The Third Narmada Extrados Bridge in India
Delivery Method: Design-Build
Owner: National Highway Authority of India
Contractor: Larsen and Toubro, ECC Division, India
Authority Engineer: Aarvee Associates, India
Design Consultant: Precast BridgeTech, Co. Ltd., Thailand
Independent Checking Consultant: Roughan & O’Donovan, Ireland
Date of Services: January 2010 to June 2013
Total Construction Cost: $25 million
Precast BridgeTech Co. Ltd. (PBT) is an engineering firm incorporated in Thailand, dedicated to bridge design. PBT’s services cover the complete bridge engineering process from concept development to detailed design, construction engineering and field supervision, and often offer optimized detailed design with their use of advanced technology tools and experience.
Engineers at Precast Bridge Tech. have been involved in large design/build bridge projects, mostly in India. They have specialized in value engineering and detailed design of span-by-span and cable stayed/ extradosed girder bridges, for which they used the Bridge Designer software successfully to optimize design and manage construction.
These projects are documented in the News page of the Interactive Design web site and on Precast Bridge Tech. upcoming web site.
The engineers at Precast Bridge Tech. have been using the Bridge Designer software for several years and a variety of Bridge Design projects, from span-by-span to balanced cantilever, extradosed and cable-stay bridges.
For precast segmental bridges, they use BD2 in order to evaluate structures under construction, including time-dependent staged construction and final stress redistributions and deflections due to time dependent material properties of concrete and steel.
Precast Bridge Tech. used Bridge Designer for construction analysis of this project.
The bridge is 1.344km long (96m+8x144m+96m long). The superstructure has 4 lane carriageway and single walkway and consists of precast segmental three-cell box girder with two planes of cable stays. Link-Saddles are utilized to transmit the stay forces to upper pylon. Expansion joints are located at mid span of every other span. Upper and lower pylons are built with cast-in-place construction. Foundation consists of 12×1.5m diameter drilled shafts. The construction period is 30 months.
Precast Bridge Tech’s Managing Director, Egemen Ayna says “The bridge articulation is such that we have a mid-span expansion joint (EJ) at every other span. Mid-span EJ utilizes a steel shear key that can only transmit vertical shear and torsion. It is installed after all SDL loads are placed and all stay stressing and post-tensioning are completed. It does not experience any permanent loads apart from shear associated with level adjustment at the EJ cantilever tip with strong backs and creep redistribution of expansion cantilevers. Creep distribution of expansion cantilevers at either side of EJ are carefully and accurately determined by using BD2 computer model of two continuous frames, separated by an EJ in between (a frame is two pylons combined with CIP closure at mid-span).”
Precast Bridge Tech Role:
Our role for Their Narmada Extradosed Bridge is the detailed or final design of the bridge and its construction support, said Egemen A. Ayna.
Because the bridge is on a straight line alignment, PBT engineers were able to use the 2D engine of Bridge Designer for planar analysis of the construction stages. Modeling this way for 2D analysis allowed them to create a model on a straight line, with equivalent cross-section properties as the three void actual bridge section, one line of stays and each post-tensioning tendon representing multiple tendons with similar elevation and plan-view layouts.
The Designer Larsen & Toubro describes this bridge as “a greener bridge”. From L&T’s web site:
“This is the first bridge of its kind in Gujarat. And it also has another distinction: the bridge is greener than many others, built with a commitment to the key principles of sustainability – Recycle, Re-use, Reduce. At the site, all surplus concrete was used in casting of crash barriers. These barriers are built for the road as well as for the survey reference pillars. A small step in recycling and ruins that has contributed to conservation of resources.”
See L&T web site: http://www.larsentoubro.com/corporate/a-greener-bridge/
The bridge models include balanced cantilever stages at each pier. The Bridge Designer software computes construction deflections and stresses under all load cases (self-weight, superimposed loads, post-tensioning initial and final forces, creep, shrinkage and any temperature effects such as linear and non-linear thermal gradients). The software provides the capability to assemble all cantilever “sub-structures” at the proper dates according to the contractor schedule, and computes the continuity effects for all load-cases as well as final conditions (support reactions, deflections and stresses) after long-term redistributions according to the local code (material properties, concrete and steel short and long term rheological properties.
Bridge Designer models post-tensioning tendons in 3D in all cases for the purpose of properly evaluating friction losses. One interesting feature of the Bridge Designer software is that it can easily convert tendon geometry from a 2D model to a 3D model, taking into account the structure horizontal curvature.
PBT engineers first created a 2D version of the model for longitudinal analysis and sizing of the post-tensioning layout. This first model was also used to evaluate long term stress redistributions (creep & shrinkage, and steel relaxation) and final deflections in order to obtain casting curves for geometry control.
The Bridge Designer User Interface provided the tools to quickly generate a 3D model. Even though only 2D (linear structure) analysis was used for this project, it is possible to quickly define an accurate 3D model of the structure as well as the Post-Tensioning tendons, usable for 3D analysis.
In addition the transverse can be accurately modeled for exact mechanical cross-section properties as well as exact geometry and topology for non-linear thermal gradient evaluation.
Precast Bridge Tech. also used the Bridge Designer software (2D engine) to evaluate transverse effects of post-tensioning, in initial and final conditions.
Transverse model with post-tensioning
The following diagrams obtained in the Bridge Designer graphical interface, show transverse force and displacement distributions under self-weight, post-tensioning, creep and shrinkage.
Axial stress on top fiber due to post-tensioning
Deflections under all applied loads
The 3rd Narmada Extrados Bridge, Baruch – India was inaugurated and subsequently opened to traffic on March 7, 2017.
See article in Indian media from last October: http://deshgujarat.com/2016/10/04/new-narmda-river-bridge-in-bharuch-on-hn-8-to-be-operational-early-next-year/