Residential Wind Damage in Hurricane Katrina Improved Building Codes and Construction Practices-

Residential Wind Damage in Hurricane Katrina
Preliminary Estimates and Potential Loss Reduction through
Improved Building Codes and Construction Practices
October 3, 2005
Prepared by
LSU Hurricane Center
Suite 3225 CEBA Building
Louisiana State University
Baton Rouge, LA 70803
Phone: (225) 578-4813
Fax: (225) 578-7646
Funded in part by Solutia, Inc.
i
PREFACE
This study has been conducted by the LSU Hurricane Center. The views presented here are
those of the report authors (shown below), who are solely responsible for the analysis and
content of this report.
Marc L. Levitan, Ph.D.
Director, LSU Hurricane Center
Charles P. Siess, Jr. Associate Professor
Department of Civil and Environmental Engineering
Louisiana State University
Ms. Carol Hill, P.E.
Graduate Research Assistant
Department of Civil and Environmental Engineering
Louisiana State University
The analysis makes use of wind field data from Hurricane Katrina initially gathered by field
researchers from Texas Tech University and Florida International University and others, with
subsequent analysis by the NOAA Hurricane Research Division and Applied Research
Associates (ARA). The data sharing among these research groups is gratefully acknowledged.
Loss estimates and effectiveness of mitigation options were determined using FEMA’s HAZUSMH
Hurricane Wind Model, MR1 Release 39 (copyright 2004, FEMA).
Loss estimates are highly dependent on the maximum wind speeds in a hurricane and the
geographical extent of hurricane force winds. There are currently several researchers on the
ground in Louisiana and Mississippi gathering as much information as they can find on the
intensity and reach of Katrina’s wind field. As new data becomes available, a reanalysis of the
estimated losses may yield somewhat different results.
Partial funding for the study was provided by Solutia, Inc, which is also gratefully
acknowledged.
ii
EXECUTIVE SUMARY
In order to provide context for consideration of changes to residential building codes and
construction practices in Louisiana, a quick study of the estimated wind damage caused by
Hurricane Katrina was conducted. As Louisiana did not experience the most intense winds from
Katrina due to its track over the very eastern part of the state, a ‘what if’ analysis was also
conducted with a Katrina-like storm shifted slightly to the west, bringing more of the windfield
over Louisiana. Both of these storm events were then used to investigate the impacts of several
different changes in building code requirements/construction practices. Mitigation measures
considered were opening protection (impact resistant shutters or laminated glass systems),
improved connection of the roof deck, installation of hurricane straps, and secondary moisture
protection of the roof deck. Damage estimates and the effectiveness of various mitigation
measures were made using FEMA’s HAZUS-MH Hurricane Wind Model, a state-of-the-art risk
assessment program for analyzing hurricane losses.
The number of residential structures in Louisiana that sustained damage from wind and winddriven
rain during Hurricane Katrina was estimated to be near 273,000, 16% of the total building
stock in Louisiana. Had Katrina made landfall slightly west of New Orleans instead of just east
of the city, the number of damaged and destroyed buildings in Louisiana would have been
doubled, to well over half a million residences.
Investigation of mitigation options showed all to be effective individually. When considered
together as a package, these four combined mitigation measures reduced the estimated losses by
over 75%.
The loss reduction estimates using different mitigation techniques do not include such additional
benefits as reduction in human suffering, reduced disruption of communities and local
economies, reduced emergency response costs, and other significant but difficult to quantify
losses.
These preliminary results support the need for reforms in building code requirements and
construction practices in Louisiana.
1
RESIDENTIAL LOSS ANALYSIS
The HAZUS-MH Wind Model was used to simulate the wind field experienced during Hurricane
Katrina. The model inputs were based upon a preliminary study of surface winds produced by
Applied Research Associates (ARA). The ARA modeled storm was based on National
Hurricane Center Forecast/Advisories (through NHC Advisory 27), H*Wind analyses of wind
speeds and radius to maximum winds from NOAA’s Hurricane Research Division (HRD) and
ground-measured wind speeds from the Florida Coastal Monitoring Program (FCMP) and
reporting airports.
The modeled wind field for Hurricane Katrina in Louisiana is shown in Figure 1. A large scale
map of the affected portion of Louisiana is shown in Figure 2. Wind speeds represent peak gusts
at 33 ft height over flat open terrain, the standard reporting methodology for wind speeds.
Figure 1 – Estimated Hurricane Katrina Peak Gust Wind Speeds
Figure 2 – Estimated Hurricane Katrina Peak Gust Wind Speeds (Southeast LA view)
2
To simulate the landfall of Hurricane Katrina on a more damaging path, each of the points used
to define the track of Hurricane Katrina was shifted 0.7 decimal degrees (approximately 40
miles) to the west. This track puts New Orleans on the right front side of the storm, where it will
experience more severe winds and flooding. This hypothetical Katrina-West scenario was
analyzed with the HAZUS Wind Model to produce a wind field for purposes of comparison with
the actual Katrina track. The modeled wind field for the adjusted storm track in the state of
Louisiana is shown in Figure 3. A larger scale map of the southeast corner of Louisiana is
shown in Figure 4.
Figure 3 – Hypothetical Katrina-West Scenario Peak Gust Wind Speeds
Figure 4 – Hypothetical Katrina-West Scenario Peak Gust Wind Speeds
(Southeast Louisiana view)
3
The HAZUS-MH Hurricane Model uses a structural load vs. resistance methodology to calculate
damage experienced in a hurricane. The wind speeds discussed previously are the most
important inputs to determine the wind loads on the structures. The damage results provided by
HAZUS include five damage states: none or very minor, minor, moderate, severe, and
destruction. Qualitative descriptions for each of these damage categories are given below in
Table 1.
Table 1 – HAZUS Damage States for Residential Construction
Qualitative Damage Description
Roof
Cover
Failure
Window
Door
Failures
Roof Deck
Missile
Impacts on
Walls
Roof
Structure
Failure
Wall
Structure
Failure
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.
≤2% No No No No No
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.
>2% and
≤15%
One
window,
door, or
garage
door
failure
No 15% and
≤50%
> one and
≤ the larger
of 20% &
3
1 to 3
panels
Typically 5
to 10
impacts
No No
Severe Damage
Major window damage or roof sheathing
loss. Major roof cover loss. Extensive
damage to interior from water.
>50%
> the larger
of 20% &
3 and
≤50%
>3 and
≤25%
Typically
10 to 20
impacts
No No
Destruction
Complete roof failure and/or, failure of
wall frame. Loss of more than 50% of roof
sheathing.
Typically
>50% >50% >25%
Typically
>20
impacts
Yes Yes
A building damage and loss analysis was completed for Hurricane Katrina using the default
settings for the HAZUS program. The state of Louisiana was used as the study region.
According to the HAZUS databases, the study area contains approximately 1,719,000 total
buildings (based on 2000 census data). From the HAZUS databases, residential buildings
constitute approximately 99% of the buildings in the study region.
The output of the HAZUS model provides building counts for each of damage categories shown
above. The Hurricane Katrina analysis results are given in Table 2. Counts represent the
estimated total number of affected buildings in Louisiana, and percentage figures represent the
percent of buildings in each of the categories. Maps of the areas affected by Hurricane Katrina
winds are shown in Figures 5 and 6. Figure 5 presents a graphical depiction of residential
properties modeled by HAZUS that sustained at least minor damage, delineated by census tract,
and Figure 6 shows properties experiencing at least moderate damage. The total estimated
damage from wind and rain (including costs to repair and loss of contents) is $10.0 billion.
4
Table 2 – HAZUS Modeled Residential Damage for Hurricane Katrina
Reporting Basis Minor
Damage
Moderate
Damage
Severe
Damage Destruction
Total
Buildings
Affected
Building Count 153,250 75,227 24,604 20,402 273,483
Percentage of
Buildings in LA 9% 4% 1% 1% 16%
Figure 5 – Estimated Percentage of Residential Properties Sustaining at Least Minor
Damage during Hurricane Katrina (HAZUS)
Figure 6 – Estimated Percentage of Residential Properties Sustaining at Least Moderate
Damage from Hurricane Katrina (HAZUS)
5
A HAZUS analysis was also conducted for the simulated Hurricane Katrina on a more westerly
track. The estimated damage results are given in Table 3 below. This storm is shown to cause
much more damage. It effects more than twice as many buildings and destroys several times as
many as indicated by the Katrina estimate
Table 3 – HAZUS Modeled Residential Damage for Hypothetical Katrina-West Scenario
Reporting Basis Minor
Damage
Moderate
Damage
Severe
Damage Destruction
Total
Buildings
Affected
Building Count 143,601 172,393 134,368 125,132 575,493
Percentage of
Buildings in LA 8% 10% 8% 7% 33%
These damage estimates are shown graphically in Figures 7 and 8. Figure 7 shows the modified
Katrina track HAZUS results for residential properties sustaining at least minor damage,
delineated by census tract. Figure 8 shows the percentage of residential properties modeled by
HAZUS as experiencing at least moderate damage for this storm scenario.
Figure 7 – Percentage of Residential Properties Sustaining at Least Minor Damage for
Hypothetical Katrina-West Scenario (HAZUS)
6
Figure 8 – Percentage of Residential Properties Sustaining at Least Moderate Damage for
Hypothetical Katrina-West Scenario (HAZUS)
ANALYSIS OF EFFECTIVENESS OF MITIGATION MEASURES
Both the actual Hurricane Katrina track and the modified track west of New Orleans were
analyzed with the HAZUS Hurricane Model to explore the effectiveness of improved building
codes and construction practices in mitigating wind damage. The mitigation options are based
upon strategies incorporated in the Dade County South Florida Building Code, described below
(see the HAZUS Technical Manual for more details).
􀂃 Protection of Building Openings
Windows and doors are the weak spots in the wall envelope. Requiring debris impact
resistant windows and doors or debris impact protective coverings (shutters) prevents
most window and door failures. This helps keep the wind and rain out of the building,
reducing structural damage, damage to finishes, and damage to contents.
􀂃 Improved Roof Sheathing Attachment
Better attachment of the plywood or OSB roof sheathing to the roof structure through
appropriate fasteners and closer fastener spacing helps prevent sections of the roof deck
from being lifted off by the wind. This reduces progressive failures and wind and water
from penetrating the building envelope.
􀂃 Improved Roof-Wall Connections
Installation of metal ‘hurricane clips’ or’ hurricane straps’ provides a continuous load
path from the roof to the foundation, helping prevent catastrophic roof uplift failures.
􀂃 Secondary Waterproofing to Roof Joints
Sealing the joints between the sheets of roof decking provides a second line of defense
against roof leaks, even if the roof coverings are damaged or destroyed.
7
Reduction in Building Damage Through Mitigation
The four mitigation strategies were applied to each hazard scenario individually to assess the
effectiveness of each option. A combined analysis was also performed to provide an
understanding of the effectiveness of the combination of mitigation measures. Results for this
analysis are given in the same format as for the Katrina analysis, with building counts in each
damage state and the percentage of buildings statewide that are classified in each damage state.
Additionally, the percent reduction from the basic Hurricane Katrina analysis is calculated for
each damage state.
Note that reduction in damage is not modeled by selecting secondary waterproofing for roof
joints, but results for waterproofing are included in the economic loss analysis. The results of the
mitigation analysis are given for Hurricane Katrina in Tables 4-7. Mitigation analysis results for
the hypothetical Katrina-West track (not shown here) resulted in similar percent reductions.
Table 4 – Hurricane Katrina Mitigation Analysis Results – 100% Implementation of
Opening Protection for Residential Buildings
Reporting Basis Minor
Damage
Moderate
Damage
Severe
Damage Destruction
Total
Buildings
Affected
Residential
Building &
Contents Loss
(Billions)
Percent
Reduction
from Katrina
Estimate
Katrina Estimate
Damaged Building
Count
153,250 75,227 24,604 20,402 273,483 $10.0
Building Count – 100%
Opening Protection 159,304 69,911 14,234 4,342 247,791 $5.5 45%
Table 5 – Hurricane Katrina Mitigation Analysis Results – 100% Implementation of Roof
to Wall Straps/Clips for Residential Buildings
Reporting Basis Minor
Damage
Moderate
Damage
Severe
Damage Destruction
Total
Buildings
Affected
Residential
Building &
Contents Loss
(Billions)
Percent
Reduction
from Katrina
Estimate
Katrina Estimate
Damaged Building
Count
153,250 75,227 24,604 20,402 273,483 $10.0
Building Count – 100%
Straps/Clips 154,486 78,252 27,718 13,029 273,485 $8.9 11%
8
Table 6 – Hurricane Katrina Mitigation Analysis Results – 100% Implementation of
Upgraded Roof Deck Attachment for Residential Buildings
Reporting Basis Minor
Damage
Moderate
Damage
Severe
Damage Destruction
Total
Buildings
Affected
Residential
Building &
Contents Loss
(Billions)
Percent
Reduction
from Katrina
Estimate
Katrina Estimate
Damaged Building
Count
153,250 75,227 24,604 20,402 273,483 $10.0
Building Count – 100%
Upgraded Roof Deck
Attachment
109,787 44,429 17,778 18,474 190,468 $8.0 20%
Table 7 – Hurricane Katrina Mitigation Analysis Results – 100% Implementation of all
Four Mitigation Options for Residential Buildings
Reporting Basis Minor
Damage
Moderate
Damage
Severe
Damage Destruction
Total
Buildings
Affected
Residential
Building &
Contents Loss
(Billions)
Percent
Reduction
from Katrina
Estimate
Katrina Estimate
Damaged Building
Count
153,250 75,227 24,604 20,402 273,483 $10.0
Building Count – 100%
All Mitigation
Strategies
113,938 29,959 3,451 1,985 149,334 $2.1 79%
Reduction in Economic Loss through Mitigation
Using the mitigation strategies outlined above, HAZUS was also used to provide cost estimates
of direct losses. Direct property damage losses are the estimated costs to repair or replace the
damage caused to the building and its contents. Information from the HAZUS databases
indicates that the Louisiana study region consists of 1,718,706 buildings with an aggregate total
replacement value (excluding contents) of $235.9 billion (2002 dollars). Residential buildings
make up 86% of the building value in the state of Louisiana, with total replacement value of
$203.3 billion.
The analysis for Hurricane Katrina estimates total losses (including business interruption) in the
state of Louisiana of $13.1 billion. Residential losses make up 89% of the total loss at $10
billion. Mitigation strategies were evaluated to assess the economic effectiveness of
strengthening the residential building stock in the study area. Mitigation alternatives shutters on
exterior openings, straps/clips at the roof to wall connection, upgraded roof deck attachment,
secondary waterproofing for roof joints for single family, multi-family and mobile home
9
dwellings. Each of these mitigation strategies was applied to 100% of the study region building
stock individually to assess the economic loss reduction effectiveness. An analysis combining
all available mitigation strategies was also completed.
Table 8 provides economic loss information modeled by HAZUS for Hurricane Katrina. These
values represent modeled residential building and contents losses in the state of Louisiana.
Percent of total replacement value represents the total residential losses normalized by the total
replacement value of residential buildings in the state ($203.3 billion). Percent reduction reflects
the reduction in economic losses for each mitigation option from the default analysis. Reduction
in economic losses was seen at approximately the same percentages for each mitigation option in
the hypothetical Katrina-West simulation.
Table 8 –Residential Economic Loss Values for Hurricane Katrina Alone and Katrina with
Mitigation Measures
Analysis Type
Residential
Building &
Contents Loss
(Billions)
Percent of State
Total
Replacement
Value
Percent
Reduction from
Katrina Estimate
Hurricane Katrina Estimate $10.0 4.9% N/A
Mitigation – 100% Opening Protection $5.5 2.9% 45%
Mitigation – 100% Straps/Clips $8.9 2.8% 11%
Mitigation – 100% Upgraded Roof Deck $8.0 2.7% 20%
Mitigation – 100% Secondary
Waterproofing $8.9 4.4% 11%
All 4 Mitigation Options Selected $2.1 1.0% 79%

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