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post tensioning in arch girder

 
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jebincet
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PostPosted: Mon Mar 11, 2013 9:22 am    Post subject: post tensioning in arch girder Reply with quote

Hi all,

Good day!!!
The project i'm working now is a bridge with post tensioned Tie girder. I'm in the contractor side and we have assigned a consultant to verify the designs done by client's consultant. As per the client's design, there is an upward camber due to the post tensioning of about 500mm. But, our consultant says that the post tensioning in an arch beam causes a reverse effect unlike other post tensioning applications. They say, when the strands are tensioned through the ducts of the arch girder, a downward force is acted and there happens a downward deflection which is causing a negative effect. Can any experts in the post tensioning field give an expert opinion in this matter?? Please.....
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bsec
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PostPosted: Mon Mar 11, 2013 4:20 pm    Post subject: Re: post tensioning in arch girder Reply with quote

Dear jebincet,

What is a Tie Girder ? What is arch girder ? what is the span length for the bridge ? How many lanes it is carrying ? Is it a simply supported span or a continuous span ? Can you please give a sketch of the said girder and also the prestressing details so that the shape is clear ?

Best Wishes

Alok Bhowmick


It is not
jebincet wrote:
Hi all,

Good day!!!
The project i'm working now is a bridge with post tensioned Tie girder. I'm in the contractor side and we have assigned a consultant to verify the designs done by client's consultant. As per the client's design, there is an upward camber due to the post tensioning of about 500mm. But, our consultant says that the post tensioning in an arch beam causes a reverse effect unlike other post tensioning applications. They say, when the strands are tensioned through the ducts of the arch girder, a downward force is acted and there happens a downward deflection which is causing a negative effect. Can any experts in the post tensioning field give an expert opinion in this matter?? Please.....
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suresh_sharma
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Joined: 23 Mar 2011
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PostPosted: Wed Mar 13, 2013 1:30 am    Post subject: Reply with quote

It all depends on the profile of the duct. If the profile of strand is curvilinear i.e on the bottom layer of the arch at the crown  and top layer at the springing level of the arch, then what designer is suggesting is ok but if the the profile is parallel to the axis of arch then what your consultant is saying is correct. But profile is not likely to be parallel to the axis of the arch. From submission of your query it is clear that profile is curvilinear because you have stated  that camber is upwards.

I have never designed PT beam and only supervised some small works and made a mild study on the topic.

It would be nice if some real expert steps in to  deal with the matter in question.
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jebincet
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PostPosted: Wed Mar 13, 2013 4:55 am    Post subject: Reply with quote

Hi Er. Alok Bhowmick  and Er. Suresh Sharma,

Attaching a diagram showing the elevation of the bridge. Its a single span (122m ) suspension bridge in which the deck is post tensioned. The construction sequence is like this, first the deck is erected, then it is post tensioned. Then the steel arch is erected, then the hangers are installed, then they are tensioned. Now, the doubt i asked is while the second stage of the construction sequence. ie, during post tensioning of the deck(two girders are there in the deck and the longitudinal post tensioning is done in the girder only). while post tensioning, the deck is in arch shape, so the profile of the tendon also is arch (parallel to the longitudinal axis of the girder). But, as this girder is curved upwards, the tendon also is curved upwards. But, for a straight beam when it is prestressed, the curve of the tendons is given downwards to cater the sagging moment(or lets say, downward deflection). But, here, the moment is still sagging but, the initial curvature of the tendons is given(due to the initial shape of the girder) upwards. SO, during post tensioning, it will cause an adverse effect??????



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bsec
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PostPosted: Wed Mar 13, 2013 7:57 am    Post subject: Reply with quote

Dear Mr jbincet,

The bridge type you have shown is called "Network Arch Bridge" and not suspension bridge. As far as the deflection pattern of the tie girder is concerned, as I understand from the sketch, the girder is originally placed in an arched profile (with 2m upward camber at centre). Therefore when prestressing force is applied, the prestressing cables will have a tendency to become horizontal. So it will give a downward radial thrust to the beam, causing a 'downward' deflection of the girder.

Best Wishes

Alok bhowmick

jebincet wrote:
Hi Er. Alok Bhowmick  and Er. Suresh Sharma,

Attaching a diagram showing the elevation of the bridge. Its a single span (122m ) suspension bridge in which the deck is post tensioned. The construction sequence is like this, first the deck is erected, then it is post tensioned. Then the steel arch is erected, then the hangers are installed, then they are tensioned. Now, the doubt i asked is while the second stage of the construction sequence. ie, during post tensioning of the deck(two girders are there in the deck and the longitudinal post tensioning is done in the girder only). while post tensioning, the deck is in arch shape, so the profile of the tendon also is arch (parallel to the longitudinal axis of the girder). But, as this girder is curved upwards, the tendon also is curved upwards. But, for a straight beam when it is prestressed, the curve of the tendons is given downwards to cater the sagging moment(or lets say, downward deflection). But, here, the moment is still sagging but, the initial curvature of the tendons is given(due to the initial shape of the girder) upwards. SO, during post tensioning, it will cause an adverse effect??????
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jebincet
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PostPosted: Wed Mar 13, 2013 10:04 am    Post subject: Reply with quote

Ok..if it so, what is the use of giving post tensioning here?? if it is making an adverse effect?? that is my doubt.. is it just to give more stiffness to the P.T. girder??
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bsec
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PostPosted: Thu Mar 14, 2013 2:40 am    Post subject: Reply with quote

Dear jebincet,

The structure is a 'tied' arch. The springing points of the arch are connected by the girder, at deck level, which acts as a 'tie' during service stage. This ensures that no horizontal thrust is transferred to the foundation due to arching action. Prestressing of the girder therefore obviously helps to counter the tensile force created by the tie action.

Best Wishes

Alok Bhowmick


jebincet wrote:
Ok..if it so, what is the use of giving post tensioning here?? if it is making an adverse effect?? that is my doubt.. is it just to give more stiffness to the P.T. girder??
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