Vincenzo Piluso

Theory and design of seismic resistant steel frames

Preface. Notation. Basic seismic design criteria. Global ductility of MR-frames. Local ductility of steel beams and beam-columns. Evaluation of the q-factor. Overall stability effects. Behaviour of connections. Seismic behaviour of semirigid frames. Structural regularity. Influence of random material variability. Influence of claddings. Index.

PLASTIC DESIGN OF SEISMIC RESISTANT STEEL FRAMES

A new method for designing moment resisting steel frames failing in a global mode is presented in this paper. Starting from the analysis of the typical collapse mechanisms of frames subjected to horizontal forces, the method is based on the application of the kinematic theorem of plastic collapse. The beam section properties are assumed to be known quantities, because they are designed to resist vertical loads. As a consequence, the unknowns of the design problem are the column sections. They are determined by means of design conditions expressing that the kinematically admissible multiplier of the horizontal forces corresponding to the global mechanism has to be the smallest among all kinematically admissible multipliers. In addition, the proposed design method includes both the influence of distributed loads acting on the beams and the influence of second-order effects. In particular, the influence of second-order effects, which can play an important role in the seismic design of steel frames, is accounted for by the mechanism equilibrium curves of the analysed collapse mechanisms. Moreover, in order to show the practical application of the proposed design procedure, a worked example is presented. Finally, the inelastic behaviour of the designed frame is compared to that obtained when the simple member hierarchy criterion or a similar rule is applied.

Plastic Design of Seismic Resistant V-Braced Frames

In this article, a new method for designing chevron concentrically braced steel frames is presented. The aim of the proposed method is the design of concentrically braced steel frames able to guarantee, under seismic horizontal forces, a collapse mechanism of global type. This result is of great importance in the seismic design of structures, because local failure modes give rise to a worsening of the energy dissipation capacity of structures and, therefore, to an higher probability of failure during severe earthquakes. With reference to the examined structural typology, the global mechanism is characterized by the yielding of tensile bracing diagonals and by the buckling of the compressed diagonals of all the stories. The proposed method is rigorously based on “capacity design approach” which requires that dissipative zones have to be designed to withstand the internal actions due to the seismic design horizontal forces and the vertical loads acting in the seismic load combination; while non dissipative zones have to be designed considering the maximum internal actions that dissipative zones, yielded and strain-hardened, are able to transmit. The new design issue covered by the proposed design procedure is the need to account for the contribution of the compressed diagonals in deriving the design axial force of non dissipative members. The seismic inelastic response of a sample structure is investigated by means of nonlinear dynamic analyses. The results carried out with reference to braced frames designed according to the proposed procedure are compared with those obtained with reference to the same structural schemes designed according to Eurocode 8.

Experimental Analysis of Bolted Steel Beam-to-Column Connections: Component Identification

In this paper, the results of an experimental program dealing with the ultimate behavior of bolted beam-to-column connections under cyclic actions are presented. The design criteria adopted for tested specimens are discussed in detail, aiming to point out how the ultimate behavior can be governed by properly strengthening the components for which yielding has to be prevented. To this scope, the component approach is adopted as a design tool for component hierarchy criteria. The aim of the paper is the investigation of the actual possibility of extending the component approach to the prediction of the cyclic response of beam-to-column joints. To this scope, the attention has been focused on the possibility to evaluate the overall energy dissipation capacity starting from the energy dissipation of the single joint components, provided that they are properly identified and their cyclic behavior is properly measured.

Failure Mode Control of X-Braced Frames Under Seismic Actions

In this article, a new method for designing concentrically braced steel frames is presented. The aim of the proposed method is the design of concentrically braced steel frames able to guarantee, under seismic horizontal forces, a collapse mechanism of global type. This result is of great importance in the seismic design of structures, because local failure modes give rise to a worsening of the energy dissipation capacity of structures and, therefore, to an higher probability of failure during severe earthquakes. With reference to the examined structural typology, the global mechanism is characterized by the yielding of bracing diagonals of all the stories. The proposed method is rigorously based on “capacity design” which requires that non dissipative zones have to be designed to withstand the internal actions coming from the seismic design horizontal forces and the vertical loads acting in the seismic load combination, while non dissipative zones have to be designed considering the maximum internal actions that dissipative zones, yielded and strain-hardened, are able to transmit [Mazzolani and Piluso, 1996]. The new design issue covered by the proposed design procedure is the need to account for the contributions of the compressed diagonals in deriving the design axial force of non dissipative members (beams and columns). The seismic inelastic response of a sample structure is investigated by means of nonlinear dynamic analyses, which have been carried out by means of the PC-ANSR [Maison, 1992] program, where the cyclic behavior of brace elements has been accurately modeled by means of the Georgescu model [Georgescu et al., 1991]. The results of dynamic inelastic analyses carried out with reference to the braced frame designed according to the proposed procedure are compared with those obtained with reference to the same structural scheme designed according to Eurocode 8 [CEN, 2000; CEN, 2003]. In particular, it is pointed out that the application of Eurocode 8 design criteria leads to premature buckling of columns, so that the proposal of a new design approach is fully justified.

Cyclic Modeling of Bolted Beam-to-Column Connections: Component Approach

The work is aimed at the prediction of the cyclic response of bolted beam-to-column joints starting from the knowledge of their geometrical and mechanical properties. To this scope a mechanical model is developed within the framework of the component approach already codified by Eurocode 3 for monotonic loadings. Accuracy of the developed mechanical model is investigated by means of the comparison between numerical and experimental results with reference to an experimental program carried out at Salerno University. The obtained results are encouraging about the possibility of extending the component approach to the prediction of the cyclic response of bolted connections.

Failure Mode Control and Seismic Response of Dissipative Truss Moment Frames

AbstractIn this paper, a new design approach for dissipative truss moment frames (DTMFs) able to guarantee, under seismic forces, the development of a yield mechanism of global type is presented and applied. In particular, DTMFs consist of a truss moment frame where the energy dissipation is provided by means of special devices located at the ends of the truss girders at the bottom chord level. The proposed design methodology is based on the kinematic theorem of plastic collapse. The method is based on the assumption that sections of truss elements and the yield threshold of dissipative parts (i.e., special devices) are known, and therefore the column sections are the unknowns of the design problem, which are obtained by imposing that the equilibrium curve corresponding to the global mechanism has to lie below those corresponding to all the possible mechanisms within a displacement range compatible with local ductility supply of dissipative elements. The design methodology has been implemented in a comput...

Influence of Design Criteria on the Seismic Reliability of X-braced Frames

According to the most modern trend, performance-based seismic design is aimed at the evaluation of the seismic risk defined as the mean annual frequency of exceeding a threshold level of damage, i.e., a limit state. A procedure for performance-based seismic assessment is herein briefly summarized and applied to concentrically “X” braced steel frames, designed according to different criteria. In particular, two design approaches are examined. The first one corresponds to the provisions suggested by Eurocode 8, while the second approach is based on a rigorous application of the capacity design criteria aiming to the control of the failure mode, as suggested in a companion article. The aim of the presented work is to focus on the seismic reliability obtained through these design methodologies. The probabilistic approach is based on an appropriate combination of probabilistic seismic hazard analysis (PSHA), probabilistic seismic demand analysis (PSDA), and probabilistic seismic capacity analysis (PSCA). Regarding PSDA, nonlinear dynamic analyses have been carried out in order to obtain the parameters describing the probability distributions of demand, conditioned to given values of the earthquake intensity measure. It is demonstrated how the proposed design method leads to a very important improvement of the seismic performances with a negligible increase of the overall building cost.

Failure Mode and Drift Control of MRF-CBF Dual Systems

In this paper, a new method for designing moment resisting frame (MRF) - concentrically braced frame (CBF) dual systems failing in global mode is presented. Starting from the analyses of the typical collapse mechanisms of such structural typology subjected to seismic horizontal forces, the method is based on the application of the kinematic theorem of plastic collapse. Beam and diagonal sections are assumed to be known quantities, because they are designed to resist vertical loads and horizontal forces, respectively. Therefore, the column sections are the only unknowns of the design problem. Column sections are obtained by imposing that the equilibrium curve corresponding to the global mechanism has to lie below all the equilibrium curves corresponding to the undesired collapse mechanisms within a displacement range compatible with the local ductility supply of dissipative elements. Such procedure has been applied to design sev- eral MRF-CBF dual systems and has been validated by means of non-linear static analyses aimed to check the actual pat- tern of yielding.

Prediction of the Rotation Capacity of Aluminium Alloy Beams

Abstract In this paper the theoretical steps of a semi-empirical method for evaluating the rotation capacity of aluminium alloy members subjected to non-uniform bending are outlined. The approach is represented by the extension to aluminium alloy members of the semi-empirical methods proposed for evaluating the rotation capacity of steel members. The moment-curvature relationship of aluminium alloy members can be conveniently represented by means of a Ramberg-Osgood type relation. This allows, with reference to the classical three-point bending test, the derivation in closed form of the curvature diagram. Furthermore, a closed-form integration of the curvature diagram can be performed, providing a relation for evaluating the rotation corresponding to the occurrence of local buckling. The rotation capacity is then computed. The final ring of this link is represented by the experimental evaluation of the non-dimensional stress corresponding to the attainment of the local buckling. The testing needs for this evaluation are outlined, and both the preliminary test results and the planned activity are presented.