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Bearings
- Thread starter Hariharan
- Start date
Briefly :
Just about all bridge bearings pre 1950 or so were metallic - (cast iron, steel, bronze, monel metal, whatever), as rockers, rollers or sliders.
For heavy loads and/or large displacements (ie most long-span bridges), metallic bearings would still be the norm.
For lighter loadings elastomerics appear to be the economic solution. There is a school of thought that suggests they require less maintenance - perhaps time will tell.
Just about all bridge bearings pre 1950 or so were metallic - (cast iron, steel, bronze, monel metal, whatever), as rockers, rollers or sliders.
For heavy loads and/or large displacements (ie most long-span bridges), metallic bearings would still be the norm.
For lighter loadings elastomerics appear to be the economic solution. There is a school of thought that suggests they require less maintenance - perhaps time will tell.
I agree with Austim's comments and offer the following.
PTFE bearings are quite common as are POT bearings. Both types are used on long span steel stuctures and only when either the load or displacement require them. The reason? They are expensive to manufacture. Here in the midwest the price for complete in place PTFE sliding bearings is approximately $6000 each. These bearings are characterized by steel sole plates and a 1/8" stainless steel plate with mirror finish which sets on a Polytetrafloroethylene (PTFE) surface (hard plastic!). This accomondates the expansion. the bearings are quite large and therefore very stable. POT bearings are similar in allowing expansion through PTFE and stainless steel but are manufactured to handle larger loads.
PTFE bearings are quite common as are POT bearings. Both types are used on long span steel stuctures and only when either the load or displacement require them. The reason? They are expensive to manufacture. Here in the midwest the price for complete in place PTFE sliding bearings is approximately $6000 each. These bearings are characterized by steel sole plates and a 1/8" stainless steel plate with mirror finish which sets on a Polytetrafloroethylene (PTFE) surface (hard plastic!). This accomondates the expansion. the bearings are quite large and therefore very stable. POT bearings are similar in allowing expansion through PTFE and stainless steel but are manufactured to handle larger loads.
- Thread starter
- #4
Thanks, austim, Qshake.
I understand that Pot bearings for a concrete bridge of about 30
metre span cost about $650 here in India. I don't know the
details, but was curious because the total amount spent on
bearings would be quite substantial for a long structure. I
understand that the life of an elastomeric bearing is about 25
years or so. Bridges and other infrastructure items will have a
useful life of over 50 years. In such a case, replacement of
bearings is a very difficult and expensive proposition.
We have just designed metal bearings for a jetty approach trestle
(which is also a bridge structure) and the expected cost of the
metal bearing is a fraction of the cost of the pot bearing.
Considering the number of bearings required, the cost saving is
substantial. No maintenance / replacement issues expected.
I was trying to get the opinion of members of this forum on the
relative merits of the two types of bearings.
M. Hariharan
I understand that Pot bearings for a concrete bridge of about 30
metre span cost about $650 here in India. I don't know the
details, but was curious because the total amount spent on
bearings would be quite substantial for a long structure. I
understand that the life of an elastomeric bearing is about 25
years or so. Bridges and other infrastructure items will have a
useful life of over 50 years. In such a case, replacement of
bearings is a very difficult and expensive proposition.
We have just designed metal bearings for a jetty approach trestle
(which is also a bridge structure) and the expected cost of the
metal bearing is a fraction of the cost of the pot bearing.
Considering the number of bearings required, the cost saving is
substantial. No maintenance / replacement issues expected.
I was trying to get the opinion of members of this forum on the
relative merits of the two types of bearings.
M. Hariharan
Hi, again, Harihan.
Many engineers outside India might be surprised to read that pot bearings had been selected for a 30m span bridge. Common practice elsewhere would be to use simple laminated elastomeric pads, which would be substantially cheaper.
Pot bearings require very close tolerances for satisfactory performance (hence the higher costs), and would not be the automatic first choice of most bridge designers, except for particular situations.
Replacement of elastomeric bearings does not have to be a major problem. The bearings are usually detailed to facititate removal and replacement.
Can you be entirely confident that the sliding surfaces in your steel bearings will be working as new after 25 years of marine exposure ?
One of my friends (and an occasional client) is a manufacturer of bridge bearings (elastomeric pads, pot bearings, sliding PTFE bearings - the full range). (Some of the work that I have done for him has related to design of pot bearings for use in India). He has considerable experience of manufacture in India; much of the remaining commentary is basically his (credit where due etc.
).
The Indian standard specifications for bridge bearings appear to be very different from those in use elsewhere, and contribute significantly to higher costs.
Thus, for example, the testing requirement is for each batch of bearings to include two additional bearings for testing (to three times design load) and to be subsequently discarded regardless of test performance.
I understand that the Indian standard for elastomeric bearings limits the elastomer to certain products from specific manufacturers. Presumably there are sound local reasons for doing that, but it would appear that simply specifying the material tests that had to be met would widen the competition and reduce the final costs.
The use of pot bearings where simple laminated pads would suffice is either a matter of local fashion or of distrust in pads. Perhaps there have been some instances of unsatisfactory performance in locally produced pad bearings?.
Many engineers outside India might be surprised to read that pot bearings had been selected for a 30m span bridge. Common practice elsewhere would be to use simple laminated elastomeric pads, which would be substantially cheaper.
Pot bearings require very close tolerances for satisfactory performance (hence the higher costs), and would not be the automatic first choice of most bridge designers, except for particular situations.
Replacement of elastomeric bearings does not have to be a major problem. The bearings are usually detailed to facititate removal and replacement.
Can you be entirely confident that the sliding surfaces in your steel bearings will be working as new after 25 years of marine exposure ?
One of my friends (and an occasional client) is a manufacturer of bridge bearings (elastomeric pads, pot bearings, sliding PTFE bearings - the full range). (Some of the work that I have done for him has related to design of pot bearings for use in India). He has considerable experience of manufacture in India; much of the remaining commentary is basically his (credit where due etc.
). The Indian standard specifications for bridge bearings appear to be very different from those in use elsewhere, and contribute significantly to higher costs.
Thus, for example, the testing requirement is for each batch of bearings to include two additional bearings for testing (to three times design load) and to be subsequently discarded regardless of test performance.
I understand that the Indian standard for elastomeric bearings limits the elastomer to certain products from specific manufacturers. Presumably there are sound local reasons for doing that, but it would appear that simply specifying the material tests that had to be met would widen the competition and reduce the final costs.
The use of pot bearings where simple laminated pads would suffice is either a matter of local fashion or of distrust in pads. Perhaps there have been some instances of unsatisfactory performance in locally produced pad bearings?.
hi
i have worked in the northeastern US in past. there most of the bridges we designed were concrete deck slab on steel superstructure. there we commomly used steel laminated elastomeric bearings for spans upto 30m (90ft) for longer spans we used pot bearings. from what i have seen here in india, most of the bridges are cip/precast/prestressed or post tensioned box girders bridges. they result in higher reactions at the supports as compared to a bridge superstructure bridge.
abhay
i have worked in the northeastern US in past. there most of the bridges we designed were concrete deck slab on steel superstructure. there we commomly used steel laminated elastomeric bearings for spans upto 30m (90ft) for longer spans we used pot bearings. from what i have seen here in india, most of the bridges are cip/precast/prestressed or post tensioned box girders bridges. they result in higher reactions at the supports as compared to a bridge superstructure bridge.
abhay
- Thread starter
- #7
The bridge I was referring to was multiple span continuous
beam segmental prestressed concrete bridge. The reaction
on the bearing would have been quite large. The reference
was primarily intended to give an idea of the cost. Most
bridges of comparable span probably have laminated pads. I
am not very conversant with the practice of bridge engineers,
would not be able to comment on the bearing specs in Indian
standards. Will try to find out.
Regarding the steel bearing for marine application, we wanted
the girders to function as longitudinal ties under wave action,
but retain the capability for thermal expansion. The bearings
were custom designed. The sliding surface is steel. The
movement due to thermal effects is a few millimeters. The
bearings consist of a pipe inside a pipe to restrain translation
beyond thermal effects. The inner pipe is anchored inside the
girder. The outer pipe is connected to the supporting beam.
The outer pipe has a curved bearing surface which permits the
reaction to be at the centre of the upper pipe when the girder
deflects. This resulted in minimum stresses in the pipes. The
contact surfaces at the sides are stainless steel. Since the
movement is very small, we do not anticipate any problem
with respect to lifetime performance. This system permits the
girder to transmit a significant amount of longitudinal load to
adjacent spans, thus spreading the wave loading effect over a
number of spans. I am not sure if I could describe the details
in text.
beam segmental prestressed concrete bridge. The reaction
on the bearing would have been quite large. The reference
was primarily intended to give an idea of the cost. Most
bridges of comparable span probably have laminated pads. I
am not very conversant with the practice of bridge engineers,
would not be able to comment on the bearing specs in Indian
standards. Will try to find out.
Regarding the steel bearing for marine application, we wanted
the girders to function as longitudinal ties under wave action,
but retain the capability for thermal expansion. The bearings
were custom designed. The sliding surface is steel. The
movement due to thermal effects is a few millimeters. The
bearings consist of a pipe inside a pipe to restrain translation
beyond thermal effects. The inner pipe is anchored inside the
girder. The outer pipe is connected to the supporting beam.
The outer pipe has a curved bearing surface which permits the
reaction to be at the centre of the upper pipe when the girder
deflects. This resulted in minimum stresses in the pipes. The
contact surfaces at the sides are stainless steel. Since the
movement is very small, we do not anticipate any problem
with respect to lifetime performance. This system permits the
girder to transmit a significant amount of longitudinal load to
adjacent spans, thus spreading the wave loading effect over a
number of spans. I am not sure if I could describe the details
in text.
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