Pour Strip in PT Flat Plate

[mrengineer]

I am working on design of 11 story condo with 8" PT flat plate. Shape/column layout is wacky, but lets say for talking purposes that building is a 240' long trapezoid, with building width 136' on one end, 72' on the other end. The area where I would like to place a pour strip has max column spacing approx 28'. Would you use a narrow pour strip in the middle of the bay (most likely requiring shoring to be left in place for 30 days minimum), or use a wide pour strip and cantilever from the columns at either side to the pour strip?

 

[dik]

What is the purpose of the pour strip? Why 30 days?

Dik

 

[mrengineer]

Each half of the building will have a c.i.p. core area for stairs and elevator. Need pour strip so that elastic shortening due to post-tensioning is not restrained by the core walls. Also need to allow for shrinkage of the slab.

 

[dik]

I'm not sure about shrinkage, but if the cores are the problem with post-tensioning, is it possible to isolate the cores, stress the slab and then do a pour strip at the cores; this might bring the casting of the pour strip to about day 14 or less. What is the approximate total elastic shortening of the slab between the cores?

 

[mrengineer]

We are in the very early stages of this project, so I haven't done much number crunching. The core areas are about 112' apart, so I would imagine that there would be about 1" of elastic shortening due to PT. I am aware that temporarily isolating the core areas from the slab is a possibility. My concern with that idea is doesn't that make it more difficult for the contractor with regards to stability of the structure until the core area is hard connected to the slab? At any rate, since the configuration of the building does not readily lend itself to a building expansion joint, my gut feeling is that a pour strip somewhere near the building is a necessity.

 

[dik]

Approx shrinkage? = (.2/4000)*112*12 approx 0.1"? assuming that the slab is stressed to approx 200 psi... and Ec approx 4000 ksi?

Can you sleeve/wrap your wall steel through the slab with with urethane foam? and not bother to use pour strip?

Dik

 

[dik]

oops... shortening, not shrinkage

 

[mrengineer]

The urethane foam idea has merit-I have also seen details using a splice sleeve that is grouted later. So I gather that even though there is temporarily no hard connection between the core and the rest of the structure, the amount of potential movement of the structure is obviously small compared to what you would have with a pour strip, and is of little or no concern? Then all I have to do is convince myself that I don't need an E.J. due to 240' length. (Notice L775's thoughts on this in thread 507-146604).

 

[dik]

One of the first post-tensioned slabs I did was about 250' long with no EJ; with PT, I never thought it would be needed. What I learned from that project was that I'd spec'd that the strands be tensioned for each end... First time on site, when I learned that the second stressing with poly sheaths had no elongation... and quickly allowed stressing from one end only... friction losses were nada!

Dik

 

[mrengineer]

I am more concerned with restraint cracking due to concrete shrinkage and PT induced elastic shortening than I am due to friction losses. But it sounds like the 240' length is within your comfort zone based on experience. I appreciate your thoughts on the matter.

 

[rapt]

mrengineer,

Normally I would provide the pouir strip at about .2 - .25 * span from one end of the central span, where the moments are smallest. This means propping the longer slab until they are joined.
Placing it at midspan will have serious effects on your deflections, as the pour strip length will be plain RC and will crack much more than the PT slab. The wide pour strip will be even worse for this. I seriously doubt that the 8" slab would work in this span with a central pour strip. Then, I think 8" is probably too thin for these spans anyway, especially if the end span is 28', but the average moment design approach in ACI will allow you to justify it (too bad about statics).

Sleveing the bars in cores is too much trouble. Pour strips around coreds are a possibility but a lot of top steel would be needed as well as propping.

The amount of shrinkage restraint you will save will be about 20% after about 1 month. But you will still get 80% of shrinkage + 100% of temperature change + creep. Depending on the temperature at the time of pouring a slab and the temperature in the building after it is finished, this could be more than the shrinkage restraint.

Without the cores this shortening would not be a problem except at the bottom couple of floors, but with the cores it will be significant throughout the height of the building.

240' is longer than I would allow without an expansion joint with flexible restraints such as small columns. I would definitely have an expansion joint with stiff cores towards each end, but then I do not like unsightly cracks. You could solve it with extra untensioned reinforcement instead of the joint.

 

[dik]

Sleeving is easy... just slip a chunk of pipe insul over the rebar... Dik

 

[mrengineer]

rapt:

1) You seem to be saying that midspan pour strip (even if it is narrow) would result in adverse deflections since pour strip is RC. With a narrow pour strip, since there might still be the same total number of uniform tendons, I would think that rather than the deflection being primarily affected by the uniform direction it is much more affected by the fact that the tendons perpendicular to the pour strip are discontinuous at the pour strip?

2) Are you saying that aside from the pour strip issue, the 8" slab thickness is marginal? I actually have a few 28'x30' bays. In looking at numerous PCA case histories, it appears that 8" slabs are not uncommon for 30' spans.

3) You mentioned the possibility of using extra untensioned reinforcing in lieu of an E.J. I realize one can come up with calculations for the reinforcing, but any idea on magnitude of that reinforcing? In going thru the literature, it appears that some engineers like to have something like #4 @ 24" to 30" or so each way throughout the slab for improved crack control.

 

[DTGT2002]

Working as a formwork and shoring engineer, I would certainly prefer the short cantilevered slabs at the pour strip, allowing the removal of initial shoring at the pour strip. If you start with that mindset, everyone should be saved a few headaches, as the owner and general contractor will likely push or be pushed to demand such a configuration for ease of shoring and release of the floors at a later date.

We have formed many (read over a hundred) of similare flat plates and suitable designs have been created to adequately deal with this situation and with acceptable results. With the narrow span, the impact is seen by the owner and GC as higher costs. We do not allow significant work on floors where our shoring and reshoring operations are in place. Additionally, if the pour strip is designed such that the formwork and shoring never strip until the pour strip is placed and cured, the amount of shoring is dramatically increased, raising costs significantlyboth in material and the labor required to place and remove the shoring. If efforts are made on the front end to design a structure that performs as required but is more constructable, that design effort can be rewarded with a nice cost savings over the other option. Of course, you could present the bad option, and reap the cash to redesign in the more constructable manner, but now that you have the knowledge to decide the best course, you are ethically bound to do it the preferred way, right?

Good luck!

With either option, PT slab reinforcement must be rigourously checked and installed on good formwork/shoring to minimize eccentric loads that will fight you when placing the pour strip. Thats another reason to use the shorter cantilevered slabs.

Daniel Toon

 

[mrengineer]

DTGT2002:
A short cantilever at both ends of the bay results in wide pour strip. Does that present much of a problem for the contractor?

 

[rapt]

Dik.

If they are core walls there are a lottt of bars to sleve, plus there has to be a slip layer between the slab and the core walls to allow the floor to slip. It is very messy.

Plus the core walls cannot be slipped/climbed in advance or even poured above the floor until the sleves are grouted. If he wants a 1 month separation it is not feasible.

 

[rapt]

mrengineer

1 In the direction perpendicular to the pour strip the pour strip part of the slab is RC and cracked. You cannot assume that the slab is fully PT in the design. The RC section and its cracking has to be allowed for. If you use a wide pour strip this is significantly worse and 8" will definitely not work.

2 Yes, I am saying 8" marginal if you do the calculations properly (not by the structuyally incorrect ACI average moments method).

3 Bars at 24' to 30" give no useful crack control at all. 8" is good. The amount needed is too variable to guess at. You would have to calculate it for your conditions depending on temperature change and shrinkage and amount of restraint expected.

 

[pappyirl]

mrengineer

I agree with rapt, an 8" slab does seem to be marginal in my experience. Long term deflection in the end span would be quite large.

Providing untensioned reinforcement to control cracking over the entire slab seems un-economical. This would have to be placed top and bottom I imagine most contractors in this situation would sooner increase the rebar and RC the entire slab is depth wasn't a limitation.

Again the movement joint is on the limit. I would normally consider a movement joint on slabs over 80m.

As rapt pointed out in relation to the vertical sleeved dowel bars, the contractor would not be able to pour his wall until the required shrinkage and time had elapsed. This would not be acceptable. An alternative detail is placing sleeved dowel bars horizontally at midpoint of the slab at one of the cores. The dowel bars allow two way horizontal movements and are grouted up after the required shrinkage has taken place. If the walls are slipped a coupled bar is used instead of an L bar.

To answer your question, the pour strip should be placed at a 1/4 of the span length where there is theoretically the lowest moment. A wide pour strip is not an option. For a 28' span an 8" RC section would not work unless the slab is stepped or the span at this location is less.