Prestress Precast I-girder - horizontal sweep 4

[Bridge_engr]

Hi all,

What are the potential causes if a precast prestressed (post tensioned) I-girder slender beam deforms in the lateral direction after release of the prestress force:
I know some of the reasons are:

eccentric tendons in the lateral direction;
imperfections in the casting of girder;
unbalance force transfer


How much role does slenderness of the beam has in deforming in the lateral direction?
any thoughts?

 

[KootK]

The more slender the girder is in weak axis bending, the less able it will be to resist lateral movement induced by the sources that you mentioned.

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]

precast prestressed (post tensioned) I-girder...

Are you sure it is POST-tensioned? Usually PRE-tensioned for a precast I, or sometimes a combination of PRE and POST, however, such combination girders are usually not too slender.

EDIT: For a PRE-tensioned slender girder, the pattern and method of release can effect lateral sweep.

 

[cooperDBM]

As Ingenuity says it's sounds like you might be talking about a pre-tensioned girder (initially at least), particularly your reference to release. If that is the case, you should have a look at the references listing in section 8.4.1 of the PCI Design Handbook about lateral stability. A larger than normal initial sweep to the girder may make lateral stability during lifting and transport more of an issue.

In case you're in Canada, section 3.9.3 of the CPCI Design Manual, 5th ed., gives more detail and an example based on the PCI references. You can obtain a free electronic version of the Manual from CPCI at

http://www.cpci.ca/en/resources/technical_publications/.

Lateral stiffness will certainly effect the degree of sweep. Any asymmetry may cause a sweep including the order in which strands are released. A sweep can also be caused by external influences like wind or sun exposure one one side. Those are usually temporary but may cause some built-in distortion due to unbalance shrinkage and creep on the still greenish concrete.

 

[Bridge_engr]

Thank you all for the useful feedback.

Let me add some info which may be helpful in the discussion and might answer your comments as well:

The precast girder is a 41m long, 2.1m deep and yes it is post tensioned with 3 tendons. After the release of the prestress force while the girder remains on the temporary bed, we've noticed lateral deformation occurred in some of the girders ranging from 10mm to 60mm with one exceptional case of 150mm. Now some of the strongest reasons for this behavior I believe are:

1-eccentricity of tendons (perhaps occurred during placement of tendons and during casting of concrete);
2-girder end restraint conditions at the base of the girder (after prestressing, concrete at the base of the girder broke as the girder was perhaps not able to move);
3- low friction value

Considering the point at 1 above, PCI allows tolerance for duct placement as 6mm and 6mm for concrete casting imperfections with a total (lets say) allowable eccentricity of 12mm. Even if I consider 12mm straight eccentricity, the deformations occurred at site are still higher than if it is calculated with 12mm lateral eccentricity. My question is: Does slenderness of the girder has any role in the increased deformation?

 

[Bridge_engr]

KootK
Yes I agree with your point, but I believe there will be no bending if tendons has zero eccentricity? even the girder is slender.

Ingenuity
I did not find any reference for slenderness criteria to be used for precast prestressed beams. I mean is there any slenderness ratio criteria based on which we can call a girder section to be slender?

cooperDBM
Thanks for the references. But i understand these references are related to lateral stability during handling, transportation and erection which is also important.
You are right any asymmetry (girder imperfections) will affect the sweep. And yes the external influences such as temperature are being considered but they have very little effect. The girders are still in the precast yard area and are on the ground.

Thank you so much for your comments.

 

[rapt]

I had this problem many years ago on some post-tensioned precast I beams.

The Bed was found to be put of alignment, a nice gentle curve!

We made them straighten it!

 

[KootK]

Are you concerned that the girders may be buckling as a result of the post-tension anchorage forces applied at the ends? That should not happen so long as the tendons are made to travel laterally in unison with the girder. Any reason that wouldn't be the case here?

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.

 

[Bridge_engr]

KootK
Yes you are correct, it should not happen if there is no lateral eccentricity.

The fact that large sweep values are observed after release of the force which indicates one of the following and these could not be verified as far as i know:
1-lateral eccentricity of the tendons;
2-casting tolerances;
3-unbalance force application at the ends;
4-end bearing conditions (this was spotted after application of force as the girder showed concrete cracked at the ends)

I am not too sure though, upto how much extent this problem is related to the section design of the girder and how the slenderness of the girder would contribute to such large values. I didn't find any related references

 

[Bridge_engr]

I think this ACI 318-14 R9.2.3.2 clause is quite related to this issue:

If the duct size is not excessively larger than the prestressed reinforcement, the axial prestress force cannot buckle the member.

 

[KootK]

If the duct size is not excessively larger than the prestressed reinforcement, the axial prestress force cannot buckle the member.

Right. So how large is the duct relative to the reinforcement? And is it ungrouted?

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.

 

[Bridge_engr]

KootK,

In my case, the duct size is not excessively large and therefore it is not the reason. The grouting was done after the application of force.

With this clause,I was thinking that the behavior of prestressed member as compared to that of column or other compression members is different. The prestress force will not allow the member to buckle as the compressive force is internally applied along the length of the member whereas in other normal cases, force is applied externally to the members. Is it right?

 

[KootK]

The prestress force will not allow the member to buckle as the compressive force is internally applied along the length of the member whereas in other normal cases, force is applied externally to the members. Is it right?

I agree with the result but not the mechanism. In post-tensioned concrete, the axial force is usually delivered externally at the ends of the member. It is the fact that the tendons are forced to buckle along with the concrete that keeps things stable. As the member would tend to drift laterally, the tendons tend bring it back in line.

Are your post-tensioned tendons draped? Any chance the balancing load from the post-tensioning is causing the member to laterally torsionally buckle?

I thought that these kinds of members were generally pre-tensioned at the fabrication plant and then post-tensioned in place in the field. What's the reasoning for doing the post-tensioning at the plant? I have little experience with bridge structures so you'll have to forgive my lack of understanding with that.



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]

I thought that these kinds of members were generally pre-tensioned at the fabrication plant and then post-tensioned in place in the field. What's the reasoning for doing the post-tensioning at the plant?

In my (limited) experience with precast bridge girders, some precasters do not have the stressing bed capacity to provide the required total prestress. In such situations, we have maxed-out the precasters PRE-tensioning capacity and supplemented with grouted (bonded) POST-tensioning. Also, for deep section precast girders the harped strand hold-down forces for pre-tensioned strand can be significant for the casting beds/forms, so draped PT is able to solve such issues. The OP's experience/reasoning may be different - like site-cast where a bed is a non-stressing bed etc.

Any chance the balancing load from the post-tensioning is causing the member to laterally torsionally buckle?

Can you expand on this?

 

[KootK]

Thanks for the explanation Ingenuity.

Can you expand on this?

It seems to me that post-tensioning generally puts the concrete into flexure, even as it just sits in the yard. Where there's flexure, there's usually potential for lateral torsional buckling and compression flange lateral displacement. The tendons probably help resist this to some degree as they do for column buckling effects but I don't think that it's straight, AOK situation in the same kind of way.

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.

 

[KootK]

Or maybe it's impossible to have LTB in the absence of torsional end restraint... More thinking required.

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.