ROOFTOP EQUIPMENT WIND LOAD AND ITS · PDF filerooftop equipment wind load and ... rooftop...

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 488 “Mitigation Assessment Team Report: Hurricane Charley in Florida” April 2005. Chapter 5, Pg 5-64.



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

15

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)


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)


c

b

17

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)


Middle of RoofPeak Shear Loads

0

10

20

30

40

50

60

1 2 3 4 5 6

Leg

Forc

e (lb

s)


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)


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)


19

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 Wind Speed, Mean Hourly (mph)


22



z0 (ft) α zg (ft)








115

C

33

C

33

115.00


60

60.00

3

3.00

95.14

139.20



Wind Speed, New Avg. Time (mph)






New Exposure Type




23



z0 (ft) α zg (ft)








140

C

33

C

33

140.00


60

60.00

3

3.00

115.82

169.47Wind Speed, New Avg. Time (mph)






New Exposure Type






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







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







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.

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