Stefano Sorace

Non-linear dynamic modelling and design procedure of FV spring- dampers for base isolation

Abstract A viable non-linear dynamic design procedure for fluid viscous spring-dampers used in base-isolation systems of building structures is presented herein. The procedure consists of a preliminary design phase developed for a single-degree-of-freedom idealisation of the structural system, and in a final verification phase. The damping coefficient values ensuring achievement of the basic performance levels hypothesised in the design problem are estimated at the first stage. An explicit relationship interpolating the damping coefficient demand curves traced out as functions of damper loss factor, and dynamic input frequency composition and amplitude, is proposed as the basic design equation to this aim. This relationship, derived from previous analytical studies, is further validated herein against the results of experimental forced-vibration tests conducted on a steel frame mock-up. A first demonstrative application of the procedure is then carried out on a simple building with very stiff superstructure. Doubled calibration of the damping coefficient under normative-generated and pulse-type input ground motions, and evaluation of the influence of the pre-stress load imposed on several types of fluid viscous devices in the manufacturing process, are included. Selected case studies are discussed in the accompanying paper.

Analysis and Demonstrative Application of a Base Isolation/Supplemental Damping Technology

As a concluding step of several studies on a special base isolation/supplemental damping system, where pressurized fluid viscous spring-dampers are coupled to steel-Teflon sliders, the system was applied for the first time to a demonstrative strategic building in Italy. A final experimental campaign was developed to assess the interference of the dissipative actions of the two component devices. The campaign confirmed the linear additive combination implicitly assumed in relevant numerical models. The design and performance evaluation analyses performed on the building showed that maximum base displacements were only just below 45 mm, for the basic design earthquake level. As a result, very simple joints for all the facilities were used. For the same earthquake level, reduction factors of 2.48 and 2.12 on the superstructure response accelerations were obtained for the two main directions in plan as compared to peak ground acceleration. Low base displacement values, and a totally elastic superstructure response also emerged for the maximum earthquake level considered, as well as for the most demanding Italian historical near-fault ground motions introduced as inputs in the final verification analyses. The cost of the building structure resulted to be around 10% lower than the cost of a fixed-base traditional design, as well as of a base isolated structure incorporating high damping rubber bearings.

Shaking Table and Numerical Seismic Performance Evaluation of a Fluid Viscous-Dissipative Bracing System

A shaking table campaign was carried out on a 2:3-scale, two-story steel frame structure retrofitted by a dissipative bracing system incorporating pressurized fluid viscous spring-dampers. Up to 1.16 g peak ground accelerations were imposed in the most severe of the 33 tests developed. The response was always elastic, with maximum interstory drift ratios limited below 0.62%. The protection technology, in fact, features high dissipative capacities and produced equivalent linear viscous damping coefficients up to 37.5%. A numerical enquiry carried out on the test structure in its original unbraced configuration showed interstory drift reductions from about 80% to about 90% when passing to dissipative braced conditions. A final performance-based analysis developed in terms of interstory drifts and beam and column rotations, in compliance with the criteria formulated in ASCE/SEI 41-06 Standard, emphasized three through five enhancements of building performance in retrofitted conditions for the four earthquake levels examined.

Motion control-based seismic retrofit solutions for a R/C school building designed with earlier Technical Standards

The study of two motion control-based seismic retrofit solutions for a low-rise reinforced concrete school building is presented in this paper. The building was assumed as a benchmark structure for a Research Project financed by the Italian Department of Civil Protection, and is representative of several similar public edifices designed with earlier Technical Standards editions, in Italy as well as in other earthquake-prone European countries. The two solutions refer to the alternative earthquake protection strategies based on the concepts of supplemental damping and seismic isolation, respectively. Namely, they consist in the installation of: (1) a dissipative bracing system incorporating pressurized fluid viscous spring-dampers; and (2) a base isolation system including double friction pendulum sliding bearings. The structural characteristics of the building, and a synthesis of the investigation campaigns developed on it, are initially presented. The mechanical parameters, dimensions, locations and installation details of the constituting elements of the two protective systems are then illustrated, along with the performance assessment analyses carried out in original and rehabilitated conditions according to a full non-linear dynamic approach. The results of the analyses show a remarkable enhancement of the seismic response capacities of the structure for both retrofit hypotheses. This allows reaching the mutual high performance levels postulated in the two rehabilitation designs with remarkably lower costs and architectural intrusion as compared to traditional rehabilitation interventions designed for the same objectives.

Non-linear dynamic design procedure of FV spring-dampers for base isolation — frame building applications

Abstract The non-linear dynamic design procedure of fluid viscous spring-dampers proposed in the accompanying paper is applied to two selected case studies, represented by a reinforced concrete and a steel five-storey frame building with identical global dimensions. The fundamental vibration periods of the two structures in fixed-base conditions, equal to 0.58 s and 1.08 s, respectively, fall within the range of technical interest for use of base isolation. The reliability of the analytical relationship by which the damping coefficient is estimated in the preliminary design phase is further verified by comparing its predictions with the loss factor values calculated from the results of numerical integration of the equations of motion. The final verification phase of the procedure is then developed with regard to a double design performance objective, for which immediate occupancy and life safety levels are targeted under the “basic design” and “maximum considered” earthquakes, respectively.

Dissipative Bracing-Based Seismic Retrofit of R/C School Buildings

A dissipative bracing system incorporating pressurized fluid viscous spring-dampers has been studied for many years by the authors as an alternative seismic protection strategy for new and existing frame structures. As a concluding stage of this activity, a set of demonstrative case studies was examined to illustrate the enhancement of seismic performance and the economic advantages guaranteed by the system in actual design applications. The paper offers a selection of two among these case studies, both concerning retrofit interventions of reinforced concrete school buildings designed with earlier Seismic Standards editions, representative of a large stock of similar edifices built during the 1970s and earlier 1980s. The following aspects are presented and discussed in detail: the mechanical parameters, layouts and locations selected for the constituting elements of the system; the architectural refurbishment projects developed to properly incorporate the structural interventions and improve the appearance of the buildings; highlights of the installation works completed in one of the two case studies; and a synthesis of the performance assessment analyses in original and rehabilitated conditions, developed according to a full non-linear dynamic approach. The results of the analyses show a remarkable enhancement of the seismic response capacities of both structures. This allows reaching the high performance objectives postulated in the retrofit designs, with much lower costs and architectural intrusion as compared to traditional rehabilitation interventions designed for the same objectives.

Seismic performance assessment and base-isolated floor protection of statues exhibited in museum halls

A study concerning the evaluation of seismic response of statues exhibited in art museums, and a base-isolated floor strategy for their enhanced protection, are presented in this paper. Attention is particularly focused on statues made of small tensile strength materials, whose behaviour is simulated by a smeared-crack finite element approach. Seismic performance is assessed by referring to four levels specially postulated herein, and namely: (1) Rest conditions; (2) No rocking; (3) Damage control; and (4) Collapse prevention. The response is investigated via incremental dynamic analysis, by progressively increasing the amplitude of the ground motion histories adopted as inputs, and by relating output data to the limit conditions fixed for the above-mentioned performance levels. The assessment procedure is applied to a demonstrative case study, represented by a marble statue to be exhibited in the museum wing situated at the ground level of a medieval castle in Italy, according to an architectural hypothesis of partial rebuilding and reuse of the stronghold. The design solution for the base-isolated floor consists in a system of double-friction pendulum isolators. The finite element model constitutive laws and parameters, the dynamic analyses carried out in fixed-base and base-isolated floor conditions, and the practical implementation of the assumed performance assessment criteria are reported for the statue examined, along with a selection of technical details of the floor design.

Analysis, Design, and Construction of a Base-Isolated Multiple Building Structure

The analysis and design of a multiple residential building, seismically protected by a base isolation system incorporating double friction pendulum sliders as protective devices, are presented in the paper. The building, situated in the suburban area of Florence, is composed of four independent reinforced concrete framed structures, mutually separated by three thermal expansion joints. The plan is L-shaped, with dimensions of about 75 m in the longitudinal direction and about 30 m along the longest side of the transversal direction. These characteristics identify the structure as the largest example of a base-isolated “artificial ground” ever built in Italy. The base isolation solution guarantees lower costs, a much greater performance, and a finer architectural look, as compared to a conventional fixed-base antiseismic design. The characteristics of the building and the isolators, the mechanical properties and the experimental characterization campaign and preliminary sizing carried out on the latter, and the nonlinear time-history design and performance assessment analyses developed on the base isolated building are reported in this paper, along with details about the installation of the isolators and the plants and highlights of the construction works.

Advanced Materials and Technologies for Structural Performance Improvement

1Polytechnic Department of Engineering and Architecture, University of Udine, Via delle Scienze 206, 33100 Udine, Italy 2Department of the Built Environment, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands 3Department of Civil Engineering, KU Leuven, Kasteelpark Arenberg 40, 3001 Leuven, Belgium 4Department of Civil and Environmental Engineering, University of Florence, Via S. Marta 3, 50139 Florence, Italy 5Department of Civil and Environmental Engineering, Northeastern University, 441 Snell Engineering Center, 360 Huntington Avenue, Boston, MA 02115, USA 6ELSA Laboratory, Institute for the Protection and Security of the Citizen, Joint Research Center of the European Commission, Via E. Fermi 2749, 21027 Ispra, Italy 7Department of Civil, Geo and Environmental Engineering, Technical University of Munchen, Arcisstrasse 21, 80333 Munchen, Germany

Damped Interconnection-Based Mitigation of Seismic Pounding between Adjacent R/C Buildings

viscoelastic model for the numerical time-history analysis of the dynamic impact problem is proposed and implemented in the finite element model of the buildings. The results of the assessment enquiries carried out in current conditions, and a damped interconnection-based mitigation solution based on the incorporation of pressurized fluid-viscous dissipaters across the inadequate separation gaps, are presented. Evaluations of the benefits provided by the retrofit intervention, and some of its technical installation details, are finally offered.