Download Loss-Consistent Categorization of Hurricane Wind and Storm Surge

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.
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