BOLTED CONNECTIONS FOR EASILY REPAIRABLE SEISMIC RESISTANT STEEL STRUCTURES

Recent years have brought significant advances in the design capabilities and construction practices of steel structures. These were partially caused by technological development and a direct effect of the research community efforts towards the mitigation of the earthquake induced damage. Making the traditional structural systems more resilient is one of the directions taken but, more and more, solutions with reduced post-earthquake repair costs are preferred. Steel structures are particularly malleable in the modern spirit of integrating devices which render the structure as “low-damage” or “easily repairable”. The recent earthquakes of Japan and New Zealand have demonstrated the feasibility and the advantages of such structural typologies. The current work presents an investigation on two steel structural solutions, including thus both moment resisting and braced frames, which have the potential of being easily used in practice, with minimal alteration of the design and erection procedures and improved post-earthquake economic benefits. The thesis focuses on (i) bolted connections of detachable short links for eccentrically braced frame and second, and (ii) on bolted friction connections for moment resisting frames. The main objective is to facilitate the application of these structural solutions in practice by enhancing the knowledge of their relevant bolted connection design and behavior

Finite element simulations on the ultimate response of extended stiffened end-plate joints

The design criteria and the corresponding performance levels characterize the response of extended stiffened end-plate beam-to-column joints. In order to guarantee a ductile behavior, hierarchy criteria should be adopted to enforce the plastic deformations in the ductile components of the joint. However, the effectiveness of thesecriteria can be impaired if the actual resistance of the end-plate material largely differs from the design value due to the potential activation of brittle failure modes of the bolt rows (e.g., occurrence of failure mode 3 in the place of mode 1 per bolt row). Also the number and the position of bolt rows directly affect the joint response. The presence of a bolt row in the center of the connection does not improve the strength of the joint under both gravity, wind and seismic loading, but it can modify the damage pattern of ductile connections, reducing the gap opening between the end-plate and the column face. On the other hand, the presence of a central bolt row can influence the capacity of the joint to resist the catenary actions developing under a column loss scenario, thus improving the joint robustness. Aiming at investigating the influence of these features on both the cyclic behavior and the response under column loss, a wide range of finite element analyses (FEAs) were performed and the main results are described and discussed in this paper

Finite Element Analysis of Composite Replaceable Short Links

Eccentrically braced frames (EBF) with detachable short links are an efficient solution for buildings in seismic areas owing to their high energy dissipation capacity and ductility and ease of repair in the earthquake aftermath. Past studies revealed that short links can develop shear overstrength (i.e. Vu/Vp, where Vu is the ultimate shear strength and Vp the corresponding plastic resistance) larger than the value recommended in EC8 (i.e. Vu/Vp =1.5). One of the factors causing the higher shear overstrength is the presence of axial restraints that leads to the development of tensile forces in the link at large levels of rotation. Another reason for higher shear overstrength is the composite slab that can resist the shear distortion together with the short link. Within the DUAREM project, full scale pseudo-dynamic experimental tests were carried out on 3D EBF allowing thus the investigation of replaceable links considering two arrangements: (i) steel solution – the link was uncoupled from the slab (ii) composite solution – the slab and link are connected. The aim of this paper is to present the results of finite element analyses (FEAs), based on calibrated models and the comparison between the obtained results and the experimental tests performed within DUAREM project. The numerical investigation carried out aims to evaluate the shear overstrength and the level of axial force in the link for both tested configuration

Seismic response of steel Moment Resisting Frames equipped with friction beam-to-column joints

The use of supplementary dissipative devices is an effective design strategy to improve the seismic response of structures. Friction devices inserted into beam-to-column joints can be a viable solution to optimize the seismic performance of steel Moment Resisting Frames (MRFs). The joints equipped with such devices are full rigid but very ductile partial strength, whose resistance can be easily calibrated to match the EC8 design moments with negligible overstrength. On the contrary, EC8-compliant MRFs equipped with traditional full-strength joints are often characterized by large beam overstrength due to the need to satisfy both serviceability and stability checks that lead largely oversizing the columns. In this paper, three design criteria for MRFs equipped with friction beam-to-column joints are described and examined by means non-linear static and dynamic analyses. The discussion of the results highlights the benefits of friction joints as well as the effectiveness of the examined design criteria

Comparison Between Different Design Strategies for FREEDAM Frames

The seismic design rules currently recommended by modern codes are based on the so-called capacity design principles, which allow damage to occur in the ductile parts of the structure, such as beam ends. Therefore, in the aftermath of design earthquakes, plastic deformations at member or connection level will imply high repair costs. Innovative structural solutions to reduce the damage in the structure have been proposed in the last decades. In particular, the so-called supplementary energy dissipation strategy allows to increase the dissipative capacity of structures equipping it with special damping devices. In case of substitution of dissipative zones with dissipative devices the strategy takes the name of substitutive strategy. This is the case of Moment Resisting Frames (MRFs) investigated in this paper, where traditional dissipative zones, i.e. beam ends have been equipped with innovative free from damage frictional devices called FREEDAM dampers. However, the current version of EC8 does not provide any guidelines for the design of MRFs equipped with this type of friction joints. For this reason, in this paper two new design criteria for this structural system are proposed, where the first one is based on the application of the beam-to-column hierarchy criterion (BCHC) while the second one exploits the Theory of Plastic Mechanism Control (TPMC), used with the scope to design structures exhibiting at the collapse a mechanism of global type. A comparison between both the design approaches and the results of the new rules are herein examined by means of nonlinear static analyses