Concrete Pan Joists 1

[abusementpark]

Is the concrete pan joist system still an economical system for concrete structures?

It seems like it was widely used in the past, but it is not as popular today. Is a flat slab and beam system more economical? Seems like you would save a lot of formwork.

 

[JAE]

While a concrete pan joist system (especially the skip-pan system) is my favorite structure, I don't think it is as economical as others. There is a lot of shoring and formword but it ends up being a very malleable system for architects, is straightforward in design, and is easily adjusted both during design and after construction for unique situations, openings, etc.

This system seems to be more prevalent in the southern US. In Texas where I used to work it was very common.

 

[msquared48]

It was very prevalant as a school design problem or project when I was in college in the mid to late 60's, but, in all the time I have spent in structural engineering since then, I never used one system. Not one.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[DaveAtkins]

In my 24 year career, I have used the system just once. One of its chief benefits is the entire roof or floor system is the same depth--no deep beams for the mechanical engineer to contend with.

I think its drawback is that it is labor intensive--and labor is more expensive than materials these days.

DaveAtkins

 

[BAretired]

I used it on several institutional projects in the mid fifties and early sixties. I have never used it on commercial or design-build projects because it was not favored by my clients, or more correctly, by contractors working for my clients.

From a material point of view, it seems like an extremely economical system but, unless a contractor can get repeated use of pans which he has purchased, he must rent them from others. This, together with high labor costs has more or less ruled out the pan joist and waffle slab system in my locale.

BA

 

[hokie66]

Agree with the others. We used pans for one way joists and waffle slabs in Virginia in the 60's, mostly for institutional projects. I don't think they are common today.

 

[a2mfk]

SE company I used to work for did several building using this system in a specific area of central Florida. It was part of a very large office and mixed use complex, and I believe the contractor had some type of relationship with the owner and this was their decision. I think the land development company retains ownership of the buildings, so maybe they were convinced this was a good long term investment and they had a GC familiar with this type of construction. I would estimate the majority of these buildings were constructed around 1990-2005...

I only got to help out on one, seems like a very interesting system and if you are going concrete this is a pretty economical way to do it.

 

[TXStructural]

Wide module/skip joist is still extremely common in Texas and the surrounding states. It provides long, stiff spans. It is being used in the Devon Energy tower now going up in OKC.
It is a bit more labor intensive than flat plat, and takes taller floor-to-floor heights, due to the depth of the joists. Where there are height restrictions and more columns are not a problem, flat plate is attractive.

The first project I designed was wide module, and I see projects using it all the time.

See

http://crsi.org/index.cfm/engineering/floor

 

[PUEngineer]

I work for an Indiana firm. We do most of our work in Indiana and the surrounding states. I've personally done three wide module systems in the last two years. Two in Indiana and one in Illinois. These are all institutional buildings of the 4 to 5 story range where floor to floor heights weren't an issue.

As far as the OP's question regarding economics versus beam and slab system. It depends on the column layout/spans. We ran several framing scenarios for estimation, in last project, with beam and slab systems versus the wide module system. The wide module system ended up cheaper per square foot.

 

[Eldorado]

I have and continue to use a skip joist design on projects, mainly schools, here in Texas. Labor costs are significantly lower here and pans are usually reused more than they should be by the concrete contractors. The floors for the AlamoDome here in San Antonio are formed with a concrete joist system. Since new pans were going to be made, the forms were made a little deeper than typically available for the project.

 

[JAE]

Eldorado - that's right - the Alamodome used 24" high pans with 4 1/2" slab. It was a skip pan system with joists at 5'-1" o.c. (56" wide pans with 5" joist widths).

 

[BAretired]

So far as I know, the skip pan system has not been used in my area. It sounds like a good system. As I recall, we had 24" and 30" wide pans with variable height. I always felt the joists were too close together.

In the skip pan system, I assume you could vary the joist width to suit requirements, so it could be 56" + 6" for a 5'-2" c/c spacing. Or perhaps 56" + 8" for a 5'-4" c/c spacing?

The waffle slab has eye appeal and should be very economical for a two way system. Again, it would be nice if the pans could be larger than those I have seen.

BA

 

[abusementpark]

From a material point of view, it seems like an extremely economical system

I'm not seeing offhand why it saves material. Is there a structural reason why? Maybe for longer spans the flat slab becomes too thick?

takes taller floor-to-floor heights, due to the depth of the joists

Why exactly is this? It seems like the structural depth of the floor would still be governed by the beam depth, which using the same grid for comparison, shouldn't vary much in depth between systems, right?


Thanks all for the responses. Do the pans come in standard spacings/modules?

 

[hokie66]

abusementpark,

That's right, flat slabs and flat plates, when they get quite thick for long spans, have a lot of concrete that is not needed, and dead load that is detrimental. Thus the voids in pan systems. I saw an old variation on this theme recently, in Turkey. A common method of construction there is to build a flat form, build sections of clay tiles where the concrete is not required for strength, then place the reinforcement and concrete ribs. They leave the clay tiles in place, so don't get much benefit of reduced dead load.

 

[abusementpark]

That's right, flat slabs and flat plates, when they get quite thick for long spans, have a lot of concrete that is not needed, and dead load that is detrimental.

So maybe for spans > 25 feet the flat slab becomes excessive in thickness and a concrete pan joist system could be more feasible?

 

[hokie66]

That varies with location, but I think reinforced flat slabs can go to 28 ft economically, and post-tensioned to 32 ft. Where I am, we tend to use band beams (wide, flat beams) and one way slabs, with spans about 34 ft.

 

[JAE]

The Alamodome joists spanned 42 feet with the 24" + 4 1/2" = 28 1/2" total depth joists.

 

[RHTPE]

A somewhat belated response: I worked for one of the US's largest concrete subcontractors for many years. My observation is that the refinement of post-tensioned concrete has nudged one-way (pan) and two-way (dome or waffle) slab construction out of the picture for many applications. It is simply a matter of economics - P/T flat slabs require less labor overall.

Pan or dome slabs still have their place - typically in structures requiring very stiff floors. Custom dome slabs showed up on several semiconducter projects we were involved in, as the floor of a chip plant must be as vibration-free as possible - I believe the floor thickness was 36" overall - 30" deep domes with a 6" topping. Columns were massive and on a very tight grid (20' or so). Joist spacing varied to suit the projects and were spaced around 6 feet centers.

Most of the metal forms for one-way systems do NOT produce a finish suitable for being exposed in the finished structure. New FRP or steel dome forms can produce an attractive structural system, but little is being designed utilizing them.


Ralph
Structures Consulting
Northeast USA

 

[TXStructural]

Since the controlling influence on long spans is often vibration and deflection, having deep ribs in the form of pan joists give you excellent long-span performance and economy.

The newest thing is "voided slabs" with a flat bottom and top, but using any of a variety of void-forming components inside. This is similar to the filigree systems of the past. One-way systems use hollow tubes, two-way systems use spheres or such in the middle-middle portion of a conventional 2-way design - with the intent being to remove unneeded dead weight.

 

[RHTPE]

TXStructural - I've seen the recent articles about Voided Slabs. When my carer started about 4 decades ago, we were involved in the construction of a parking garage that was designed by a local engineer using his patented system - it was then called a Tube Slab. A man by the name of A.J. Macchi held the patent. The "new" voided slabs are simply a variation on the same theme, the primary differences being today's better recognition of concrete placement technigues needed, and the different styles of voids utilized. Back then we simply used capped Sonotubes. Dayton actually developed hold-down hardware for the Sonotubes.

Funny how some concepts cycle through over the years.

Ralph
Structures Consulting
Northeast USA