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Month: January 2014
DESIGN OF HYSTERETIC DAMPED BRACES TO IMPROVE THE SEISMIC PERFORMANCE OF STEEL AND R.C. FRAMED STRUCTURES
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.
MULTIAXIAL PRESTRESS OF REINFORCED CONCRETE I-BEAMS
We analyze the effects of multiaxial prestress on the limit behavior of I-shaped reinforced concrete elements subjected to combined axial load and bending. We provide a collection of design charts and moment – curvature diagrams for reinforced concrete I-beams with laterally and longitudinally prestressed flanges. The given results highlight the special ability of the active prestress technique in enhancing the strength and ductility properties of seismic resistant columns and shear walls.
DYNAMIC BEHAVIOUR OF CRACKED GRANITE AND MARBLE COLUMNS RETROFITTED WITH STEEL COLLARS
Marble and heavy stone columns are widely diffused in ancient churches and historical buildings in all the Mediterranean area. Their good mechanical properties allowed carrying great load values, while their bright colours and aesthetical characteristics have been used by a lot of ancient architects to achieve structural solutions with great visual impact.
Despite their good compressive strength, marble columns could be damaged from environmental effects (e.g. long- term effects or thermal loads), which could crack the structural members. In this way, the slenderness of the column increases drastically and the presence of an imposed ground shaking could be critical, since the column will be more vulnerable to rocking motion and to overturning risk.
This paper focuses on the rocking behaviour of cracked granite and marble columns subjected to a pulse type ground shaking. The effect of the presence of circular collars is analysed by means of a mechanical model. The overturning spectra are determined including the presence of the collars, showing their efficiency in reducing the overturning risk. Comparisons are shown with numerical analyses and a simplified design method is proposed.
STRUCTURAL CAPACITY OF MASONRY WALLS UNDER HORIZONTAL LOADS
In the present work we describe a procedure for determining the ground acceleration that activates the in-plane mechanism of a wall panel with openings. The method, based on the assumption that the material is unilateral (namely a No-Tension material in the sense of Heyman), leans on the kinematic theorem of limit analysis. By working with the kinematic theorem, we admit singular strains representing concentrated fractures; in other words we allow for strong discontinuities in the displacements. In recent papers we adopted finite elements with strong discontinuities and search for local minima of the energy (in proximity of equilibrium trajectories) by minimizing the energy through descent, both with respect to the displacements and with respect to the position of the jump set. In this paper we propose a similar, though simplified, strategy to explore compatible mechanisms having free discontinuities. The numerical implementation of the proposed approach is discussed through illustrative examples, on examining collapse mechanisms of masonry structures subject to vertical and seismic loads.