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Loss-Consistent Categorization of Hurricane Wind and Storm Surge Damage for Residential Structures Carol J. Friedland, P.E., Ph.D.1, Marc L. Levitan, Ph.D.2 1 Assistant Professor, Department of Construction Management and Industrial Engineering, Louisiana State University, Baton Rouge, LA, USA, [email protected] 2 Associate Professor, Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, LA, USA, [email protected] ABSTRACT This paper presents the application of a new damage scale to describe the level of damage to residential buildings in wind, flood and combined wind and flood events. While the development of the Wind and Flood (WF) Damage Scale is the topic of a separate document in preparation, this paper provides a brief background of the need for a damage scale that can describe the combined damage caused by wind and flood hazards and shows application of the new scale for assessment of residential building damage after Hurricanes Katrina and Ike. The basis of the WF Damage Scale is the HAZUS Wind Damage Scale, which is expanded and modified to address levels and types of damage associated with hurricane storm surge and other inundation flooding. INTRODUCTION No single scale exists that adequately describes all aspects of hurricane damage, and the scales that do exist are not compatible for direct comparison of damage states. Several damage scales categorize wind damage from hurricanes, but none capture the full range of behavior in detail. Scales such as the Saffir-Simpson scale and Enhanced Fujita (EF) scale relate wind speed with degree of damage for particular building types. The Saffir-Simpson scale provides a general range of building damage, but does not provide sufficient detail to be applied to specific buildings. The Enhanced Fujita scale, developed for tornadoes, has been utilized by some to describe hurricane wind damage; however, the degree of damage (DOD) indicated for specific wind speeds may not be appropriate for buildings constructed to higher coastal standards. FEMA’s HAZUS-MH Wind Model [1] contains a damage scale that describes typical levels of damage caused by hurricane winds (and ensuing damage from wind/rain intrusion); however, damage caused by storm surge is not considered. HAZUS and other sources have developed loss curves that describe increasing damage as a function of wind speed for various types of construction, but continuous scales are generally not useful for categorizing observed damage. While the wind engineering community generally uses discrete scales to describe building damage in terms of physical parameters, methods used to categorize flood damage typically consist of single variable-dependent continuous damage functions that relate the depth of floodwaters to building damage defined in terms of economic loss, expressed either in terms of currency [2] or as a percentage of the building value[3]. Some research has focused on identifying collapse potential of residential buildings as a function of flood depth and velocity (e.g.,[4]), but such studies generally provide only an empirical function, above which structural failure occurs and below which only inundation damage is present. A few discrete scales do exist that describe flood consequences in terms of physical damage[5]; however, they do not adequately segregate the higher levels of damage. Further, as evidenced by Hurricanes Katrina, Rita and Ike, the source of damage (wind or flood) is often in question. A new scale is needed that will allow assessment of damage for the entire structure, rather than having to classify wind or flood as the source of the damage, as the cause may not always be readily apparent. An initial attempt at such as scale was developed immediately after Hurricane Katrina to aid in field damage data collection [6]. However, subsequent field investigations and analysis of the hurricane damage data have revealed significant shortcomings to that first combined damage scale, and have highlighted the need for a more comprehensive and loss-consistent categorization. The WF Damage Scale considers the combined damage from hurricane wind and flood on a loss consistent basis. Regardless of the damage mechanism, the physical damage is described in a manner that accounts for the presence or absence of structural damage, as well as treating repairable non-structural damage on an economically consistent basis. This damage scale has been constructed using damage scales from both the HAZUS-MH Wind and Flood [7] models, as well as guidance from the US Army Corps of Engineers, the European Macroseismic Scale (EMS-98)[8], and others. WIND AND FLOOD DAMAGE SCALE Damage states associated with hurricane storm surge are often much more significant than damage that is described by either wind or flood categorization schemes. The HAZUS-MH model describes hurricane wind damage on a five category scale: no or very minor damage, minor damage, moderate damage, severe damage, and destruction. One of the indicators for the highest wind damage state, “destruction,” is defined as “loss of more than 25% of roof sheathing.” While this level of damage may result in significant loss from water intrusion or may lead to damage caused by internal pressurization, this is clearly a very different damage state than “partial collapse” or “complete collapse” of a structure, commonly experienced along storm surge-affected coastlines. The two collapse damage classifications are typically used in the assessment of earthquake damage to buildings and have been employed by researchers assessing damage to buildings after tsunami events [9]. The WF Damage Scale categories were developed through enhancement and modification of the standard FEMA HAZUS-MH Wind Model damage classifications to include both collapse and partial collapse categories (Figure 1), as well as to provide detailed guidance in the assessment of combined damage. No Damage or Very Minor Damage Minor Damage Moderate Damage Severe Damage Very Severe Damage Partial Collapse Collapse Range of Damage Considered by FEMA’s HAZUS-MH Wind Model Range of Damage Typically Associated with Hurricane Storm Surge Figure 1: Damage categories associated with hurricane wind and storm surge for residential buildings The WF Damage Scale is presented in Table 1, where the shaded WF indicators represent the damage signatures that require corresponding classification, while non-shaded cells are general values for each damage state. Figures 2-5 demonstrate the application of the scale using examples from damage assessments completed after 2005 Hurricane Katrina and 2008 Hurricane Ike. While the housing types encountered in coastal Mississippi and Galveston, Texas, are markedly different, the scale is applied to consistently describe observed damage. As in Table 1, the shaded WF indicators in Figures 2-5 represent the damage signatures that require corresponding classification, while non-shaded cells provide general values for each damage state. The following details explain the assessment for each structure: • WF-0 Katrina and Ike – No visible damage, no floodwater impacted the buildings • WF-1 Katrina – Minor cladding damage to side of house, no flooding within the building • WF-1 Ike – Breakaway wall damage with no damage to building • WF-2 Katrina – Appurtenant structure (porch) failure, with damage to main building • WF-2 Ike – Cladding damage, does not expose the structure interior, building wrap not intact • WF-3 Katrina – “Wash through” of cladding, structural system still in place, minor roof deck failure (<25%) • WF-3 Ike – Approximately 25% roof deck failure • WF-4 Katrina – Structural damage (repairable) to porch and portions of the building • WF-4 Ike – Roof deck damage exceeding 25% (roof deck is also removed from backside of the building) • WF-5 Katrina – Unrepairable structural damage to more than 25% of the building • WF-5 Ike – Elevated structure is racked • WF-6 Katrina and Ike – Total structural failure of the building WF DAMAGE SCALE APPLICATION FOR HURRICANES KATRINA AND IKE Building attributes and damage were classified for nearly 2000 residential structures following Hurricanes Katrina and Ike based on review of high definition video and still images collected during field reconnaissance missions in coastal Mississippi and Galveston, Texas. Figures 6 and 7 provide summary attribute and damage information for both datasets, including (a) number of stories, (b) foundation type, (c) approximate base floor elevation, (d) calculated depth of flooding (calculated as surge elevation minus approximate base floor elevation, (e) WF damage state from the video review (not including flood depth), and (f) WF damage state including flood depth. Attribute details that were not able to be assessed for destroyed buildings are indicated, and base floor elevations for unassessed buildings are included in the < 1 ft category in Figure 6. The buildings were assumed to be of wood construction, as all buildings with exposed structure appeared to be wood framed. Table 1: Wind and Flood Damage Scale [10] Damage State WF-0 WF-1 WF-2 WF-3 WF-4 WF-5 WF-6 Qualitative Wind Damage Description Qualitative Surge/Flood Damage Description No Damage or Very Minor Damage Little or no visible damage from the outside. No broken windows, or failed roof deck. Minimal loss of roof cover, with no or very limited water penetration. No Damage or Very Minor Damage No floodwater impacts the building. Minor Damage Maximum of one broken window, door or garage door. Moderate roof cover loss that can be covered to prevent additional water entering the building. Marks or dents on walls requiring painting or patching for repair. Moderate Damage Major roof cover damage, moderate window breakage. Minor roof sheathing failure. Some resulting damage to interior of building from water. Minor Damage Breakaway walls or appurtenant structures (staircases carports, etc.) damaged or removed without physical damage to remaining structure. No floodwater impacts the building. Severe Damage Major window damage or roof sheathing loss. Major roof cover loss. Extensive damage to interior from water. Very Severe Damage Complete roof failure and/or, failure of wall frame. Loss of more than 25% of roof sheathing. Partial Collapse House shifted off foundation, overall structure racking, unrepairable structural damage (structure still partly intact). Collapse Total structural failure (no intact structure). Moderate Damage Some wall cladding damage from floodborne debris or high velocity floodwater. Breakaway walls or appurtenant structures (staircases carports, etc.) damaged or removed with physical damage to remaining structure. Severe Damage Removal of cladding from "wash through" of surge without wall structural damage. Very Severe Damage Failure of wall frame, repairable structural damage to any portion of the building or cases of unrepairable structural damage, not to exceed 25% of the building plan area. Partial Collapse House shifted off foundation, overall structure racking, unrepairable structural damage to > 25% of the building plan area. Structure is still partly intact. Collapse Total structural failure (no intact structure). Roof Cover Damage Window/ Door Damage Roof Deck Failure Foundation Damage Appurtenant Structure Damage Wall Cladding Damage Wall Structure Failure Roof Structure Failure Structural Damage "Stillwater" Flood Depth ≤2% No No No No No No No No None >2% and ≤15% One window, door, or garage door failure No Slab, pile scour with no apparent building damage Yes, without damage to building Minor cladding damage with building wrap intact No No No None >15% and ≤50% > one and ≤ the larger of 20% & 3 1 to 3 panels Yes Yes, with damage to building Moderate cladding damage that does not expose structure interior, building wrap not intact No No No > 0 ft and <2 ft (one story) or <5 ft (two+ stories) >50% > the larger of 20% & 3 and ≤50% >3 and ≤25% Yes Yes "Wash through" damage No No No >2 ft (one story) or >5 ft (two+ stories) >10 ft Typically >50% >50% >25% Cracked slab with visible deformation Yes Yes Yes Yes Any repairable structural damage or ≤25% unrepairable damage Typically >50% >50% >25% Racking of elevated structure Yes Yes Yes Yes Unrepairable structural damage (>25%) >10 ft Typically >50% >50% >25% Yes Yes Yes Yes Yes Total structural failure >10 ft Hurricane Katrina Example Hurricane Ike Example WF Damage Indicators Roof Cover Damage Window/ Door Damage WF-0: No Damage or Very Minor Damage WF Rating Roof Deck Failure Foundation Damage Appurtenant Structure Damage Wall Cladding Damage Wall Structure Failure Roof Structure Failure Structural Damage "Stillwater" Flood Depth Roof Cover Damage Window/ Door Damage WF-1: Minor Damage Roof Deck Failure Foundation Damage Appurtenant Structure Damage Wall Cladding Damage Wall Structure Failure Roof Structure Failure Figure 2: WF-0 to WF-1 Damage Classification Examples for Hurricanes Katrina and Ike ≤2% No No No No No No No No None >2% and ≤15% One window, door, or garage door failure No Slab, pile scour with no apparent building damage Yes, without damage to building Minor cladding damage with building wrap intact No No Structural Damage No "Stillwater" Flood Depth None Hurricane Katrina Example Hurricane Ike Example WF-2: Moderate Damage WF Rating WF Damage Indicators Roof Cover Damage >15% and ≤50% Window/ Door Damage > one and ≤ the larger of 20% & 3 Roof Deck Failure 1 to 3 panels Foundation Damage Yes Appurtenant Structure Damage Yes, with damage to building Wall Cladding Damage Moderate cladding damage that does not expose structure interior, building wrap not intact Wall Structure Failure Roof Structure Failure WF-3: Severe Damage No Structural Damage No "Stillwater" Flood Depth > 0 ft and <2 ft (one story) or <5 ft (two+ stories) Roof Cover Damage Window/ Door Damage Roof Deck Failure Foundation Damage Appurtenant Structure Damage Wall Cladding Damage Wall Structure Failure Roof Structure Failure Figure 3: WF-2 to WF-3 Damage Classification Examples for Hurricanes Katrina and Ike No >50% > the larger of 20% & 3 and ≤50% >3 and ≤25% Yes Yes "Wash through" damage No No Structural Damage No "Stillwater" Flood Depth >2 ft (one story) or >5 ft (two+ stories) WF Rating Hurricane Katrina Example Hurricane Ike Example WF Damage Indicators WF-4: Very Severe Damage Roof Cover Damage Window/ Door Damage WF-5: Partial Collapse Figure 4: WF-4 to WF-5 Damage Classification Examples for Hurricanes Katrina and Ike >50% Roof Deck Failure >25% Foundation Damage Appurtenant Structure Damage Wall Cladding Damage Wall Structure Failure Roof Structure Failure Cracked slab with visible deformation Structural Damage Note: the backside of the roof also experienced roof deck failure, causing > 25% roof deck loss Typically >50% "Stillwater" Flood Depth Roof Cover Damage Window/ Door Damage Yes Yes Yes Yes Any repairable structural damage or ≤25% unrepairable damage >10 ft Typically >50% >50% Roof Deck Failure >25% Foundation Damage Appurtenant Structure Damage Wall Cladding Damage Wall Structure Failure Roof Structure Failure Racking of elevated structure Yes Yes Yes Yes Structural Damage Unrepairable structural damage (>25%) "Stillwater" Flood Depth >10 ft WF Rating Hurricane Katrina Example Hurricane Ike Example WF Damage Indicators Roof Cover Damage Window/ Door Damage Roof Deck Failure WF-6: Collapse Foundation Damage Appurtenant Structure Damage Wall Cladding Damage Wall Structure Failure Roof Structure Failure Figure 5: WF-6 Damage Classification Examples for Hurricanes Katrina and Ike Typically >50% >50% >25% Yes Yes Yes Yes Yes Structural Damage Total structural failure "Stillwater" Flood Depth >10 ft Hurricane Katrina Number of Stories (a) Not Assessed 42% 1 Story 43% 2 Story 12% Slab 31% (b) Hurricane Katrina Approximate Base Floor Elevation 3 - 6 ft 8% Piles 6% Raised Floor 21% Not Assessed 45% 3 Story 0% Hurricane Katrina Foundation Hurricane Katrina Calculated Flood Depth Above Floor 0 ft 6% 8 - 10 ft 5% < 1 ft 64% > 11 ft 27% 1 - 2 ft 23% (c) 2 - 6 ft 17% 7 - 10 ft 40% (d) Hurricane Katrina WF Damage State Physical Damage Only WF-6 42% Hurricane Katrina WF Damage State Physical + Flood Damage WF-0 2% WF-0 21% WF-1 2% WF-2 9% WF-6 42% WF-1 14% WF-5 5% (e) WF-4 WF-3 5% 7% < 2 ft 10% WF-3 35% WF-2 6% (f) WF-5 5% WF-4 5% Figure 6: Hurricane Katrina building inventory and damage characteristics Based on a review of Figure 6, the foundation type and structure elevation appear to have significantly contributed to the amount of damage sustained from Hurricane Katrina’s storm surge. Figure 6(d) shows approximately 70% of buildings were flooded by more than 7 feet. Over half of the buildings in the building database experienced some level of structural damage (WF-4 to WF-6), resulting in significant debris generated from damaged and destroyed buildings. Both the hazard environment and the coastal construction techniques employed in Galveston, Texas, limited the amount of damage experienced in the study area. Galveston did not experience the most severe wind, surge and wave forces; however many residences on Bolivar Peninsula were completely destroyed as a result of Hurricane Ike and areas that were most heavily impacted were inaccessible at the time of the field survey. Differences in damage severity noted in Figure 6 and 7 (e) and (f) show the usefulness of the inclusion of flood hazards in overall building damage assessments. Significant portions of the Hurricane Katrina dataset were subject to storm surge inundation, which is reflected in the increased levels of damage observed when the flood hazard is included. The Hurricane Ike dataset in Galveston, Texas, was exposed to less severe storm surge as a result of a lowered hazard environment and the predominance of pile foundations. As shown in Figure 7 (e) and (f), few changes in damage state were observed as a result of inclusion of surge inundation depth. The WF Damage Scale does allow assessment damage (including collapse and partial collapse) without identification of the mechanism of the damage. Hurricane Ike Number of Stories 3 Story 2% Hurricane Ike Foundation Not Assessed 2% Not Assessed 5% 1 Story 50% 2 Story 46% (a) Piles 95% (b) Hurricane Ike Calculated Flood Depth Above Floor Hurricane Ike Approximate Base Floor Elevation Not Assessed 2% < 8 ft 2% 2 - 4 ft 1% > 10 ft 11% < 2 ft 0% > 4 ft 2% 8 - 10 ft 85% (c) (d) Hurricane Ike WF Damage State Physical Damage Only WF-2 10% Hurricane Ike WF Damage State Physical + Flood Damage WF-0 24% WF-2 11% WF-1 63% (e) 0 ft 97% WF-0 23% WF-1 63% (f) Figure 7: Hurricane Ike Building Inventory and Damage Characteristics CONCLUSIONS This paper has presented the application of the new Wind and Flood Damage Scale. The primary need that the WF Damage Scale addresses in hurricane damage assessments is the ability to describe damage to residential buildings, regardless of the damage mechanism. This ability allows for a more complete understanding of the overall level of damage caused by hurricanes. The WF Damage Scale provides a method to describe total building damage on a loss consistent basis, allowing description of the overall levels of building damage. These descriptions of damage have multiple applications, including loss of life estimates, emergency management planning and response activities, and improved understanding of the overall performance of residential buildings exposed to hurricane conditions. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] FEMA, Multi-hazard loss estimation methodology - hurricane model, HAZUS-MH MR2 Technical Manual, Federal Emergency Management Agency, Washington, D.C., 2006a. E. C. Penning-Rowsell and J. B. Chatterton, The benefits of flood alleviation: a manual of assessment techniques, Aldershot: Gower (1977). USACE, Generic depth-damage relationships for residential structures with basements, Economic Guidance Memorandum 04-01, U.S. Army Corps of Engineers, Washington, D.C., 2003. CH2M Hill, Potential flood damages, Willamette River System, Department of the Army Portland District, Corps of Engineers, Portland, OR, 1974. I. Kelman, Physical flood vulnerability of residential properties in coastal, eastern England, PhD Thesis, University of Cambridge, 2002. J. A. Womble, S. Ghosh, B. Adams, and C. Friedland, Advanced damage detection for Hurricane Katrina: integrating remote sensing and VIEWSTM field reconnaissance, Hurricane Katrina Special Report Series MCEER, Buffalo, NY, 2006. FEMA, Multi-hazard loss estimation methodology – flood model, HAZUS-MH MR2 Technical Manual, Federal Emergency Management Agency, Washington, D.C., 2006b. G. Grünthal ed, European macroseismic scale 1998 (EMS-98) Cahiers du Centre Européen de Géodynamique et de Séismologie, Luxembourg, 1998. H. Miura, A. C. Wijeyewickrema, and S. Inoue, Evaluation of tsunami damage in the eastern part of Sri Lanka due to the 2004 Sumatra earthquake using remote sensing technique, in: 8th U.S. National Conference on Earthquake Engineering (8NCEE), (San Francisco, CA), 2006. C. J. Friedland, Residential building damage from hurricane storm surge: proposed methodologies to describe, assess and model building damage, PhD Thesis, Louisiana State University, 2009.