Abstract

Developing a continuum model of a lattice structure means expressing a complicated structure into a simpler one, and this is done by determining equivalent stiffness of the original structure, which can further be used to transform into desired simpler geometry for multiple benefits. Objective of this paper is to find a reliable method for determination of equivalent stiffness of lattice girder, comparing all the techniques suggested by various researchers. In present work the actual lattice girder structures and their equivalents were analysed using Finite Element Method tool Strand7. The primary focus is on lattice girder, which is a primary rock support system employed in tunnels and caverns. The equivalent stiffness so obtained is used as input parameter in numerical analysis tool STRAND7 for analysis of equivalent lattice beam. The maximum deflection and bending stress at centre of equivalent lattice beam were calculated and compared. In an attempt to develop a more accurate equivalent beam model a novel approach which is modification of the Method -B has been developed by authors. This Noval approach is the most convenient approach to develop an equivalent section with an extremely uncompromised representation of stress development and deformation characteristic. In this approach, the focus was made on the physical resemblance of a proposed equivalent beam with the actual lattice girder, leading us to adopt a beam of I-cross-section. This approach yielded similar stress and deflections in the equivalent beam as that of the actual lattice girder.

Introduction

In tunneling, Lattice girder poses some similar function as steel arch sup-port, as it serves as an element of temporary lining to support excavation initially and sometime as a part of permanent lining, offer an extremely prudent alternative to other support technique for an extensive variety of ground condition[1][2]. Lattice girder is utilized as a NATM support to prevent the development of initial deformation until the point when it is completely cured shotcrete is poured instantly after drilling. It is a combination of truss and beam element and it transfer load to the lower bar through the bending and axial force. Lattice girder has a three dimensional structure and length / width ratios can be 10 to 15.

They can be fully integrated with the shotcrete lining and form optimum bond to the ground supported. LG acts as immediate and reliable ground support in the excavation area [3][4]. The main advantages of lattice girder over conventional steel arch supports are its low weight per meter length of the girder and, a more efficient bond with shotcrete making it economical than other supports.

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