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Recent severe ice storms across the United States severely damaged trees resulting in extensive electrical power outages. Furthermore, trees and branches can fall on nearby roads, blocking traffic flow and reducing the safety of drivers. In this study, trees subjected to ice loads were analyzed using the finite element method and Monte Carlo simulation to develop analytical fragility curves. Two-dimensional, fractal trees were constructed with randomly generated geometric and mechanical parameters for four deciduous tree species: Acer saccharum, Tilia americana, Fagus grandifolia, and Quercus alba. Two load case scenarios were considered – with and without the effects of leaves – which were then subjected to varying ice accumulation thicknesses. The resulting fragility curves suggest that leaves have a substantial impact on tree branch damage under ice loads, which is significant because of the increase in unseasonably early ice storms due to climate change. [ABSTRACT FROM AUTHOR]

Apparent weakening is the softening of a structure's apparent stiffness by adding negative stiffness to the overall system, which has the benefit of reducing the peak accelerations and base shears induced in the structure due to a seismic event. Apparent weakening is an elastic effect that does not reduce the main structural strength. To attain this behavior, negative stiffness can be introduced through the integration of negative stiffness devices. This study presents a novel smooth negative stiffness device (SNSD) consisting of cables, pulleys, and tension springs. A nonlinear mathematical model of the force-deflection behavior of the SNSD was developed and used to determine the optimal geometry for such a device. A prototype device was designed and fabricated for installation in a bench-scale experimental structure, which was characterized through static and dynamic tests. This paper also presents a numerical study on two other device configurations designed to achieve different force-deflection relations for use in an inelastic model building subject to a suite of historic and synthetic ground motions. In both the experimental prototype and the numerical study, the SNSDs successfully produced apparent weakening, effectively reducing accelerations and base shears of the structures. [ABSTRACT FROM AUTHOR]

It is well established that the lateral bending stiffness of thin panels is considerably enhanced by judicious use of ribs or stiffeners. This increase in stiffness is primarily due to a disproportionate increase in the second moment of area, and because relatively little mass is added, stiffened panels are especially appealing in an aerospace engineering context. In experiments it is relatively straightforward to measure bending deflections (especially in a cantilever configuration) and natural frequencies using laser vibrometry and associated signal-processing tools. This paper reports on a parametric study in which bending stiffness and natural frequencies are measured for a set of rectangular panels, conveniently manufactured using a three-dimensional (3D) printer. Both isogrid (stiffeners in a triangular configuration) and orthogrid (stiffeners in a rectangular configuration) structures were made and tested, with certain nondimensional parameters held constant (for example, total mass and length-to-width ratio) to facilitate easier comparison. The boundary condition considered was one edge clamped and the other edges free (simple cantilever) with a consistent plan area aspect ratio, and with the height and arrangement of stiffening ribs providing the major parametric variation. [ABSTRACT FROM AUTHOR]

The motion of a slender rigid block resting on an isolated base supported by rolling bearings is analyzed. This paper investigates what happens when the ground upon which the isolation system rests is subjected to a pulse-like acceleration, representative of floor motion in a building. The isolator-block system exhibits planar motion: the block may rotate about either of its bottom corners, and the isolated base may displace both horizontally and vertically. The rolling surfaces are concave and may have an arbitrary geometry, which imposes a kinematic constraint relating the base's vertical and horizontal displacements. During an impact when the block rocks from one corner to the other, the preimpact and postimpact translational (isolator) and rotational (block) velocities are related through the principle of impulse and momentum. The response of the system is sensitive to the ground-motion parameters (amplitude and frequency), and regions of Safe (no overturning) and Unsafe (overturning) outcome are plotted in the excitation parameter plane. Boundaries between these regions are inherently sensitive to the displacement capacity of the isolation system but not to the mass ratio (block to base). [ABSTRACT FROM AUTHOR]

Rolling isolation systems (RISs) have been used extensively to protect bridges, buildings, and equipment from earthquakeinduced ground and floor motions. RISs are attractive in a performance-based design context because horizontal acceleration design requirements may easily be met by adjusting the slope of the rolling surface. However, in the process of reducing horizontal accelerations, the isolator transmits vertical accelerations due to the pendulumlike motion. These vertical accelerations may exceed tolerable limits if left unchecked. In this study, the influence of the rolling surface profile on peak vertical accelerations is assessed through experiments and simulations. RISs with different radii of curvature are fabricated and then tested for a wide range of ground-motion frequencies and amplitudes. The experiments and simulations show that vertical accelerations are not negligible and should not be ignored in the design of RISs. [ABSTRACT FROM AUTHOR]