Aurelio Ghersi

Influence of bi-directional ground motions on the inelastic response of one-storey in-plan irregular systems

This paper examines the influence of bi-directional seismic excitations on the inelastic behaviour of in-plan irregular systems having one symmetry axis, schematised as one-storey models with resisting elements arranged along two orthogonal directions. Their strength is designed by means of the standard application of the modal analysis and by a procedure already proposed by the authors with reference to asymmetric models subjected to unidirectional ground motions. The stochastic nature of the seismic excitation is considered by analysing the structural inelastic response to 30 pairs of artificially generated accelerograms matching the elastic response spectrum proposed by Eurocode 8 for hard layer soil. The secondary horizontal seismic component is scaled to different values so as to examine the influence of its intensity on the ductility demand. The analyses show that the inelastic response is affected only in a minor way by the contemporary presence of the principal and secondary components of the seismic action, although the results are more scattered and significant increases of ductility demand in the elements along the asymmetric direction may sometimes arise. The proposed design procedure is almost always able to reduce the ductility demand of the resisting elements along the asymmetric direction to values comparable to those required by torsionally balanced systems. In most cases the adoption of Eurocode 8 provisions to combine the effects of the two seismic components allows the limitation of the orthogonal elements ductility demand.

N-M-V INTERACTION DOMAINS FOR BOX AND I-SHAPED REINFORCED CONCRETE MEMBERS

This research proposes an approximate physical model to evaluate the N-M-V interaction resistance domains for box- and I-shaped concrete cross sections. The model subdivides the concrete beam in layers, with nearly constant stress fields, and determines the internal forces satisfying equilibrium of the cross section, applying the static theorem of plasticity. The resisting contribution of the web longitudinal reinforcement is also considered by including stress fields of variable inclination on the longitudinal element direction. The proposed model leads to a conservative evaluation of bearing capacity and allows for the numerical acquisition of the N-M-V interaction resistance domains of the indicated cross sections. The obtained N-M-V failure surfaces are in good agreement with experimental test evidence performed by other researchers.

Static vs. Modal Analysis of Asymmetric Buildings: Effectiveness of Dynamic Eccentricity Formulations

The use of modal analysis appears necessary in order to reduce both displacement demand under weak seismic events and ductility demand under strong earthquakes. Static analysis can be effective only if used with proper values of additional eccentricities. To overcome the inaccuracy of the code formulations, the authors propose a simple procedure that gives the exact values of these eccentricities and discuss the influence of the main parameters that govern the structural behavior. They also point out the difficulty in evaluating some parameters (stiffness radius of gyration, structural eccentricity) in the case of multistory buildings and discuss the validity of simplified formulations proposed to overcome this problem. The effectiveness of static analysis, applied to three-dimensional multistory structures with properly evaluated corrective eccentricities, is analyzed with reference both to regularly asymmetric multistory schemes and to an actual irregularly asymmetric structure (the main building of the Faculty of Engineering at the University of Catania, Italy).

A Capacity Design Procedure for Columns of Steel Structures with Diagonals Braces

According to modern seismic codes, in concentrically braced frames the seismic input energy should be dissi- pated by means of the hysteretic behaviour of braces while all the other members (i.e. beams and columns) have to remain elastic. Accordingly, the design internal forces of braces are determined in these codes by elastic analysis of the structure subjected to seismic forces obtained by the design spectrum. The internal forces of the non-dissipative members, instead, are calculated by means of specified rules for the application of capacity design principles. According to some recent nu- merical analyses, the yielding or buckling of columns may take place before braces achieve their axial deformation capac- ity. This paper investigates the reasons of this unsatisfactory behaviour and proposes technological suggestions and a de- sign procedure to improve the seismic performance of columns of building structures with diagonal braces.

Design of Metallic Cold-Formed Thin-Walled Members

This book approaches cold-forming techniques and the specific problems of cold-formed thin-walled members. It discusses the problems that may be covered in the evaluation of the strength of cross-sections and the buckling resistance of members, and highlights the theoretical basis and the design approach necessary to overcome them. Of particular benefit is a Windows-based program accompanying the book that allows the reader to easily evaluate the strength of a cross-section and the buckling resistance of a member for both steel and aluminum.

Formulation of design eccentricity to reduce ductility demand in asymmetric buildings

A critical analysis of the large number of papers about the seismic behaviour of asymmetric buildings shows some concordant results: the modal analysis correctly predicts their elastic dynamic response to seismic records, while it overestimates deck rotation in the inelastic range. On this basis, the authors propose to design asymmetric structures by twice repeating the modal analysis: the first one with the actual mass distribution, so as to cover the elastic behaviour; the second one by considering the centre of mass displaced towards the centre of rigidity by a design eccentricity, so as to fit the inelastic response. In order to assess a formulation for the design eccentricity that reduces the maximum ductility demand, the paper statistically analyses the inelastic response of an idealised one storey building, symmetric about one direction, to different sets of accelerograms (both natural and artificial) and compares it to that of the corresponding balanced building; the analysis is repeated many times, so as to evaluate the influence of the different geometrical and mechanical parameters governing the inelastic response. The proposed approach and formulations prove to be effective in evaluating the effects of asymmetry, thus providing a design criterion which limits the ductility demand of asymmetric schemes without relevant increments of structural costs.

Statistical analysis of seismic behaviour of steel frames: Influence of overstrength

Abstract The influence of structural overstrength on the seismic behaviour of steel structures and on the assessment of the q factor has been studied by means of a suitable procedure, based on the selection of a significant set of homogeneous historical earthquakes and on the statistical analysis of the dynamic inelastic response of the structure to these accelerograms. Six schemes of steel frames, proportioned according to different design criteria and therefore with different amounts and distribution of overstrength, have been analysed. Three values of the behaviour factor have been considered for each one. The large number of results, obtained by more than 4800 dynamic analyses, has been elaborated in a statistical way, providing damage level indexes which express the probability of yielding and collapse and the plastic level reached, both for the single section and the whole frame. Interesting conclusions have been obtained. In particular, the effectiveness of the capacity design criteria in reducing the probability of collapse of frames has been highlighted. Similar good results have been obtained even when the same overstrength is given to all columns, without any particular attention to its distribution, while the increment of the beam resistance was not favourable in any case. Moreover, a very large improvement in safety has been obtained with just a little increase in the column strength over that supplied by the application of the capacity design criterion. The global behaviour appears thus connected to a new parameter, herein defined and named “effective behaviour factor”, which takes into account the mean overstrength both of beams and columns

Application of Nonlinear Static Method with Corrective Eccentricities to Steel Multi-storey Braced Buildings

Nonlinear static methods may be not very effective in the assessment of 3D building structures because sometimes they do not provide an accurate estimate of the deck rotations. In order to overcome this shortcoming, the authors of this chapter proposed a nonlinear static approach for asymmetric structures that is performed applying the lateral force with two different eccentricities (named corrective eccentricities) with respect to the centre of mass of the deck. In this chapter the effectiveness of the corrective eccentricity method is verified with reference to four five-storey mass eccentric steel buildings in which the seismic force is sustained by frames equipped with buckling restrained braces.

Prediction of the Seismic Response of Steel Frames with Concentric Diagonal Bracings

Concentrically braced frames and eccentrically braced frames are efficient seismic resistant systems but they are generally prone to develop storey collapse mechanisms. As underlined in some previous papers with regard to eccen- trically braced frames, this tendency depends on the overstrength factors and damage distribution capacity factors result- ing from the use of common design procedures. In this paper a previously proposed procedure which predicts the height- wise distribution of the damage at collapse of eccentrically braced frames is applied to concentrically braced frames. To apply this procedure, proper definitions of the overstrength factors and damage distribution capacity factors are derived. The effectiveness of the proposed procedure is tested on frames with concentric diagonal bracings characterised by differ- ent storey numbers and designed by common design procedures. The target response is provided by nonlinear dynamic analyses. The seismic input is constituted by ten artificial accelerograms. The paper proves that the proposed procedure is able to predict accurately the nonlinear dynamic response of systems in which the damage is mainly restricted to a few storeys. In the other cases, some not negligible scattering between actual and expected values of damage can be found at some storeys.

Application of Nonlinear Static Method with Corrective Eccentricities to Multistorey Buildings: A Case Study

Nonlinear static methods are not always effective in the assessment of three-dimensional building structures because their estimate of the torsional response is not always reliable. To overcome this shortcoming, some of the authors have proposed a double application of the nonlinear static analysis by means of lateral forces applied to points of the deck characterised by different eccentricities with respect to the centre of mass. These eccentricities, named “corrective eccentricities”, were calibrated in such a way that the displacements of the edges of the deck evaluated by the two nonlinear static analyses equal those evaluated by nonlinear dynamic analysis. Analytical expressions of the corrective eccentricities were derived and presented in a previous study based on a parametrical investigation conducted on a wide set of one-storey systems subjected to bidirectional ground motions. In this chapter, the peculiarities of the application of the proposed nonlinear static method to multistorey buildings are discussed first. Second, as an example, the method is applied to predict the seismic response of a five-storey asymmetric building. The structure of the analysed building is constituted by reinforced concrete frames arranged along two orthogonal directions. The cross sections of the structural element are equal at all storeys. To test the effectiveness of the proposed method, the in-plan distributions of the deck displacement and drift are evaluated by nonlinear dynamic analysis and compared to those resulting from the nonlinear static method with corrective eccentricities. The obtained results show that the use of the corrective eccentricities leads to a suitable estimate of the maximum dynamic displacements and drifts.