1,721,043 research outputs found
Pseudo-force method for a stochastic analysis of nonlinear systems
No full textNonlinear systems, driven by external white noise input processes and handled by means of pseudo-force theory, are transformed through simple coordinate transformation to quasi-linear systems. By means of Ità ́ stochastic differential calculus for parametric processes, a finite hierarchy for the moment equations of these systems can be exactly obtained. Applications of this procedure to the first-order differential equation with cubic nonlinearity and to the Duffing oscillator show the versatility of the proposed method. The accuracy of the proposed procedure improves by making use of the classical equivalent linearization technique
Prevention of brittle failure for steel connections utilizing special devices
Full text linkThe present paper proposes the use of a special design procedure devoted to the prevention of brittle failure for
welded steel connections. In particular, reference is made to steel frame structures made up of beam elements
with I-shaped cross-sections where the connections between columns and beams are usually welded and/or
bolted. The proposed new design procedure consists of four subsequent analysis steps; it is based on the identification
and analytical definition of new appropriate brittle safe domains defined in the N, V, M space and on
the use of suitably designed devices (LRPD), recently proposed by the authors, belonging to the class of the RBS
connections. The latter are able to impose prefixed values of internal forces on selected beam element crosssections,
avoiding any modification of the elastic stiffness features of the involved beam. The main novelties
of the present study consist in the introduction of new brittle safe domains for I-shaped cross-sections, in the
specialization of the LRPD optimal design to the present context and in the definition of a special design strategy
which allows to contextually obtain structures safe from the risk of brittle failure as well as being able to dissipate
an appropriate amount of plastic strain energy. The numerical application, devoted to plane steel frames, confirms
the sound reliability of the procedure and the great flexibility of the utilized LRPDs
On the Limit Behaviour of Moment Resisting Connections Under Uncertainties
Full text linkMoment resisting connections are mainly designed to transfer bending moments and
shear forces. Generally speaking, the design strength of a moment resisting connection can be
classified as full-strength (moment capacity of the connection equal to or greater than that of
the connected member) or partial-strength (the moment capacity of the connection less than
that of the connected member). Similar remarks can be made regarding the stiffness defining
connection rigid or semi-rigid if compared to the stiffness of the connected member. In the
past, full-strength connections have been widely adopted especially in moment resisting frames
and their structural performance relied on the proper behaviour of welding. However, the
research following the 1994 Northridge and 1995 Kobe earthquakes demonstrated the lower
than expected performance of welded connections, stimulating the onset and development of
pre-qualified connections to be adopted especially in seismic areas. Among these connections
the most studied ones are those belonging to the Reduced Beam Section (RBS) typology, being
the so-called “dogbone” connection the most adopted. The dogbone presents a bending
strength and a flexural stiffness lesser than the ones of the original structural member.
Recently, the authors proposed a special device suitably designed to realize an innovative
moment resisting connection for steel beam elements belonging to the RBS typology. Such a
device, called Limited Resistance Plastic Device (LRPD), is constituted by three different
portions: the central one is devoted to the onset and development of plastic deformations and
presents geometrical dimensions reduced with respect to those of the original structural
member; the external ones are devoted to recover the stiffness of beam-device system to that of
the original structural member and present greater geometrical dimensions. This latter remark
allows to affirm that, from a connectivity point of view, the stiffness of LRPD at the columnbeam
interface, is greater than the one of the original structural member. Another fundamental
remark is that the structural connections are intrinsically characterized by uncertainties related
either to geometrical or to material ones. Usually, the effect of uncertainties is covered by the
use of safety coefficients and the analyses are performed referring only to the nominal values
of the geometrical and mechanical characteristics. However, in order to perform a more
complete interpretation of the mechanical behaviour of the studied connections, a nondeterministic
analysis approach can be used. Aim of the paper is the characterization of the
structural behaviour of the referenced connections (“dogbone” and LRPD) taking into account
the main geometrical uncertainties and that related to the material strength by performing
suitably Monte Carlo simulations and by determining the relevant M-N domains. Starting from
the described characterization, different commercial steel profiles will be considered in order to
build a series of M-N domains useful to quantify the safety level and the range of usability of
the two different RBS approaches. Finally, the implemented applications will lead to
demonstrate the greater reliability of LRPD compared to the classical dogbone
Simplified Computational Procedure for the Design of Special Moment Resisting Steel Connections
No full textA simplified computational procedure for determining the optimal design of special
moment resisting steel connections is proposed. The connection, recently patented by the authors,
is the so-called Limited Resistance Plastic Device (LRPD); it appertains to the class of the
Reduced Beam Sections (RBS) which mainly constitute appropriate beam portions devoted to
receiving plastic deformations, but it also possesses additional and innovative features related
with its resistance and stiffness. The optimal design of the LRPD usually requires the solution of a
strongly nonlinear minimum volume problem. On the grounds of the known expected LRPD
mechanical features, a simplified computational procedure is defined. Reference will be made to
the device applied to I-shaped steel beam elements subjected to a plane force system. Some
numerical applications are presented, regarding the comparison between the results obtained by
means of the proposed simplified strategy and those obtained by the solution to the optimal design
problem. Furthermore, a larger application devoted to a plane frame proves the reliability of the
devices as receptors of plastic deformations
Filter approach to the stochastic analysis of MDOF wind-excited structures
No full textIn this paper, an approach useful for stochastic analysis of the Gaussian and non-Gaussian behavior of the response of multi-degree-of-freedom (MDOF) wind-excited structures is presented. This approach is based on a particular model of the multivariate stochastic wind field based upon a particular diagonalization of the power spectral density (PSD) matrix of the fluctuating part of wind velocity. This diagonalization is performed in the space of eigenvectors and eigenvalues that are called here wind-eigenvalues and wind-eigenvectors, respectively. From the examination of these quantities it can be recognized that the wind-eigenvectors change slowly with frequency while the first wind-eigenvalue dominates all the others in the low-frequency range. It is shown that the wind field can be modeled in a satisfactory way by taking the first wind-eigenvector as constant and by retaining only the first eigenvalue in the calculations. The described model is then used for stochastic analysis in the time domain of MDOF wind-excited structures. This is accomplished by modeling each element of the diagonalized wind-PSD matrix as the velocity PSD function of a set of second-order digital filters with viscous damping driven by white noise of selected intensity. This approach makes it easy to obtain in closed form the statistical moments of every order of the structural response, taking full advantage of the Ito calculus. Moreover, in the proposed approach, it is possible to reduce the computational effort by appropriately selecting the number of wind modes retained in the calculation
Yield Behaviour of Welded I-Shaped Steel Cross-Sections
Full text linkThe limit behaviour of I-shaped welded steel cross-sections subjected to axial force, shear, and bending moment is a crucial matter to ascertain the reliability of framed structures constituted by non-standard beam elements. International standards provide an approximate solution to the problem, and other studies have proposed improved approximate formulations to ascertain the real features of the relevant cross-sections. The present paper is devoted to enhancing the problem of the limit behaviour of plane I-shaped welded steel cross-sections subjected to axial force N, shear T and bending moment M; therefore, new appropriate formulations are proposed in order to define suitable new domains, both in planes N,T, N,M, and M,T and in the space N,T,M. The material is assumed as elastic-perfectly plastic and the Von Mises limit condition is adopted as the resistance criterion. The elastic stresses are described by the Navier formula and the Jourawski formula. The limit stress condition related to the contemporaneous presence of the acting forces is defined as the one that, at each point of the cross-section, fulfils the Von Mises limit condition as equality. The formulation is rigorously devoted to factory-made welded I-shaped steel cross-sections. Some numerical examples are reported in the application stage and useful comparison are carried out, with the results being obtainable by the application of the classical known standard formulae, proving the reliability and effectiveness of the determined domains
Mode-superposition correction method for deterministic and stochastic analysis of structural systems
No full textThe role played by the modal analysis in the framework of structural dynamics is fundamental from both deterministic and stochastic point of view. However the accuracy obtained by means of the classical modal analysis is not always satisfactory. Therefore it is clear the importance of methods able to correct the modal response in such a way to obtain the required accuracy. Many methods have been proposed in the last years but they are meaningful only when the forcing function is expressed by an analytical function. Moreover in stochastic analysis they fail for white noise excitation. In the paper a method able to give a very accurate response for both deterministic and stochastic input is presented. This method is based upon the use of Ritz vectors together with the classical modal analysis. Numerical applications for both deterministic and stochastic inputs show the great accuracy of the proposed method. © 2001 Civil-Comp Ltd. and Elsevier Science Ltd. All rights reserved
Limited Resistance Rigid Perfectly Plastic Hinges for Steel Frames
No full textThe paper concerns the proposal of a new special device for steel frames that can be utilized as external constraint as well as internal one connecting the structure beam elements. The
device is designed as a rigid perfectly plastic hinge characterized by suitably chosen stiffness and resistance. Its constituting material is steel. The elastic stiffness and the limit resistance, treated as independent of each other, are fixed with the aim of satisfying special features required to the structure. The proposed device is thought as a sandwich section with wing thickness appropriately variable. Its dimensions are designed so that it can exhibit the fixed independent stiffness and limit resistance. More devices can be utilized for a given frame, and each one can be designed in order to exhibit different stiffness and resistances. These features allow the designer to define a structure with optimal requisites both for serviceability conditions and for severe limit states. The device is connected to the end cross section of the relevant beam element, so that any modification in the shape of the beam elements is avoided. The effected applications are related to simple portal steel frames and they show the great effectiveness and wide applicability of the proposed device
Indagini teoriche e sperimentali su dissipatori ad attrito per edifici in acciaio in zona sismica
No full textThe present paper presents the results of experimental analyses carried out to study the behaviour of slotted bolted connections (SBC) as friction steel dampers. The devices consist of bolted joints with slotted holes, which can be used in both steel and RC framed structures or, alternatively, into beam-column and column-foundation joints.
The most suitable configurations to increase friction are investigated utilizing steel S355J0 plates and class 10.9HV high-strength bolts, under controlled pre-load and considering different types of surface treatment.
Experimental tests are carried out making use of a universal testing machine, which allows mak-ing displacement-controlled tests. Once the pre-load frictional force is reached, the plates begin to slide each other through the hollow holes, dissipating energy through friction.
The paper shows the results obtained during the tests in terms of frictional force vs. displacement, friction coefficient values and variation pre-load in bolts. The results are compared with the theo-retical values derived from the calculation
Welded section defence by LRPD devices
Full text linkThe present paper concerns a special application of some recently proposed structural devices, called LRPD, able to protect the welded sections of frame steel structures from undesired brittle collapse ensuring the good expected ductile behaviour. Standard I-shaped cross-sections are treated, and the proposed devices are suitably considered as moment resisting connections between beams and columns. At first the domain representing the brittle safe conditions is defined in the N,V,M space; then a sample plane frame subjected to seismic load conditions is studied and it is proved that, equipping the structure with the proposed devices suitably designed, the generalized stresses at the welded sections remain within the relevant brittle safe domain and the structure is able to dissipate a significant amount of plastic dissipation energy
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