Concrete Framing Systems For Educational Facilities 4

[abusementpark]

I'm working on a new school project. Normally, the structure on a building like this would be primarily structural steel, maybe with tilt-up concrete exterior walls. However, with the way that steel prices have surged in recent years, the contractor is pushing for an entire cast-in-place concrete structure, even the roof. Has anyone here designed a cast-in-place concrete school or similar structure with 25' to 30' spans?

Obviously, there are a lot different options for the type of concrete system used: conventional vs. post-tensioned, flat plate vs. beam/slab, pan joist, etc. If anyone has some recommendations for the best system, I'd like to hear them. We are assuming we'll be able to get enough shear walls such that the columns won't need to be relied on in the LFRS.

 

[milkshakelake]

I haven't done schools with concrete specifically but I've done 25' to 30' spans with flat slab. I don't see a fundamental difference between a school and a residential/commercial building. Check your code (I use ACI) for minimum thickness. For punching shear problems, stud rails are the norm here instead of drop panels. Flat slab with columns is the norm here in New York City. I generally use 12x24 columns regardless of span; there's a textbook for economical concrete structures that says to use as few column sizes as possible (I think by CRSI).

Post-tensioning, waffle slabs, and beam/slab systems are unheard of here but very common in many places. Unlike structural steel which is very standardized on a national level, concrete systems depend on where you're located and what contractors typically do there. If you specify post tensioning but only one contractor has done it like 20 years ago, it's not a viable system for that area, not to mention the difficulty of getting someone to make accurate shop drawings. I would ask the contractor about the most economical option and start from there.

 

[KootK]

I'll throw my hat in the ring with beam and slab:

1) Lots of future space flexibility.

2) Fairly simple and repetitive formwork.

3) Nice mech run space.

4) Easy roof sloping potentially.

5) I'm thinking mild reinforcing but could be PT easily enough.

 

[abusementpark]

milkshakelake - What slab thickness range do you typically end up using for conventional flat plate with those span? And what type of software do you use for analysis?

I’m in the south US and there seems to be a good variety of systems. We’ll definitely be engaging the contractor, but wanted to do some research on the various options.

 

[milkshakelake]

ACI 318-08 Table 9.5c: 25' span two way slab -> 10". 30' span two way slab -> 12". Try to line up columns and avoid transfer slabs and beams; two way slab isn't great for transfer columns in my opinion due to punching shear. But let's say your school is 3 stories; a transfer slab at the 2nd floor would be about 14" to 16".

Check a few panels by hand or Excel w/ direct design method; it's just plugging in numbers. Highly loaded areas, like assembly spaces and hallways, might need special attention regarding punching shear. Bottom rebar will be two ways consistently, like a welded wire mesh but with rebar, and additional bottom rebar only where needed. Top rebar is only needed at column strips, middle strips, and incidental areas with negative moment. I put top rebar next to all shear walls just as a matter of safety, even if not strictly needed; at least there will be some guaranteed shear transfer. If you're unconfident about the top rebar, you can make it like a mesh similar to the bottom rebar but the school owner might not like paying for the additional rebar. These are some very basic guidelines before picking a system; there's a bit more to it, and CRSI has good books on the matter.

For software, I use SAFE. It can do long term deflection and cracking analysis. If you have a pretty regular layout, you might not need any software, and do everything w/ direct design method and hand calculations for punching shear. For lateral design, I use ETABS, but that part of it isn't any different than a steel building w/ concrete shear walls. Stud rails calculated w/ Decon software. Edit: There might also be a version of ETABS that does the long term stuff; I'm just used to SAFE for that.

 

[abusementpark]

milkshakelake - That is all great information. Thanks!

 

[KootK]

...pan joist...

Is that still a viable system in some markets? My first concrete building was pan joist and I thought that it was a great system for somewhat regular, long-ish span floors. That said, I've not seen nor heard of a new build with pan joints in two decades.

 

[Rabbit12]

I find it almost unbelievable we are in a position where low-rise/single story concrete buildings are being considered due solely on cost.

 

[abusementpark]

KootK - I’ve seen the pan joist system used a few times in my area over the past couple decades. I haven’t really studied the economy of it.

 

[JLNJ]

We normally design schools as with CMU bearing walls at the exterior and corridors and hollowcore plank spanning from corridor to exterior walls. I'd rather deal with masons than exterior precast wall people any day.

 

[KootK]

I'd rather deal with masons than exterior precast wall people any day.

#MeToo. That's sad given that, sometimes, I am the precast wall people.

 

[JLNJ]

When I wrote that I was thinking, "This might incur the wrath of KootK."

Gratuitous photo of a terrible precast column is below. I took this photo yesterday.

 

[KootK]

This might incur the wrath of KootK.

I'm curious to know though, why do you prefer to deal with masons vs precasters? Baring that atrocious looking column precast quality is usually pretty good in my experience. My guys don't even avail themselves of the cover relaxations that are available.

 

[Brad805]

Koot, I suspect the primary advantage to masonry is they figure it out on site as they go. Those of us in the precast world are about as tedious as the steel detailers.

 

[KootK]

Yeah, the cost of some precast wall panels tends to be trivial compared to the man hours involved in having to answer my 10,000 questions en route to shop drawing submittal.

 

[JLNJ]

My blood pressure raising issues with precast are the following (albeit from a relatively small sample size):
[ol 1]
[li]Composite, non-composite, or partial? There does not appear to be consensus regarding the design parameters and methodology for the treatment of connectors. [/li]
[li]"We know your spec read 3/4" max camber, but we can't control that so you get what you get. We might be able to ship you a new piece of hollowcore, but it will be weeks."[/li]
[li]"Sorry that your panel bows out like that and doesn't fit up at the corners. We can't really control that. We might be able to ship you another wall panel, but it will be months."[/li] [li]Sorry! We forgot to reduce the cast panel width by the width of the joint. We didn't find it until 20 panels were erected and the man door opening was blocked by a column. We can make new panels but it will be many months.[/li]
[/ol]

Generally, erected frame columns and beams seem to go the smoothest, hollowcore planks next in quality, with wall panels a distant third.

In defense of precast, I will say that precast shops are usually pretty good, while meaningful CMU reinforcement shops can be tough to come by.

 

[hokie66]

The OP asked for suggestions on cast in place systems, and KootK gave him one scheme which has merit.

 

[KootK]

My blood pressure raising issues with precast are the following (albeit from a relatively small sample size):

Thanks for elaborating.

Yeah, estimating wall bowing and %composite is a hassle on the design end to.

Precasters ought to be able to commit to an estimate of plank camber though. That's weak.

 

[Brad805]

JLNJ, we are making a lot of progress on the partially composite panel design. Dr. Marc Macguire (Utah) is doing a lot of valuable research and Dr. Edward Losch updated his software last year to take that into account the new research. We still need more research to show how the two skins will interact if lateral loads are applied.

The fit issue you mention with precast is quite sad, but it seems to happen. The NA industry is slow to adopt some of the 3D tools used in the steel detailing, and some work between many different CAD files.