Adaptive numerical method algorithms for nonlinear viscous and bilinear oil damper models subjected to dynamic loading

Adaptive numerical method algorithms are presented for the numerical simulation of the hysteretic behaviour of nonlinear viscous and bilinear oil dampers within a finite element program for nonlinear dynamic analysis of frame structures under earthquake excitations. The adaptive algorithms are applicable for computing high-precision solutions for nonlinear viscous and bilinear oil dampers with valve relief that are typically represented mathematically with a nonlinear Maxwell model. The algorithms presented possess excellent convergence characteristics for viscous dampers with a wide range of velocity exponents and axial stiffness properties. The algorithms are implemented in an open source finite element software, and their applicability and computational efficiency is demonstrated through a number of validation examples with data that involve component experimentation as well as the utilization of full-scale shake table tests of a 5-story steel building equipped with nonlinear viscous and bilinear oil dampers

Rate-dependent model for simulating the hysteretic behavior of low-yield stress buckling-restrained braces under dynamic excitations

Buckling-restrained braces (BRBs) are often idealized with rate-independent simulation models. However, under dynamic loading, BRBs featuring low-yield point steel exhibit rate-dependency that may lead to appreciable amplifications of the BRB forces. This paper proposes a new rate dependent model for simulating a BRB's response under dynamic excitations. The proposed model consists of a displacement-dependent asymmetric Menegotto-Pinto material law and a velocity-dependent bilinear oil damper model. The calibration process of the proposed model is also presented. Two approaches are demonstrated in which the proposed model can be utilized within a nonlinear frame analysis program. A comparative study based on test data from full-scale shake table tests of a five-story steel building equipped with BRBs underscores that if their rate-dependency is neglected then the BRB local force demands may be significantly underestimated. This may also lead to erroneous predictions of lateral story drift demands as well as absolute floor accelerations during earthquake shaking

Seismic Assessment and Retrofit of Pre-Northridge High Rise Steel Moment Resisting Frame Buildings with Bilinear Oil Dampers

This paper presents quantitative information on the effectiveness of seismic retrofit solutions using bilinear oil dampers for seismically deficient existing tall steel buildings. For this purpose, a benchmark 40-story steel space moment-resisting frame building is studied that represents 1970s design practice in North America. Rigorous seismic performance assessment based on ASCE 41 recommendations reveals a high collapse risk for the existing building. The local engineering demand parameters are comprehensively assessed to quantify the impact of seismic retrofit on steel columns and column splices, which are particularly vulnerable due to the time of construction. Multiple retrofit schemes are explored with numerous damping levels and vertical damping distribution methods. The dampers are designed via a recently developed multi-degree-of-freedom performance curves method. A new balanced vertical damping method is proposed to account for the effects of frame inelasticity. This strongly depends on the supplemental damping level, and it determines the effectiveness of the employed vertical damping distribution method. The results indicate that the proposed retrofit strategies can minimize the collapse risk of the tall building. It is shown that the balanced vertical damping distribution method provides the most uniform drift distribution along the building height. Despite the reduction in story drift ratios, the axial force demand in exterior columns remains relatively high in the bottom stories regardless of the seismic retrofit solution. On the other hand, bilinear oil dampers produce relative constant forces despite exhibiting higher velocity demands than expected

A hybrid seismic isolation system toward more resilient structures: Shaking table experiment and fragility analysis

The effectiveness of various vibration control strategies has always been a debate among structural engineers. Seismic base isolation systems and passive dampers are recognized as two of the most economical devices which have passive mechanisms in reducing the structural vibration and responses. To this end, comprehensive biaxial shake table testings have been carried out on a building frame with and without a proposed base isolation system. The proposed device has a novel combined isolation mechanism at the structure's base. By different methods of testing, natural frequencies and viscous damping for the frame model with and without the proposed system were identified. Both structures were intensively tested under various earthquake motions, and various structural responses were recorded. The experimental results indicated that the newly proposed system is very effective in controlling the vibration of building structures and can be used to increase the seismic resilience metrics. As a complementary investigation, the incremental dynamic analysis (IDA) was conducted to develop the seismic fragility curves under both near-field and far-field strong ground motions (SGMs). The fragility estimations indicated that the proposed system has a higher collapse margin ratio (CMR) compared to conventional fixed-base frames