Post-Tensioning of Bridge Segments - Methodology

[flamby]

I have a situation where a penstock (water pipe of 6m dia) is to be carried across a creek, 50m wide. I am thinking of designing a PT girder for this.

Can someone suggest a "standard" erection scheme for this? I have no experience in segmental PT construction and trying to understand how to hold the things up before post tensioning. I have to only design the beam. Construction will be done by specialty firm but I need to understand how they may go about the launch.

Thanks.

Ciao.

 

[Zambo]

This should turn into an interesting thread. Usually an overhead or underslung launching gantry is a good option for segmental construction, but probably not economic for only one span. What are the dimensions of the "creek". Would scaffolding be possible for most of the span with some steel I beams or plate girders to span the centre section?

Zambo

 

[Qshake]

I think ZAMBO is right: this should be interesting.

If the application is precast, there may be problems in shipping the precast section on sight. Also, if the precast section is of substantial bulk (buld tee) there will be problems in erecting the single girder along with stability during erection.

If the application is cast-in-place a significant amount of shoring will be required to support the girder's wet concrete. And then there is the overhead gantry method and the cable towers and so on...

It has always amazed me that in building a bridge of some sort, we accept using a cable application for erection but not for final design. Here is a case that seems just right for cable stay design or even suspension. There are a lot of short span suspension bridges for utility applications. Utimately, the cable and suspension methods will also afford more freeboard too. That should be a benefit to the water company or whoever the client is.

Regards,
Qshake

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[Zambo]

I guess Flamby has some kind of design brief. If segmental rather than beams the weight of each segment should be less - maybe 30 tonne (but of course any length of segment could be used to either increase or decrease the weight. If a scaffold (better described as heavy duty falsework) was used then a crane with enough capacity to reach to the middle of the span would be needed. This then depends on the size of the creek and how much backfilling could be used to get the crane nearer to the placement point.

If using a heavy duty falsework there would be moveable support frames needed on the towers so that the segments can move to accommodate shortening during prestressing.

Zambo

 

[flamby]

Thanks Zambo and Qshake for your comments.

One clarification, that we are still in the Planning stage of the project and actual design-work may be on the cards one year down the line. Hence PT, cable stayed (as suggested by Qshake) or even traditional steel truss girder - all options are open at the moment. The primary task is to plan a feasible solution. I have not even seen the site as yet.

As per maps, this is a penstock which is running along a steep hill contour, and the "creek" is actually a drain which runs only during Monsoon (June to September in India).I guess, the bed of the drain is so covered with boulders, erecting scaffolding is dooomed to be a full scale project. The PT is client's idea, a State owned power company. I, personnay would have gone for steel bridge for this. What is your idea on this?

Qshake,

Can you give me any references (about 50m span) of cable bridges for utilities? This will help me impress the clients.

Thanks again.

Ciao.

 

[letrab]

Hi Flamby,

This is a seriously large pipe and consquently likely to be purpose made.
It may be useful to consider the pipe as the structure and omit additional supporting structures as you have a 1 to 10 depth to span ratio.

Regards,
letrab

 

[Zambo]

Letrab,

I guess this is back to Qshake's suggestion that the pipe be cable supported, this would probably be more cost effective than strengthening the pipe to span 50m without a supporting system. But Flamby is still stuck with ideas on the construction method.

Zambo