Evaluation of Safety of Reinforced Concrete Buildings to Earthquakes

National Science Foundation Grant GK-3637

Application of reliability-based robustness assessment of steel moment resisting frame structures under post-mainshock cascading events

This paper proposes a reliability-based framework for quantifying structural robustness considering the occurrence of a major earthquake (mainshock) and subsequent cascading hazard events, such as aftershocks that are triggered by the mainshock. These events can significantly increase the probability of failure of buildings, especially for structures that are damaged during the mainshock. The application of the proposed framework is exemplified through three numerical case studies. The case studies correspond to three SAC steel moment frame buildings of three, nine, and 20 stories, which were designed to pre-Northridge codes and standards. Two-dimensional nonlinear finite-element models of the buildings are developed with the Open System for Earthquake Engineering Simulation framework (OpenSees), using a finite length plastic hinge beam model and a bilinear constitutive law with deterioration, and are subjected to multiple mainshock-aftershock seismic sequences. For the three buildings analyzed herein, it is shown that the structural reliability under a single seismic event can be significantly different from that under a sequence of seismic events. The reliability based robustness indicator shows that the structural robustness is influenced by the extent to which a structure can distribute damage

Design and Probabilistic Analysis of Mid- and High-rise Wood Buildings Subjected to Earthquake Excitations

Mass timber products, such as the glulam or cross laminated timber (CLT), are less frequently used construction materials at present for mid-rise and high-rise buildings. The feasibility and possible advantages of applying timber materials for constructing mid- and high-rise buildings are under investigation. One of the issues that needs to be addressed for the use of heavy timber materials is the safety of such constructions under seismic excitations. To address this issue, the nonlinear inelastic seismic responses and capacity curves of a wood buildings must be assessed. For this, the 10-, 15- and 20-storey buildings are designed using heavy timber structural members considering the requirements stipulated in applicable Canadian design codes and standards. When considering the buildings under unidirectional ground motion, the structural capacity curves along the structural axes in the horizontal plane are identified using well accepted approaches such as the incremental dynamic analysis (IDA) and nonlinear static pushover analysis (NSPA). The capacity curve is used as the basis to develop equivalent nonlinear inelastic single-degree-of-freedom (SDOF) system. The equivalent SDOF system is then employed for the structural reliability. The results indicate that the estimated reliabilities of the designed timber buildings are similar to those of steel frame structures designed according to Canadian practice. To consider the effect of the bidirectional ground motions on the building responses and their seismic reliability, a procedure is proposed in develop the capacity surface based on the results from the IDA and NSPA. Also, a procedure is proposed to establish equivalent nonlinear inelastic two-degree-of-freedom (2DOF) system based on the capacity surface. The use of the equivalent 2DOF system largely simplifies the reliability analysis of the buildings under bidirectional ground motions. The analysis results indicate that the failure probabilities under bidirectional ground motions are about 3 to 8 times greater than those obtained under unidirectional ground motions. Therefore, the consideration of bidirectional ground motions in assessing the reliability of building under seismic ground motions can be important for seismic risk modeling and emergency preparedness

Probabilistic Evaluation of the Adaptation Time for Structures under Seismic Loads

In this paper, a probabilistic approach for the evaluation of the adaptation time for elastic perfectly plastic frames is proposed. The considered load history acting on the structure is defined as a suitable combination of quasi-statical loads and seismic actions. The proposed approach utilizes the Monte Carlo method in order to generate a suitable large number of seismic acceleration histories and for each one the related load combination is defined. Furthermore, for each load combination the related adaptation time is determined, if any, as the optimal one for which the structure is able to shakedown under the unamplified applied actions. A known generalized Ceradini's theorem is utilized. The adaptation time values obtained with reference to all the generated seismic acceleration histories for which the shakedown occurs allows us to define the related cumulative conditioned probability function and, therefore, to identify the optimal adaptation time as the one with a probability not lower than a suitably assigned value

The application of reliability methods in the design of stiffened FRP composite panels for marine vessels

The use of composite laminate materials has increased rapidly in recent years due to their excellent strength to weight ratio and resistance to corrosion. In the construction of marine vessels, stiffened plates are the most commonly used structural elements, forming the deck, bottom hull, side shells and bulkheads. This paper presents the use of a stochastic approach to the design of stiffened marine composite panels as part of a current research programme into developing stochastic methods for composite ship structures, accounting for variations in material properties, geometric indices and processing techniques, from the component level to the full system level. An analytical model for the solution of a stiffened isotropic plate using a grillage analogy is extended by the use of equivalent elastic properties for composite modelling. This methodology is applied in a reliability analysis of an isotropic (steel) stiffened plate before the final application for a reliability analysis for a FRP composite stiffened plate

Seismic progressive collapse of reinforced concrete frame structures using the applied element method

Collapse of reinforced concrete structures under earthquakes is the main reason for life loss. Thus, avoiding structural collapse under strong earthquakes is the aim of seismic codes. The aim of the current study is to lead to an improved understanding of the seismic progressive collapse behaviour of reinforced concrete frame structures and to identify the most important parameters that should be considered in seismic progressive collapse analysis. The Applied Element Method, AEM, is an innovative method for direct progressive collapse simulation, in which strong geometric nonlinearity, element separation and collision can automatically be considered. Most previous studies focused on side-sway collapse modes only and indirectly checked for vertical collapse modes. A validation of the AEM for seismic progressive collapse simulation has been carried out. The AEM models of three different frame structures have been validated by comparing the analytical and experimental results. The results have indicated that the AEM can simulate the structure response from linear range up to collapse reasonably well. Sensitivity studies have been conducted to rank the material parameters most important to the collapse process in terms of the time at incipient collapse and to investigate their effects on the possible failure modes. The results show that the most important parameters are the parameters that can alter the failure mode. An investigation on the effect of inclusion of the vertical ground motions on the collapse capacity and the possible failure modes has been performed. Considering vertical ground motions in collapse assessment of irregular frame structures has led to a decrease in the collapse capacity and to modifications in the possible failure mechanisms resulting in vertical rather than side-sway collapse modes. A correlation study for investigation of the effect of using different intensity measures, fifteen spectrum and structure based intensity measures, for scaling far- and near-field ground motions for seismic assessment of mid-rise frame structures has been carried out. Employing intensity measures that account for the spectral shape has led to a considerably better correlation with the engineering demand parameters than utilizing intensity measures that are based on a single spectral value or a combination of two spectral values

MODEL UPDATING AND STRUCTURAL HEALTH MONITORING OF HORIZONTAL AXIS WIND TURBINES VIA ADVANCED SPINNING FINITE ELEMENTS AND STOCHASTIC SUBSPACE IDENTIFICATION METHODS

Wind energy has been one of the most growing sectors of the nation’s renewable energy portfolio for the past decade, and the same tendency is being projected for the upcoming years given the aggressive governmental policies for the reduction of fossil fuel dependency. Great technological expectation and outstanding commercial penetration has shown the so called Horizontal Axis Wind Turbines (HAWT) technologies. Given its great acceptance, size evolution of wind turbines over time has increased exponentially. However, safety and economical concerns have emerged as a result of the newly design tendencies for massive scale wind turbine structures presenting high slenderness ratios and complex shapes, typically located in remote areas (e.g. offshore wind farms). In this regard, safety operation requires not only having first-hand information regarding actual structural dynamic conditions under aerodynamic action, but also a deep understanding of the environmental factors in which these multibody rotating structures operate. Given the cyclo-stochastic patterns of the wind loading exerting pressure on a HAWT, a probabilistic framework is appropriate to characterize the risk of failure in terms of resistance and serviceability conditions, at any given time. Furthermore, sources of uncertainty such as material imperfections, buffeting and flutter, aeroelastic damping, gyroscopic effects, turbulence, among others, have pleaded for the use of a more sophisticated mathematical framework that could properly handle all these sources of indetermination. The attainable modeling complexity that arises as a result of these characterizations demands a data-driven experimental validation methodology to calibrate and corroborate the model. For this aim, System Identification (SI) techniques offer a spectrum of well-established numerical methods appropriated for stationary, deterministic, and data-driven numerical schemes, capable of predicting actual dynamic states (eigenrealizations) of traditional time-invariant dynamic systems. As a consequence, it is proposed a modified data-driven SI metric based on the so called Subspace Realization Theory, now adapted for stochastic non-stationary and timevarying systems, as is the case of HAWT’s complex aerodynamics. Simultaneously, this investigation explores the characterization of the turbine loading and response envelopes for critical failure modes of the structural components the wind turbine is made of. In the long run, both aerodynamic framework (theoretical model) and system identification (experimental model) will be merged in a numerical engine formulated as a search algorithm for model updating, also known as Adaptive Simulated Annealing (ASA) process. This iterative engine is based on a set of function minimizations computed by a metric called Modal Assurance Criterion (MAC). In summary, the Thesis is composed of four major parts: (1) development of an analytical aerodynamic framework that predicts interacted wind-structure stochastic loads on wind turbine components; (2) development of a novel tapered-swept-corved Spinning Finite Element (SFE) that includes dampedgyroscopic effects and axial-flexural-torsional coupling; (3) a novel data-driven structural health monitoring (SHM) algorithm via stochastic subspace identification methods; and (4) a numerical search (optimization) engine based on ASA and MAC capable of updating the SFE aerodynamic model

Transient Thermal Stresses in FG Porous Rotating Truncated Cones Reinforced by Graphene Platelets

none5siThe present work studies an axisymmetric rotating truncated cone made of functionally graded (FG) porous materials reinforced by graphene platelets (GPLs) under a thermal loading. The problem is tackled theoretically based on a classical linear thermoelasticity approach. The truncated cone consists of a layered material with a uniform or non-uniform dispersion of GPLs in a metal matrix with open-cell internal pores, whose effective properties are determined according to the extended rule of mixture and modified Halpin–Tsai model. A graded finite element method (FEM) based on Rayleigh–Ritz energy formulation and Crank–Nicolson algorithm is here applied to solve the problem both in time and space domain. The thermo-mechanical response is checked for different porosity distributions (uniform and functionally graded), together with different types of GPL patterns across the cone thickness. A parametric study is performed to analyze the effect of porosity coefficients, weight fractions of GPL, semi-vertex angles of cone, and circular velocity, on the thermal, kinematic, and stress response of the structural member.Masoud Babaei; Faraz Kiarasi; Kamran Asemi; Rossana Dimitri; Francesco TornabeneBabaei, Masoud; Kiarasi, Faraz; Asemi, Kamran; Dimitri, Rossana; Tornabene, Francesc