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Mumbai foot bridge collapse - reasons?

 
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abhio
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Joined: 08 Mar 2010
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PostPosted: Sat Mar 16, 2019 6:02 am    Post subject: Mumbai foot bridge collapse - reasons? Reply with quote

Dear all,

I have seen some photos of the bridge at CST after the failure. It is seen that the bridge trusses are intact, but about 14m length of the deck has collapsed. This deck (as seen from the portion still standing) was in the form of precast planks on steel beams. The beams spanned from truss to truss, at each bottom chord node of the truss. It appears that the connections of the ends of each beam has failed simultaneously for 7 beams.

What would be the mechanism for such a simultaneous failure? I should think that the deck acted to tie together the beams, and the failure spread from one overloaded member to the next, until some weakness in the decking allowed it to fail without overloading the next beam (it is seen that there is no longitudinal reinforcement, and probably no structural topping for the precast slabs).

Hope seniors can shed some light on the matter.

Regards,

A S Oundhakar
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N. Prabhakar
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Joined: 25 Apr 2009
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PostPosted: Sun Mar 17, 2019 5:34 am    Post subject: Reply with quote

Dear Mr. Oundhakar and Sefians,

In My opinion, the sudden collapse of the bridge deck is due to shear failure of the cross beam joint at the truss location due to high corrosion.  It is not the case of flexural failure due to overloading of the beams supporting the bridge deck as such failures give enough warning with large deflection before collapse.  As the beam joint with the truss is not accessible due to bridge deck laid over it, there may not be any maintenance done over a period of 30 years to stop corrosion at the joint location.  The corrosion at the joint must have caused thinning down of supporting members considerably  at the joint location which has resulted in sudden shear failure.

With best wishes,

N. Prabhakar
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abhio
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PostPosted: Sun Mar 17, 2019 5:55 am    Post subject: Reply with quote

Dear Prabhakar Sir,

Thank you for your insights. It is clear that this is a connection failure, probably due to fatigue compounded by corrosion. However, I am still mystified why multiple beams failed simultaneously and at both ends, if there was no effective longitudinal tying between different spans. I hope you could guide us.

Respectful regards,

Abhijeet Oundhakar
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N. Prabhakar
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PostPosted: Sun Mar 17, 2019 6:22 am    Post subject: Reply with quote

Dear Mr. Abhijeet Oundhakar,

I am equally mystified how all the beam supports failed simultaneously.  It appears, the joint supporting members have corroded more or less equally over a period of time as the environmental conditions are same at all the supports.  The forensic tests of several of these joints may give a clue about the equal corrosion, there by the sudden simultaneous failure.

With best wishes,

N. Prabhakar
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N. Prabhakar
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PostPosted: Mon Mar 18, 2019 5:28 am    Post subject: Reply with quote

Dear Mr. Oundhakar,

Your observation about the effective longitudinal tying between different spans is in fact provided by the bond developed  between precast deck slab and floor tiles which are laid over it with mortar bedding. It appears that the precast concrete deck slab is simply supported, without any connection, on the cross steel beams provided between the trusses.  

The  mechanism of failure is that one of the cross steel beam  alone has suddenly collapsed due to shear failure of corroded parts at the support joints, there by the concrete deck slab and tiles over the collapsed steel beam  is over hanging from the adjacent steel beam due to effective tying provided by bedding of the tiles over the deck slab.   This leads to increased load on the adjacent steel beam from 1 span load to 1.5 span load which immediately causes adjacent steel beam with over hanging deck slab and tiles over it also to collapse due to  shear failure at support joints.  This mechanism continues immediately with series of collapse one after the other, of all the adjacent steel beams and the deck slab and tiles over it.

I would like to see in this posting views of other members of SEFI on this important structural detail.

With best wishes,

N. Prabhakar
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vegad
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Joined: 25 Dec 2013
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PostPosted: Mon Mar 18, 2019 6:06 am    Post subject: Reply with quote

It was due to 'growing population' - as told by moral and technical experts of nation.


And as population itself is an act of god; the experts may come to conclusion that the failure was also an act of God.

The experts are advocates of 'terrorism of irresponsibility, terrorism of non-accountability'.
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abhio
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PostPosted: Mon Mar 18, 2019 6:49 am    Post subject: Reply with quote

Sir,

Politicians' greatest skill is in reaching positions of responsibility and then in avoiding all responsibility. However, I would suggest that this forum isn't the right place to discuss/ curse them.

Let us leave the politicking to politicians, and try to understand the failure so that we can learn how to avoid such failures in the future.
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vegad
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PostPosted: Mon Mar 18, 2019 10:09 am    Post subject: Reply with quote

Dear Abhio,

Thank you for your request. The technical discussion meets close to 'dead end' by the reasoning provided by Prabhakar Sir.

I would like to add - absence of plan truss, which might be one of the reason to trigger the mechanism already on verge of failure.

its easy to fall in love to allopath's approach - symptomatic one - while discussing solutions.

The reason it appears that i spilled out of technical discussion is that a quick RCA of the problem does not allow the omission in discussions of the governing offices, civic admins (controlling funds) and approach of the elected civic representatives (controlling decisions) towards the infrastructure and its maintenance.

We have paid the civic system with the taxes, lives of citizens and careers of so many sensible engineers who themselves and their family have toiled hard to make them one.

We have been charged for smart cities and we are not even getting basic and functional ones.
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sushil_singh
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PostPosted: Tue Mar 19, 2019 8:21 am    Post subject: Re: Mumbai foot bridge collapse - reasons? Reply with quote

It appears to me that the cross beams do not sit on the Main bottom girder but sideways with a connection cleat. The load transfer from "Most" of the dead load and "All" of the live load happens through this cleat. The frequent addition/ deletion of flooring on the top adds to the problem. Cleats can fail multiple ways primary being the weld failure due to corrosion. The cascading effect on the adjacent panel once crosses 1.5 times( A probable factor of safety adopted) will fail the adjacent panel.
Frankly speaking, Most if not all the foot over bridges in Mumbai over railway lines are this type of design where the entire pathway hangs from the Truss.

Sad part was to see a JCB trying to dismantle the bridge parts right below it with no safety parameters in place.......

abhio wrote:
Dear all,

I have seen some photos of the bridge at CST after the failure. It is seen that the bridge trusses are intact, but about 14m length of the deck has collapsed. This deck (as seen from the portion still standing) was in the form of precast planks on steel beams. The beams spanned from truss to truss, at each bottom chord node of the truss. It appears that the connections of the ends of each beam has failed simultaneously for 7 beams.

What would be the mechanism for such a simultaneous failure? I should think that the deck acted to tie together the beams, and the failure spread from one overloaded member to the next, until some weakness in the decking allowed it to fail without overloading the next beam (it is seen that there is no longitudinal reinforcement, and probably no structural topping for the precast slabs).

Hope seniors can shed some light on the matter.

Regards,

A S Oundhakar



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