Steel chevron concentrically braced frames are expected to dissipate seismic energy by yielding of the brace under tension, while both beam and columns behave elastically. Besides the strength, also the stiffness of the brace-intercepted beam plays a key role to avoid unfavourable mechanisms. However, no codified requirements are provided to assure adequate beam rigidity. In order to examine this aspect, in the first part of this paper the main results of a numerical parametric study devoted to investigate the mutual interaction between the beam vertical deflection and the brace ductility demand are described. The second part of this article investigates the efficiency of both EC8 and AISC341-10 seismic provisions on the global performance of chevron bracings, particularly focusing on the design of the beam of the braced bays. The results of incremental dynamic analyses performed on several structures confirm the primary importance of the flexural stiffness of the beam.
controventi concentrici
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
Controventi innovativi del tipo «RSS»: regole di progetto ed affidabilità sismica
In this paper, a new design methodology for concentrically braced frames is presented aiming at the control of the failure mode by means of a rigorous application of the capacity design principles. The design goal is the prevention of the out-of-plan buckling of columns. In fact, the use of the recommendations provided by Eurocode 8 /1/, which have been substantially adopted by OPCM 3431 /2/, leads to the design of structures which are not able to exploit all the possible dissipative zones due to the premature out-of-plan buckling of columns. Whith reference to three bay concentrically braced frames having only one brace element for each braced bay, the proposed design methodology is applied both including and without including the local weakening strategy (Reduced Section Solution or RSS). The behaviour of concentrically braced frames designed according to above methodologies is compared with the one resulting from application of Eurocode 8 design rules. To this scope, the Jalayer and Cornell approach leading to the evaluation of the mean annual frequency of exceeding a given limit state is applied, so that the obtained results accounts for the record-to-record variability.
This paper is available in Italian only.
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