A Displacement-Based Design (D.B.D.) procedure is adopted for the retrofit of framed structures by inserting hysteretic damped braces (HYDBs) in order to attain, for a specific level of seismic intensity, a designated performance level. To check the reliability of the design procedure, two six-storey buildings are considered as having steel and r.c. framed structures, which, originally designed in a medium-risk seismic region, have to be retrofitted as if in a high-risk seismic region. To avoid high deformability of the steel structure at the damage limit state (SLD) and brittle behaviour of the r.c. structure at the life-safety limit state (SLV), two retrofitting structural solutions are examined: additional diagonal braces; HYDs supported by the additional diagonal braces. Nonlinear dynamic analyses under real ground motions are carried out by a step-by-step procedure. The frame members and the HYDs are idealized by a bilinear model; an elastic behaviour is considered for the braces.
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Evaluation of the equivalent viscous damping (EVD) coefficient for steel concentrically brace frame structures with hollow rectangular section (HRS): direct displacement based design (DDBD)
The work presented here concerns the assessment of the corrected equivalent viscous damping coefficient for concentric braced steel frames, referring to the direct displacement based design method (DDBD) described at Section 3. First, concentric braced steel frames and displacement based design are described, with particular attention given to the evaluation of the damping coefficient. The models realized to perform non-linear analysis and the procedure used to determine the corrected equivalent viscous damping coefficient are then described (Sections 4 and 5). Finally, the results obtained through non-linear dynamic analysis are presented and used to propose a relation between the damping coefficient, the slenderness of braces and the ductility level.
Seismic design methods of concentrically braced steel frames
SUMMARY – The work described here is addressed to the evaluation and the comparison of different seismic design methods of multistory steel frame structures with concentric braces. In particular, a multistory frame is designed through both the simplified method currently proposed by the recent code provisions (OPCM 3431, DM 24/01/2008, Eurocode 8 ) and an alternative simplified one, based on the partial contribution due to the brace under compression. The assessment of the seismic response of the two structures is then performed by means of static analyses, where the non-linearities of the geometry and of the materials are included, in order to compare the related seismic performances. In addition, the seismic response of the frame designed with the new method is assessed through non-linear dynamic analyses and incremental dynamic analyses, to particularly highlight the properties of the alternative adopted method, which permit to achieve less conservative solutions, more economical in terms of amount of material and characterized by more effective collapse mechanisms.
For this paper is available an extended abstract after the text in Italian