Bryant G. Nielson

Analytical Seismic Fragility Curves for Typical Bridges in the Central and Southeastern United States

Seismic fragility curves for classes of highway bridges are essential for risk assessment of highway transportation networks exposed to seismic hazards. This study develops seismic fragility curves for nine classes of bridges (common three-span, zero-skew bridges with non-integral abutments) common to the central and southeastern United States. The methodology adopted uses 3-D analytical models and nonlinear time-history analyses. An important aspect of the selected methodology is that it considers the contribution of multiple bridge components. The results show that multispan steel girder bridges are the most vulnerable of the considered bridge classes while single-span bridges tend to be the least vulnerable. A comparison of the proposed fragility curves with those currently found in HAZUS-MH shows a strong agreement for the multispan simply supported steel girder bridge class. However, for other simply supported bridge classes (concrete girder, slab), the proposed fragility curves suggest a lower vulnerability level than presented in HAZUS-MH.

Response Sensitivity of Highway Bridges to Randomly Oriented Multi-Component Earthquake Excitation

In two-dimensional and single axis three-dimensional finite element analyses, the ground motion incidence angle can play a significant role in structural response. The effect of incidence angle for three-dimensional excitation and response is investigated in this paper for response of highway bridges. Single-degree-of-freedom elastic and inelastic mean spectra were computed from various orientation techniques and found indistinguishable for combinations of orthogonal horizontal components. Probabilistic seismic demand models were generated for the nonlinear response of five different bridge models. The negligible effect of incidence angle on mean ensemble response was confirmed with a stochastic representation of the ground motions.

Sensitivity of Load Distribution in Light-Framed Wood Roof Systems due to Typical Modeling Parameters

AbstractBecause failure of roof systems in past high wind events has demonstrated the consequences of not maintaining a continuous load path from the roof to the foundation, many studies have been conducted to better understand this load path, including how loads are distributed in the system. The current study looks to add to the current knowledge of the vertical load path by focusing on uplift loads and by considering the sensitivity of different modeling parameters. This is done by developing and assessing load influence coefficient contours for various roof-to-wall (RTW) connections. An analytical model of a light-framed wood structure using finite-element software is developed. The model has a gable roof system comprised of fink trusses and is modeled in a highly detailed fashion including the explicit modeling of each connector/nail in the system. The influence coefficient plots indicate that the distribution of loads is indeed sensitive to the overall stiffness of RTW connections but is not overly ...

Seismic Fragility Methodology for Highway Bridges

Bridge fragility curves, which express the probability of a br idge reaching a certain damage state for a given ground motion parameter, play an important role in the overall seismic risk assessment of a transportation network. For various reasons, analytical methodologies for developing these fragility curves have experienced an increase in demand and popularity. However, these methodologies have yet to adequat ely account for all major contributing bridge components in the generation of analyt ical bridge fragility curves. There have been some studies which have shown tha t for certain bridge types, neglecting to account for all of these components can lead to a misrepresentation of the bridges’ overall fragilities. In this study, an expanded methodology for the generation of analytica l fragility curves for highway bridges is presented. This methodology considers the contribution of the major components of the bridge, such as the columns, bearings and abutm ents, to its overall system fragility. In particular, this methodology util izes some probability tools to directly estimate the bridge system fragility from the i ndividual component fragilities. It is illustrated using a bridge whose construction and configuration ar e typical to the Central and Southeastern part of the United States and the results are presented and discussed herein. As these analytical methodologies are improved, t he fragility information they generate will be more reliable and more likely t o be implemented into transportation network loss estimation models and retrofit prioritization stra tegies.

Behavior of Light-Framed Wood Roof-to-Wall Connectors Using Aged Lumber and Multiple Connection Mechanisms

Ensuring a complete load path in light-framed wood structures is critical in providing proper load transfer during wind and seismic events. Roof-to-wall metal-connectors are often used to accomplish this goal. Currently, the design and construction community utilizes published metal connector capacities found in manufacturer documentation or other design aids to properly size connectors for a given demand. However, with the ever-increasing trend of using recycled dimensioned lumber and in situ retrofits of older buildings, the limited supply of research in this area is becoming more apparent. At present, the state-of-the-practice does not provide guidance to account for changes in material properties in structurally sound reclaimed or aged lumber. This study uses dimensioned lumber that was extracted from decommissioned buildings, originally built in the 1960s, to examine the effect of reclaimed dimensioned lumber on the design strength of connections by using metal-connectors. Through the use of testing ...

Probabilistic Descriptions of In-Situ Roof to Top Plate Connections in Light Frame Wood Structures

Failure of roof-to-wall top plate connections is a major cause of damage in light frame wooden structures subjected to high wind loads. Numerous post-storm investigations revealed that these connections fail below design wind loads [FEMA 2005; FEMA 2006; Kareem 1985]. This damage history highlights the need for understanding their resistance and behavior under such extreme loads. Earlier research studies replicated roof-to-wall top plate connections in the laboratory to study their ultimate capacity, but failed to capture the actual capacity of the connections in their as built in-situ condition. Furthermore, little effort has been made in the past to identify the probability distributions that describe the response and the ultimate capacity of these connections. This knowledge is needed to set the appropriate structural resistance criteria for the structural members and develop risk-consistent and reliable structural design methodologies for low-rise, wood-framed buildings. In this study, the uplift capacities of existing roof-to-wall toe nailed connections are determined and their associated probability distributions are presented. About 81 existing roof-to-wall top plate connections in light-framed wood residential structures built between 50 and 60 years ago were tested in accordance with ASTM D1761-2000 to determine their uplift performance. The results provided a robust statistical sample to assess the uplift capacity and probability distributions for typical toe-nailed roof-to-wall top plate connections in these residential structures. Results show that connections using 2-16d toenails have a mean ultimate capacity of 1.52 kN (342 lbs) and a coefficient of variation (COV) of 0.35. This COV value is found to be approximately twice that found from previous laboratory based studies lending motivation for conducting in-situ tests. Furthermore, cyclic testing of these connections also provided load-displacement relationships from which stiffness values were approximated. Results show that capacity and stiffness can plausibly be modeled as jointly lognormal.

Scaling Bias and Record Selection for Quantifying Seismic Structural Demand

AbstractLack of sufficient earthquake records for a given site and hazard level is a frequent challenge in seismic response assessment. This paper considers the response of several simple nonlinear...

Uncertainty Quantification in Analytical Bridge Fragility Curves

Robust analytical techniques that quantify the uncertainty in response and damage due to earthquake excitation are of great value for design, emergency response, and infrastructure management purposes. Researchers have proposed different methods for generating analytical bridge fragility curves based on nonlinear dynamic analysis. This paper examines the analytical damage fragility methodology previously implemented for typical bridges in the Central and Southeastern US and California. A unified analytical fragility method, based on the bridge component approach, is investigated that combines aspects of probabilistic seismic demand analysis and Latin hypercube bridge population sampling. The procedure separates uncertainty arising from aleatory randomness in the ground motion, epistemic uncertainty pertaining to bridge component damage, and uncertainty pertaining to the bridge parameters. The unified methodology is demonstrated for a multiple-span box girder bridge common in California.

Which ground motion intensity measure is most appropriate for conditioning demand models for bridge portfolios

Probabilistic seismic demand analyses are central to performance-based evaluation of structures and seismic risk assessments. The anticipated structural response and demand under earthquake loading is often characterized using a tool known as a probabilistic seismic demand model (PSDM). However, the degree of uncertainty in the model is dependent on the ground motion intensity measure (IM) used for conditioning the response (e.g. peak ground acceleration, spectral acceleration). Vulnerability assessments of general classes, or portfolios of structures, are becoming more essential because of their use in risk assessment packages such as HAZUSMH, and hence the need for identification of optimal IMs increases. Appropriate intensity measures for general classes of bridges are evaluated as a part of this study, and the conditions under which various conclusions are valid. The influence of characteristics of the demand analysis on selecting an IM is assessed, such as the use of synthetic or recorded ground motions. The results are intended to offer guidance for appropriate intensity measure selection for probabilistic seismic demand models of bridge portfolios, which will considerably enhance future structural performance evaluations and regional risk assessments for transportation networks.