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G. Radhakrishnan

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PostPosted: Sat Sep 24, 2011 9:38 am Back to top

Though steel girders are required to be maintained periodically, those are far better in view of safety when compared with PSC girders. Those who are creating assets are not bound to maintain the same in our system, like Open line & Construction. Quality of works differs from contractors to contractors, Engineers to Engineers, Place to Place etc even though the Specifications, Codes, Manuals to be followed are same.

The main advantages in steel girders viz, problems like corrosion can be overcome with protective coating, possibilities of strengthening wherever or whenever or whatever required, easy replacement by steel girders if it warrants, temporary girders of similar spans available in case of emergencies etc

The span of PSC girders mostly are not matching with the conventional steel girder span of 12.2, 18.3, 24.4, 30.5 etc. If any PSC girder fails the alternate arrangement is not confined but we know failure in PSC will be a sudden.

In our Zone, concept of providing PSC girders in place of steel girders during GC is 100% implemented resulting steel girder bridges in MG have been rebuilt with PSC in BG.

Within short period of time, we are facing leaching problems in PSC girders (water is coming from the deck slab to bottom of PSC solid girders) which will lead to corrosion in tendons also and failure in near future.

Due to non availability of bridge staff (No intake for a quite long period) SSE/Bridges are engaging contract system for inspection also. (I have seen such tenders in SWR) Noticing the problems itself now becoming a question mark

At this juncture, what are the precautions to be taken in case of failure or emergencies in PSC girder bridges?
Also, what are the remedial measures being adopted in case of such leaching problems anywhere?

            
            
            
- G. Radhakrishnan  ... 24-09-11

Narasimha Murty Gannavarapu
EXECUTIVE ENGINEER(CON)

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PostPosted: Sat Sep 24, 2011 6:01 pm Back to top

In the good olden days, for higher spans, steel girders were pre-dominantly used. After the advent of pre-stressed concrete, its use in higher span bridges increased tremendously. Main reason behind use of PSC girders is due to its initial economical cost
Main reasons for not preferring steel girder bridges are:
(i) Its initial cost is more.(ii) It requires recurring expenditure in painting.
In contrary to that it is believed that PSC girders are cheaper and
does not require any type of maintenance.
One more merit of PSC Girder Bridge is about its facility of providing the same track structure like in approaches.
Demerits:
As far as demerits of the PSC girders are concerned, these are-
(i)Brittle failure nature. (ii)Less life. (iii)Very difficult strengthening/repair/ restoration
Concrete is brittle in failure. It fails all of a sudden without giving any warning. Failure of Mondovi Bridge at Goa is a burning example.
Contrary to this , steel girders which are ductile in nature gives ample warnings
before failure

Since concrete is very strong in compression,PSC construction has gained momentum.
But unlike RCC ,PSC requires lot of quality control .High durability has to be maintained during construction.The girder should be impervious .otherwise water will percolate in to the member. Since the cables are heavily stressed they will prone to corrosion at a faster rate due to ingressed water.
Sri Radhakrishnan has quoted that the girders are leaking heavily indicates the poor quality in construction which will spoil the girders within short period.
Remedies: (i)Observing the camber at regular intervals inorder to know the soundness of the girder.
(ii)If substantial decrease in camber is noticed ,external pre-stressing is to be resorted through future cablels .
(iii)If any loss of pre-stress is taken place ,it can be noticed near end blocks in the form of shear cracks at 45 ° angle.Hence end blocks have to be observedc periodically.

Railway Board vide their letter No.2005/CE-1/BR-11/8 dtd 28.05.2009 has instructed to adopt steel girders for spans more than 24.40 m.
The reasons as mentioned in the Board,s letter are (1)There are no detailed guide lines for inspection and defects identification of PSC girders (ii)In case of any eventuality on such bridges,Rly. do not have any immediate temporary restoration measures availble up to 80 ft.temporary span arrangements.(iii) At the same time steel bridges are known to have longer life than others and IR have many number of examples.
            
            
            
- Narasimha Murty Gannavarapu EXECUTIVE ENGINEER(CON) ... 24-09-11

G. Radhakrishnan

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PostPosted: Mon Sep 26, 2011 8:50 am Back to top

Regretted to highlight that without developing suitable guidelines for inspection & maintenance, we have switched over to PSC girders! and most of the steel girder bridges have been converted in to PSC.

General comment during inspection is “What is there to see in PSC when compared with steel” without considering the consequences!!

Recently constructed PSC girder bridges during GC have started giving symptoms about their health. Next generation will have to face a lot of problems with poor quality PSC girders. At that time, a new technology may be implemented in lieu of PSC and like our early steel girders all PSC girders constructed before xxxxx shall be replaced!!!
            
            
            
- G. Radhakrishnan  ... 26-09-11

RAM KUMAR SINGH
Sr. DEN/E/NGP/SECR

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PostPosted: Tue Sep 27, 2011 12:19 pm Back to top

Railway has realised the problem soon and has again shifted to steel girders for span more than 24.4m.
            
            
            
- RAM KUMAR SINGH Dy. CE/BR/Spl works/SECR/BSP ... 27-09-11

G. Radhakrishnan

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PostPosted: Wed Sep 28, 2011 5:53 am Back to top

Good.

In the meantime a numbers of PSC girders have been erected with spans more than 24.4m

Specific guidelines are required to be framed and circulated for inspection, maintenance, monitoring the noticed defects, method of recording camber, temporary arrangements, action in case of emergencies including loss of camber, method of replacement in case of failure in the existing PSC girders
            
            
            
- G. Radhakrishnan  ... 28-09-11

Narasimha Murty Gannavarapu
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PostPosted: Fri Sep 30, 2011 12:38 pm Back to top

An article on "INSTRUMENTATION TECHNIQUES TO MONITOR
LOSS OF PRE-STRESS AND CORROSION OF STEEL
IN PRE-STRESSED CONCRETE " can be seen in this link which is useful for bridge maintenance wing:

http://www.rdso.indianrailways.gov.in/uploads/files/1296817034676-bs_36.pdf
            
            
            
- Narasimha Murty Gannavarapu EXECUTIVE ENGINEER(CON) ... 30-09-11

G. Radhakrishnan

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PostPosted: Sat Oct 01, 2011 3:09 pm Back to top

From the above quoted Article, the following sentences are reproduced here.

Concrete structures are supposed to give trouble free and maintenance free service, if constructed properly. Normally, durability aspects are usually lost sight of. Because of which, many times, concrete structures develop defects during their service.

It will be worthwhile to note that no direct method is available to measure loss of pre-stress.

It can be noted that V.W. load cell system is suitable for new construction and not for existing bridge.

Vibrating Wire Strain Gauge is suitable for use on new as well as existing bridges. Not much of literature on actual use of above instrumentation system is available either in India or abroad

No instrument / technique is available, at present, which can measure the extent of corrosion of steel


It will not be incorrect to state that the instrumentation of bridges is still at a primitive stage in India.


Hence, we cannot go for instrumentation of each bridge due to cost affair, SERC or other similar organization to be invited and also instrumentation alone will not solve our problems. We may have to go for rehabilitation, strengthening or replacement after instrumentation.

Due to leaching defect, the reinforcements including tendons will corrode with a period of time which will lead to loss of pre-stress. No instrumentation is further required for corrosion or loss of pre-stress. What will be the remedial action, once corrosion started?

In steel girders, the main problem is corrosion which can be controlled. Even, camber loss girders are still surviving in other topics of this forum.
            
            
            
- G. Radhakrishnan  ... 01-10-11

V. B. Sood
Director (B & S)

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PostPosted: Sun Oct 02, 2011 9:16 am Back to top

There are few things to be considered:

1. The PSC girders are not as fragile as they are normally presumed to be. These are designed as ductile structures as the normal steel/RCC structures are and the cracks will be there in structure for long time before actual collapse takes place, except in the case when the PSC tendons are not grouted properly and snap due to corrosion.

2. Yes, the understanding of PSC girders is on lower side to desired, partly because of lack of training and partly because of lack of difficult nature of subject. But it cannot be said that these were introduced without preparations as the will have the problem of chicken-and-egg situation - whether to first construct PSC, which is a material of choice world over, or first create awareness and inspection methodology. I would say that the approach was OK that way and if the organization cannot get people to learn the things in 30 years, there is no excuse about anything.

3. Our bridge management is not in that great shape. Except for the steel girder inspection and management, which to me is the easiest to learn and master, we are lacking in almost all aspects. The floods and scour is a very much neglected field, sub structure inspection is a joke, underwater inspections have assumed some sort of priority only after killing scores of people on this account. And the less said about modern fields like NDT, the better it is. I had an opportunity to inspect some old arches and I saw old inspection records for 25 years or so and was astonished to see the diverging views that learned engineers have taken over the years and almost none had given a solution that could restore the bridges to a respectable shape. This much for the oldest type of the bridges that we are using. So, the issue of PSC is not that dire when seen in light with its older peers.

4. The problem in the particular case appears due to the slab connecting the I girders. These slabs are the last item of construction and the time constraints can cause the problems mentioned. But the problem will most likely be in RCC and not PSC components. If this is the case, you can cheer up as RCC is lot easier to maintain and rehabilitate than PSC, and also the water might not be affecting the tendons. We had dealt with some such cases on WR and I had made a film on the same which can be seen here:

http://wiki.iricen.gov.in/doku/doku.php?id=training_films

If the problem is different, please let us know.
            
            
            
- V. B. Sood Director (B & S) ... 02-10-11

G. Radhakrishnan

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PostPosted: Wed Oct 05, 2011 12:18 pm Back to top

In my case is different. PSC solid girders of 10 spans of 9.14m each in a bridge constructed during the year 1998, are having leaching defect (from the top of deck to bottom) in almost all the girders. Seepage of water, after raining, can be visually seen. Also, at the junction of retaining wall connecting deck slab in so many bridges including ROBs such leaching defects were noticed
            
            
            
- G. Radhakrishnan  ... 05-10-11

G. Radhakrishnan

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PostPosted: Wed Oct 12, 2011 7:33 am Back to top

In PSC box girders, the open spaces are mis-used by public. I found that one PSC box girder ROB is used as mini BAR. Shall we close the openings provided in the box girders.

On seeing such mischievous we get warning as "danger ahead!"
            
            
            
- G. Radhakrishnan  ... 12-10-11

A.P. MALIK
DyCPM/C/III

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PostPosted: Wed Oct 19, 2011 1:52 pm Back to top

There are many factors that go into deciding the material to be used in construction. First and foremost of these is the reality and facts as they exist, instead of what we think they should be or assume them to be. Our assumptions must as close to reality as possible.

And the fact is, PSC is not the material for our bridges. Our quality control, handling equipments, maintenance resources are not fit for PSC girders. For spans of 9.15m and above, we should stick to steel girders, the tried, the tested and the elegant elements. For sleepers over them we may approach SC to allow timbers, as volume would be much less now, as for everything else we have switched over to PSC sleepers.

Unlike road, we can not divert traffic at will. We get very limited time for repairs and replacement. Any failures, unlike in a road bridge, are potential disaster for us. Our sites are where it is very difficult to take cranes to handle the weights involved. Just to give you an idea, total weight to be handled in case of PSC 9.15 m span is 82 tonnes (minimum component weight being 20/30 T, depending on whether pre or post-tensioned) as gainst about 9 tonnes for a 9.15m steel span.
            
            
            
- A.P. MALIK DyCPM/C/III ... 19-10-11

Avijit Mujumder

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PostPosted: Tue Oct 25, 2011 3:14 pm Back to top

Apart from above defficiencies, the height of deck in case of PSC girder is another detrimental factor which can not be ignored as the approach of bridges is to be raised. In case of tall piers, seismic loads on heavier superstructure(PSC girders) has considerable effect on substructure & it's foundation.
            
            
            
- Avijit Mujumder  ... 25-10-11

G. Radhakrishnan

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PostPosted: Mon Nov 21, 2011 7:23 am Back to top

The two merits mentioned by EXECUTIVE ENGINEER/CON for the use of PSC girders also having the following dis-advantages

(i) The “Main reason behind use of PSC girders is due to its initial economical cost” is applicable for new construction only. In open line, replacement of steel girder with PSC girder will cost more due to expensive launching scheme, prolonged speed restriction & mega line block

(ii) One more merit of PSC Girder Bridge is about its facility of providing the same track structure like in approaches. But, without adhering the manual provisions, the same track structure of approach (LWR) has been continued on the PSC girder bridges. If anything happens then only responsibility will be fixed and also circulars will come. So, PSC girder facilitates for mis-leading the LWR manual provisions
            
            
            
- G. Radhakrishnan  ... 21-11-11

Narasimha Murty Gannavarapu
EXECUTIVE ENGINEER(CON)

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PostPosted: Mon Nov 21, 2011 4:52 pm Back to top

Mr.Radhakrishnan has expressed view in the maintenance point of view.
Apart from certain dis-advantages ,PSC girders have numerous advantages also undoubtedly.
Since steel girders had long history and stood for the test of time and more over the steel is manufactured at steel plants with highest quality control, no doubt they are superior and stronger to PSC girders.But the disadvantage with them is the maintainability of track over those girders.Last decade P-Way Engineers suffered a lot on that score.Then only PSC girders have come in to picture.
Regarding replacement of steel girders with PSC,it is restricted upto 6.10m only and over and above they will be cast-in-situ only.
As far as continuing of LWR over PSC girder bridges are concerned , certain Railways have officially undertaken the same on trial basis and are giving better results.Offcourse Konkan Railway has already continued their all LWRs on girder bridges provided with bearings and no any un-toward incidents have been noticed .
Manuals are suggestive only and does not restrict the betterments.Due to this flexibility ,correction slips are being issued as and when needed.
After all we the scientific and engineering community should always think for better changes and accordingly experiments have to be continued.
Gaps between single rails have been provided for several years with a view that linear expansion will create problems until LWRs have been discovered and proved it was wrong.
Rutherford ‘s quantum theory regarding electrons revolving round the nucleus was proved wrong till Bohr ‘s experiment revealed that they will revolve in fixed orbits after 60 years .That is sscience and changes will always takes place.
As far as PSC girders are concerned ,the main problem with them is with the quality at site.They require high standards.Very few contractors aware of the same and they thought that quality is the responsibility of the railway.
Un like steel ,concrete is a heterogeneous mix and has to be made at site ,maintaining quality and achieving uniform quality is a toughest task for the Constn. Engineers.Knowing all these facts , Railway Board has restricted its use up to 24.40m girders.
            
            
            
- Narasimha Murty Gannavarapu EXECUTIVE ENGINEER(CON) ... 21-11-11

G. Radhakrishnan

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PostPosted: Mon Nov 21, 2011 10:58 pm Back to top

I have focused in maintenance point of view only.

In Pamban Bridge, we have replaced the PSC (MG) girders with steel girders by gantry method.

Steel girder spans above 6.10m are being converted in to PSC by adopting suitable launching scheme in traffic condition.

PSC girders itself may require replacement!

Better changes (in the material of construction) are always welcome. At the same time, technology for maintaining the same also to be developed duly considering the vulnerable factors such as quality of construction, environmental condition, workmanship etc.,

Better understanding is essentially required when we changeover to a new system

As mentioned that very few contractors are taking care of quality, very few Engineers are worried about quality and hence very few bridges are in good condition

Concrete steps are required to be taken to safeguard our assets of concrete bridges

Steel girders >>> Concrete girders >>> Composite girders >>> ?
            
            
            
- G. Radhakrishnan  ... 21-11-11
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