Performance of Embedded Gravel Roof
Systems in Extreme Wind Loading
Photo Courtesy of Gene Whiting
Forrest Masters, PhD and Kurt Gurley, PhD
UNIVERSITY OF FLORIDA ∙ CIVIL AND COASTAL ENGINEERING ∙ www.ce.ufl.edu
Motivation for Study
Pursuant to Section 2 of SB 2836,
Section 2. (1) Before eliminating gravel or stone roofing
systems in the Florida Building Code, the Florida Building
Commission shall determine and document:
(a) Whether there is a scientific basis or reason for
eliminating this option
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Motivation for StudyUPDATED
• Other (not UF) researchers are investigating whether
– There is an available alternative that is equivalent in cost and
durability (FRSA to speak on this issue);
– Eliminating this option will unnecessarily restrict or eliminate
business or consumer choice in roofing systems (FRSA to speak
on this issue); and
– Eliminating this option will negatively affect the nesting habitat of
any species of nesting bird (Fish and Wildlife Conservation Commission)
• Full text of the bill may be found at:
www.myfloridahouse.gov/Sections/Bills/billsdetail.aspx?BillId=36579
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Approach
• A defensible rationale for eliminating gravel roofing in
Florida must include reasonable evidence that
– Gravel blow-off occurs at or below design-level event wind
speeds
– The wind carries gravel over a distance and with sufficient
velocity to damage the buildings downwind
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Topical Overview
1.
2.
3.
4.
5.
Post Hurricane Observations
Gravel Scour and Blow-Off
Gravel Transport
Gravel Impact on Downwind Structures
Conclusion (so far)
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Post Hurricane Observations: Sources
• Government sources
– Federal Emergency Management Agency (FEMA)
– National Institute of Standards and Technology
• Industry sources
– Florida Roofing, Sheet Metal and Air Conditioning Contractors
Association (FRSA)
– Roofing Industry Committee on Weather Issues (RICOWI)
• Academic Journals (e.g., ASCE Structural Engineering,
Wind Engineering and Industrial Aerodynamics)
• The authors requested information from stakeholders at the
August 27, 2007 Hurricane Research Advisory Committee
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Evidence for Elimination
• Numerous studies of post-hurricane damage specifically
cite roof gravel as a significant source of damaging debris
• Both low-rise and high-rise gravel roof systems with and
without parapets have been documented as a primary
source of window breakage and subsequent water
penetration and roof system loss from internal
pressurization
• Minor (1994) presents a synopsis of such observations
over many years, including Hurricanes Celia (1970),
Frederick (1979), Allen (1980), Alicia (1983), and Andrew
(1992).
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Evidence for Elimination
• RICOWI field investigation teams (Croft et al. 2006)
documented gravel scour and blow-off in Hurricanes
Charley and Ivan (2004)
• The SouthTrust Bank Building in Pensacola “lost its gravel
surface BUR… Gravel from this roof was blown downwind
onto the Judicial Center. This gravel almost certainly
caused the window damage noted there.”
• Damage to cars was also attributed to windborne gravel
generated from roof of the Baptist Hospital Medical Facility
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Evidence for Elimination
• FEMA 488 attributed breakage of tempered glass windows
to BUR gravel generated by 100-115 mph winds during
Hurricane Charley
• At a hospital in Arcadia, several windows at the intensive
care area were broken, in part, by aggregate from the
hospital’s roofs. Three of the eight intensive care rooms
were taken out of service, and windows were broken in
other patient rooms
• At a school in Port Charlotte, aggregate flew over an 11”
parapet and traveled a considerable distance downwind
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Evidence for Elimination
• FEMA 489 noted gravel blow-off during Hurricane Ivan
• FEMA 490 refers to roofing aggregate as a major cause of
window breakage, including essential facilities (e.g., during
Hurricane Frances windborne gravel broke several of the
patient room windows at Indian River Memorial Hospital)
• The report recommends the removal of aggregate systems
from essential facilities and the development of technically
based criteria for aggregate surfacing in other applications.
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Evidence for Elimination
• Notre Dame researchers Kareem and Bashor (2006)
studied glass and cladding failures in New Orleans after
Hurricane Katrina
• Found roof gravel at the site of many broken windows, and
documented gravel blow-off from inspected roofs (Kareem
2006, personal communication July 29, 2007).
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Evidence against Elimination
• Whiting Construction has observed that other than minor
scouring on the roof corners, gravel roofs perform
adequately during extreme winds. Over several decades,
thousands of these roofs were installed in SE Florida,
including areas affected by Hurricanes Andrew (1992),
Frances (2004), Jeanne (2004) and Wilma (2005)
• RICOWI’s wind investigation report on Hurricane Katrina
(2007) concluded that “aggregate and stone blowoff was
not identified as a major problem in this investigation.”
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Evidence against EliminationNEW
• Increased parapet height reduces the maximum suction
near the upstream corner of the rooftop and broadens the
low-pressure region
• This phenomenon was attributed to the vortex cores
becoming broader and lying farther above the roof
• Baskaran et al. (2007) and RICOWI conducted post-storm
damage assessments after Hurricane Charley (2004) and
found that a 3 ft parapet height is of sufficient to prevent
stone blow-off
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Evidence against EliminationNEW
• “If gravel is to blame for so much damage why are there
gravel covered roofs on Jackson Memorial Hospital in
downtown Miami? How did the gravel stay on in Hurricane
Andrew & Wilma?” [personal communication with Gene Whiting, 1/21/2008]
• Typically post-storm damage assessments, journal articles,
etc. focus on underperforming systems not success stories
• Compelling evidence may not have surfaced?...
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Post Hurricane Observations: Summary
• Independent observations from different investigating
groups over many storms go back to at least 1970
• Gravel blow-off is not just anecdotal or a rare event
unworthy of careful consideration
• However, these reports have not produced statistically
quantifiable results concerning the performance of gravel
roof systems
• It is unclear if gravel blow-off is a problem that affects one
in ten roofs or one in a thousand
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Gravel Scour and Blow-OffUPDATED
• Two major comprehensive wind tunnel studies:
– National Research Council of Canada (1970s). Dow Chemical of
Canada Limited sponsored a series of tests at to investigate roof
gravel scour and windborne debris generation
– Colorado State (late 1990s). Developed a theoretical model for
the UN Internationale Decade for Natural Disaster Reduction
Programme
• Note that no full-scale experiments investigations of gravel
performance have been made. However, during FIU
preliminary investigation of vortex suppression technologies
gravel blow-off was observed
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Gravel Scour and Blow-Off: NRC
• In the first round of tests, Kind (1974a) found that the
critical wind speed at which stone motion began is
proportional to the square root of the nominal stone size
• In the second round of tests, found that the lowest wind
speed thresholds required to cause gravel scour and blowoff occurred when the winds
traveled diagonally over the
corner parapet (i.e. where the
WIND
walls on a full-scale structure
are oriented 45º from the
mean wind direction
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Gravel Scour and Blow-Off: NRC
• These tests did not consider the effects of the building
shape (only the roof itself was tested)
• Kind (1974b) followed with a second series of experiments
using three 1:10 scale warehouse/factory building models
with four interchangeable parapets of varying height
• Tests were performed in the National Aeronautical
Establishment 30 ft x 30 ft wind tunnel, calibrated to
produce a open exposure terrain conditions
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Gravel Scour and Blow-Off: NRCNEW
• Kind (1974a)
– Pea Gravel
– ¾ in Natural Gravel
– ¾ in Crushed Limestone
• Kind (1974b) provided sieve results for natural gravel used
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Gravel Scour and Blow-Off: NRC
• During testing, wind speeds were gradually increased, and
research personnel recorded four critical gust speeds:
Threshold
Vc1
Vc2
Vc3
Vc4
Gravel Behavior
first stone motion observed
scouring occurs more or less indefinitely
gravel propelled over windward parapet
gravel propelled over leeward parapet
OUR FOCUS
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Gravel Scour and Blow-Off: NRC
•
•
Results of these experiments were condensed into a
rational procedure to estimate critical design gust speeds
Reproduced on the next slide for a nom. 0.63” gravel size
–
–
•
Sieving of aggregate samples provided by FRSA indicated that
> ½” is a reasonable assumption for a “large” gravel size
The most comprehensive experimental investigation of
minimum breakage velocities for windborne gravel impacts on
glazing was conducted using a “representative” projectile mass
of 5.55 grams (Harris 1978)
Choosing a smaller nominal size would have resulted in a
lower wind speed threshold
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Gravel Scour and Blow-Off: NRC
•
•
A range of building (0-60 ft) and parapet heights (6”-3 ft)
are tabulated below
These are rooftop speeds
Tall parapets: greatest uncertainty in model
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Gravel Scour and Blow-Off: NRC
•
•
Next, the rooftop wind speeds were converted (increased)
to equivalent ASCE 7 Basic Wind Speeds
Assumed Exposure B conditions
Windward
Leeward
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Gravel Scour and Blow-Off: NRCNEW
•
•
•
•
In 1990, NCFI Polyurethanes reviewed the Kind and
Wardlaw documents in depth and performed design
calculations based on their methodology
Found that the methodology did not produce results
that agreed field experience, which included hundreds
of on-site inspections
Asserts that it is not appropriate for loose-laid gravel on
spray foam roofing systems
Personal communication: Roger Morrison (3/11/08)
UNIVERSITY OF FLORIDA ∙ CIVIL AND COASTAL ENGINEERING ∙ www.ce.ufl.edu
Gravel Scour and Blow-Off: CSU
• Wills et al (1998) developed and tested a theoretical model
for the flight speed threshold for compact objects
• Compares favorably with the NRC studies
Sieve Size
in
0.093
0.187
0.374
0.492
0.630
0.748
0.984
mm
2.36
4.75
9.50
12.50
16.00
19.00
25.00
Threshold of Flight
m/s
9.8
13.9
19.7
22.6
25.6
27.8
31.9
mph
22
31
44
51
57
62
71
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Gravel Transport: Holmes (2002)
• Following Wills et al. (1998), Holmes (2002) developed a
theoretical flight model for several idealized debris shapes,
including a compact projectile
• The basis of the models is that once gravel takes flight,
drag forces continues to accelerate it
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Gravel Transport: Holmes (2002)
Rooftop Wind Speed = 84 mph, Gravel Diameter = 0.63 in, Gravel Mass = 5.4 g
Velocity (mph)
60
40
20
Distance Traveled (ft)
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0.2
0.4
0.6
0.8
1
Time (s)
1.2
1.4
1.6
1.8
2
100
50
0
0
Vertical Fall (ft)
0
-20
-40
-60
0
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Gravel Transport: Holmes (2002)
• While it is not possible for the gravel to achieve the full wind
speed, it can reach approximately 50% of that value in less
than two seconds, traveling horizontally almost 100 ft and
falling about 60 ft
• Only horizontal drag forces were considered. Holmes
(2004) subsequently evolved this model to account for the
effects of vertical air resistance, which were found to be
significant. In reality, the vertical air resistance would
extend the time of flight shown
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Gravel Transport: FEMA (2003)
• Applied Research Associates numerically modeled the
expected number of aggregate impacts on a building
downwind of a gravel roof for a range of common low-rise
residential and commercial structures located in a suburban
exposure
• Developed for FEMA’s risk assessment software, HAZUSMH, and has been approved by the Florida Commission on
Hurricane Loss Projection Methodology (FEMA 2003)
• Developed independently of the Holmes (2002) model
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Gravel Transport: FEMA (2003)
• Model provides enveloped results based on
– Four roof area / height combinations
– Gravel diameters linearly distributed from 0–0.87 in, which
approximately bounds the gradation requirements found in ASTM
D 1863-03
– Gravel depth set to 1.6 in, approximately three times the standard
depth for built-up roofs installed in Florida (Johns Manville 2004a,
2004b). This choice affects the supply of windborne debris but
not the propensity for gravel to take flight in extreme winds.
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Gravel Transport: FEMA (2003)
110
150 mph
100
140 mph
2
Impacts per hour per m at 2 m
90
80
130 mph
70
60
120 mph
50
40
110 mph
30
20
100 mph
10
0
0
20
40
60
80
100
120
140
160
Center-to-Center Spacing between Buildings (m)
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Gravel Impact
• It has been shown that gravel blow-off occurs at wind
speeds less than the design level requirements for the
State of Florida and that gravel, once airborne, continues to
accelerate before reaching the ground or striking a
structure downwind
• Now we consider the effects of unprotected, non-impact
resistant fenestration to gravel impact
• Numerous studies have been conducted on
annealed/tempered (e.g., Beason 1974, Harris 1978) and
laminated (e.g., Pantelides et al. 1993, Ji et al. 1998, Saxe
et al. 2002, Dharani et al. 2004) glazing
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Gravel Impact
• Variation of the target’s surface area has been shown to
have little effect on the mean minimum breaking velocity
(Minor 1974)
• Minor et al. (1976) also found that the presence of a
uniform wind pressure on the glazing affects the character
of the breakage but does not lower the missile speed
required to break glass
• The most comprehensive set of results are found from a
series of experiments conducted by Harris (1978) and
Minor et al. (1978) at Texas Tech University
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Gravel Impact: TTU
• Under the direction of Minor, Harris (1978) conducted tests
on 257 samples of annealed and tempered glass of varying
thickness to determine the missile impact velocities
required to break glass
• A 5.55 gram steel ball, representative of an “average” large
size aggregate from a conventional tar and gravel roof was
chosen for the projectile
• Regression analysis was performed on the results to
determine missile impact velocities associated with a 5%
probability of failure.
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Gravel Impact: TTU
Glass
Thickness
(in)
3/16
1/4
5/16
3/8
1/2
3/4
Annealed
Rooftop Gust Speed
m/s
mph
10.2
9.5
8.6
10.9
11.8
17.3
23
21
19
24
26
39
Mom.
kg · m/s
0.057
0.053
0.048
0.061
0.065
0.096
Intermediate Temper
Rooftop Gust Speed
Mom.
m/s
mph
kg · m/s
-10.9
-----
-24
-----
-0.060
-----
Highly Tempered
Rooftop Gust Speed
Mom.
m/s
mph
kg · m/s
20.0
-19.6
18.9
15.2
16.6
45
-44
42
34
37
Range of Wind Speeds: 19-39 mph
Range of Wind Speeds: 37-45 mph
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0.111
-0.109
0.105
0.085
0.092
Gravel Impact: TTU + Holmes (2002)
Glass
Thickness
(in)
3/16
1/4
5/16
3/8
1/2
3/4
Annealed
Rooftop Gust Speed
m/s
mph
10.2
9.5
8.6
10.9
11.8
17.3
Intermediate Temper
Rooftop Gust Speed
Mom.
m/s
mph
kg · m/s
Mom.
kg · m/s
23
21
19
24
26
39
0.057
0.053
0.048
0.061
0.065
0.096
-10.9
-----
-24
-----
-0.060
-----
Highly Tempered
Rooftop Gust Speed
Mom.
m/s
mph
kg · m/s
20.0
-19.6
18.9
15.2
16.6
45
-44
42
34
37
0.111
-0.109
0.105
0.085
0.092
Comparing to Holmes (2002)
Rooftop Wind Speed = 84 mph, Gravel Diameter = 0.63 in, Gravel Mass = 5.4 g
Velocity (mph)
60
40
20
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
d (ft)
Time (s)
100
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1.8
2
Conclusion
• Research shows that roof gravel used in built-up roofing is
susceptible to blow-off in wind speeds lower than the
design (basic) wind speeds stipulated for the Florida
• At the onset of hurricane force winds,
– A portion of roof gravel will become airborne
– A portion of that airborne gravel will impact structures downwind
– A percentage of the airborne gravel that impacts glazing has
achieved sufficient momentum to cause damage
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