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PT effects on shoring
3

PT effects on shoring

PT effects on shoring

(OP)
In a two way PT floor the distributed tendons are usually stressed (100%) first.  This would create a line of uplift at midspan and a line of downward force along the column lines.  This increase in force is not accounted for in the shoring designs of most contractors.  Although the loading is only temporary (until the banded tendons are stressed) is it something I should be concerned about?  Has anyone considered or heard of contractors considering this subject?  Thank you.

RE: PT effects on shoring

i've never heard of this being considered.  if you cut a free body, the load applied hasn't changed.  

 

RE: PT effects on shoring

(OP)
I am unclear as to how the applied load hasnt changed.  The horizontal force in the tendon is creating upward and downward forces in the slab, basically redistributing the slab weight towards the tendon high points.

RE: PT effects on shoring

so?  if you take bandline with it's tributary, it'll still the same load is it was prior to stressing.

RE: PT effects on shoring

(OP)
I am concerned about the loads on the shores.  If the shores are placed at every 4 feet they are designed to take a 4' x 4' tributary weight of the slab.  The shores and slab below are designed to distribute that weight out.  If only the distributed tendons are stressed it will decrease the shore loads in the middle of the span and increase the shore loads near the column line.  The shores and slab below near the column line were designed to distribute a 4' x 4' weight of slab, but are now seeing a larger force due to the redistribution of slab weight...  

RE: PT effects on shoring

There's a flaw in your understanding:


The stressed tendons do not induce load... they precompress parts of the slab to increase the flexural capacity.  Concrete still weighs 150pcf.  

You might be confused if you're using the load balance method and draw a "reverse loading" diagram on a slab strip.  That's just something to make the math work.

Think about it... if you have a simple-span PT beam, stress the tendons to balance 100% of the dead load, then remove the supports, would the beam float?  

The concrete still has the same dead load.

RE: PT effects on shoring

JKStruct is right. You only induce a lateral load from prestressing, not a vertical load.

RE: PT effects on shoring

I think what he is asking is if the entire slab is supported on shoring and the distributed tendons are stressed, part of the slab between the banded region is no longer fully supported by the shores since the prestressing is now taking some of the weight. This would therefore place an additional downward vertical reaction on the banded areas where the shores are still firmly in place below, therefore these shores are now receiving more load than what they were before.

  

RE: PT effects on shoring

yes there will be an increase in force in the shoring under transverse beam lines, but I've never heard of this being accounted for.
(analogous to a precast bridge girder with camber, only supported at the ends since the center has lifted off the ground)

RE: PT effects on shoring

swivel, JKStruct, and slickdeals are incorrect.  The prestressing does change the loading distribution on the props.  The props should be designed with adequate overload capacity to account for more load being taken along the column lines before the slab carries the load itself.

RE: PT effects on shoring

has anyone ever seen this taken into account in shoring shops or discussions with the shoring engineer?  i see where he's going with it, but i still don't think it really matters.   

RE: PT effects on shoring


I'm with Hokie on this one.  

I've designed AASHTO I-beams and Bulb-Tees using strand layouts to produce an upward camber to counter the dead load and superimpose dead load effects of a CIP slab.

Think of the strands as an eccentric axial load on a column. (horizontal in this case) The force of the strands produces internal moment in the concrete member and results in deflection.

Hope this is helpful.

RE: PT effects on shoring

(OP)
I agree, the shores along column lines should be designed to support the overload force from the tendons.  Now, what about the slabs below?  

The increased shore load along the column lines is accompanied by a decreased shore load at the mid spans.  The column lines below (through the loading path) would see the majority of the slab weight above regardless of where the force is placed, (say 80%/20% for column line load/midspan line load).  This would mean that the uneven distribution of load from the shores above would not affect the slab design below.  Is this an accurate way of thinking?

RE: PT effects on shoring

When all of the tendons have been stressed, the slab actually lifts off the form.  If (a) 100% of the distributed tendons are stressed before any of the banded tendons and (b) the slab is supported by closely spaced shores, then the shores near the column lines (high point of the drape) will gradually take more load as stress is applied.

This is usually not the case, however.  Normally, the tendons are stressed sequentially in both orthogonal directions until all tendons are fully stressed.  

In all PT slabs I have seen, the forms have the capability to span from column line to column line without intermediate supports.  Thus the shores supporting the forms are capable of carrying virtually all of the load at or near the column lines.

Best regards,

BA

RE: PT effects on shoring

I also agree with hokie66. I have had slabs lift enough that the shores below fall out. I don't know of anyone ever taking this into account. Concrete has the ability to redistribute loads so the overload shouldn't be a problem.

RE: PT effects on shoring

Do you have a copy of the California Shoring Manual?  It is more for bridge structures, but there is a section relating to this.    I think it can be found on the California DOT's webiste for free download.

It will depend on the situation of the floor  (continuous, or simple span) and whether there is stage construction (continuous spans with hinges). In the latter, it defintely needs to be accounted for.   In your case, with a 2 way floor, it should be accounted for, because I think your stressing sequence will cause some dead load redistribution.

RE: PT effects on shoring

I agree with BAretired that the strands in both directions should be stressed gradually.  However they are stressed, the loading on the props around the columns will get progressively greater.  You want the props to take the load at this stage, not the column, as early age punching shear problems are to be avoided at all costs.

RE: PT effects on shoring

i've always seen distribtuted first then bands.

i see what happens, but on the jobs i've been, tables have run both in the banded direction on some jobs and the uniform directions on others.  

can we get someone who works on shoring and table design in here?  because a lot of folks who are in here probably do the slab design and not the shoring.   

RE: PT effects on shoring

hokie66,

I agree that the stressing order needs to be considered. If the distributed tendons are stressed first then the band zone needs to be fully supported by the props. Preferrably there shopuld be partial stressing in each direction to reduce the effects, and the banded direction should be partially stressed first, before it is loaded by the distributed direction. This would remove the prop problem! But to get aprtial stressing done properly on site is probably wishful thinking.

RE the punching shear comment, to get the load of the slab itself to the props instead of the column itself after the stressing is completed in both directions would be impossible. For the load to transfer to the props around the column there would have to be separation at the column (so punching shear has already failed).

Another one you you banded/distributed designers.
PTI and associated parties suggest that the banded tendons should be in the long span direction. This is absolutely incorrect and completely illogical structurally. The banded direction should be the short direction. This is the way the slab wants to act. Doing it the other way is going against nature!!

RE: PT effects on shoring

rapt,

We are in agreement as to everything except my "punching shear comment."  My reasoning is that the prestressing applies a gradually increasing downward load to the slab, formwork, and props in the vicinity of the column prior to any release of the forms.  This load does not transfer completely to the column until the props are loosened.  Depending on the time of stripping and backpropping (if any), there can be maturity issues with the concrete.

RE: PT effects on shoring

My experience is that bands typically run in the long direction of the building and not related to span lengths.

RE: PT effects on shoring

Just a side comment here.  Mortgage providers in my location have become increasingly nervous about post tensioned buildings.  The reason for this is presumably the high cost of repairing such buildings in the event of broken tendons.  

A number of buildings constructed in Alberta twenty to thirty years ago, reinforced with unbonded post-tensioned tendons have had corrosion problems due to water entering the plastic sheath.  In one case, some cables reportedly burst through the floor into the living space.  The repair cost was extremely high. Today, the technology is better but the nervousness remains.

It has become common practice for mortgage lenders in Alberta to make the mortgage conditional on a professional engineer signing a form stating that there are no prestressing cables in the building or anywhere on site. Substitute wording is unacceptable to them.  

Just before I retired last June, I received a request to sign such a form for a client who was seeking financing for a wood frame building he had constructed.  He was quite upset when I refused on the basis that I could not guarantee the absence of cables on his site.  I'm not sure if he was able to find another engineer to sign the document.

 

Best regards,

BA

RE: PT effects on shoring

Very interesting, BA.  Sounds like the US and Canada need to start using bonded tendons like we do in Australia.  I have been convinced of this for years since hearing a strand break one night in an empty office building.

RE: PT effects on shoring

I agree, hokie66.  Bonded tendons are the way to go.

Best regards,

BA

RE: PT effects on shoring

Ron9876,

Well logically it should be related to span lengths and not long direction. PTI recomment long span direction but it should be short span direction.

Hokie66,

You beat me to it. basically, the rest of the world has converted or is in the process of converting to bonded PT. Countries where the PT industry is mature tend to have moved to bonded PT. Countries starting out in PT often start as unbonded because it looks easier but they convert to bonded as they mature. UK, Middle East and Asia have all converted predominately to bonded PT with only new startup comapnies using unbonded. UAE authorities like Jafza and DM have finally specified bonded only. There is only one laggard. A little like the SI/US units issue!

I have been warning about this for years, but the PTI and other interested parties in USA continue to deny that there is a problem with unbonded PT buildings in the face of evidence such as yours. If Ingenuity is still monitoring Engtips, he might like to butt in here and give some real insights into the problems as he is repairing them all the time in Hawaii (not ones that he built himnself).

RE Punching Shear,
There may be some load taken by the forms around the column but I would have thought that the column shortening required to make sure that significant load actually gets into the shores near the columns would not occur and that most of it would go straight to the column. It would be interesting to see some test results on it.

RE: PT effects on shoring

rapt,

You may be right about the load going into the concrete column immediately whether it is desired or not.

RE: PT effects on shoring

2
My firm specializes in the design of concrete formwork, shoring and reshoring for over 34 years.

I had an article published in "Concrete Construction" many years ago about PT transfer loads on shoring.

We would look at this as the load being distributed between the I.P. points of the cable; since a two-way slab shoring system usually has a lot of shoring in this area then it is not a problem (sometimes it is though; needs to be checked).

Our experience is that it is a much bigger problem with a one-way slab system since the shoring at a column line will usually end up with all of the load transferred to it.

Additionally, a PT beam that is tensioned but is supported by a PT girder that is not tensioned will create very large loads in a small area.

I will be glad to discuss this further if you request.

Sincerely,
Randy
 

RE: PT effects on shoring

Randy,

Good to hear that your company designs for the PT imposed vertical forces.  Unfortunately, that may not be universal in the formwork fraternity.

RE: PT effects on shoring

Hokie66,
Your right; a lot of shoring designers do not take it into account.  They love to put notes on their plans that it is the contractors responsibility.

Thankfully, there are usually enough shores in a bay and enough redundancy in a concrete floor system so they can get by with it, however risky.

Many years ago we were involved in a PT beam and slab project and I happened to be at the jobsite.  During the stressing the wood shores were actually vibrating and humming!  That is when I started investigating the problem; that was in 1974.  As I previously said, it is especially sensitive on a one-way system.

We use a load balancing method of determining the actual transfer.  Although it is not exactly perfect, it is very close to what should be expected.  

Randy
 

RE: PT effects on shoring

@Randy,
Do you still have a copy of your article on Concrete Construction? Would you be able to send a link for it?

RE: PT effects on shoring

There is no link; I'm an old guy that did it before the internet.  However, I will send you a .pdf if you would like.

Please send your request to Bordnereng@att.net
I will be glad to send it to you.

Randy
 

RE: PT effects on shoring

Dear Slickdeals;

I specialize in the design of shoring/reshoring as does everyone in my firm.  I started my firm in 1989 to provide design services to contractors.  I was an active member of ACI-347 for many years and chaired the committe for 4 years.
This doesn't make me the guru but I have designed about 7 million square feet of shoring/reshoring every year since
1973.  There is a huge demand in the private sector for engineers that are willing to learn the business/take the risk.  Actually, with good contractors the risk is very manageable.  

When I started my firm there was not much need for a PE sealed drawing.  I must have hit it at the right time since I never marketed my company and had more work than I could ever handle.
Randy
 

RE: PT effects on shoring

Attached is an article I wrote concerning this subject for "Concrete Construction" in the late 80's.
Please note that it was directed towards a specific audience and I was showing the concept and what can occur.
A competent PT designer can use better methods to determine the actual reactions due to PT stressing.

Randy

RE: PT effects on shoring

I also wanted to mention; the cover photo of the magazine was one of our parking garages in Houston.  I worked for a concrete construction contractor at that particular time.
Randy

RE: PT effects on shoring

Thanks for sharing your article, Randy.  It does indeed show the concept.

RE: PT effects on shoring

cool.  you learn something new everyday.   

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