J. W. van de Lindt

Effect of Mainshock-Aftershock Sequences on Woodframe Building Damage Fragilities

AbstractAlthough aftershocks have the potential to cause severe damage to buildings and threaten life safety, their effect in seismic risk analysis is not explicitly accounted for in modern building design codes, nor in emerging methodologies such as performance-based seismic design. The ultimate objective of this study is to systematically integrate aftershock hazard into performance-based earthquake engineering (PBEE) through analytical studies with structural degradation models derived from publicly available experimental data. In this paper, the first step is made by introducing a procedure to compute the probability of a mainshock-damaged woodframe building entering different damage states as a result of aftershock. Aftershock fragilities are developed by performing incremental dynamic analysis (IDA) using a sequence of mainshock-aftershock ground motions. To compute the seismic response of the damaged building, IDA is performed using a sequence of mainshocks of different intensity levels combined wi...

Design estimates of surface wave interaction with compliant deepwater platforms

The extreme behavior of surface waves as they encounter and pass compliant deepwater platforms is an important class of problems for offshore engineers attempting to specify the platform deck elevation. In this study analytical expressions for the probability density and cumulative distribution functions that utilize empirical coefficients in an attempt to accurately model surface wave runup and airgap problems are presented. The analysis focuses upon interpreting the tails of the measured data histograms using two parameter Weibull distribution models. The appropriate empirical constants, assumed to be solely dependent upon the significant wave height, were evaluated and compared for all the test data. Based upon a small select set of data, for a mini-TLP and two Spar platforms, the airgap problem was found to be adequately modeled using a Rayleigh distribution. Further, for the seven seastates analyzed, the Weibull shape parameter was nearly constant and the data confirmed that the exclusive fit of the scale parameter assuming dependence only on the significant wave height was a reasonable approach for modeling the wave runup. Finally, by combining these models with a Poisson return model for each storm the associated reliability estimates for various deck heights were estimated.

Experimental Study of Collapse Limits for Wood Frame Shear Walls

AbstractUnder extreme earthquake loading, light-frame wood building collapse is often caused by excessive interstory drifts at one or more story levels, leading to catastrophic P-Δ failure once the shear walls, and subsequently the entire structure, become unstable. This soft-story collapse mechanism has been observed in numerous earthquakes. Current performance-based seismic design methods for light-frame wood buildings typically uses very conservative drift levels to represent the near collapse deformation of wood frame building systems, such as the 3% drift limit used in ACSE Standard 41 corresponding to a collapse prevention performance target. A series of full-scale collapse loading tests on wood shear walls was conducted in this study to identify the ultimate drift level at which P-Δ collapse will occur and is described in this paper. It was concluded that laterally braced wood shear walls can remain stable up to ∼7–10% interstory drift, depending on the magnitude of the vertical loading. If one con...

Quantifying Changes in Structural Design Needed to Account for Aftershock Hazard

AbstractAftershocks have the potential to cause severe damage to buildings and threaten life following a major earthquake. However, their effect on seismic hazards is not explicitly accounted for in modern building design codes, nor in emerging methodologies such as performance-based seismic design. The objective of this study was to develop a methodology that can quantify the changes that would be needed in the structural design of a building to account for aftershock (AS) hazards and illustrate it using a basic nonlinear model of a building. In other words, what changes to a structural design would be needed such that the building has the same collapse probability for the combined mainshock and aftershock (MS + AS) hazard as the collapse probability for the original building, subjected to the mainshock (MS) only? The total collapse probability is computed using a combination of seismic fragility results convolved with the two types of hazard curves, namely, a typical hazard curve and an AS hazard curve....

Estimating extreme tendon response using environmental contours

The environmental contour technique was used to estimate the extreme in-line responses of deep-water TLP tendons designed for the Gulf of Mexico. The simulated response estimates were then used to estimate failure probabilities and reliabilities utilizing a deterministic displacement limit state. The reliability of individual tendons and tendon systems is directly associated with their respective probabilities of failure. By designing for environmental contours identified using this technique the resulting design will be more likely to approach the intended target reliability. In this article the environmental contour theory is explained and then used to estimate the extreme tendon responses in two examples reflecting practical design uncertainties. Experimental data from large scale model tendon experiments was introduced in order to assess the numerical prediction. In the first example the problem of uncertainty associated with pretensioning of the individual tendons is investigated. Although the amount of uncertainty due to the change in tension is not known the use of contour inflation is illustrated as a means to compare the numerical prediction with the experimental data. The second example addresses the uncertainties associated with the fluid/structure interaction. The placement of tendons in close proximity results in the amplification of the tendon motions. At present, no adequate hydrodynamic model exists which can be used with confidence in design practice. Again contour inflation is explored as a means to compensate for this phenomena and to quantify in a global sense the impact of this uncertainty on design.

Characterizing random wave surface elevation data

The definition and subsequent use of dimensional and dimensionless parameters to characterize various nonlinear aspects of ocean surface waves has again become a matter of great interest to the offshore community. The desire to ascertain whether laboratory simulations are adequately representing the surface waves found in the oceans and the concern over the mechanisms behind platform response phenomena, like ringing, has driven this resurgence of interest. This paper presents a depth independent characterization of single design waves, from which improved estimates of localized wave crest front and back slopes follow that are consistent with discrete time series analysis. Characterization of the nature of the entire wave data recorded requires a combination of spectral parameters and probabilistic models in addition to those used in the design wave characterization. A new expression for the direct evaluation of the kurtosis from knowledge of the spectral bandwidth, the relationship between some of the common spectral parameters, and some modified spectral parameters are presented and discussed. Three illustrative examples are presented. The first example provides a detailed examination of wave data measured from a series of random amplitude and random phase tests in a large model basin. The second presents estimates of the various parameters for the Pierson-Moskowitz and Wallops wave spectrum models. The third example investigates the use of the spectral peakedness ratio for comparing data with selected wave spectrum models. The examples illustrate how the formulae can provide a comprehensive local and global parametric characterization of surface wave elevation data.

Energy-Based Similitude for Shake Table Testing of Scale Woodframe Structures

The development and refinement of performance seismic design is underway, thus understanding the dynamic behavior of woodframe structures has become critical. Although several full scale shake table tests have been performed, many details associated with load transfer/path and behavior of varying systems remains to be investigated. This short technical communication presents the results of a study whose objective was to scale a woodframe structure to one-half scale using similitude theory, something that has eluded researchers to date. It is widely felt that woodframe structures cannot be scaled because there is no way to scale a naturally occurring fibrous material with non isotropic properties. However, because the dynamic response of wood shearwalls (and thus woodframe structures) is dominated by the behavior of the sheathing-to-framing connectors, an energy-based similitude was developed at the connector/fastener (nail) level. Shake table tests were performed for both the full-scale prototype and half-scale model. Peak displacements at roof level for the prototype and model were found to be very close, i.e., within 2%, for the largest simulated ground motion and only within 30% for the smallest simulated ground motions. While the displacement time series scaled very well, the resulting damage did not scale.The development and refinement of performance seismic design is underway, thus understanding the dynamic behavior of woodframe structures has become critical. Although several full scale shake tabl...

Implementation of Plan Irregularity Rapid Visual Screening Tool for Wood-Frame, Single-Family Dwellings

A plan irregularity rapid visual screening method for seismic performance assessment of wood-frame, single-family dwellings is presented. Results from 124 samples were compared with (a) building-specific, nonlinear time-history analysis and (b) FEMA 154 and ASCE 31 Tier 1. Verification using two houses damaged in the 1994 Northridge Earthquake is presented. The method includes effects of shape, torsional forces from eccentricity, and is based on conservative values of shear wall capacities and a nonlinear time-history analysis. The method is relatively more conservative than ASCE 31 Tier 1 and FEMA 154, and provides conservative but reasonable predictions of actual earthquake damage.

A Time Variant Approach to Performance-Based Engineering

This study presents an overview of an approach that identifies a design earthquake in terms of a critical response spectrum at a site in order to estimate the time variant reliability of the structure against exceedance of a specified inter-story drift. This is accomplished by coupling a technique similar to Probabilistic Seismic Hazard Analysis (PSHA) with performance-based reliability. Once the critical response spectrum is identified in terms of earthquake magnitude, M, and site-to-source distance, R, Monte Carlo Simulation (MCS) is used to account for spectral scatter, , and structural resistance uncertainty present in the mass, stiffness, and damping of the model. An illustrative example is included for clarity. This example combines MCS with response spectrum analysis to estimate the static reliability of the structure. Then, the return period of the design earthquake is determined by definition and the time interval of interest is modeled as Poisson with a mean proportional to the length of the interval, and the time variant reliability of the structure determined.

An Inverse-Reliability Approach to Generating Composite Seismic Response Spectra

Abstract This study presents an Inverse-First Order Reliability Method (FORM) approach to generating site-specific composite seismic response spectra. The proposed technique searches the surfaces of Inverse-FORM generated contours for the earthquake statistically most likely to produce the target spectral acceleration at the desired structural period, as modelled by the appropriate attenuation relation. Identification of this spectral acceleration level over a range of periods produces a composite design response spectrum consisting of spectral acceleration levels most likely to occur and all lying on the surface of the n-year contour, hence possessing the desired n-year return period. This composite seismic response spectrum can be used in design and analysis of structural systems having variable levels of complexity. The result is an n-year composite, seismic design response spectrum, found using Inverse-FORM generated contours that are generated in a discrete four-dimensional space. The random variables associated with this four-dimensional space are earthquake magnitude, site-to-source distance, attenuation relation uncertainty, and fault of origin. This study outlines the technique and presents a hypothetical example for a region threatened by three behaviour ally different earthquake source mechanisms. An illustrative example is presented for target spectral accelerations of O.lg and 0.4g, having two different return periods. The number of different earthquakes contributing to each spectrum varied between three and nine, depending on the specifics.