Per Erik Austrell

Simulation of the response of base-isolated buildings under earthquake excitations considering soil flexibility

The accurate analysis of the seismic response of isolated structures requires incorporation of the flexibility of supporting soil. However, it is often customary to idealize the soil as rigid during the analysis of such structures. In this paper, seismic response time history analyses of base-isolated buildings modelled as linear single degree-of-freedom (SDOF) and multi degree-of-freedom (MDOF) systems with linear and nonlinear base models considering and ignoring the flexibility of supporting soil are conducted. The flexibility of supporting soil is modelled through a lumped parameter model consisting of swaying and rocking spring-dashpots. In the analysis, a large number of parametric studies for different earthquake excitations with three different peak ground acceleration (PGA) levels, different natural periods of the building models, and different shear wave velocities in the soil are considered. For the isolation system, laminated rubber bearings (LRBs) as well as high damping rubber bearings (HDRBs) are used. Responses of the isolated buildings with and without SSI are compared under different ground motions leading to the following conclusions: (1) soil flexibility may considerably influence the stiff superstructure response and may only slightly influence the response of the flexible structures; (2) the use of HDRBs for the isolation system induces higher structural peak responses with SSI compared to the system with LRBs; (3) although the peak response is affected by the incorporation of soil flexibility, it appears insensitive to the variation of shear wave velocity in the soil; (4) the response amplifications of the SDOF system become closer to unit with the increase in the natural period of the building, indicating an inverse relationship between SSI effects and natural periods for all the considered ground motions, base isolations and shear wave velocities; (5) the incorporation of SSI increases the number of significant cycles of large amplitude accelerations for all the stories, especially for earthquakes with low and moderate PGA levels; and (6) buildings with a linear LRB base-isolation system exhibit larger differences in displacement and acceleration amplifications, especially at the level of the lower stories.

Characterisation of an Elastomer for Noise and Vibration Insulation in Lightweight Timber Buildings

Regulations regarding impact and airborne sound insulation for lightweight timber constructions have become increasingly stringent due in particular to complaints by inhabitants. Accordingly, some building techniques frequently use elastomers at junctions so as to reduce low frequency noise. Development of accurate predictive tools (involving exact material properties) by using numerical methods such as the finite element (FE) method is needed in tackling flanking transmission problems during the design phase of buildings. The present research concerns the characterisation of an elastomer, presenting an accurate method for extracting its material properties from the manufacturers data sheet (properties there being often linked to such structural effects as shape factors and boundary conditions of samples and tests). The properties were extracted by comparing results obtained by analytical calculations, FE simulations, and mechanical testing, separating geometry and material dependence and ultimately serving as input to commercial FE software for setting up the aforementioned prediction tools.

Load and Response Prediction Using Numerical Methods in Acoustic Fatigue

A numerical procedure for load and response prediction in the context of acoustic fatigue is investigated on a model problem. Contrary to design guidelines, where the load needs to be specified (for example, based on experiments), the procedure used herein consists of simulating the load with computational fluid dynamics and then using the simulated load as a load input to a finite element simulation of the exposed structure. The model problem studied is a ramped backward-facing step with a thin aluminum panel fitted downstream of the step, parallel to the flow. The vortices generated in the wake of the step impose a time-varying load on the aluminum panel. The numerical results on the load and response are compared to experimental results. The load is simulated with large-eddy simulations with a wall function. The mean reattachment length, load intensity, and spectrum compare well with the measurements, with the exception of a somewhat overpredicted cutoff frequency. The panel response prediction compare...

Analytical modelling of hysteresis heating in rolling contact of rubber covered rollers

Abstract In a previous paper, an approximate model for the contact between a rubber covered roller and a rigid roller was developed as analytical functional relationships connecting geometric parameters and material properties of the rubber to nip properties such as maximum contact pressure, etc. in a two-dimensional relationship. The results from that development are used in this work, with the objective to provide a means of estimating the temperature rise due to hysteresis heating. In the heat conduction modelling, one-dimensional steady state heat conduction is assumed. The heat source is a power input coming from internal friction developed during the rolling contact. The power input is expressed by the hysteresis in the rubber represented by a loss angle of the material together with the peripheral speed and some parameters taken from the previously developed contact model. The temperature distribution is calculated in accordance with temperature and heat flow boundary conditions.

Two-dimensional elastic contact model for rubber covered rollers

Abstract A model for the contact between a rubber covered roller and a rigid roller was developed using analytical functions. The model, being two-dimensional, connects the line load, geometric properties (roller radii, rubber thickness) and the shear modulus of the rubber to nip properties in the roller contact. The output from the model is the indentation of the rigid roller into the rubber, the nip maximum pressure, the nip width and the surface strain in the centre of the nip. Moreover, the shape of the pressure distribution is also given as an analytical expression. The basic assumption relies on the work of Parish (−58 and −61), but a development of this work was performed, showing the influence of the rubber shear modulus and also how the surface strain in the nip can be described. The functional relationships are based on least squares fitting of analytical functions, depending on two master variables, to a large number of finite element analysis results.

Optimization of wind turbine foundations for poor soil

One of the most important constraints on large-scale civil engineering projects is economic feasibility. All projects are restricted by the cost of construction and materials. A cost analysis of alternative foundation designs for wind turbines is done in this work to aid the choice of design solution. The suitability of different foundation types are compared including a novel solution, with a circular raft surrounded by a water tank for soil with poor properties. In this work cost and behavior comparisons are done for two foundation solutions, namely a piled raft and a circular raft surrounded by a water tank. An observed soil profile found in Gothenburg region, Sweden is used. The soil profile is implemented in the FE software Abaqus to find out the validity of using the different types of foundations on the mentioned type of soil. In terms of settlement and tilting it is shown that there is a good effect of using a water tank surrounding the ordinary circular raft for the actual soil profile in the Gothenburg region. For the cost issue, the analysis was carried out to calculate the whole foundation cost for a circular raft surrounded by a water tank and also for a piled raft. It is shown that using the new foundation system decrease the foundation cost compared to using piled raft with pile length = 28 m and one meter square pile. The effect of dynamic loads was also investigated and the results showed that the complete system, using circular raft surrounded by a water tank as a foundation, successfully avoids resonance through the rotor excitations. (Less)

Passive load control in backward-facing step flow by using chevrons

The ability of chevrons at the top edge of a backward-facing step to reduce downstream surface pressure fluctuations is investigated numerically. Three different chevron configurations are compared against a baseline case without chevrons. Low frequency reduction in the surface pressure fluctuations is observed for two of the configurations. The chevrons do not appear to have a significant effect on the flow as the mean reattachment length for all configurations is nearly constant and there is only a small increase in streamwise turbulence for one configuration with the other configurations unchanged.

Non-Linear Behaviour of Base-Isolated Building Supported on Flexible Soil under Damaging Earthquakes

Seismic isolation is a strategy to reduce damage of structures exposed to devastating earthquake excitations. Isolation systems, applied at the base of buildings, lower the fundamental frequency of the structure below the range of dominant frequencies of the ground motion as well as allow to dissipate more energy during structural vibrations. The effectiveness of the base-isolated buildings in damage reduction has been confirmed numerically for the models of structures with fixed supports. The aim of the present paper is to show the results of the non-linear analysis of the response of a base-isolated building supported on soft soil incorporating soil-structure interaction. The detailed study has been conducted for the building equipped with high damping rubber bearings used as isolation devices. The results of numerical simulations demonstrate that soil flexibility has a significant influence on the behaviour of isolated base of the structure. Considering the flexibility of soil significantly affects the rigid superstructure response lowering its potential to reduce structural damage.