PT effects on shoring 3

[ReginaldBoigard]

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.

 

[ron9876]

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

 

[BAretired]

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

 

[hokie66]

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.

 

[BAretired]

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

Best regards,

BA

 

[rapt]

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.

 

[hokie66]

rapt,

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

 

[RHB51]

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

 

[hokie66]

Randy,

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

 

[RHB51]

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

 

[slickdeals]

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

 

[RHB51]

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

 

[RHB51]

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

 

[RHB51]

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

 

[RHB51]

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

 

[hokie66]

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

 

[swivel63]

cool. you learn something new everyday.