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Monday, June 13, 2011

PSC Girder Bridge Construction - construction of a bridge- bridge construction method

This article is based on my training I got from IVRCL infrastructures limited. (BOT Project Salem to Kumarpalyam on NH-47 in the State of Tamilnadu for 53 kms.)

Parts of bridge:-
Sub-Structure:
            Open foundation
            Pile foundation
Super-Structure:
            Pier
            Pier Cap
            Pot Bearing and Pedestal
            Girders
            Deck Slab
            Road Surface 










FOUNDATION
            The structures used for transfering and distributing the dead load of super-structures, piers, etc along with live loads likely to come on the bridge to soil underneath are known as foundation.
Open Foundation:
·                   Individual sloped footings are used in this project in the places where the soil has sufficient bearing capacity.Generally it is practicable for a depth upto 5m and is normally convenient above water table.Because of heavy loaded columns steel reinforcement is provided in both directions in concrete bed.100mm thick PCC (M-15) is laid below foundation after the exaction and dewatering.


Pile Foundation:
     Pile foundation is generally preferred in following conditions:-
1.When foundation soil is loose and when hard stratum is available at a depth of around 10m to 15m.
      2.When the structure is tall and heavy.
3. Dead load and live load coming from the structure is very heavy and the distribution of load is uneven.
      Because of the sewage water passed on that area, there is full of clay soil. So that they removed the water in that area and changed the path of sewage water and filled a layer of 2m soil in that area to make it stronger.Cast in situ concrete piles are used in this project. A bore is dug into the ground by inserting a casing.This bore is then filled after placing reinforcement.In some areas bore will touch hard rock within 4m.In that situation they made it as open foundation and they redesign.After 28 days of casting of piles deflection test is done with the help of sand bags to find how much load it can take.Above a set of pile foundation (8piles for main carriage way & 5piles for Service lanes) open foundation is made.






Super-structure:
It is the part of bridge over which the traffic moves safely. It consists of Pier, Pier cap, Pedestal, Bearings and Deck with Road surface.
Pier and Pier Cap:
·         Pier and pier cap is used to transfer the load from girders to the foundations.
·         Their numbers depends upon the length of bridge and the span between the piers.



PEDESTAL & BEARINGS:

·         Pot bearing, Polished stainless steel (mirror finishing) welded to the bottom side of the rectangular sliding plate.It is a Structural bearing required to connect the decks to piers.They are capable of transmitting forces while absorbing the structure’s deformations and rotations.

Site Installation:
1) Place the pot bearing in its position level it with timber edges. Because of bearings allowing horizontal displacements it should be checked that the arrow painted on the slide plate is pointing in the correct direction.
2) GROUTING: Install the form for grouting the space between pier and pot bearing (Pedestal). Grout the space between pier and pot bearing. Fill in the recesses checking that the level is the correct.
3) FORMING OF DECK: The formwork of the deck is placed embedding the upper dowels of the bearing.
4) REMOVAL OF FIXING PLATES: Once the formwork has been removed, the bearing is definitively installed. Remove the lateral fixing plates of the bearing in order to allow its free movement.
·         In future if they want to change the bearings, they provide a hydraulic jack in between girder and pedestal and they will change it.

DECK WITH ROAD SURFACE:
            Deck is the top portion of the bridge. The road way is formed on the deck. It is made up of RCC and PSC girders.
GIRDER:
·         A girder is a support beam used in construction.
·         Two types of girders used in this project.
1.      RCC Girder, 2. PSC Girder
 1. RCC GIRDER:
·         RCC girder is used in curves of bridge.
·         In curves 3span continuous is provided.
      It is suitable for span length of 20-25m.


 2. PSC GIRDER:
·         In Pre Stressed Concrete Girder they provide 10 ducts (cables) inside each girder with 12 strands in each duct of High Tensile Steel (HTS). It can be build till span length of 45m. 




CABLE DATA
·         Total Cable Length = 38m
·         Cable extension = 137mm
·         No of strand per cable = 12
·         Ultimate tensile strength = 1890 kg/cm ²

STRETCHING DETAILS
·         Jack force = 170ton
·         Cable nos 1-7 are stage 1 cables and these will be stretched in order from both ends with a jacking force of 170tons.
Stage 1 cables will be stressed neither before 10days from the date of casting of girder nor minimum cube strength of concrete is 400 kg/cm ², whichever is later
·         Cable nos 8-9 are stressed after 28 days of casting of deck slab , but before placing of wearing coat.

·         Dummy cable “D” shall be stressed only in emergency after all the tendons have been stressed and stressing results verified by department, In case it is not required to be stressed , tendons may be pulled out and duct hole grouted.

EXPANSION JOINT:
·         These joints are provided to allow for expansion of the slabs due to rise in slab temperature above the construction temperature of the cement concrete. It also permits the contraction of slabs.
·         These joints are provided at interval of 40m.
·         They are provided in full depth of member.
·         The gap width for this type of joints is from 20 to 25mm.
·         Neo Pran pad is provided as a Filling material in the expansion joints to avoid infiltration of rain water and ingress of stone grits.
GROUTING:
·         Grouting is done to fill the voids and space in between the strands.
·         Cement, Ceebex100 admixture for grout expansion and Ice water are used as grouting material.
·         Ratio :- 1bag Cement : 100g Ceebex100
·         To reduce the heat of hydration, ice water is used otherwise because of heat, cement will settle in beginning itself. It will not reach the other end.



FINISHING WORKS:-
·         15mm thick mastic asphalt for wearing coat over deck slab.
·         50mm thick Bituminous concrete over wearing coat with proper slope to drain rain water.
·         Anti Crash barrier
·         Safety Kerb
·         Hand Rails are erected on the sides of the bridge along the length of the bridge. They serve as guard for the users and prevent the possibility of any user falling by slip.
·         Median is provided in to prevent the head on collision between vehicles moving in opposite direction.
·         Drainage pipe to drain off rain water.
·         Painting
·         Lighting
EQUIPMENTS USED:
·         Multi Purpose Excavators – For site clearance
·         Backactor – For foundation excavation
·         Earth Movers
·         Pile Driving Equipments
·         Hydraulic cranes
·         Concrete Pump
·         Boom Placer – Concrete placing. Advantages:-
1) Low labor requirement,
2) 70m3 discharge per hour,
3) 36m length with 4 booms.
·         Needle Vibrators




Why companies hiring the equipments?
·         Only because of maintenance problems they are hiring the equipments.
·         If they buy it by their own means they have to maintain a separate team to manage the equipments.
·         As a civil engineer we don’t know the problems in equipments. Vehicle operators may cheat us. So if we hire vehicles the maintenance should be made by the renting companies.

CONCRETE BATCHING PLANT:
            A plant used to make concrete using various components such as Sand , Aggregate , Water , Cement and Admixtures.
Grade of cement: 43G OPC
Grade of concrete:
1.      For PCC – M10
2.      For RCC – M25
3.      For Pile Foundation – M35
4.      For PSC Girders – M45
5.      For Kerbs – M20

For Service loops they provide the RE-Panel walls:-
·         Reduction in project cost
·         Quicker and Easier
·         More stronger (It will take any kind of loads)
·         Repair works can be done easier.
    
     Advantages of Girder Bridge:-
·         It is cheaper than steel bridge.
·         Maintenance cost is low.
·         It is durable.
·         It is compact and presents a neat appearance. 


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