Evaluation of the structural performance of tetragonal lattice girders

This paper presents an evaluation of the structural performance of tetragonal lattice girders for NATM tunnel construction. H-shaped steel ribs and triangular lattice girders have been widely used as a temporary support for NATM tunneling. H steel ribs have high flexural stiffness and strength, but they may induce unexpected internal gaps after shotcreting, because of their geometric characteristics. To overcome the problem, triangular lattice girders were developed. However, the lattice girder revealed frequent local failure at the joints of each bar. Moreover, those two supports show the quite low connection strength of members. Recently, a new type support, the tetragonal lattice girder, was developed to overcome the weaknesses of former supports. In this study, the structural performance of the new type support was studied by analytical and experimental methods. For the evaluation, 4-point flexural analysis, the conventional evaluation method, and arch analysis including stability analysis were performed. In addition, a 4-point flexural experiment was performed to evaluate the ultimate load carrying capacity, and observe whether local failure occurs or not. In the experimental study, a connected model was also considered as well as a unit model, in order to check the rationality of the connection method of the girder. All analytical and experimental results were compared with the structural performance of former supports.