Maximum Tendon Length 3

[struggle66]

Good Day to you all,

Eg. TR43 recommends maximum slab tendon length to be 60 m. What will happen if I go longer than that with careful consideration of friction loss?

Another question

Is vibration only serviceability requirement? At what extent will it affect the structural integrity? In one of our company trailer ramp, the vibration is quite obvious.

Thanks for your replies.

 

[Ingenuity]

The longest tendons I have personally stressed (meaning me hands-on with the stressing pump switch and 1 ton ram) is 1,000 ft. Took several hours to multistress 31 strands and get 6 foot of elongation. Not a pleasant way to spend hours inside a box girder bridge.

But I assume your structure is a building.

With "...careful consideration of friction loss" I think you will be fine, assuming your friction calcs are based upon the correct parameters, and double-end stressed tendons.

You may loose some economic efficiency if the tendons are too long, meaning that the amount of prestress uplift (balanced loads) at the centre bays may results in having to 'up' your prestress quantity.

Usually the concrete pour AREA (think concrete finishing) or placement VOLUME (supply or pump limitations) will govern the max length between pours. If your building is too long you will have other issues than just PT losses - like shrinkage/creep/thermal effects, where permanent joints or pour strips may be justified.

We recently reconfigured tendons to an existing PT slab that was 60m (200 ft) long slab that was originally stressed at an intermediate CJ and live-end at the pour 2 slab edge. We destressed the tendons (at live end and CJ) then restressed from both ends (changing the old dead-ends to new live-ends). 60m double live is somewhat common for unbonded PT.

Are your tendons unbonded or bonded?

 

[rapt]

Struggle,

You will have to consider restraint to shortening effects in a lot of detail. Long tendons in bridges are not a problem because restraint is often removed and if there is any it is designed for.

In buildings, it is often ignored or given a cursory look.

The longer the building the more serious the effects of restraint due to shortening due to shrinkage, temperature variation creep and axial shortening due to prestress.

This last one is the smallest of the lot. Shrinkage and temperature variation cause very significant restraint effects in most buildings.

And NONE of the design software considers it fully.

 

[struggle66]

Rapt, Ingenuity Thanks

Here in Singapore only main structural consultant decides whether to have a permanent joint or not. Sometimes we PT specialist dose not have choice but to follow them.

What about the shortest tendon length? any limit? I havnt seen any.

BTW any comment on the vibration?

 

[KootK]

Some good PTI information on short tendons: Link. I can't help much with the vibration impacts on ultimate strength part I'm afraid. At extreme level of vibration, I'd have to think that fatigue issues would start to crop up (Link). It's not a problem that I've experienced or designed for personally however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ingenuity]

Tendons shorter than 20 ft (6m) are not usually recommended due to cost. You wish to amortize the costs of the anchorages over the longest possible length to keep your average tendon length as high as possible.

Do NOT do short tendons like the following photo. Case of structural consultant placing a line through the drawing as saying "PT slab - by others", then holding the PT supplier/designer to the word, resulting in tendons that are less than 2m long.

Also, with power-seating monostrand jacks (as used in the North American unbonded market) the stated wedge-seating loss is usually 1/4", BUT commonly field measurements have it closer to more than 3/8" and on short tendons this needs to be accounted for.

 

[KootK]

Oh! That is so going in my stupid *&%! photo library. Are you sure that's not photo-shopped.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ingenuity]

KootK

Nope. I personally took the photo - no digital manipulation.

I was not involved in the project in any way. I was in Sydney, Australia at the time (2013/2014) and drove past the job site regularly an the way to another project, so as you do, I stopped and took a look when the site was closed. It was a new car dealership being constructed and I was working with an architect on another dealership so I mentioned this one to him, and he said it too was his project so I asked him who the structural consultant was - it was one of the very, very large consulting firms! Nothing against mega firms (I used to work for some large consulting firms) but this was a big disconnect between design/construction for this consultant.

 

[Trenno]

Haha that old gem! I remember you posted that on a thread I started a few years ago.

 

[rapt]

And when you look at the extensions in Ingenuity's pretty picture with the bonded dead ends, extension will be about 10-12mm on jacking and draw-in loss will be 8-10mm after release of the jacks, so there is not much force there! A good way to waste money!

The link in KootK's post to the Bondy article is referring to unbonded PT for the 35' effect of draw-in. In the Bonded world (just about everywhere outside North America and some poorer European countries), the draw-in effect would be over about 6-8m depending on L/D.

Some tendons in the other direction might have been a bit more useful!

 

[struggle66]

RAPT,

Well said

"A good way to waste money"

I will be using this phrase to convince people.

 

[rapt]

Struggle66,

I have copyright on that one!

RE your comment that the consultant tells you where to put the permanent expansion joints. Any joint spacing is possible. You just have to allow for the restraint effects in design and detailing. As long as you provide adequate crack control in the slabs and design the connections to the columns and the columns themselves for any axial effects and moments induced, there is no technical limit. Whether it is economical or not is another matter!

The normal spacing limits of about 40 - 50m are assuming you do not have really strong restraint and you do not make special allowance for restraint effects.

I think the Dubai Airport terminal 1 is about 1000m long with no permanent expansion joints. The RC slabs are reinforced for full restraint, so minimum reinforcement would have to be between .6 and 1%.

When you start adding PT, you have to provide the crack control for something like this using extra reinforcement as PT is awful at providing crack control from restraint as the tendons (bonded of course in Singapore) are too far apart to provide crack control once the concrete is cracked. So you would do your normal PT design for loading assuming the concrete tensile strength is 0 for deflection design as there can be no tension stiffening with the levels of stress you will get from high restraint stresses. Then you would have to add extra reinforcement which combined with the tendons will provide your crack control in the restrained condition.
You still have to have construction joints to limit pour sizes and with PT these have to be positioned to ensure that the tendon lengths are logical taking into account stressing access and prestress friction losses. This is also affected by span lengths in critical spans and the type of stressing access again in those spans.

RE Vibration, unless you have more stringent requirements than normal, L/D ratios of 40 for PT flat slabs are normally considered to give adequate vibration performance. If you have special requirements you would have to do a vibration study on the building and slabs.

 

[RHTPE]

One consideration for tendon lengths that I have not yet seen mentioned - where does one store the 'yet to be encased' portions of the tendons for the portion(s) of the structure not yet built?

My experience is primarily with buildings & parking garages, but one of our careful considerations when determining construction joint locations involved finding space for the portions of the tendons required for the formwork deck that has not yet been constructed.


Ralph
Structures Consulting
Northeast USA

 

[Teguci]

That photo reminds me of a client who required a P/T strand to be wrapped around (a big oval embedded about 6" from the edges) the top flange of a precast double tee.

 

[Ingenuity]

...where does one store the 'yet to be encased' portions of the tendons for the portion(s) of the structure not yet built?

For UNbonded monostrand tendons it is a consideration, but for bonded tendons (and the OP is in a bonded PT market) it is lesser so, as for multi-pour placement the bonded tendons are cut (on site) to match the length of each separate pour and coupled using coupler anchorages and compressions swages at each construction joint.

Also, for bonded tendons, a single (or twin ram) center-hole jack is usually used, whilst for UNbonded a open-throat (saddle) jack must be used to access the intermediate pour tendons without 'threading' the jack over the remaining length of unplaced strand.

Photo of typical ram/jack used for bonded slab-system stressing:

 

[rapt]

Teguci

If you are talking about a tendon that has two anchors on the same face and has a 180 degree return bend at the other concrete face, that is a standard detail with unbonded tendons. Friction would be too high to do it with bonded and dead end anchors in bonded are relatively cheap so it is not as necessary, whereas in unbonded you basically have a live end at each end, so to do it with 2 separate tendons would require 4 anchorages.