Franklin T. Lombardo

Empirical Approach to Evaluating the Tornado Fragility of Residential Structures

AbstractTornado-induced wind load modeling has advanced significantly in recent years, but comparison of the experimental or numerical models to observed tornado damage is limited. This paper descr...

Discussion of “Ultimate Wind Load Design Gust Wind Speeds in the United States for Use in ASCE-7” by Peter J. Vickery, Dhiraj Wadhera, Jon Galsworthy, Jon A. Peterka, Peter A. Irwin, and Lawrence A. Griffis

sign error in Eq. (16). This was a typographical error in the paper; all calculations were done using the correct sign (+). The writers also thank the discusser for providing a historical account of the derivation of Barbero’s equation, although it should be noted that in his original paper Barbero cited both the Zhan and Ylinen work. In the literature on pultruded profiles, the Barbero and Tomblin (1994) equation is most often cited. It has been shown to represent experimental data. It is used in design codes (CNR 2008) and in textbooks (Bank 2006). The focus of the paper was to investigate appropriate resistance factors and the related reliability that could be used for pultruded columns with a variety of different material properties. The writers also thank the discusser for providing a curve-fitting equation for the experimental data but wish to point out that the approach presented is not curve fitting. It is based on two well-accepted analytical equations for global (Gere and Timoshenko 1997) and local (Kollár 2003) buckling in pultruded profiles and has a single fitting parameter that is calibrated from experimental data for design equations to account for the interaction between local and global buckling. The methodology used is not particularly different from that used in other design codes.

Characterization and comparison of aerodynamic roughness lengths using ground-based photography and sonic anemometry

In this study, two methods used to estimate surface terrain conditions (z0) surrounding Automated Surface Observing System (ASOS) stations in tropical cyclone-prone regions were analyzed. The first method utilized ground-based photography from the National Oceanic and Atmospheric Administration Hurricane Research Divisions Tropical Cyclone Wind Exposure Documentation Project (WEDP). The second applied a modified effective z0 method using ASOS wind data. Comparisons reveal that WEDP z0 estimates are typically larger in magnitude and do not account for changes in upstream z0. Variability in the z0 estimates showed a distinctly skewed nature in the probability distributions, which may have some physical meaning. It was also shown that standardized maximum 1-min sustained wind speeds using both the WEDP and MM2010 median z0 estimates for short fetch lengths can have differences as large as 15%. The MM2010 z0 estimates were also compared with single weighted mean z0 estimates to be incorporated in the next version of the U.S. wind loading standard and suggest that a single weighted mean z0 value does not effectively capture the variability of the terrain with respect to wind direction.

Characterization of Joint Wind and Ice Hazard in Midwestern United States

AbstractReliability-based design of infrastructure requires the probabilistic assessment of jointly occurring natural hazards. As infrastructure design practices evolve, it is important that multiple hazards be considered—for example, wind and storm surge in a hurricane, an earthquake generating a tsunami, or strong winds and high temperatures leading to wildfires. These jointly occurring hazards in some cases can be more devastating than a single hazard. This paper describes the assessment of two jointly occurring hazards: wind and ice. For locations in the United States, wind and ice [e.g.,xa0freezing rain (FR)] are hazards whose properties are of interest for the design of transmission lines and other energy infrastructure (e.g.,xa0wind turbines). Using archived meteorological data from measurement stations in the Midwest, occurrences of icing conditions and the wind speeds that accompany them were identified and input into a simple ice accretion model. Observations from wind speed and ice thickness were f...

Development of Empirically-Based Fragilities of Residential Damage in the 2011 Joplin, Missouri, Tornado

Performance-based engineering (PBE) is a methodology that requires specification on a range of performances or target reliabilities for structures of different importance. Information on these ‘performance levels’ require a probabilistic assessment of the potential factors that may influence a design, including information on the hazard, load, resistance, loss estimates, expert opinion and public perception. This paper describes one such probabilistic assessment in the development of empirically-based fragility functions for tornadoes using damage assessment data and a tornado wind field model for the 22 May 2011 Joplin, MO tornado. The damage assessment data was collected during field surveys of more than 1,240 structures in the aftermath of the tornado, using provisions of the Enhanced Fujita (EF) Scale to assess the damage. The wind field model was developed from the tree-fall patterns noted in the damage path of the tornado. Fragility functions were developed for the Degrees of Damage (DOD) associated with One- and Two-Family Residences in the EF Scale. The empiricallyderived fragility functions were progressive in nature, with median wind speeds varying from 33.6 m/s for initiation of visible damage to 85.2 m/s for complete destruction. These functions were compared to existing fragility functions for straightline winds to evaluate potential differences in failure mechanisms for structures exposed to tornadoes. Wind speeds associated with the median failure probability were used to estimate load factors, defined as the square of the ratio of the straightline wind speed to the tornado wind speed. Structures tended to fail at lower wind speeds in tornadoes than in straightline winds, with load factors between 1.32 and 1.51. A fragility assessment in the context of PBE naturally requires attribution and quantification of all uncertainties. Uncertainties in the both the damage and wind speed estimation in the development of fragilities are quantified and assessed using Monte Carlo methods. Preliminary results show variance in fragility parameters is higher for higher damage states but all damage states have relatively low coefficients of variation.

Analysis of Washington, DC, Wind and Temperature Extremes with Examination of Climate Change for Engineering Applications

AbstractClimate projections suggest the frequency and intensity of some environmental extremes will be affected in the future due to a changing climate. These projections raise questions regarding the treatment of future extreme environmental loading for the design of buildings and other structures. One of the more uncertain questions is possible changes in the properties of extreme wind. For this paper, extreme wind events for nearly 70xa0years from the Washington, DC, area are analyzed from the three major airports [(1)xa0International Airport at Dulles; (2)xa0Washington, District of Columbia, Reagan Airport; and (3)xa0Baltimore/Washington International Airport]. Uncertainties in estimation of extreme wind speeds without considering climate change are identified. Analysis disregarding climate change revealed that thunderstorms control design wind speeds for Washington, DC. As thunderstorms are then important, climate projections with respect to thunderstorms are also introduced. Possible strategies for long-ter...

Mapping Return Values of Extreme Wind Speeds

Structures subjected to wind loads must be designed to perform adequately from the points of view of stress and serviceability. Wind loading specified for design is based in part on the wind speeds affecting the site of interest. A particular quantity of interest in design is the N-year extreme wind speed, regardless of its direction, at a location of interest, defined by its longitude and latitude. Wind maps consisting of isotachs for N-year extreme wind speeds defined in building codes and standards are therefore required for structural design purposes. Alternatively, numerical versions of maps can be developed wherein automatic interpolations are performed that yield the N-year speeds at points defined by longitude and latitude. The raw data to be analyzed to develop the map are irregular time series of wind speeds above a specified threshold at multiple wind reporting stations. This work presents a two-stage approach to creating the map. The first stage involves the estimation of the parameters of an extreme value distribution at each station. In the second stage an interpolant based on the estimated parameters is created so that the N-year extreme wind speeds may be estimated at the geographical coordinates of interest. Standard errors and confidence bounds for the estimates are calculated using a non-parametric bootstrap algorithm. Results are presented for a region within Kansas, and those results are compared to the ASCE 7-10 Standard over the same region.

Constructing Probable Wind and Water Damage Sequences from Timelines — The Technical Perspective

Hurricane Katrina (2005) brought unrivaled destruction to buildings along the Mississippi Coast, generating thousands of property insurance claims. Hurricane Katrina had been a Saffir-Simpson Category 5 storm only 18 hours prior to making landfall at the Category 3 level along the Louisiana/Mississippi state line. At landfall, it still contained enormous potential for storm surge — far greater than the Category 3 landfall designation (based only on sustained windspeed) might indicate. As a result, damage from storm surge tended to be far more severe than damage from wind action, though many buildings sustained some levels of damage from both storm surge and wind. Exclusion of flood damage from standard property insurance policies has resulted in partial or full denial of many Hurricane Katrina-related damage claims, as District Court rulings in Mississippi have considered flood damage to include damage from storm surge. An unparalleled series of legal challenges has followed — pitting homeowners against insurers and requiring resolution of such questions as: 1 Was the residence destroyed by wind or water? 2 How much wind damage, if any, was sustained prior to the eventual destruction of the residence by storm surge? Settlement of these legal challenges has necessitated the separation of probable wind damage and storm-surge damage. The separation of damages is particularly complicated when structures have been completely destroyed or when engineers have been asked to determine wind and water damages long after critical evidence has been removed from the site. However, additional significant data concerning region-wide wind-speed and storm-surge levels (and the timing thereof) may not be available until several months after the hurricane, once detailed analysis of available region-wide observations has concluded. These regional timeline data provide a valuable resource for engineers in the reconstruction of probable wind/surge damage sequences some time after the hurricane damage has been removed.

A Plan for Characterizing Uncertainties in Extreme Environmental Loads with Climate Change Considerations: Wind Speed and Wave Height as Case Studies

Current projections suggest the frequency and intensity of some environmental extremes will be affected due to a changing climate; thereby raising questions regarding the treatment of future extreme environmental loading for the design of buildings and other structures. The estimation of future environmental loading is benefitted from the knowledge of the various uncertainties at each step of the estimation process. Possible changes due to climate add yet another uncertainty into environmental load estimation for which research has been limited. Characterization of extreme environmental loading therefore poses serious challenges to the engineering community. For this work, two environmental variables will be considered, wind speed and wave height in a case study approach. Uncertainties in observations and climate projections will be identified for both variables. These uncertainties will be characterized using best available knowledge and subsequently will be attributed and quantified with respect to a “typical” extreme environmental load estimation process in engineering.

Investigation of Thunderstorm Winds on an Instrumented Building

The Texas Tech Wind Engineering Field Research Laboratory (WERFL) building is equipped with over 200 sensors to measure the pressure on the building. Since its inception, many pressure time histories have been documented. However, a full length thunderstorm record has never been fully studied for its effects on the building and its comparisons with larger scale synoptic winds. On August 24, 2007 a moderate thunderstorm downdraft occurred at the WERFL building with a peak gust approaching 50 mph. An investigation of this thunderstorm is contained within this paper. The large increase in wind speed and air density/pressure accompanied by downburst passage may cause increased pressures on the building. Also, the turbulent and thermodynamic characteristics of thunderstorm winds contribute to large vertical (upward) velocities which may in turn lead to stronger uplift pressures. These pressures on the building may lead to increased damage probabilities on low-rise structures. Comparisons with a non-thunderstorm record at a similar angle of attack are also carried out.