The location of existing bridge is in the northern gateway of Trichy city. The bridge connects Samayapuram toll gate with Thrivanalikovil and Srirangam towns. The intensity of traffic through the existing narrow bridge is inadequate for present day traffic.
The heavy vehicles from Trichy to Samayapuram, Turaiyr,Musiri, Namakka, Alem, Lalgudia and Ariyalur are diverted from Trivanaikovil to NH-45 four lane bypass pads, as the existing bridge is narrow and weak.During peak hours, passengers find it difficult to cross the bridge due to heavy traffic congestion. This existing bridge serves as the means of transporting agricultural products to Trichy city from northern districts. Hence,reconstruction of the bridge is very essential for road users for free flow of traffic.
Specifics
The bridge is across River Coleroon at Srirangam TamilNadu. The total length of bridge is 792.0m. It is 4-lane bridge with footpath on either side, with total width of 21.45m. The span arrangement is 24 spans of 33m.The superstructure is with Bow String Arch, with partially prestressing (for tie member) substructureportal frame, with twin piers over pile foundation with 1200mm dia bored cast-in-situ piles.
Geographics
Trichy is the fourth largest municipal corporation in Tamil Nadu,and, the fourth largest urban agglomeration in the state. The city covers an area of 146 Sq. Kmwith a population of 7,52,066 in 2001.It is rated to be one of the fastest growing cities in TamilNadu. During, the last couple of decades, the life style of people, and their commuting habits have undergone radical changes, due to steep hike in the cost of urban land in Trichy, and, the revolutionary changes in major engineering equipment manufacturing, high-pressure boiler manufacturing steel plant and chemical industries.
During peak hours, passengers find it difficult to cross the bridge due to heavy traffic congestion. Moreover, Sriranga Temple – Pilgrims and tourists come to this, and, it is considered as the most important place in Trichy District. Sincethese towns are located on the banks of river Kaveri all the developments, settlements have been concentrated between this river and around.
The project bridge is across river Coleroon at Srirangam Tamilnadu. The bridge location is very close to ancient SriRangam temple, and, in view of its attention was paid to aesthetics of the bridge structure, Bow string Arch was selected accordingly. Most of the bowstring arch bridges in our country is with 2-lane & footpath. This is one of the widest Bridge with Bow String Arch.The analysis of portal frame for substructure, carried out with Staad Pro software for various loads and combinations, including wind & Seismic as per IRC Codes.The piles were analysed for vertical forces and horizontal forces with Soil Structure interaction, considering scour levels, with Staad Pro analysis.
B V NAGESH, Principal Consultant, Nagesh Consultants
The proposed bridge is connected to important places like Rock fort, Srirangam, Renganather Temple, Thriruvanaikovil, Akilandeswari etc.
Approach Methodology
Stage – I : Field Investigation:
• Conducting Topographic Survey
• Soil Investigation as per NIT.
• Conducting Traffic Survey as per NIT
• Identifying the Utilities.
Stage – II : Detailed Analysis and Designs
• Preparation of GAD and discussing with the authorities for approval of the same.
• Detailed Analysis of Super Structure and Sub Structures.
• Design of Super Structure, Substructure and Foundations as per IRC Codes and specifications.
• Preparation of Detailed Drawings.
• Preparation of Detailed BOQ and cost estimates.
• Preparation of Tender documents.
Structural System of Bridge
24 No of 33m c/c spans are proposed to suit the Existing Pier location and Length of the Bridge. The overall length of the Bridge from Abutment to Abutment works out to 792m.
Super Structure
Super structure is proposed with Bow string arch girder. C/C distance between expansion joint is 33m, and effective span is 30.75m. Tie members are with cast in situ post-tension girder, other elements are cast in situ RCC member.
Entire super structure shall be cast and cured before the post tensioning operation. Staging provided for tie member shall be removed only after stressing operation.The deck system is analysed using STAAD-PRO software with 2 models i.e., for deck slab as continuous beam of width3-D analysis for remaining elements of super structure.
The Pot cum PTFF bearings are proposed as per IRC: 83 part III. The Jack locations are proposed at 0.75m from C/L of bearing below the end cross girder, and, the load combinations are as per IRC: 6-2010, and, design is carried out as per the guidelines of IRC: 112-201, and, references are made to Euro code, IS codes.
Deck Slab
The slab is supported on cross beams, which are 3.075m apart. 11 cross beams are proposed including 2 at the bearing locations which will rest on the tie beams spaced at 17.25m. Hence, the slab panel size is 3.075 x 17.25, essentially one-way slab. The slab is idealized as one-way continuous slab of 1m width and analysed using Staad Pro adopting effective width of dispersion method.
Codes & Standards
The design of various components of the bridge, in general are based on provisions of IRC/IS codes
IRC: 5 – 1998: Standard Specifications and Code of Practice for Road Bridges, Section I – General Features of Design.
IRC: 6 – 2000: Standard Specifications and Code of Practice for Road Bridges, Section II – Loads and stresses.
IRC: 112 – 2000: Standard Specifications and Code of Practice for ConcreteRoad Bridges.
LOADING:
Dead Load (DL)
The unit weight for Dead Loads calculation has been considered by adopting unit weights as per IRC: 6 – 2000(Standard Specifications and Code of Practice for Road Bridges, Section II – Loads and Stresses).
1) RCC -25 KN/m3
Super Imposed Dead Load (SIDL)
1. Wearing coat -2 KN/m2
2. Crash barrier -7.75 KN/m
3. PCC in footpath/median filling–22kN/m3.
Footpath Live Load
As per Cl: 206.3, IRC: 6-2010 – 5 kN/m2
Vehicular Live Load (LL)
As per IRC: 6, deck slab is analysed for the following vehicles and whichever produces the severe effect has been considered in the design. Following combinations are used:
1) Three class A vehicle.
2) One 70R tracked + one classes A vehicle.
3) One 70R bogie + one class a vehicle.
Transverse Analysis of Deck Slab
The deck slab is idealized as one-way slab of 1m width supported over cross girders supports are assigned at cross girder location panel size works out to 3.075 x 17.25.
Dead load analysis:
Dead load analysis is carried out for 3 cases:
At footpath location.
At Median location.
At Carriage way.
(A) At Footpath Location:
a) Self weight of slab = 0.275 x 25 = 6.875kN/m2.
b) Super imposed dead load:
i) Due to hand rail – 7kN/m
ii) Due to PCC filling in footpath = 0.25 x 22 = 6.16kN/m2.
iii) Footpath live load = 5kN/m2.
(B) At Median Location:
a) Due to Self weight = 0.275 x 25 = 6.875kN/m.
b) Due to median filling =0.30 x 22 = 6.60kN/m
= 13.475kN/m
(C) At Carriage way:
i) Due to Selfweight = 0.275 x 25 = 6.875kN/m2
Due to Wearing Coat = 2.00kN/m2.
= 8.875kN/m2
Analysis for Vehicular Live Load
Vehicular live load analysis is carried out using staad pro with moving load option for following vehicles.
1. Class A vehicle.
2. Class 70R wheeled.
3. Class 70R Tracked vehicle.
Post processing is carried out for 3 supports and 3 spans for above vehicles and reinforcement detailing is carried out for critical values.
Effective width of dispersion
For interior span beff= α x a (1-a/l) +b1———Formula 2
b1= 0.25+2×0.075 =0.40 ( Width of dispersion parallel to support upto the top of slab.
α =2.6, As per clause 305.16.2 IRC: 21-2000, for continuous span with l/b>2.
a= C.G of the load from nearest support.
Calculation of ‘a’
Vehicle moved on the deck with an increment of 0.25m. Load position with respect to critical value of bending moment both for support and span is fond and C.G of the loads corresponding to this location is found .and then C.G of the wheel loads distance from nearest support is taken as ‘a’ specimen calculation for all the 3 types of vehicles are explained in the following.
Impact Factors and Partial Safety Factors:
Impact factor is considered as per cl.208, IRC:6-2010.Following load factors are considered for structural design, IRC:6-2010
i) Dead Load and SIDL except W.C = 1.35
ii) Wearing coat =1.75
iii) Footpath live load = 1.50
Internal Cross Girder
Concrete Grades and Reinforcements:
Concrete: –
For RC Deck Slab (M – 40)
Reinforcement: –
HYSD bars (Grade Fe – 415) conforming to IS: 1786 are provided as reinforcement.
Clear Covers: –
The minimum clear cover of 40 mm to reinforcement has been considered indesign as the site is under “moderate” exposure condition.
Bearings
POT cum PTFE Bearings are proposed as the order of loading is high. 4 bearings are proposed per each span with articulation of Fixed, Guided and Free Bearings.
Sub Structure
Turning pier substructure system was proposed. The twin pier system was analysed for following loads and load combinations –
1) Dead load from superstructure.
2) Live load from superstructure.
Following vehicular loads were considered
i) 2 class -A
ii) 4 Class A
iii) 70R vehicular + 2 Class A
iv) 2 – 70R vehicles
Vehicles on one span & both span was considered.
3) Longitudinal forces as per IRC:78.
4) Wind load.
5) Seismic forces.
6) Water current forces.
Load Combination
- Load comb 99 dl+ll on one span with if+bf+ff.
- Load comb 100 dl+ll on both span with if+bf+ff
- Load comb 101 dl+ll on one span without if+bf+ff.
- Load comb 103 dl+ll with if+bf+ff+wind on ss+(wind on ss+ll)+wind on pierbeam.
- Load comb 104 dl+ll without if+bf+watercurrent+ff+wind on ss+(wind on ss+ll)+wind on pierbeam + wind on pier stem
- Load comb 105 dl+seismic trans(dl)+0.2vll+0.5bf+ff+sf+water current.
- Load comb 106 dl+seismic(longi) dl +watercurrent
Foundation
Pile Foundation with 4 Nos of 1200mm dia RCC Bored cast in situ Piles under each columns are proposed with Pile Cap at Top. The Founding Levels/Pile termination levels are fixed based on the required Pile capacity. In general Piles are terminated by socketing depth of 5dia of pile into disintegrated Rock or 0.5 dia into hard Rock.
Cement concrete pavement with 75 mm of uniform Thickness is proposed in M30 grade of Concrete. Deck is proposed with 2.5% camber in transverse direction to drain off the water with drainage spouts proposed at the edge of Kerb. Footpath with Hand rails is proposed on both side of bridge to facilitate pedestrian movements.
Encounters
The challenges faced are as follows:
- Analysis of Bow String Arch for 4-lane & footpath on either side, with 2 nos of Bow-Strings.
- The critical design of the foundation, as the bridge is proposed very close to old existing single lane steel girder bridge with pile foundation.
Fact File
Client: Tamil Nadu Highways-Trichy
Structural Consultant: Nagesh Consultants
Contractor: P.K.Mookanambalam & Co
Steel Tonnage:
a) Reinforcement Steel – 3200 MT
b) High Tensile Steel – 150 MT