A Resource for the State of Florida
HURRICANE LOSS REDUCTION
FOR HOUSING IN FLORIDA:
ROOFTOP EQUIPMENT WIND LOAD AND ITS MITIGATION FOR BUILDINGS IN HURRICANE
PRONE REGIONS
A Research Project Funded by The State of Florida Division of Emergency Management
Through Contract # 06RC-A%-13-00-05-261
Prepared by:
Arindam Gan Chowdhury, PhD & Jimmy Erwin, Research Graduate Department of Civil and Environmental Engineering
Florida International University
In Partnership with: The International Hurricane Research Center
Florida International University
August 2007
2
TABLE OF CONTENTS
CHAPTER PAGE
EXECUTIVE SUMMARY…………………………………………………………………………………. 3
1. PROBLEM STATEMENT .................................................................................................................. 3
2. RESEARCH BACKGROUND ............................................................................................................ 6
3. RESEARCH OBJECTIVES ................................................................................................................. 8
4. METHODOLOGY AND EXPERIMENTATION.............................................................................. 10
5. RESULTS AND DISCUSSION ......................................................................................................... 15
6. COST BENEFIT ANALYSIS ............................................................................................................ 19
7. FUTURE WORK................................................................................................................................ 27
REFERENCES ............................................................................................................................................. 28
3
ROOFTOP EQUIPMENT WIND LOAD AND ITS MITIGATION FOR
BUILDINGS IN HURRICANE PRONE REGIONS Executive Summary
Rooftop equipment is a general term used to describe components such as condensers,
exhaust hoods, HVAC units, and communications equipment that are typically mounted on the
roof of structures. This type of equipment is subjected to wind loads that must be considered
when designing the anchorage connection between the particular component and the roof.
Damage reconnaissance studies conducted during the 2004 and 2005 hurricane seasons
witnessed widespread rooftop equipment failures. The aim of this research is to develop
mitigation techniques that will reduce the wind loading on mechanical rooftop equipment and the
forces transferred to the roof supporting structure by the equipment through the use of
aerodynamic retrofits such as installation of wind screens.
1. Problem Statement
Rooftop equipment is subjected to high wind loads during extreme wind events such as
hurricanes and a structural system is needed to resist and transfer the loads. The equipment,
structural system, and anchorage connections to the roof members need to be carefully designed
to prevent failure during sever storms. Wind-induced failure of rooftop equipment during a
hurricane may result in large openings in the roof that will allow water to penetrate into the
building, puncturing of the roof membrane, again allowing water infiltration, and detached
rooftop equipment can pose considerable threats as windborne debris [Reinhold, 2006].
Secondary effects associated with rooftop equipment failure may lead to extended delays in
restoring occupancy and function of the building because significant drying out time may be
4
needed or difficulties may arise from exposed electrical and plumbing connections resulting from
such failures [Reinhold, 2006]. With significant roofing damage and secondary water damage
occurring related to the poor performance of rooftop equipment during extreme wind events,
high maintenance and costly repair works (Fig. 2) are often needed.
Figure 1. Rooftop AC Units and Structural Framing System
Figure 2. Repair of Rooftop AC Units and Structural Framing System after Damages from Hurricane Wilma in 2005
Damage reconnaissance studies conducted during the 2004 and 2005 hurricane seasons
witnessed widespread rooftop equipment failures (Fig. 3). In the aftermath of the 2004
5
hurricanes Charley, Frances, Ivan, and Jeanne, the Federal Emergency Management Agency
(FEMA) deployed Mitigation Assessment Teams (MATs) to perform field observations of
building performance and damage caused by the storms. Rooftop mechanical and electrical
equipment was listed by the MATs as one of the key modes of observed building failure, causing
millions of dollars worth of damage and even impeding recovery efforts by impacting critical
and essential facilities [FEMA 490, 2005]. Similar results were found following Hurricane
Katrina (2005); the FEMA MAT identified wind impacts to rooftop equipment as one area
requiring “additional attention” from designers, architects, and contractors [FEMA 549, 2006].
Following hurricanes Katrina and Rita (2005), the National Institute for Standards and
Technology (NIST) also recognized the impact of rooftop equipment failure on major buildings
and windborne debris damage caused by rooftop equipment detachment in all locations of
reconnaissance [NIST 1476, 2006].
Figure 3. Rooftop Equipment Damage During 2004-2005 Hurricane Seasons (Cont.)
Image Source: FEMA 549 “Mitigation Assessment Team Report: Hurricane Katrina in the Gulf Coast.” July 2006. Chapter 5, Pg 5-80.
Image Source: FEMA 489 “Mitigation Assessment Team Report: Hurricane Ivan in Alabama and Florida” August 2005. Chapter 5, Pg 5-61.
6
Figure 3. Rooftop Equipment Damage During 2004-2005 Hurricane Seasons 2. Research Background
There is very limited research available to provide designers with guidance about the
wind-induced forces exerted on rooftop equipment. ASCE 7 Standard is the source adopted by
all the latest national model codes and standards for wind loading information [Reinhold, 2006].
Image Source: FEMA 489 “Mitigation Assessment Team Report: Hurricane Ivan in Alabama and Florida” August 2005. Chapter 5, Pg 5-58.
Image Source: FEMA 488 “Mitigation Assessment Team Report: Hurricane Charley in Florida” April 2005. Chapter 5, Pg 5-64.
Image Source: FEMA 489 “Mitigation Assessment Team Report: Hurricane Ivan in Alabama and Florida” August 2005. Chapter 5, Pg 5-60.
Image Source: FEMA 489 “Mitigation Assessment Team Report: Hurricane Ivan in Alabama and Florida” August 2005. Chapter 5, Pg 5-59.
7
In the 2002 and 2005 editions of ASCE 7, the force applied to rooftop equipment is computed
using equations (6-25) and (6-28), respectively, and is given by:
)(lbAGCqF ffz= (1)
where qz is the velocity pressure evaluated at height z of the centroid of area Af using the
appropriate exposure category, G is the gust-effect factor, Cf is the force coefficient, and Af is the
projected area normal to the wind except where Cf is specified for the actual surface area [ASCE
7-02, ASCE 7-05].
ASCE 7-02 was the first edition of the reference standard that sought to address the
proper design loads for wind-sensitive rooftop systems. The provisions of ASCE 7-02 require
the use of Figure 6-19 to determine the value for the force coefficient, Cf, to be used in Eq. 1.
Based on the ASCE 7-02 commentary, the ASCE 7 committee was vague about providing
guidance for dealing with the increased loads exerted on the equipment because there is no basis
to make a recommendation [ASCE 7-02]. Additionally, uplift forces on the rooftop equipment
are not considered in the methodology at all.
ASCE 7-05 commentary mentions that because of the small size of the rooftop equipment
in comparison to the building, it is expected that the wind force will be higher than that predicted
by ASCE 7-02 due to higher correlation of pressures across the structure surface, higher
turbulence on the building roof, and accelerated wind speed on the roof. ASCE 7-05 commentary
also mentions that research [Hosoya et al., 2001] has shown high uplifts on the top of the rooftop
air conditioning units, although the net uplift on the units was not measured. The consensus of
the committee is that uplift forces may be a significant fraction of the horizontal force. Hence
uplift load should also be considered by the designer.
8
ASCE 7-05 uses Figure 6-21, which is same as Figure 6-19 in ASCE 7-02, but specific
changes were added by providing a section that dealt with wind loads on structures and
equipment for low-rise buildings, that is, buildings less than or equal to 60 ft [ASCE 7-05,
Reinhold, 2006]. These changes were based on the results of a wind tunnel study conducted by
Hosoya et al. [2001], which modeled a 4x4x4 ft air conditioning unit mounted at three different
locations on the Texas Tech University (TTU) field site building. From the ASCE 7-05
commentary, the revised methodology requires a new factor ranging from 1.0 to 1.9, depending
on the size of the equipment, to be multiplied with the force determined using Eq. 1. Thus the
force should be increased by a factor of 1.9 for units with area Af less than 0.1Bh (10% of the
building area). Because the multiplier is expected to approach 1.0 as Af approaches that of the
building (Bh), a linear interpolation is included as a way to avoid a step function in load if the
designer wants to treat other sizes. The research only treated one value of Af (0.4Bh).
In nearly all cases, the area of the equipment will be less than 10% of Bh [Reinhold,
2006], meaning that a force factor of 1.9 will almost always be used. The implication is that
forces calculated for rooftop equipment on low-rise structures will nearly double when using the
ASCE 7-05 methodology versus the ASCE 7-02 methodology. Due to the limited research and a
significant level of uncertainty, experts in the wind engineering field recommend further
increasing the forces determined using ASCE 7-05 with a factor of safety of 2 for regular
structures and a factor of safety of 3 for critical and essential facilities [Barista, 2007; Reinhold,
2006].
3. Research Objectives
As the latest edition of the ASCE 7 standard is adopted by the national building codes,
the load increase from ASCE 7-02 to ASCE 7-05 translates to significantly higher costs in the
9
design and construction of framing and anchorage for rooftop equipment for new buildings.
Furthermore, existing buildings requiring new rooftop equipment may need retrofits to increase
the strength of existing roof members to withstand higher wind loads transferred by the
equipment and the structural framing. Better and cost-effective alternatives are to reduce the
wind loading on the equipment itself by aerodynamic retrofitting or to dampen wind loading
transferred to the roof structure by structural retrofitting. Potential retrofits may include, but are
not limited to, aerodynamic edge shapes, wind screens, and elastomeric dampers.
The primary focus of this research is to develop a baseline for cost-effective techniques to
mitigate wind loading on rooftop equipment, lowering the cost of framing and anchorage,
reducing the chances of rooftop equipment failure, and saving potential losses associated with
roof damage and water infiltration. The research objectives are stated as:
• Full-scale testing of rooftop equipment on a low-rise structure to measure the
effects of hurricane-induced wind.
• Developing techniques to alleviate severe wind loads on the rooftop equipment
and reactions transferred to the roof structure by the equipment.
To achieve these goals, the scope of work for this project consists of the following tasks:
1. Conduct preliminary rooftop equipment wind loading and force measurements
with the current Wall of Wind windstorm simulator setup in order to establish the baseline loads
exerted on the equipment.
2. Test and assess the effectiveness of aerodynamic mitigation techniques such as
retrofitting rooftop equipment with wind screens.
3. Perform cost-benefit analysis to evaluate the effectiveness of the retrofit methods.
10
The broader impact of this proposed research work is to make the structural community
aware of the impact from the higher wind loads and also suggest a path for development of
efficient techniques to mitigate these loads cost effectively. With improved load determination,
more awareness during design, and proper mitigation techniques, damage related to rooftop
equipment failures during extreme wind events will be minimized.
4. Methodology and Experimentation
Reduced scale model testing on rooftop equipments is not feasible due to the small size
of the units. Thus wind tunnel testing would not be appropriate for this purpose. To overcome
wind tunnel constraints, the current study aims to test rooftop equipment subjected to design-
level hurricane force winds at full scale, using the Wall of Wind windstorm simulator (Fig. 4a)
operated at Florida International University (FIU). For this testing, typical air conditioning
condenser units are mounted on the roof of a 10x10x10 ft building model (Fig. 4b). Consistent
with general practice the rooftop units are mounted to a steel frame and the steel frame is
anchored to the roof supporting members of the test building by six legs. Both the building and
the rooftop equipment are fully engulfed by the wind flow generated by the Wall of Wind.
Based on the findings of Hosoya et al. [2001] and the failure observations reported by FEMA
[2005, 2006], NIST [2006], and others, the wind-induced effects of interest for the rooftop
equipment are the shear force, the axial force, and the overturning moment.
Instrumentation of the rooftop equipment consists of six Omega LC402 force transducers
(Fig. 5) to measure the axial forces, and six Omega LC101 force transducers (Fig. 5) to measure
the shear forces; from the equipment’s geometry and the measured forces, the overturning
moment can be determined. The Wall of Wind data acquisition system records loading time
histories for each of these transducers.
11
Figure 4. (a) 6-fan Wall of Wind at FIU, (b) One-story building model
Figure 5. Force Transducer Instrumentation for Rooftop Equipment Testing
LC 402
LC 101
WIND
2
4
6
1
3
5
12
The steel support frame is mounted at two different locations on the building structure: at
the windward edge of the building, and with the centerline of the equipment at a distance of 5’-
0” away from the roof edge. The rooftop equipment are mounted perpendicular to the wind flow.
Three AC units are placed on the steel supporting frame and the tests comprised of placing the
frame with the AC units at two different locations (#1 and #2) as illustrated in Fig. 6.
Figure 6. Rooftop Equipment Mounting Configurations (Plan View)
1
2
3
1’
2’
3’
5’–0”
Frame Centerline
Location #1
Frame Centerline
Location #2
Wall of Wind Flow Field
Building Structure
13
A control test without any retrofit technique was performed for each setup to establish the
baseline forces exerted on the rooftop equipment by running the Wall of Wind hurricane
simulator at 4400 rpm and 4000 rpm for the top and bottom engines respectively. Once the
baseline forces were known, the effectiveness of mitigation techniques was sought. For this
project the mitigation technique selected was the provision of a perimeter wind screen around the
AC units to alleviate the wind loading on the units and the structural frame. The screen was
fabricated from metal grating with a porosity of approximately 50%. The screen was installed
around the rooftop equipments and the test assembly was subjected to wind loading using the
Wall of Wind engine RPM profile and time duration identical to that of the baseline
measurement tests. Figure 7 shows test configurations with and without the wind screen.
Force time histories for shear and axial loading on the bottom of each leg of the structural
frame were recorded for the wind screen installed condition and then compared to the baseline
measurements to determine the effectiveness of the retrofit technique for reducing the loads
exerted on the rooftop equipment and transferred to the roof supporting structure.
14
Figure 7. Rooftop Equipment Set-up: (a) Middle of Roof without Screen, (b) Middle of Roof with Screen, (c) Edge of Roof without Screen, (d) Edge of Roof with Screen, (e) Edge of Roof without Screen (Side View), (f) Edge of Roof with Screen (Side View).
a b
c d
e f
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5. Results and Discussion
Wind loads on the rooftop equipment are transferred to the structural frame with pin-
connected legs in the form of shear and axial forces. With the given engine RPM profiles used
for this project the Wall of Wind fans generated a maximum wind speed of 120 mph which is
equivalent to a Category 3 hurricane and the forces generated at the bottom of each leg of the
frame were recorded with and without the wind screen retrofit technique. Figure 8 shows
selected loading time histories and Figs. 9 and 10 show the comparison of peak wind induced
loads for all the 6 legs (shown in Fig. 5) with and without screen for the two testing locations.
.
Figure 8. (a) Axial Compressive Loading Time History, (b) Axial Tensile Loading Time History, (c) Shear Loading Time History (Cont.)
Middle of RoofLeg 1
Axial Force Time History
-450
-400
-350
-300
-250
-200
-150
-100
-50
00 20 40 60 80 100 120 140 160 180 200
Time (sec)
Forc
e (lb
s)
Without WindscreenWindscreen
a
16
Figure 8. (a) Axial Compressive Loading Time History, (b) Axial Tensile Loading Time History, (c) Shear Loading Time History
Middle of RoofLeg 1
Wind-Induced Shear Force Time History
-30
-20
-10
0
10
20
30
40
50
60
0 20 40 60 80 100 120 140 160 180 200
Time (sec)
Forc
e (lb
s)
Without WindscreenWindscreen
Middle of Roof Leg 4
Axial Load Time History
-100
-50
0
50
100
150
200
250
300
350
400
0 20 40 60 80 100 120 140 160 180 200
Time (sec)
Forc
e (lb
s)
Without WindscreenWindscreen
c
b
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Figure 9. (a) Peak Axial Loading -- AC Units at Middle of the Roof, (b) Peak Shear Loading -- AC Units at Middle of the Roof
Middle of RoofPeak Axial Loads
-600
-400
-200
0
200
400
600
1 2 3 4 5 6
Leg
Forc
e (lb
s)
Without WindscreenWindscreen
Middle of RoofPeak Shear Loads
0
10
20
30
40
50
60
1 2 3 4 5 6
Leg
Forc
e (lb
s)
Without WindscreenWindscreen
18
Figure 10. (a) Peak Axial Loading -- AC Units at Edge of the Roof, (b) Peak Shear Loading -- AC Units at Edge of the Roof
Edge of RoofPeak Axial Loads
-600
-400
-200
0
200
400
600
1 2 3 4 5 6
Leg
Forc
e (lb
s)
Without WindscreenWindscreen
Edge of RoofPeak Shear Loads
0
10
20
30
40
50
60
70
80
1 2 3 4 5 6
Leg
Forc
e (lb
s)
Without WindscreenWindscreen
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A comparison between the peak loading with and without the wind screen is presented in
Table 1. Percentage reductions in peak loadings are also shown. Signification reductions have
been achieved with the wind screen which shows the effectiveness of the screen as a retrofit
method to reduce damage of rooftop equipment and secondary losses due to roof membrane
damage, water infiltration, debris generation (dislodged units) during hurricane events. Wall of
Wind full scale testing also showed that the wind screen and its connections didn’t have damage
from high wind loading up to 120 mph.
Table 1: 6. Cost-Benefit Analysis Cost and benefit analysis was performed to evaluate the economic effectiveness of the
proposed retrofit technique by installation of a wind screen to reduce the rooftop equipment
Middle of Roof
1 -414.01 -204.21 50.7% 56.06 28.88 48.5%2 233.48 82.79 64.5% 42.64 24.99 41.4%3 -540.29 -332.43 38.5% 41.25 27.50 33.3%4 370.38 112.51 69.6% 31.39 24.52 21.9%5 -546.36 -265.53 51.4% 27.02 20.17 25.4%6 323.36 73.84 77.2% 43.64 21.04 51.8%
Average: 58.6% Average: 37.0%
Edge of Roof
1 -360.09 -186.56 48.2% 39.79 30.08 24.4%2 179.66 23.10 87.1% 32.23 27.33 15.2%3 -565.78 -364.79 35.5% 74.30 44.83 39.7%4 359.59 133.10 63.0% 39.98 31.15 22.1%5 -466.12 -247.91 46.8% 34.91 28.06 19.6%6 272.48 44.49 83.7% 45.79 25.15 45.1%
Average: 60.7% Average: 27.7%
Percent Reduction
Percent Reduction
Peak Axial Load (lbs)Percent
Reduction
Peak Shear Load (lbs)Percent
ReductionWindscreen
Without Windscreen Windscreen
Peak Shear Load (lbs)
Leg Without Windscreen Windscreen Without
Windscreen
Without Windscreen Windscreen
LegPeak Axial Load (lbs)
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loading. Table 2 shows the cost of the AC units (cost is based on new AC units), structural
frame, labor and also the material and labor cost for the wind screen. The wind screen cost was
evaluated as approx. 20% of the cost of the rooftop equipments and the structural framing.
Table 2: Rooftop Equipment and FrameMaterialsUnit 1 $1,500.00Unit 2 $1,500.00Unit 3 $1,500.00Rooftop Equipment Mounting Frame $400.00
LaborInstallation of Frame and Equipment $1,500.00Misc. $700.00
TOTAL $7,100.00
WindscreenMaterialsSteel and Expanded Metal $310.00
Labor16 Hours @ $70 per hour for fabrication $1,120.00Installation
TOTAL $1,430.00 For the benefit analysis, a numerical example has been presented to show the pressure
loading on rooftop equipment to comparing effects of moderate Category 1 (sustained wind
speed taken as 90 mph), Category 3 (sustained wind speed taken as 115 mph), Category 4
(sustained wind speed taken as 140 mph) hurricanes. Tables 3, 4, 5 show the conversions of 1-
min sustained wind speeds (as considered in Saffir Simpson Hurricane Scale) to 3-sec gust wind
speeds as considered by ASCE 7-02 and ASCE 7-05. The converted 3-sec gust wind speeds are
then used to calculate rooftop equipment pressure loading as per ASCE 7-05 as shown in Tables
6, 7, 8.
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Table 3: Conversion of moderate Category 1 sustained wind speed to 3s gust wind speed
WIND SPEED CONVERSION
z0 (ft) α zg (ft)
Exposure B (Suburban Terrain) 0.49 7.0 1200
Exposure C (Open Terrain) 0.066 9.5 900
Exposure D (Coastal Areas) 0.016 11.5 700
z0 = surface roughness depending upon terrain category
α = Power Law coefficient dependant on surface roughness
zg = gradient height dependant on surface roughness
Wind Speed Conversion for Height and Terrain change
90
C
33
C
33
90.00
Wind Speed Conversion for Averaging Time change
60
60.00
3
3.00
74.46
108.94Wind Speed, New Avg. Time (mph)
Original Wind Speed (mph)
New Averaging Time (sec)
New Wind Speed (mph)
Original Exposure Type
Original Height above GL (ft.)
New Exposure Type
New Height above GL (ft.)
Original Averaging Time (sec)
New Averaging Time (sec)
New Wind Speed, Mean Hourly (mph)
Original Averaging Time (sec)
22
Table 4: Conversion of moderate Category 3 sustained wind speed to 3s gust wind speed
WIND SPEED CONVERSION
z0 (ft) α zg (ft)
Exposure B (Suburban Terrain) 0.49 7.0 1200
Exposure C (Open Terrain) 0.066 9.5 900
Exposure D (Coastal Areas) 0.016 11.5 700
z0 = surface roughness depending upon terrain category
α = Power Law coefficient dependant on surface roughness
zg = gradient height dependant on surface roughness
Wind Speed Conversion for Height and Terrain change
115
C
33
C
33
115.00
Wind Speed Conversion for Averaging Time change
60
60.00
3
3.00
95.14
139.20
New Wind Speed, Mean Hourly (mph)
Original Averaging Time (sec)
Wind Speed, New Avg. Time (mph)
Original Wind Speed (mph)
New Averaging Time (sec)
New Wind Speed (mph)
Original Exposure Type
Original Height above GL (ft.)
New Exposure Type
New Height above GL (ft.)
Original Averaging Time (sec)
New Averaging Time (sec)
23
Table 5: Conversion of moderate Category 4 sustained wind speed to 3s gust wind speed
WIND SPEED CONVERSION
z0 (ft) α zg (ft)
Exposure B (Suburban Terrain) 0.49 7.0 1200
Exposure C (Open Terrain) 0.066 9.5 900
Exposure D (Coastal Areas) 0.016 11.5 700
z0 = surface roughness depending upon terrain category
α = Power Law coefficient dependant on surface roughness
zg = gradient height dependant on surface roughness
Wind Speed Conversion for Height and Terrain change
140
C
33
C
33
140.00
Wind Speed Conversion for Averaging Time change
60
60.00
3
3.00
115.82
169.47Wind Speed, New Avg. Time (mph)
Original Wind Speed (mph)
New Averaging Time (sec)
New Wind Speed (mph)
Original Exposure Type
Original Height above GL (ft.)
New Exposure Type
New Height above GL (ft.)
Original Averaging Time (sec)
New Averaging Time (sec)
New Wind Speed, Mean Hourly (mph)
Original Averaging Time (sec)
24
Table 6: Calculation of Rooftop Equipment Load for moderate Category 1 hurricane
ROOFTOP EQUIPMENT LOADING CALCULATIONS -- ASCE 7-05 DESIGN WIND LOADS Chimneys, Tanks, Rooftop Equipment, & Similar Structures; Fig. 6-21 in ASCE 7-05 Velocity = 108.94 mph Importance = 1.00 (0.77, 1.0, 1.15) h = 2.03 D = 2.42 Kzt = 1.00
Kd = 0.90 0.9 for rectangular equipment, 0.95 for round equipment
G = 0.85 h/D = 0.84 q (psf) = (0.00256) * Kz *Kzt * Kd *V^2 * I Cf = 1.30 p (psf) = qz*G*Cf F (lbs) = qz*G*Cf*Af
EXPOSURE "B" EXPOSURE "C" Height Kz q p Height Kz q p
ft "B" psf psf ft "C" psf psf 10 0.85 23.2 25.6
30 0.70 19.2 21.2 30 0.98 26.9 29.7 50 0.81 22.2 24.5 50 1.09 29.9 33.0 75 0.91 24.9 27.5 75 1.19 32.6 36.0 100 0.99 27.0 29.9 100 1.27 34.6 38.2 125 1.05 28.8 31.8 125 1.33 36.3 40.1 150 1.11 30.3 33.5 150 1.38 37.7 41.6 175 1.16 31.7 35.0 175 1.42 38.9 43.0 200 1.20 32.9 36.4 200 1.46 40.0 44.2 250 1.28 35.1 38.8 250 1.53 42.0 46.4 300 1.35 37.0 40.9 300 1.59 43.6 48.2 350 1.41 38.7 42.7 350 1.65 45.1 49.8 400 1.47 40.2 44.4 400 1.69 46.3 51.2 450 1.52 41.5 45.9 450 1.74 47.5 52.5 500 1.57 42.8 47.3 500 1.78 48.6 53.7
25
Table 7: Calculation of Rooftop Equipment Load for moderate Category 3 hurricane
ROOFTOP EQUIPMENT LOADING CALCULATIONS -- ASCE 7-05 DESIGN WIND LOADS Chimneys, Tanks, Rooftop Equipment, & Similar Structures; Fig. 6-21 in ASCE 7-05 Velocity = 139.2 mph Importance = 1.00 (0.77, 1.0, 1.15) h = 2.03 D = 2.42 Kzt = 1.00
Kd = 0.90 0.9 for rectangular equipment, 0.95 for round equipment
G = 0.85 h/D = 0.84 q (psf) = (0.00256) * Kz *Kzt * Kd *V^2 * I Cf = 1.30 p (psf) = qz*G*Cf F (lbs) = qz*G*Cf*Af
EXPOSURE "B" EXPOSURE "C" Height Kz q p Height Kz q p
ft "B" psf psf ft "C" psf psf 10 0.85 37.9 41.9
30 0.70 31.3 34.6 30 0.98 43.9 48.5 50 0.81 36.2 40.0 50 1.09 48.8 54.0 75 0.91 40.6 44.9 75 1.19 53.2 58.8
100 0.99 44.1 48.8 100 1.27 56.5 62.4 125 1.05 47.0 52.0 125 1.33 59.2 65.4 150 1.11 49.5 54.7 150 1.38 61.5 68.0 175 1.16 51.8 57.2 175 1.42 63.6 70.2 200 1.20 53.8 59.4 200 1.46 65.4 72.2 250 1.28 57.3 63.3 250 1.53 68.5 75.7 300 1.35 60.4 66.7 300 1.59 71.2 78.7 350 1.41 63.1 69.7 350 1.65 73.6 81.3 400 1.47 65.6 72.4 400 1.69 75.7 83.6 450 1.52 67.8 74.9 450 1.74 77.6 85.7 500 1.57 69.9 77.2 500 1.78 79.3 87.6
26
Table 8: Calculation of Rooftop Equipment Load for moderate Category 4 hurricane
ROOFTOP EQUIPMENT LOADING CALCULATIONS -- ASCE 7-05 DESIGN WIND LOADS Chimneys, Tanks, Rooftop Equipment, & Similar Structures; Fig. 6-21 in ASCE 7-05 Velocity = 169.47 mph Importance = 1.00 (0.77, 1.0, 1.15) h = 2.03 D = 2.42 Kzt = 1.00
Kd = 0.90 0.9 for rectangular equipment, 0.95 for round equipment
G = 0.85 h/D = 0.84 q (psf) = (0.00256) * Kz *Kzt * Kd *V^2 * I Cf = 1.30 p (psf) = qz*G*Cf F (lbs) = qz*G*Cf*Af
EXPOSURE "B" EXPOSURE "C" Height Kz q p Height Kz q p
ft "B" psf psf ft "C" psf psf 10 0.85 56.2 62.1
30 0.70 46.4 51.2 30 0.98 65.0 71.8 50 0.81 53.6 59.3 50 1.09 72.4 80.0 75 0.91 60.2 66.6 75 1.19 78.8 87.1
100 0.99 65.4 72.3 100 1.27 83.7 92.5 125 1.05 69.7 77.0 125 1.33 87.8 97.0 150 1.11 73.4 81.1 150 1.38 91.2 100.8 175 1.16 76.7 84.8 175 1.42 94.2 104.1 200 1.20 79.7 88.1 200 1.46 96.9 107.1 250 1.28 85.0 93.9 250 1.53 101.6 112.2 300 1.35 89.5 98.9 300 1.59 105.5 116.6 350 1.41 93.5 103.4 350 1.65 109.0 120.5 400 1.47 97.2 107.4 400 1.69 112.1 123.9 450 1.52 100.5 111.1 450 1.74 114.9 127.0 500 1.57 103.6 114.4 500 1.78 117.5 129.9
27
Based on the above results, the difference in wind loadings on rooftop equipment (on a
single-story building located in Exposure C open terrain) for a moderate Category 4 hurricane
and a moderate Category 1 hurricane is approximately 58%. The difference in wind loadings on
rooftop equipment for a moderate Category 3 hurricane and a moderate Category 1 hurricane is
approximately 38%. From the experimental results as presented in Table 1, the wind screen has a
potential of reducing the effect of wind loading by two to three levels of hurricanes (i.e., Cat 3 to
Cat 1 or Cat 4 to Cat 1 on Saffir Simpson Scale) and thus can prove extremely helpful to reduce
rooftop equipment damage. The wind screen retrofit technique may cost approx. 20% of the
actual equipment, structural framing, and installation. In addition to preventing damage of
rooftop equipments such retrofit technique will have other advantages such as (i) prevention of
roof damage, (ii) elimination of water infiltration thus preventing losses to building contents,
mold growth, dry wall saturation, (iii) prevention of windborne debris that may result from
detached rooftop equipments.
7. Future Work
The future work on damage mitigation of rooftop equipment will include: (1) study of
uplift pressure on rooftop equipment by using Setra 265 pressure transducers installed on the
roof and the rooftop equipment to measure the pressure distribution, (2) use of different types of
wind screens (parametric studies: porosity, size, distance from AC units), (3) use of Elastomeric
damping devices to mitigate wind load effects and performing a cost-benefit analysis, (4) testing
under wind-driven rain injected into the flow field to reveal whether or not the presence of water
causes an increase in the loadings, (5) comparison of mean wind loads on rooftop equipment
obtained from Wall of Wind testing and design wind loads calculated using ASCE 7-05.
28
References 1. American Society of Civil Engineers, ASCE 7-02 (2002), Minimum Design Loads for
Buildings and Other Structures. 2. American Society of Civil Engineers, ASCE 7-05 (2005), Minimum Design Loads for
Buildings and Other Structures. 3. Barista, D. (2007), “9 Tips on Anchoring Rooftop Equipment for High-Wind Events.”
Building Design and Construction University. < http://www.bdcnetwork.com/university/article/CA6410045.html> (5 June 2007).
4. Federal Emergency Management Agency (2005), “Summary Report on Building
Performance: 2004 Hurricane Season,” FEMA 490. 5. Federal Emergency Management Agency (2006), “Mitigation Assessment Team Report:
Hurricane Katrina in the Gulf Coast,” FEMA 549, Chapter 5 and Appendix E. 6. Hosoya, N., Cermak, J. E., Steele C. (2001), “A Wind Tunnel Study of a Cubic Rooftop AC
Unit on a Low Building,” Americas Conference on Wind Engineering, American Association for Wind Engineering.
7. National Institute of Standards and Technology (2006), “Performance of Physical Structures
in Hurricane Katrina and Hurricane Rita: A Reconnaissance Report,” NIST Technical Note 1476.
8. Reinhold, T. A. (2006), “Wind Loads and Anchorage Requirements for Rooftop Equipment,”
ASHRAE Journal, Vol.48, No. 3, p. 36-43.