Reducing the sweep of a prestressed box beam.

[Syr111]

A prestressed beam on one of my projects has a sweep of 1.5 inches which is more than the 1 inch tolerance. The prestressed beam fabricator is planning on reducing the sweep by post tensioning the beam. Has anyone used post tensioning to reduce sweep in prestressed beams using this technique, and if so did it work?

 

[KootK]

I haven't done this but I'd have no issue with it so long as the engineering checks out. The lateral curvature of the PT will introduce additional prestress losses that may need to be accounted for. Also, you don't want durability issues due to cracking on the concave side of the beam when it is put into tension. What is the application here? Bridge?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[gwynn]

I haven't done this either, but you could potentially do this with post tensioning on both sides of the centroid so that the increase in P/A makes up for the M/Sy from the off centre post tensioning needed to correct the sweep. In other words, without increasing the chances of the concrete cracking.

 

[Syr111]

Thank you guys for your input.

KootK, yes this is for a bridge. Its actually the fascia beam of the bridge. I am researching on how I can tackle this problem using AASHTO LRFD. My starting point is a transverse post tensioning calculations that I did on one of my projects, very similar to example one of this link:

http://www.fhwa.dot.gov/bridge/prefab/if09010/appa.cfm

Thank you again for your help!

 

[cooperDBM]

What would be the consequence of accepting the sweep without the P/T? Are the boxes spaced out? Since it's the exterior beam do you expect the sweep to be unacceptably visible? The small increase in torsional load could be checked but it's probably small compared to the live load torsion the boxes are designed for. You should rule out the do-nothing option before changing the state of stress in one of the beams, making it unsymmetrical vertically. It could affect the long-term (creep) camber/deflections of that box and possibly cause a twist.

If you were to go ahead you should make sure the deck is cast shortly after the post-tensioning to reduce the chance for unwanted twisting.

 

[Syr111]

the beam is an adjacent box beam 3' wide by 3.25' high and 104 ft long. the post-tensioning force applied is 21 Kips which reduced the sweep to 5/8". I want to demonstrate the following.

1.show that the force required to take out the excess deflection does not exceed the allowable cracking capacity of the beam.

2. Design calculations showing that the transverse tendon anchor plates and surrounding concrete are not overstressed.

Any good examples or instructions that can guide me would be helpful.

 

[cooperDBM]

In calculating the force required use the latest concrete test you have for the beam (presumably 28 day) and add a bit more. Your concrete strength will probably be quite a bit higher than the specified strength, leading to greater stiffness (Ec).

 

[chicopee]

Aren't ducts needed within the concrete beam for post tensioning?

 

[Syr111]

yes you are correct I forgot to mention that that there are five ducts in this beam. only the middle duct was used to adjust the sweep on this beam.

Using LRFD the allowable concrete strength is 0.6 x fc' with fc' = 10 ksi, so I get an allowable concrete strength of 6 Ksi. Now the force applied is 21 kips, also keep in mind that this force was applied only at the middle duct, which means it will be distributed on the diaphragm of the beam which has an area of 518 in2 (14" x 37"). 21,000 lbs / 518 in2 = 40.5 psi. That's what I get.

is that all I have to do, to check that concrete will not crush at the ends? am I missing any thing?

 

[hokie66]

I think there is some confusion here as to direction of the proposed post-tensioning. I assume this is transverse post-tensioning as Syr111 indicated, but some comments seem to be talking about eccentric longitudinal prestress. As long as there is sufficiently stiff anchorage for the transverse post-tensioning, I would see no problem in straightening the beam in that manner.

 

[cooperDBM]

Thanks Hokie, I did misunderstand that.

 

[Mixtli]

Hi all,

I would investigate first what caused the sweep. Is this pre or pos tensioned, it pre, it could have been an imbalance on the stressing force among the strands.....

Cheers