08-10-2016, 03:24 PM
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I. INTRODUCTION
1 1. Need
There is a need for a reliable and economic Mobile Inspection Platform (MIP) for the inspection of Bridges. We have in our country, a large number of road bridges. Every year new road bridges are added to the list of existing bridges. The older bridges require maintenance to continue to permit serviceability. For proper maintenance, assessment of what is to be done is required. Inspection for assessment is to be done not only from top of the carriageway, but also from the side and bottom of the deck, and also around the sub-structure. The MIP provides a very economic and convenient means of producing such inspection data. Minor maintenance can also be carried out using the MIP.
1.2. Objective
In a bridge both the sub and the super-structure are to be inspected.
(a) The top of the deck has easy access for inspection (b) The portion of the sub-structure above water level can at least be observed from the shore/footpath on the deck. For closer inspection the MIP can be used. An additional appurtenance like a `Jhula' (cage) hanging from the MIP can be used. The cage can be lowered from the stationery MIP along the vertical face of the sub-structure. However, this cage was not employed in inspection of the Iswar Gupta Setu, over river Hooghly at Kalyani, where the problem was in the super structure, specially the bearings of the suspended span, Photo 1. © For portion of the sub-structure below water level, special arrangements are to be made. (d) The inspection of the deck from underneath and the deck supporting system over the pier needs the help of the MIP. A ladder at the tip of the platform canprovide inspection access between the girders, Photo 2.
1.3. Obstruction
There are at least 2 major kind of obstructions to easy mobility of MIP.
a. The sub-structure supporting the superstructure.
b. The tall light posts at the hand railings.
1.4. Owner's Initiative
The owner of Iswar Gupta Setu over river Hooghly at Kalyani is the Public Works Department (Roads), Government of West Bengal. The design of the Mobile Inspection Platform was done at the request of PWD (Roads), Govt. of West Bengal. There are about 8 m high circular steel light poles installed at an interval on each hand railing on both sides. The design and operation of the MIP could have been simpler, had these light poles been removed temporarily. However, that is not to be. The design of the MIP permits crossing the light poles while traveling along the bridge during inspection, using simple hinged connector mechanism.
1.5. Specific Need
The equipment was designed, to satisfy the specific requirement of the Iswar Gupta Setu at Kalyani over the river Hooghly. One can always broaden the specification resulting in higher cost of the equipment. The safety must be absolute. Ours is a large country. It is preferable to design and fabricate equipment to cater to specific needs and Keep the cost down and increase margin of safety and stability.
Iswar Gupta Setu over river Hooghly at Kalyani, West Bengal was constructed around the Year 1990. The bridge is 120 m span hammer head balanced cantilever with a 15 m long suspended span at the centre. There is a 7.5 m carriageway with two 1.2 m footpaths on the two sides of the carriageway. The 52.5 m long cantilever span is prestressed concrete. The 15 m long suspended span is plain reinforced concrete.
This bridge is similar in construction to the bridge on river Brahmaputra at Bhomraguri, Tezpur, Assam. While the bridge at Bhomraguri over river Brahmaputra is 28 spans long, that at Kalyani over river Hooghly is only 5 spans long. There is another difference, each central suspended span on the bridge over river Brahmaputra is located on one side fixed and one side PTFE sliding bearings, while those over river Hooghly are located on rocker and single roller bearings. Moreover, bearings on bridge over river Brahmaputra are special to accommodate large lateral movement in seismic condition.
Due to higher flexibility of the deck and corresponding vibration in the vertical axis, in these types of bridges, rocker bearing with pins and roller bearings are generally not•uitable. Certain precautions like use of rack and pinion attached to the roller bearing could have increased the serviceability of the roller bearings. However the same were not adopted. It is obvious that though the suspended span is only 15 m long, the free bearing will cater to a movement corresponding to 120 m, the distance between the null points in the deck.
When opened to traffic, it was observed that one roller in a free bearing at about the middle of the bridge has shifted. For want of close inspection, approach to the location being difficult, actual assessment could not be done for about four years. Photos 3 to 8 are shown below for perusal.
2. CONSTRUCTION
2.1. Modules
The complete MIP consists of several basic modules, as shown in Fig. 1.
(a) Carriage Bottom, (b) Carriage TOP, © Hinged Connectors, (d) Column with Main Ladder, (e) Platform & Platform Extension, (f) Ladder on Platform Extension
Description of Modules
(a) The carriage bottom will ride on the deck on four pneumatic tyred wheels. Two wheels will be riding on the footpath close to the hand railings. The remaining two wheels will be riding on the deck. There is a difference of level between the wheels riding on the footpath and those riding on the deck, to the extent of the height (standard IRC specification) of the footpath over the deck. There are four outriggers attached to the carriage bottom. The required kentledge is placed on the carriage bottom.
(b) The carriage top sits on the carriage bottom. The carriage top will bear the load of the column through the four hinged connectors. The hinged connectors are located at the upper part of the carriage top. The connectors will carry the load of the column and will transfer the same to the carriage top.
© The vertical column will hold the platform at its lower end through hinge and tie. A slant aluminium ladder is fixed inside the column. The ladder is used to go down to the platform fitted at the bottom of the column for inspection. Inside the column there are two resting platforms in front of the ladder at two levels.
(d) The horizontal platform at the bottom end of the column will clear the sub-structure at its inner edge. The bottom platform contains one telescopic platform extension, which can be pushed out, to increase the reach of the bottom platform. The extension platform is kept withdrawn while passing the piers. Both the platform and the extension platform are provided with chequered aluminium floors. All the other floors of the carriage, the resting platforms and carriage stairs are of chequered mild steel plates.
(e) The aluminium ladder at the outer end of the extension platform is foldable and will permit inspection of the inner vertical faces of the girders.
3. MATERIALS
3.1. MS RHS & SHS
All the members of the column, platform, platform extension, connector and carriage top are mainly manufactured with rectangular hollow sections (RHS) and square hollow sections (SHS) as per IS:4923.
3.2. Structurals
The carriage bottom is fabricated with MS rolled steel sections as per IS:808. The section modulus required for the members of the carriage bottom, from design considerations are beyond the range of production of RHS & SHS in this country.
3.3. Aluminium
Both the main inclined aluminium ladder for going down to the platform and the platform ladder on the extension platform arc fabricated from standard aluminium rolled sections produced in this country for making ladders .
DESIGN
5.1. Economics
The present MIP is a low cost manually operated equipment, without any frills. The whole idea during design was low cost, easy erection & operation, absolute safety and almost zero maintenance: This MIP will satisfy the requirements of specific bridge geometry. The cost is so little that another MIP can always be built to satisfy the needs of a bridge with different geometry. The insignificant cost compared to that of a bridge can permit a dedicated MIP for each medium to large span bridge.
It is almost always possible to produce an equipment to cater to the widest possible
user specification. But the cost goes up. Maintenance becomes difficult. As an example the cost of the present. MIP is about Rs 500 000/- (Rupees half a million) where as the cost of a fully hydraulic versatile MIP of foreign make is about Rs 25 000 000/- — 30 000 000/- (Rupees twenty-five — thirty million). Not only the basic procurement cost, but also the cost of maintenance and that of operation have to be added to compare the economics.
It will be economic by an order of 30 — 50 times , to divide the total magnitude of the extent of the need of the user into smaller range specific to a bridge and produce as many MIP as needed. Still the overall economy will be enormous.
Though the present MIP is designed for road bridges, the same design principles can be used to design and built an MIP suitable for the standard `track on girder' steel rail bridges.
5.2. Analysis
The MIP structure has been designed using structural analysis software. The overall factor of safety is more than adequate considering the worst wind condition as per IS: 875 (Part 3). The structural safety has been checked under the following two conditions:
The MIP is stationery with load of 4 persons and wind load for a wind speed of 55 m/sec (200 km/hr). The MIP is moving against a wind load from a basic wind speed of 15 m/sec (55 km/hr).
5.3. Sections
All the members of the column, platform, platform extension, connector and carriage top are mainly manufactured with rectangular hollow sections (RHS) and square hollow sections (SHS) as per 1S:4923. All the open faces of the RHS and SHS are sealed. There are no pockets in the structure for accumulation of rain water
(a) The provision of hollow sections has provided the highest rigidity with lowest moving mass. It is well known that hollow sections offer much better torsion resisting properties. It is also well known that resistance against buckling increases due to the improvement of `r ' the radius of gyration for a closed section over the same for a open section with comparable area of cross section.
(b) However, to the designer of construction equipment there is a very important advantage, which may not be so apparent. Due to continuous handling over the passage of time; the starting value of `14' the radius of gyration for open sections gets eroded due to distortion and corrosion of the sections. Such distortion and corrosion is comparatively much less for a closed structural. This apparent accomplishment increases the margin of safety when integrated over the life of the construction equipment.
© As far as the radius of gyration is concerned Circular Hollow Sections (CHS) offers the best advantage against pure axial loads. But in real life most of the members will-also be subjected to bending forces. The SHS & RHS holds advantage over CHS under such situation.
5.4. Detailing
With easy availability of powerful computers and matching software the calculations for the design of a structure does not present much difficulty. It is the fabrication detailing . which deserves meticulous attention. Better detailing increases the serviceability and durability of a structure with almost negligible increase in the fabrication and material costs. In this regard mention is made of the various publications by International Committee for the Development and Study of Tubular Construction (CIDECT) sponsored by the European Communities for last so many years.
5.5. Special Features
All the hinges and connections are specially designed incorporating machined elements, with increased margin of safety. The weight of the column and the platform will be carried by four (4) number of connectors attached between the column and the carriage top. At any point of time the weight of the column and the platform can be safely carried by any two (2) of the four of connectors.
In addition to the hinge and the regular tie between the column and the platform, there is a pair of additional safety tie as first line of defence and a chain to form the second line of defence, against the accidental opening out of the platform.
Each of the four pneumatic tyred wheels, carrying the carriage bottom, is mounted on its own axle. The axle, wheel rim, tyre and tubes used are chosen from the standard range used in this country in the transport industry. As such they are available as spares across the count-cr. In case of leak developing in the tube, any of the four (4) pneumatic tyres can be removed along with the hub, repaired and refitted; with the MIP in the erected condition. Sufficient clearance is provided for the purpose. The out-rigger jacks will release the wheel load.
There are two steering wheels, one at each end of the carriage bottom along the travel directions. The carriage bottom wheels are adjusted by using the steering wheel, to maintain accurate distance from the hand rails during traveling on the deck.
All the fasteners are designed using permissible strength for grade 4.6 to IS:1363 ensuring easy availability.
The whole structure is generally given two coats of epoxy primer and two coats of traffic yellow paint.
The physical and psychological safety have been inducted by the inclusion / presence of the following items:
(1) The main inclined ladder is totally enclosed inside the column.
(2) The presence of two resting platforms associated with the main inclined ladder.
(3) The presence of hand railings on both the platforms.
(4) The presence of chequered floor on both the platforms.
(5) Two additional line of defence against the opening out of the platform.
5.6. Bought Outs
The tyres used are standard pneumatic lorry tyres, available throughout the country. Two wheels riding on the footpath are at a higher level compared to the two riding on the road. As such two sets of standard lorry front wheel axle pairs including the hubs and also including the steering mechanism are. used.
6. INSTALLATION
6.1. Erection
A site is selected below the approach spans of the bridge to be inspected, where the ground clearance of the bottom of the deck is at least 2 — 3 m.
The welded structural modules are transported to the site by 2 lorries. The column, .the platform and the platform extension are kept on the ground. The carriage bottom and the rest are placed on the deck at the corresponding position.
Bolting is the main mode of assembly and erection at site. The platform and the platform extension are laid on the ground, on stools, in operating alignment. The column is erected, at the end of the platform. The carriage bottom and the rest are installed on the deck at the corresponding position. The hinged connectors are connected to the column. The platform and column • assembly is raised to match the level of the hinged connectors.
The MIP is ready. It is pulled over the deck to the required location. Slight variation of this scheme can be used at different bridge locations to suit the site
The dismantling will follow the same procedure in reverse.
6.2. Load Testing
The MIP structure has been designed to carry 4 persons. After erection, 10 persons crammed the MIP platform and extended platform. The whole equipment has been load tested with 2.5 times the permissible human load. The deflection was measured and was found to be well within the permissible limits and human comfort.
7. OPERATION
7.1. Transport
The whole unit is on pneumatic tyres. Hence pulling the carriage bottom on the deck, with the help of a lorry or a tractor moving on the bridge deck 'or using the dedicated winch mounted on the MIP is easy. The pneumatic wheels on the carriage bottom are steered using the steering wheels, to maintain accurate distance from the hand rails during traveling.
The carriage bottom can be pulled with the help a manual winch installed on the MIP. The end of the rope is attached to the hand railing at a point somewhat forward to the then location of the MIP. The MIP was transported both on the upward and downward slope slowly using a 3 toner lorry and most of the time using a tractor.
7.2. Crossing Light post
The MIP can cross a light post mounted just outside the hand railing.
The MIP is stopped 400 mm short of the light post. The two bolts one each at the top and bottom of first of the hinged connectors nearest to the light post are removed. The hinged connector is hinged ,outwards towards the light post.
The MIP is moved towards the light post such that the light post is stationed between the first and the second hinged connectors.
The first hinged connector is refitted with the bolts. The second hinged connector is opened inwards.
The MIP is moved such that the light post is stationed between the second and the third hinged connectors. And so on till it passes the light post completely.
It is observed that, at any point of time at least three hinged connectors remain connected. The light post crossing operation takes about 12 — 15 minutes.
7.3. Platform Extension
The crossing of the bridge sub-structure was done with the platform extension withdrawn.
The platform extension slides inside the main platform. The' same was withdrawn by two persons standing inside the main platform. The same can be pushed out by two persons standing inside the main platform at the end of the extension platform.
The aluminium ladder at the tip of the platform extension was pushed towards the middle of the deck ie away from the column of the MIP. The two legged ladder hinged upwards. The bottom sliding end was locked in special holders provided for the purpose.
7.4. Inspection
The MIP is stationed at a location where the bottom of the deck is to be' inspected. The screw jacks on the out riggers at the four corners of the carriage bottom is set on the footpath and the deck, using load distribution wooden blocks.
Two persons will go down the aluminium ladder inside the column to theplatform below the deck bottom level. They will push the extended platform to its out extension limit. This will permit reach up to the centre of the deck. Though the reach is half the deck width, however one the inspector is on the pier cap, he can inspect all the bearings on the pier cap.
The folding aluminium ladder on the extended platform can be raised to inspect the inner vertical faces of the girders.
For initiating traveling, folding aluminium ladder will be lowered on the extended platform. The extended platform will be pulled back on to the platform.
The MIP can travel with the inspectors in the column or on the platform.
7.5. Service Condition
In the service condition the following steps are to be ensured:
(i) The carriage will be on 4 Jacks with appropriate wedging on the 4 wheels.
(ii) The carriage will be loosely tied to the hand rails.
(iii) All 4 (four) hinged connectors will be bolted.
(iv) Total weight including sufficient kentledge will not be less than 5 t.
7.6. Transport Condition
In transport condition the following steps are to be ensured:
(a) The cari-iage will be on wheels.
(b) At least 2 (two) hinged connectors 1 (one) on each side of the centre line of the MIP, perpendicular to the traffic axis of the bridge, will always remain bolted to the column. Refer to paragraph 7.1 and 7.2 for transport.
7.7. Parking Condition
During overnight parking condition the following precautions are to be ensured:
(a) The carriage will be on 4 Jacks with appropriate wedging on the 4 wheels.
(b) The carriage will be tied to the hand rails. During stormy weather two additional guys are to be provided.
©All 4 (four) hinged connectors will be bolted.
(d)Additional kentledge of total weight 1 t @ 0.5 t on each side, over the operating kentledge will be provided.
7.8. Operating Personnel
As explained in 6.1 about 9 persons are required for the installation. A total of 10 person would be sufficient for running the MIP. The supervisor can do the preliminary survey. However, a bridge engineer will be required to inspect the bridge.
7.9. Operating Time
This is a variable quantity depending on the degree of accuracy of the inspection. However as a rough guideline, one week will be sufficient for inspection of a 3 span bridge (total length up to 360 in) including erection.
7.10. Traffic
During traveling or inspection the MIP occupies roughly 2.5 m of the 7.5 in wide carriageway beyond the footpath. Roughly 5 m is left clear for the traffic. Vehicular traffic over the bridge is permitted.
8. MAINTENANCE
The MIP requires very little maintenance. The spares are available as standard auto parts. The equipment has been field tested and has been found to be extremely easy to operate. Some greasing and periodic oiling is required for regular maintenance.
Painting after inspection of each bridge location or every two years, whichever is earlier, will increase the service life of the MIP. A budgetary provision of Rs 30 000/- (Rupees thirty thousand) per year will be sufficient to keep the MIP operating.
I. INTRODUCTION
1 1. Need
There is a need for a reliable and economic Mobile Inspection Platform (MIP) for the inspection of Bridges. We have in our country, a large number of road bridges. Every year new road bridges are added to the list of existing bridges. The older bridges require maintenance to continue to permit serviceability. For proper maintenance, assessment of what is to be done is required. Inspection for assessment is to be done not only from top of the carriageway, but also from the side and bottom of the deck, and also around the sub-structure. The MIP provides a very economic and convenient means of producing such inspection data. Minor maintenance can also be carried out using the MIP.
1.2. Objective
In a bridge both the sub and the super-structure are to be inspected.
(a) The top of the deck has easy access for inspection (b) The portion of the sub-structure above water level can at least be observed from the shore/footpath on the deck. For closer inspection the MIP can be used. An additional appurtenance like a `Jhula' (cage) hanging from the MIP can be used. The cage can be lowered from the stationery MIP along the vertical face of the sub-structure. However, this cage was not employed in inspection of the Iswar Gupta Setu, over river Hooghly at Kalyani, where the problem was in the super structure, specially the bearings of the suspended span, Photo 1. © For portion of the sub-structure below water level, special arrangements are to be made. (d) The inspection of the deck from underneath and the deck supporting system over the pier needs the help of the MIP. A ladder at the tip of the platform canprovide inspection access between the girders, Photo 2.
1.3. Obstruction
There are at least 2 major kind of obstructions to easy mobility of MIP.
a. The sub-structure supporting the superstructure.
b. The tall light posts at the hand railings.
1.4. Owner's Initiative
The owner of Iswar Gupta Setu over river Hooghly at Kalyani is the Public Works Department (Roads), Government of West Bengal. The design of the Mobile Inspection Platform was done at the request of PWD (Roads), Govt. of West Bengal. There are about 8 m high circular steel light poles installed at an interval on each hand railing on both sides. The design and operation of the MIP could have been simpler, had these light poles been removed temporarily. However, that is not to be. The design of the MIP permits crossing the light poles while traveling along the bridge during inspection, using simple hinged connector mechanism.
1.5. Specific Need
The equipment was designed, to satisfy the specific requirement of the Iswar Gupta Setu at Kalyani over the river Hooghly. One can always broaden the specification resulting in higher cost of the equipment. The safety must be absolute. Ours is a large country. It is preferable to design and fabricate equipment to cater to specific needs and Keep the cost down and increase margin of safety and stability.
Iswar Gupta Setu over river Hooghly at Kalyani, West Bengal was constructed around the Year 1990. The bridge is 120 m span hammer head balanced cantilever with a 15 m long suspended span at the centre. There is a 7.5 m carriageway with two 1.2 m footpaths on the two sides of the carriageway. The 52.5 m long cantilever span is prestressed concrete. The 15 m long suspended span is plain reinforced concrete.
This bridge is similar in construction to the bridge on river Brahmaputra at Bhomraguri, Tezpur, Assam. While the bridge at Bhomraguri over river Brahmaputra is 28 spans long, that at Kalyani over river Hooghly is only 5 spans long. There is another difference, each central suspended span on the bridge over river Brahmaputra is located on one side fixed and one side PTFE sliding bearings, while those over river Hooghly are located on rocker and single roller bearings. Moreover, bearings on bridge over river Brahmaputra are special to accommodate large lateral movement in seismic condition.
Due to higher flexibility of the deck and corresponding vibration in the vertical axis, in these types of bridges, rocker bearing with pins and roller bearings are generally not•uitable. Certain precautions like use of rack and pinion attached to the roller bearing could have increased the serviceability of the roller bearings. However the same were not adopted. It is obvious that though the suspended span is only 15 m long, the free bearing will cater to a movement corresponding to 120 m, the distance between the null points in the deck.
When opened to traffic, it was observed that one roller in a free bearing at about the middle of the bridge has shifted. For want of close inspection, approach to the location being difficult, actual assessment could not be done for about four years. Photos 3 to 8 are shown below for perusal.
2. CONSTRUCTION
2.1. Modules
The complete MIP consists of several basic modules, as shown in Fig. 1.
(a) Carriage Bottom, (b) Carriage TOP, © Hinged Connectors, (d) Column with Main Ladder, (e) Platform & Platform Extension, (f) Ladder on Platform Extension
Description of Modules
(a) The carriage bottom will ride on the deck on four pneumatic tyred wheels. Two wheels will be riding on the footpath close to the hand railings. The remaining two wheels will be riding on the deck. There is a difference of level between the wheels riding on the footpath and those riding on the deck, to the extent of the height (standard IRC specification) of the footpath over the deck. There are four outriggers attached to the carriage bottom. The required kentledge is placed on the carriage bottom.
(b) The carriage top sits on the carriage bottom. The carriage top will bear the load of the column through the four hinged connectors. The hinged connectors are located at the upper part of the carriage top. The connectors will carry the load of the column and will transfer the same to the carriage top.
© The vertical column will hold the platform at its lower end through hinge and tie. A slant aluminium ladder is fixed inside the column. The ladder is used to go down to the platform fitted at the bottom of the column for inspection. Inside the column there are two resting platforms in front of the ladder at two levels.
(d) The horizontal platform at the bottom end of the column will clear the sub-structure at its inner edge. The bottom platform contains one telescopic platform extension, which can be pushed out, to increase the reach of the bottom platform. The extension platform is kept withdrawn while passing the piers. Both the platform and the extension platform are provided with chequered aluminium floors. All the other floors of the carriage, the resting platforms and carriage stairs are of chequered mild steel plates.
(e) The aluminium ladder at the outer end of the extension platform is foldable and will permit inspection of the inner vertical faces of the girders.
3. MATERIALS
3.1. MS RHS & SHS
All the members of the column, platform, platform extension, connector and carriage top are mainly manufactured with rectangular hollow sections (RHS) and square hollow sections (SHS) as per IS:4923.
3.2. Structurals
The carriage bottom is fabricated with MS rolled steel sections as per IS:808. The section modulus required for the members of the carriage bottom, from design considerations are beyond the range of production of RHS & SHS in this country.
3.3. Aluminium
Both the main inclined aluminium ladder for going down to the platform and the platform ladder on the extension platform arc fabricated from standard aluminium rolled sections produced in this country for making ladders .
DESIGN
5.1. Economics
The present MIP is a low cost manually operated equipment, without any frills. The whole idea during design was low cost, easy erection & operation, absolute safety and almost zero maintenance: This MIP will satisfy the requirements of specific bridge geometry. The cost is so little that another MIP can always be built to satisfy the needs of a bridge with different geometry. The insignificant cost compared to that of a bridge can permit a dedicated MIP for each medium to large span bridge.
It is almost always possible to produce an equipment to cater to the widest possible
user specification. But the cost goes up. Maintenance becomes difficult. As an example the cost of the present. MIP is about Rs 500 000/- (Rupees half a million) where as the cost of a fully hydraulic versatile MIP of foreign make is about Rs 25 000 000/- — 30 000 000/- (Rupees twenty-five — thirty million). Not only the basic procurement cost, but also the cost of maintenance and that of operation have to be added to compare the economics.
It will be economic by an order of 30 — 50 times , to divide the total magnitude of the extent of the need of the user into smaller range specific to a bridge and produce as many MIP as needed. Still the overall economy will be enormous.
Though the present MIP is designed for road bridges, the same design principles can be used to design and built an MIP suitable for the standard `track on girder' steel rail bridges.
5.2. Analysis
The MIP structure has been designed using structural analysis software. The overall factor of safety is more than adequate considering the worst wind condition as per IS: 875 (Part 3). The structural safety has been checked under the following two conditions:
The MIP is stationery with load of 4 persons and wind load for a wind speed of 55 m/sec (200 km/hr). The MIP is moving against a wind load from a basic wind speed of 15 m/sec (55 km/hr).
5.3. Sections
All the members of the column, platform, platform extension, connector and carriage top are mainly manufactured with rectangular hollow sections (RHS) and square hollow sections (SHS) as per 1S:4923. All the open faces of the RHS and SHS are sealed. There are no pockets in the structure for accumulation of rain water
(a) The provision of hollow sections has provided the highest rigidity with lowest moving mass. It is well known that hollow sections offer much better torsion resisting properties. It is also well known that resistance against buckling increases due to the improvement of `r ' the radius of gyration for a closed section over the same for a open section with comparable area of cross section.
(b) However, to the designer of construction equipment there is a very important advantage, which may not be so apparent. Due to continuous handling over the passage of time; the starting value of `14' the radius of gyration for open sections gets eroded due to distortion and corrosion of the sections. Such distortion and corrosion is comparatively much less for a closed structural. This apparent accomplishment increases the margin of safety when integrated over the life of the construction equipment.
© As far as the radius of gyration is concerned Circular Hollow Sections (CHS) offers the best advantage against pure axial loads. But in real life most of the members will-also be subjected to bending forces. The SHS & RHS holds advantage over CHS under such situation.
5.4. Detailing
With easy availability of powerful computers and matching software the calculations for the design of a structure does not present much difficulty. It is the fabrication detailing . which deserves meticulous attention. Better detailing increases the serviceability and durability of a structure with almost negligible increase in the fabrication and material costs. In this regard mention is made of the various publications by International Committee for the Development and Study of Tubular Construction (CIDECT) sponsored by the European Communities for last so many years.
5.5. Special Features
All the hinges and connections are specially designed incorporating machined elements, with increased margin of safety. The weight of the column and the platform will be carried by four (4) number of connectors attached between the column and the carriage top. At any point of time the weight of the column and the platform can be safely carried by any two (2) of the four of connectors.
In addition to the hinge and the regular tie between the column and the platform, there is a pair of additional safety tie as first line of defence and a chain to form the second line of defence, against the accidental opening out of the platform.
Each of the four pneumatic tyred wheels, carrying the carriage bottom, is mounted on its own axle. The axle, wheel rim, tyre and tubes used are chosen from the standard range used in this country in the transport industry. As such they are available as spares across the count-cr. In case of leak developing in the tube, any of the four (4) pneumatic tyres can be removed along with the hub, repaired and refitted; with the MIP in the erected condition. Sufficient clearance is provided for the purpose. The out-rigger jacks will release the wheel load.
There are two steering wheels, one at each end of the carriage bottom along the travel directions. The carriage bottom wheels are adjusted by using the steering wheel, to maintain accurate distance from the hand rails during traveling on the deck.
All the fasteners are designed using permissible strength for grade 4.6 to IS:1363 ensuring easy availability.
The whole structure is generally given two coats of epoxy primer and two coats of traffic yellow paint.
The physical and psychological safety have been inducted by the inclusion / presence of the following items:
(1) The main inclined ladder is totally enclosed inside the column.
(2) The presence of two resting platforms associated with the main inclined ladder.
(3) The presence of hand railings on both the platforms.
(4) The presence of chequered floor on both the platforms.
(5) Two additional line of defence against the opening out of the platform.
5.6. Bought Outs
The tyres used are standard pneumatic lorry tyres, available throughout the country. Two wheels riding on the footpath are at a higher level compared to the two riding on the road. As such two sets of standard lorry front wheel axle pairs including the hubs and also including the steering mechanism are. used.
6. INSTALLATION
6.1. Erection
A site is selected below the approach spans of the bridge to be inspected, where the ground clearance of the bottom of the deck is at least 2 — 3 m.
The welded structural modules are transported to the site by 2 lorries. The column, .the platform and the platform extension are kept on the ground. The carriage bottom and the rest are placed on the deck at the corresponding position.
Bolting is the main mode of assembly and erection at site. The platform and the platform extension are laid on the ground, on stools, in operating alignment. The column is erected, at the end of the platform. The carriage bottom and the rest are installed on the deck at the corresponding position. The hinged connectors are connected to the column. The platform and column • assembly is raised to match the level of the hinged connectors.
The MIP is ready. It is pulled over the deck to the required location. Slight variation of this scheme can be used at different bridge locations to suit the site
The dismantling will follow the same procedure in reverse.
6.2. Load Testing
The MIP structure has been designed to carry 4 persons. After erection, 10 persons crammed the MIP platform and extended platform. The whole equipment has been load tested with 2.5 times the permissible human load. The deflection was measured and was found to be well within the permissible limits and human comfort.
7. OPERATION
7.1. Transport
The whole unit is on pneumatic tyres. Hence pulling the carriage bottom on the deck, with the help of a lorry or a tractor moving on the bridge deck 'or using the dedicated winch mounted on the MIP is easy. The pneumatic wheels on the carriage bottom are steered using the steering wheels, to maintain accurate distance from the hand rails during traveling.
The carriage bottom can be pulled with the help a manual winch installed on the MIP. The end of the rope is attached to the hand railing at a point somewhat forward to the then location of the MIP. The MIP was transported both on the upward and downward slope slowly using a 3 toner lorry and most of the time using a tractor.
7.2. Crossing Light post
The MIP can cross a light post mounted just outside the hand railing.
The MIP is stopped 400 mm short of the light post. The two bolts one each at the top and bottom of first of the hinged connectors nearest to the light post are removed. The hinged connector is hinged ,outwards towards the light post.
The MIP is moved towards the light post such that the light post is stationed between the first and the second hinged connectors.
The first hinged connector is refitted with the bolts. The second hinged connector is opened inwards.
The MIP is moved such that the light post is stationed between the second and the third hinged connectors. And so on till it passes the light post completely.
It is observed that, at any point of time at least three hinged connectors remain connected. The light post crossing operation takes about 12 — 15 minutes.
7.3. Platform Extension
The crossing of the bridge sub-structure was done with the platform extension withdrawn.
The platform extension slides inside the main platform. The' same was withdrawn by two persons standing inside the main platform. The same can be pushed out by two persons standing inside the main platform at the end of the extension platform.
The aluminium ladder at the tip of the platform extension was pushed towards the middle of the deck ie away from the column of the MIP. The two legged ladder hinged upwards. The bottom sliding end was locked in special holders provided for the purpose.
7.4. Inspection
The MIP is stationed at a location where the bottom of the deck is to be' inspected. The screw jacks on the out riggers at the four corners of the carriage bottom is set on the footpath and the deck, using load distribution wooden blocks.
Two persons will go down the aluminium ladder inside the column to theplatform below the deck bottom level. They will push the extended platform to its out extension limit. This will permit reach up to the centre of the deck. Though the reach is half the deck width, however one the inspector is on the pier cap, he can inspect all the bearings on the pier cap.
The folding aluminium ladder on the extended platform can be raised to inspect the inner vertical faces of the girders.
For initiating traveling, folding aluminium ladder will be lowered on the extended platform. The extended platform will be pulled back on to the platform.
The MIP can travel with the inspectors in the column or on the platform.
7.5. Service Condition
In the service condition the following steps are to be ensured:
(i) The carriage will be on 4 Jacks with appropriate wedging on the 4 wheels.
(ii) The carriage will be loosely tied to the hand rails.
(iii) All 4 (four) hinged connectors will be bolted.
(iv) Total weight including sufficient kentledge will not be less than 5 t.
7.6. Transport Condition
In transport condition the following steps are to be ensured:
(a) The cari-iage will be on wheels.
(b) At least 2 (two) hinged connectors 1 (one) on each side of the centre line of the MIP, perpendicular to the traffic axis of the bridge, will always remain bolted to the column. Refer to paragraph 7.1 and 7.2 for transport.
7.7. Parking Condition
During overnight parking condition the following precautions are to be ensured:
(a) The carriage will be on 4 Jacks with appropriate wedging on the 4 wheels.
(b) The carriage will be tied to the hand rails. During stormy weather two additional guys are to be provided.
©All 4 (four) hinged connectors will be bolted.
(d)Additional kentledge of total weight 1 t @ 0.5 t on each side, over the operating kentledge will be provided.
7.8. Operating Personnel
As explained in 6.1 about 9 persons are required for the installation. A total of 10 person would be sufficient for running the MIP. The supervisor can do the preliminary survey. However, a bridge engineer will be required to inspect the bridge.
7.9. Operating Time
This is a variable quantity depending on the degree of accuracy of the inspection. However as a rough guideline, one week will be sufficient for inspection of a 3 span bridge (total length up to 360 in) including erection.
7.10. Traffic
During traveling or inspection the MIP occupies roughly 2.5 m of the 7.5 in wide carriageway beyond the footpath. Roughly 5 m is left clear for the traffic. Vehicular traffic over the bridge is permitted.
8. MAINTENANCE
The MIP requires very little maintenance. The spares are available as standard auto parts. The equipment has been field tested and has been found to be extremely easy to operate. Some greasing and periodic oiling is required for regular maintenance.
Painting after inspection of each bridge location or every two years, whichever is earlier, will increase the service life of the MIP. A budgetary provision of Rs 30 000/- (Rupees thirty thousand) per year will be sufficient to keep the MIP operating.