Engineering Investigations of Hurricane Damage: Wind versus Water

This chapter describes wind principles, emphasizing hurricane wind characteristics, flow patterns around buildings, cladding and structural system loads, and basics of wind-borne debris. Several sources, including ground-mounted anemometers, dropsondes, and ocean/waterway surface buoys, indicate a hurricane's wind speed, while the Saffir-Simpson scale for hurricane intensity typically classifies hurricanes. Thunderstorm activity associated with hurricanes can produce strong winds in the form of downbursts and tornadoes. Although flow around buildings can be extremely complex, it can be broken down into several general flow patterns. Cladding loads depend on the net pressures across the element, the spatial correlations of the pressures across the area of the element, and the load sharing among adjacent fasteners in the same component. Wind tunnel testing is an accurate, time-tested method to determine wind loads on structures in straight-line wind storms such as hurricanes.

This chapter describes the processes that generate storm surge and waves, the role of each process in coastal flooding and loadings on structures, the factors that influence spatial and temporal variability in waves and water levels, and the methods for characterizing both for analysis and design. Wind and, to a lesser degree, spatial gradients in atmospheric pressure and momentum fluxes associated with waves generate storm surge, particularly in the surf zone. Hydrostatic loads are associated with the storm water level and changes in water surface elevation induced by waves; hydrodynamic loads are associated with the force of moving water, including the effects of waves. Hurricane characteristics include track, intensity, size, and forward speed. Hurricane storm surge and waves are characterized mostly through regional, basin-scale computer modeling, which can include the interactions between storm surge and waves.

This chapter provides some historical context and background for wind and water issues related to hurricane events. Many hurricanes in the last 25 years were either significant wind or storm surge events, or both. Each hurricane has provided lessons on how buildings perform under such conditions, and each has provided some insight into how newer building codes can help reduce damage from these events. The events also provide some perspective on the unknowns of a building's behavior during a hurricane. The chapter highlights various hurricane events in last 25 years, including Hurricane Hugo in 1989, Hurricane Andrew in 1992, Hurricane Charley in 2004, Hurricane Ivan in 2004, Hurricane Katrina in 2005, and Hurricane Ike in 2008.

This chapter outlines the codes and standards commonly used for the design and construction of residential and commercial buildings in locations where high winds and storm surge flooding may occur. Codes and standards are updated regularly and often modified for local jurisdictions. Both the International Building Code (IBC) and the International Residential Code (IRC) incorporate the ASCE's standard, ASCE/SEI 7-10 Minimum Design Loads for Buildings, as the reference for minimum design loads for hazards, including wind and flood hazards. The chapter discusses the summaries of the National Flood Insurance Program (NFIP) and Federal Emergency Management Agency (FEMA) flood maps because these tools form the basis for establishing flood hazard zones. The lessons learned about the performance of building components during high wind or flooding events have in many cases been incorporated into local and national building codes following historic hurricanes.

This chapter discusses the response of structures to loads from wind, storm surge, flood, and waves. Wind and water loads produce unique damage signatures due to the nature of these forces and the load paths involved. The chapter discusses the basic sources of these loads and then traces the flow of force through structures from point of application to dissipation into the ground. The loads imparted by wind are transferred from the external components, cladding, and fenestration through the structural framework and into the foundation. Wind load damage and storm surge, flood, and wave load damage have very different characteristics and are readily differentiated by the damage signature. The photographs in the chapter illustrate these damage signatures and the relative force of storm surge and wind in a region in Mississippi impacted by Hurricane Katrina.

Planning and managing hurricane damage investigations presents many logistical challenges for an engineering firm, particularly when many properties must be assessed within a short time frame. These logistical challenges are compounded by technical challenges related to quality control and consistency among engineering opinions that arise when many different engineers from one firm perform these damage assessments. The chapter introduces practical tools to assist in managing and organizing hurricane damage assessments. These project management and organization tools are most relevant where many damage assessments resulting from a single hurricane event are performed concurrently by an engineering firm because the engineering qualifications for field inspectors may vary by project and depend on engineering licensing requirements in a particular state. The engineering opinions should be communicated to the client only at the direction of the supervising professional engineer.

This chapter presents an efficient and thorough field investigation process that is facilitated by diligent research prior to the site visit and a protocol for field data collection. Once on site, the professional and forensic team conducts a repertoire of big-picture, general documentation; detailed visual observations accompanied by photographic and graphic documentation; and testing, as necessary. While conducting a field investigation, the professional and forensic team should be cognizant of safety issues that commonly result from hurricane-induced site hazards. The professional and forensic team should also be aware of the procedures for the proper usage and inherent limitations of all field equipment. Testing equipment must be properly maintained and calibrated per the manufacturer's recommendations, and proper operation of the equipment should be confirmed prior to use in the field.

This chapter provides information about the intensity of wind and flood hazards during hurricane events, methods to estimate damage, and the broader context of surrounding damage. It also presents available hazard data and models that the assessment team may use to determine underlying hazards, including one of the most accessible hurricane damage models, the Federal Emergency Management Agency's (FEMA) Hazus Hurricane Model. The chapter discusses the use of remote sensing aerial and satellite optical imagery to aid in delineating hazards for individual buildings. Many of the data are best analyzed and visualized using geographical information system (GIS) computer software, which can manage, analyze, and map multiple data layers for one or more study areas.

This chapter conveys what makes a good report, showing how a typical report should be organized and including elements that may or may not appear in every report, such as the introduction, background and information from others, observations, analysis, discussion, conclusions, recommendations, disclaimers, and reference sections, some of which may include several subsections and relevant websites. A good report is clear, well organized, and accessible to its audience. The chapter begins with a discussion of style. Useful reference books on style include The Elements of Style and The Chicago Manual of Style. The chapter argues for some suggestions that will improve a report's readability and better communicate its findings and provides general suggestions for how to organize an effective narrative.

This chapter presents case studies of hurricane investigations, illustrating the thought process for several cases where the cause or causes of damage were not immediately obvious. The case studies include a building that was completely destroyed by Hurricane Katrina in which most of the damage was caused by flood and flood-borne debris, with somewhat minor damage caused by wind. Hydrostatic pressure from flood water caused the collapse of a brick veneer and damage to the roof trusses, which usually is caused by wind. Before presenting the case studies, the chapter discusses techniques for differentiating between wind and water for buildings that are totally destroyed by a hurricane.