The existing bridges in India are more than 50 years and were designed for less volume of traffic, slower speed and lighter loads. The various factors have resulted in deterioration of bridges and the need for their maintenance, rehabilitation or replacement has increased manifold. Thus, it has become essential to have an effective system for inspection of bridges at faster speed and regular intervals. The equipment used for this purpose is known as mobile bridge inspection unit.
2. NECESSITY FOR INSPECTION OF BRIDGES
2.1 Inspection Of Bridges
It is carried out to
a)Assure that bridge is structurally safe and fit for its designated use.
b) Identify the source of trouble at earliest possible stage and take necessary preventive measures.
c) Keep the records systematically about the structure which shall be helpful for its future course of action. Timely forecast and attending the repairs will reduce their high maintenance and repair cost.
2.2Type Of Inspection
Inspection of bridges is done in three modes:-
2.2.1 Routine Inspection
It is done periodically through visual inspection.
2.2.2 Principal Inspection
If routine inspection reveals any distress in bridge structure, more intensive and detailed inspection to be done with the instruments like concrete test hammer, resistivity meter, ultrasonic instrument, permeability tester etc.
2.2.3 Special Inspection
It is required in case of unusual occurrences i.e. strong earthquake , accidents, floods, settlement of foundations, major deficiencies noticed during routine/ principal inspection and needs the involvement of experts in relevant field.
3. MOBILE BRIDGE INSPECTION UNIT
Broadly mobile bridge inspection unit is classified in two categories:-
3.1.1 Mobile Bridge Inspeciton Unit(Cage Type)
It consists of :
b)Hydraulic jacks quantity – 4 nos, which are used to level the truck descending vertically.
c)An articulated telescopic boom consisting of a telescopic main boom mounted on the turn table by means of hydraulic system.
a)The leveling of machine takes place automatically along with outrigger.
b)All movements are operated hydraulically and controlled through control panel provided in cage and on ground under bridge inspection.
c)Inter communication system is provided between cage, ground control panel and cabin.
d)Automatic safety devices are provided for its safe operation.
e)It can be used for inspection of bridges approximately 9,000 meter width and 12,000 metre in vertical downward reach.
3.1.2 Mobile Bridge Inspection Unit (Platform Type)
This type of unit is provided with working platform, heavier in weight, therefore truck chassis and stabilization system should be selected carefully to keep tyres/ stabilizers ground pressure on bridge deck within permissible limit during its translating and stabilization operation. In India this limit is taken as 5kg/cm2.
These are used during stabilization of unit and are of following types:-
a)Pneumatic tyre stabilizers.
b)Rubber bonded solid steel wheel stabilizers.
c)Stabilizers fitted with track chain.
220.127.116.11 Basic Parameters For Selection Of MBIU
The unit has been designed for its transportation at fast speed and inspects the bridges quickly. Normally one unit can be utilized effectively for inspection of bridges in three or vour adjoining states i.e. its selection should be done with great care. Following are the basic parameters and norms generally adopted in India for inspection of two lane bridge in single stetting or four lane bridge in double setting.
a)The unit should not occupy more than 3.50metre width of carriage way, leaving the balance for free flow of traffic.
b)The vehicle should be capable of running at slow speed approximately 4 metre per minute on bridge deck while platform is in working position under the bridge.
18.104.22.168 Safety Devices
The unit is provided with following safety devices for its safe operation:-
a)Over load device
It is provided on the working platform. In case of over loading on working platform, horn will give alarm sound and unit will switch over to emergency cut off position.
b)Emergency push buttons
These are provided in (a)driver’s cabin (b)main control panel (c)platform control panel (d)launching panel. In case of emergency , it cuts off electrical circuit instantaneously and stops further operation of unit.
It is used for retrieval of unit in emergency due to failure of vehicle engine, power take off or interruption in control system/hydraulic pump.
d)Emergency cut outs
These are used cot maintain appropriate load at the structure and fitted at working platform, lifting tower and main control panel.
The area of operation and load at the working platform is divided in two position (a)extended (b)retract. These switches are provided at the working platform for its restricted movement.
22.214.171.124 Lift Arrangement
It is manually operated hydraulic lift with independent pump to operate scaffolding cage up to three metre height and used for inspection of inner beam and box girder.
126.96.36.199 Descender Unit
It consists of a cage3 with two revolving rubber guide wheels for support on piers. It is operated with generating set provided with the unit. Normally two persons can be accommodated inh the cage and inspect or carry out small repairs of piers.
188.8.131.52 Repair Facilities
The working platform has suitable arrangement to provide water, compressed air, electricity connections 220 volts from external source on top of the bridge , to facilitate use of repair tools and equipment required for minor repair of bridge.
3.2 Selection Of Unit
3.2.1The mobile bridge inspection should be of repute make, proven design, structurally sound, robust in construction with emphasis on safety under all working conditions including transportation.
3.2.2 Air Suspension System
It is fitted on truck chassis of the unit. Its purpose is a)to reduce strain on truck components and increase their service life.(b) proper suspension comfort under all load conditions specially in distribution system when load increases/decreases (c)to provide consistently smooth ride under all load conditions.
The operation of MBIU is done through hydraulic system which gets the power from prime mover of truck. Besides, unit has to cover vast area under different geographical conditions for inspection of bridges therefore, truck of the unit should be of:-
2.Adequate capacity for smooth functioning of unit.
3.Follow norms of motor vehicle act applicable in India as shown below:-
i. Axle load not to exceed
a)Driven axle - 6000kg
b)Drive single axle - 10200kg
c)Drive double tandem axle - 19000kg
ii. Dimensions during transportation should not exceed
a)Length - 11m
b)Breadth - 2.5m
c)Height - 3.95m
a)Normally, operation, maintenance and repair cost of the mobile bridge inspection unit along with light commercial vehicle is Rs12 lakh per annum (approx.). Therefore, advance planning and monitoring should be done for its optimum utilization to keep the running economical.
b)It is a costly and sophisticated equipment. Therefore, skilled trained operators and mechanics should be deployed on the unit.
c)In hilly regions, roads with sharp curve, higher gradient and single/double lane bridges with low ground pressure bearing capacity normally exist. It should be preferred to procure mobile bridge inspection unit (platform type) with 7.5m working platform mounted on a single axle drive vehicle which is lighter in weight and capable to inspect double lane bridges in two settings.
d)The log book containing the utilization of unit is imported equipment. Therefore, hydraulic pump, one set of relay, power contractor, O rings etc should be kept in spare to avoid idling due to break down.
e)Electrical contact points should be deoxidized regularly to avoid break down of unit.
f)Operation of MBIU involves the safety of the unit as well as operating staff also. Therefore, manufacturer’s instructions given in operation and maintenance manual should be strictly followed.
g)Presently inspection of bridges is done visually and with concrete test hammer. The inspection instruments like resistivity meter, crack measuring meter, ultrasonic equipment, permeability tester etc should be provided with the unit to make the inspection more effective.
1. IRC Special Publication No. 35 – Guidelines for Inspection and Maintenance of Bridges