Zach Liang

Towards multiple hazard resilient bridges: a methodology for modeling frequent and infrequent time-varying loads Part I, Comprehensive reliability and partial failure probabilities

The current AASHTO load and resistance factor design (LRFD) guidelines are formulated based on bridge reliability, which interprets traditional design safety factors into more rigorously deduced factors based on the theory of probability. This is a major advancement in bridge design specifications. However, LRFD is only calibrated for dead and live loads. In cases when extreme loads are significant, they need to be individually assessed. Combining regular loads with extreme loads has been a major challenge, mainly because the extreme loads are time variables and cannot be directly combined with time invariant loads to formulate the probability of structural failure. To overcome these difficulties, this paper suggests a methodology of comprehensive reliability, by introducing the concept of partial failure probability to separate the loads so that each individual load combination under a certain condition can be approximated as time invariant. Based on these conditions, the extreme loads (also referred to as multiple hazard or MH loads) can be broken down into single effects. In Part II of this paper, a further breakdown of these conditional occurrence probabilities into pure conditions is discussed by using a live truck and earthquake loads on a bridge as an example. There are three major steps in establishing load factors from MH load distributions: (1) formulate the failure probabilities; (2) normalize various load distributions; and (3) establish design limit state equations. This paper describes the formulation of the failure probabilities of single and combined loads.

Towards multiple hazard resilient bridges: a methodology for modeling frequent and infrequent time-varying loads Part II, Examples for live and earthquake load effects

The current AASHTO load and resistance factor design (LRFD) guidelines are formulated based on bridge reliability, which interprets traditional design safety factors into more rigorously deduced factors based on the theory of probability. This is a major advancement in bridge design specifications. However, LRFD is only calibrated for dead and live loads. In cases when extreme loads are significant, they need to be individually assessed. Combining regular loads with extreme loads has been a major challenge, mainly because the extreme loads are time variable and cannot be directly combined with time invariant loads to formulate the probability of structural failure. To overcome these difficulties, this paper suggests a methodology of comprehensive reliability, by introducing the concept of partial failure probability to separate the loads so that each individual load combination under a certain condition can be approximated as time invariant. Based on these conditions, the extreme loads (also referred to as multiple hazard or MH loads) can be broken down into single effects. In this paper, a further breakdown of these conditional occurrence probabilities into pure conditions is discussed by using a live truck and earthquake loads on a bridge as an example.

PIV measurement of high-Reynolds-number homogeneous and isotropic turbulence in an enclosed flow apparatus with fan agitation

Enclosed flow apparatuses with negligible mean flow are emerging as alternatives to wind tunnels for laboratory studies of homogeneous and isotropic turbulence (HIT) with or without aerosol particles, especially in experimental validation of Direct Numerical Simulation (DNS). It is desired that these flow apparatuses generate HIT at high Taylor-microscale Reynolds numbers () and enable accurate measurement of turbulence parameters including kinetic energy dissipation rate and thereby . We have designed an enclosed, fan-driven, highly symmetric truncated-icosahedron soccer ball airflow apparatus that enables particle imaging velocimetry (PIV) and other whole-field flow measurement techniques. To minimize gravity effect on inertial particles and improve isotropy, we chose fans instead of synthetic jets as flow actuators. We developed explicit relations between and physical as well as operational parameters of enclosed HIT chambers. To experimentally characterize turbulence in this near-zero-mean flow chamber, we devised a new two-scale PIV approach utilizing two independent PIV systems to obtain both high resolution and large field of view. Velocity measurement results show that turbulence in the apparatus achieved high homogeneity and isotropy in a large central region (48 mm diameter) of the chamber. From PIV-measured velocity fields, we obtained turbulence dissipation rates and thereby by using the second-order velocity structure function. A maximum of 384 was achieved. Furthermore, experiments confirmed that the root mean square (RMS) velocity increases linearly with fan speed, and increases with the square root of fan speed. Characterizing turbulence in such apparatus paves the way for further investigation of particle dynamics in particle-laden homogeneous and isotropic turbulence.

Towards Next Generation LRFD Extreme Event Design Limit States

The American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Specification for bridges has its strength design limit state formulated and fully calibrated using the reliability-based approach. The extreme event design limit states, however, are constructed by combining the non-extreme load effects with the independently established extreme hazard load effects through professional judgment. Its margin of safety and adequacy could not be assessed quantitatively. A research project sponsored by the Federal Highway Administration (FHWA) has been carried out to explore principles and approaches for establishing multiple-hazard (MH)-LRFD based on the established rationale and reliability-based methodology of the AASHTO LRFD. An analytical framework has been established to consider the non-extreme and extreme loads on a common reliability-based platform. This paper will describe the formulation of specific extreme event design limit state (e.g. the limit state for non-extreme load with the earthquake load effects) by using the all-hazard, reliability-based approach. Future challenges towards the development of a comprehensive MH-LRFD will be described.

Effects of Reynolds Number and Stokes Number on Particle-pair Relative Velocity in Isotropic Turbulence: An Experimental Study

The effects of Reynolds number and Stokes number on particle-pair relative velocity (RV) were investigated systematically using a recently developed planar four-frame particle tracking technique in a novel homogeneous and isotropic turbulence chamber.

Lag superposition method for structural analysis through ambient vibration

Vibration analysis for non-destructive evaluation of large-scale civil engineering structures often rely on ambient vibrations as excitation sources. In this case, the input force is typically not available for establishing the transfer functions for vibration analyses. Conventional technologies dealing with random vibration signals, such as correlation analysis and/or random decrement method, may yield less accurate results when the signal-to-noise ratio of the response measurement is not sufficiently high. In this paper, a method called lag-superposition is introduced that provides a better response spectrum and more accurate results with reasonable computation speed. Formulations of Single-Degree-of-Freedom (SDOF) and Multi-Degree-of-Freedom (MDOF) approaches are presented and verified by experimental tests and numerical examples. Comparisons with other methods are also made.

Towards next generation seismic isolation technology

Publisher Summary nSeismic isolation modeled as a single degree-of-freedom (S-DOF) system has been successfully applied to vibration reduction of equipment and structures in the fields of mechanical and aerospace engineering over pervious years. More recently, the civil engineering profession has gradually introduced the technology for seismic protection of infrastructure systems—buildings, bridges, nonstructural components, and lifeline systems—against strong earthquake ground motions. It has evolved as a specialty area in earthquake engineering, known as (structural) base isolation. The application of base isolation technology in civil engineering therefore has stemmed from S-DOF models traditionally used in the mechanical/defense/aerospace industry. However, a large number of civil engineering structures are complex multiple degree-of-freedom (M-DOF) systems. Recent developments in structural dynamics, computing, and experimental capacities have shown that there are shortcomings of using oversimplified S-DOF based seismic isolation design practice for civil engineering structures. This chapter advances the concept of developing the next generation seismic isolation technology for civil engineering structures based on M-DOF models by highlighting a number of key issues not addressed by current technology. The chapter finally discusses hybrid semi-active/passive base isolator system indicating future goal of developing intelligent seismic isolation bearings.