Grillage Analysis of Prestressed Concrete Girder Deck Superstructure for NH-16 Bridge Flyover; Part-1

Abstract

The bridge superstructure flyover built on National Highway route 16 (NH-16) near Benz circle in Vijayawada to act as grade separated highway for vehicles to cross over National Highway route 65 (NH-65) is made of precast and post-tensioned concrete girders. This article is the first part of the two phase study that focuses on the structural analyses of concrete bridge girders. The bridge girders are analyzed for vehicular loads considered as per Indian Roads Congress Specifications. The IRC Class of loads of 70R and Class A and their wheel load configurations are discussed. The bridge girders along with deck are modeled as grillage girder-slab members in Staad Pro analysis software. The bending moments and shear forces are computed for each of the IRC vehicular load configurations and the envelope forces for governing load configuration are presented for each of the girders. Finally, the article summarizes the maximum flexural and shear demands on the bridge girders. The pros and cons of grillage analysis modeling for prestressed girders and the alternate superior options for precise but quick modeling are also recommended.

Introduction

I. INTRODUCTION

The bridge on NH-16 near Benz Circle in the city of Vijayawada is built as two separate carriageways each of 3-lane post-tensioned concrete girder bridges to cross over the at-grade vehicular traffic on NH-65. A bridge was identified as very necessary here due to the heavy volume of vehicular traffic on both National Highways meeting at this point. The bridge has a total length of about 240 m with an obligatory maximum span of 30m in length. The bridge supports three lanes of traffic with a total carriageway width of 11.5m. The bridge has two crash barriers one at each edge of the carriageway to make the total deck width of 12.5m. The bridge deck is supported by four post-tensioned concrete girders spaced at 3.1 m on center and supporting an overhanging slab of 1.6m. The bridge girders are connected by cross diaphragms one at each support location and one at the midspan. The concrete girders with cross diaphragms are supported on elastomer bearing pads installed on top of reinforced concrete pedestals cast monolithic with pier caps.  Figure 1 shows the sectional view of bridge girders and carriageway deck. The girders are rectangular at support locations and taper to become I-shaped along the rest of their length. Figure 2 shows the girder cross section at support location on top of pier caps.

This method of analysis is quick and reasonably accurate as such engineers prefer this method. In grillage analysis, the grillage members in both directions are assumed as 1D members with equivalent inertia and rigidly connected at their intersection. So grillage system is 2D representation of a system using D or line models of members in either direction. Line models are quick and easy to model and run the results, and they are reasonably accurate for engineering and construction purposes (Vemuri & Jonnalagadda, 2023c). The grillage system has another advantage; the force demands in girders and slab are determined with the same model, so separate analyses of girders and slab is not required. This saves  time on real projects.  

However, some of the disadvantages of grillage analysis include too conservative estimate of force demands on the girders and slab, and an inadequate estimate of torsional forces on the members (Hambly, 1991). In projects that have novel and innovative superstructure girder forms such closed or open box sections (Vineeth et al, 2019), spine and wing beams, beams with troughs for cast-in-place concrete, the grillage analysis can give quite erroneous results. Also novel precast construction methods with adapting construction sequences (Jonnalagadda & Vemuri, 2023c) and high strength materials such as Ultra-High performance concrete (Jonnalagadda & Chava, 2023) that show non-linear deformations beyond elastic range may not offer modeling grillage members without losing accuracy of analyses. Due to inaccurate torsional distribution, skew bridges may not be good candidates for grillage analysis (Raj & Phani, 2017).

Some of the alternatives for grillage analyses include finite element analysis using shell or plate models or combination of plate and shell frame member models. For Structural engineering problems, using Risa 3D® or ETabs® offers facility to model the bridge members as finite element models and optimize the mesh size to achieve accurate measure of force demands on the members. Software specific to bridge member analyses such as Midas® Bridge, Leap® Bridge, Conspan® from Bentley® are available for quick analysis of member forces in girder and slab. Grillage analysis has become an outdated modeling practice with the advent of finite element analysis bases structural analysis software products.

V. ACKNOWLEDGEMENTS

The authors would like to acknowledge the support of the office of National Highway Authority of India (NHAI), Ministry of Road Transport & Highways, Government of India for providing the drawings for the bridge superstructure.

Conclusion

This study details the analyses and design methods used in the engineering of post tensioned superstructure girders for a 3 lane bridge on NH-16 of India. The article starts with explaining the vehicular loads considered for the design of the bridge in accordance with the Indian standard specifications. The details of Grillage analyses performed in Staad Pro including bending moment and shear force diagrams for inner and outer girders are presented. The summary of factored design load demands is tabulated. Also, the pros and cons of grillage analysis are discussed, and alternate methods of modeling and structural analyses are briefly outlined. The natural follow up for this study will be on the design and detailing of reinforcement in these girders which are planned to be disseminated in a follow-up article.

References

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Copyright

Copyright © 2024 Phani Ram Gollapudi, Satish Chandra Dendukuri, Satish Brahmalla. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.