found with the 'most' information on two-way slabs is this one:

https://www.amazon.ca/Reinforced-Co...id=1641059395&sprefix=john+pao,aps,118&sr=8-1

I've never seen any good papers on more eccentric slabs (or recommended detailing practices in general). I have (and have used in the past) the reinforcing detailing manuals used by the trades.

And I've also seen lots of design examples throughout the U.S. and Canada, but most of those tend to be more 'square' shaped and get away with very conventional reinforcing layouts. I can say with confidence that designs outside my local area are far more rational than within. But as mentioned, those building layouts tend to be more forgiving to a rookie designer. Perhaps the architects in my city are simply making their buildings impossible to design?

 

[dik]

For the small stuff I do, and the bigger stuff I did, I nearly always stagger rebars, often carrying bottom reinforcing thrugh the support as shown:

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik