— Special Report —
First in a Series
Is Joplin the Next Andrew?
The Devastation Caused by Hurricane Andrew
Led to Landmark Code Changes. Will the Recent Tornadoes Lead to More?
by Tara Taffera
Eight hundred tornadoes are reported in the United States in an average
year resulting in 80 deaths and more than 1,500 injuries, according to
the National Weather Service (NWS). Yet as of June 2011, this tornado
season has proven to be anything but average. Among the deadliest U.S.
tornados, the “Dixie Outbreak” of April 27, 2011, set a modern mortality
record with more than 350 people killed, according to preliminary numbers
released by the NWS. That’s four times the average in just one day, and
includes the fatalities in Tuscaloosa, Ala. Add to that the EF5 tornado
that killed more than 150 people in Joplin, Mo., on May 22, and you have
a devastating loss of life and infrastructure.
Dr. David O. Prevatt, PE, assistant professor, department of civil and
coastal engineering for the University of Florida, is the principal investigator
of a research data collection project funded by the National Science Foundation
and the American Society of Civil Engineers to document damage to residential
buildings following the tornado that devastated Tuscaloosa and Joplin.
Prevatt points out that you never know where a tornado may strike next.
“If these storms passed five miles south of their actual tracks the damage
may have been far less than what occurred. But between the two storms
you have 11,000 homes damaged or completely destroyed,” he says.
Prevatt’s focus is to minimize that damage.
Protection against storms, including
tornadoes, requires three components:
1) Adequate wind load design;
2) Unbreakable anchors; and
3) Debris resistance.
Impact-resistant glazing systems intended for use in coastal areas
are tested according to the large and small missile requirements cited
in the building code. The most common large missile testing (below
30 feet) is done with missile D (a 9-pound wood 2-by-4 traveling at
34 mph), according to ASTM E1996, Standard Specification for the Performance
of Exterior Windows, Curtain Walls, Doors, and Impact Protective Systems
Impacted by Windborne Debris in Hurricanes. Enhanced protection for
emergency facilities may utilize missile E, a 9-pound wood 2-by-4
traveling at 55 mph.
Source: ASTM E1196
“If we can improve structural performance and reduce the amount of houses
lost from 11,000 to 5,000 then that is an enormous improvement,” he says.
“We can improve through use of better engineering details.”
Prevatt’s research focuses on the mitigation of hurricane damage to low-rise
construction and the performance of existing residential structures. He
is examining whether building practices, such as those adopted in Florida
to prevent hurricane damage, could help with protecting structures against
“In this new direction I am asking myself if this [damage] is acceptable
community-wide,” says Prevatt. “My personal answer is no but the difficulty
is trying to find what to do and what else can be done.”
Prevatt and his team are researching these two EF4 and EF5 storms to study
the construction techniques used for those buildings significantly damaged.
“For the most part I have not seen buildings that were built to current
building code—most were older structures,” he says.
Prevatt traveled to Tuscaloosa with a specific hypothesis in mind: that
certain structural retrofits and details that work with hurricane coastlines
will work in tornado alley (see box below). “To some extent there must
be some benefit we can get from impact glazing—particularly from smaller
tornadoes,” he says.
But what will encourage everyone—from homeowners to code officials to
product manufacturers—to move toward the use of impact-resistant products
in these tornado-prone regions? Or are tougher codes even the answer?
What’s the Baseline?
Before we can attempt to answer these questions, it is important to look
at existing standards.
The Federal Emergency Management Agency (FEMA) has addressed the design
and construction of safe rooms since 1997. The purpose of a safe room
is to provide a space that offers a high level of protection during an
extreme weather event such as a tornado. While these rooms may not typically
include windows, they can. FEMA 320, Taking Shelter From the Storm: Building
a Safe Room Inside Your House, is applicable to the construction of residential
safe rooms. FEMA 361 Design & Construction for Community Safe Rooms
is applicable to commercial safe rooms and includes design criteria for
community safe rooms. These standards offer information about the types
of permissible glazed openings in these spaces.
Since 2008, ICC/NSSA 500 Standard for the Design and Construction of Storm
Shelters has been available for adoption and use by any jurisdiction.
The standard applies to the design, construction, installation and inspection
of storm shelters in hurricane- or tornado-prone areas. In 2009, the International
Building Code included the ICC/NSSA 500 in the model building code.
But what if homeowners want impact-resistant glazing in their homes, and
opt against a safe room? The ICC 500 is the standard for design and construction
of storm shelters but it really doesn’t address fenestration products,
points out Ken Brenden, technical services manager for the American Architectural
Manufacturers Association (AAMA).
To fill that void, AAMA has released a voluntary specification for testing
and rating fenestration products that will be exposed to tornadoes and
similar extreme wind and rain conditions in April 2011. It worked with
the Building Enclosure Technology and Environment Council (BETEC) and
the National Institute of Building Sciences (NIBS) to gain input on the
“There are a number of test methods that exist and many were developed
for hurricane products,” says Brenden. “Tornadoes are on the same level,
if not more, than what is expected during a hurricane—but tornadoes are
much more extreme wind-wise, etc. So through expertise of our members,
we took those things under consideration in development of this document.”
AAMA 512-11, Voluntary Specifications for Tornado Hazard Mitigating Fenestration
Products, uses existing test methods and other procedures to qualify windows
and other glazed fenestration products for tornado hazard mitigation.
The newly released document provides a system for rating the ability of
windows to withstand impact, pressure cycling and water penetration, which
are generally associated with tornado conditions.
“When it came to fenestration products used in tornado structures there
was a void that existed in the architectural community,” says Brenden.
“Architects had no document to go to if they wanted a product that exceeded
The specification outlines the different levels of protection that apply
to different buildings such as, but not limited to, hospital emergency
rooms, community shelters and police/fire headquarters. These levels of
protection are specified based on requirements of the authority having
jurisdiction, and each level corresponds to different testing requirements.
The level of testing required for each of these types of facilities also
depends on the FEMA performance zone in which the building is located,
as the weather conditions and likelihood of a tornado varies depending
on the part of the country. The testing and performance requirements section
of the specification outlines the necessary test conditions for adhering
to AAMA 512. According to the specification, test specimens should be
tested for anchorage, missile impact, water testing and cycling for windows
used in hurricane-prone zones only.
|Preventing Water Damage in a Flash
When Dr. David O. Prevatt, PE, assistant professor, department of
civil and coastal engineering, University of Florida, surveyed the
damage after the Joplin, Mo., tornado he said using a water barrier
in the walls could have prevented some water damage to a building
that was damaged by a tornado.
“It appeared that had a water barrier been used it may have prevented
the premature deterioration of the wood, which may have contributed
to the failure of the building in the tornado prior to the May 22
storm,” says Prevatt. “Even relatively new structures had experienced
accelerated deterioration as a result of water damage resulting from
long-term leaks at windows and doors to the wall sheathing.”
Forrest Masters, Prevatt’s colleague at the University of Florida,
“Proper flashing is essential to reducing water intrusion,” says the
Frank Lukens, president of WinDoor Inc., adds that the focus on preventing
water infiltration began in Florida and should be a part of manufacturers’
“All exterior walls should be flashed—not just the windows and doors,”
says Lukens. “Water is a horrible thing in our minds and we have to
protect against intrusion as much as we can.”
Ken Brenden, technical services manager for the American Architectural
Manufacturers Association (AAMA), clarifies that flashing is important,
but not the only solution—and warns that flashing, like windows, can
be installed improperly.
“Windows shall be flashed and installed in accordance with the manufacturer’s
written installation instructions,” says Brenden. “Water penetration
is sure to occur if continuity is not achieved between the drainage
plane of the window system assembly and the weather-resistive barrier
on the structure.
When it comes to flashing and window installation, AAMA offers the
• AAMA 100-07, FMA/AAMA Standard Practice for the Installation of
Windows with Flanges or Mounting Fins in Wood Frame Construction;
• AAMA 200-09, FMA/AAMA Standard Practice for the Installation of
Windows with Frontal Flanges for Surface Barrier Masonry Construction
for Extreme Wind/Water Conditions; and
• AAMA 2400-10, Standard Practice for Installation of Windows with
a Mounting Flange in Open Stud Frame Construction for Low Wind/Water
In addition, AAMA’s Instal-lationMasters™ program offers training
and certification to installers of fenestration products in residential
and light-commercial structures.
Windows will require a minimum AW60 rating in accordance with AAMA/WDMA/CSA
101/I.S.2/A440 to meet the requirements of AAMA 512.
“Most single-family dwellings do not entertain an AW60 product in their
homes,” says Brenden.
He clarifies, however, that the spec makes no claim for a specific occupancy.
“It could be applicable to residential or commercial,” says Brenden. “The
current North America Fenestration Standard rates AW as the highest rated
Code and Cost Concerns
While architects and others may not entertain the idea of upgrading to
an impact-resistant product currently, the industry is divided as to whether
the recent storms will serve as a catalyst for change.
“Most window companies will use the building code as their guide or at
least as the minimum requirements,” says Brenden.
Time will tell if the building codes will move toward higher standards.
“You are seeing some positive movements, some voluntary specs, [and] some
initial steps that can lead to code changes, but I
don’t know the amount of hold it will take,” says Frank Lukens, president
of WinDoor Inc.
But Prevatt says that after surveying the damage following the recent
tornadoes, the answer is simple.
“Making the building codes stronger in itself is a large step but it only
addresses vulnerability of new buildings going forward. The more difficult
challenge is to determine what should be done to reduce the risk of damage
to existing buildings,” he says.
Prevatt believes the codes will change because of a unique factor in the
Tuscaloosa and Joplin tornadoes.
“These hit densely populated areas in the middle of the cities,” says
Prevatt. “These are towns of about 100,000 or more. It literally cut right
through the center. This is what we have to be prepared for—a low-risk
event but with very severe consequences and this is a major concern.”
That “low-risk” factor is what make some say the codes won’t change—and
“One of the reasons why the codes don’t require impact-resistant windows
outside of hurricane-prone areas is because the probability that a home
will face a storm of this severity is very low compared with the probability
a home in certain hurricane-prone regions will experience a hurricane,”
says Jeff Inks, vice president, codes and regulatory affairs, for the
Window and Door Manufacturers Association.
No matter who you talk to, people are going to make comparisons between
tornadoes and hurricanes. And when it comes to hurricanes, most will mention
Hurricane Andrew, as it was the impetus for major changes in Florida building
codes. While there are similarities between these two weather events,
Inks points out there are some pretty significant differences to note.
“In areas like Miami, the likelihood that a home will face several hurricanes
is significant,” says Inks. “That is not true with tornadoes for homes
located in regions where tornadoes can occur. Tornadoes do not fall into
the same repetitive and more predictable patterns that hurricanes do.”
Part of the other requirement related to hurricane resistance is the length
of the exposure, adds Inks.
“A tornado is a much more spontaneous and shorter event than a hurricane,
often coming and going in a matter of minutes. Hurricanes on the other
hand can expose windows to hours of high wind and flying debris in a single
event,” he says.
However, Dr. Tim Reinhold, senior vice president of research and chief
engineer at the Insurance Institute for Business and Home Safety (IBHS),
points out that the low probability of an area being hit by a tornado
more than once also will play a role in future building code decisions
and that regions that want higher standards will do so on a voluntary
“Deaths most frequently occur in EF3, EF4 and EF5 Tornadoes. These intense
storms represent less than six percent of the storms that hit the United
States and EF4 and EF5 tornadoes represent less than one percent of U.S.
tornadoes. The chances that you are going to get hit by any tornado is
less than about one in 5,000 per year. Normally we design residences for
a one in 500 to one in 700 event and critical facilities at the one in
1,500 to 2,000 probability level— mandating the design of residences for
tornadoes would be a huge change,” he says.
Valerie Block, senior marketing specialist, DuPont Glass Laminating Solutions,
suggests homeowners may start to look more at impact-resistant windows
for tornado-prone regions, but admits that it will take a while for codes
“From the residential viewpoint, we probably aren’t going to see building
codes require this in the near future but you may see people opting for
better protection based on their personal concern for safety,” says Block.
“Change will come faster if building code requirements for mandatory impact
protection are adopted at the state or local code enforcement level.”
Prevatt hopes that new research being initiated now, will result in changes
to ASCE-7 by the 2016 version of the codes.
Still, some caution against making code changes based on a few select
“The adequacy of building codes is often questioned after natural hazard
events like this,” says Inks. “But when evaluating damage there are many
factors that must be considered including what code requirements the home
was built to to begin with, whether those requirements were met and whether
installations of components like windows and roofs were done properly.
That information is critical to determining whether the current building
codes are adequate.”
Lukens says that if you look at when the Florida codes started to change,
the demand from homeowners was small until the local codes mandated impact-resistant
products. Reinhold says that for the codes to mandate impact-resistant
products in tornado areas, the areas affected would have to be expansive
as tornadoes “could” occur in a variety of states.
“The probability is one in 5,000 to 10,000 if you are not in tornado alley,”
says Reinhold. “So for someone to mandate specific requirements you are
going to jack up the price of a house by a large percent, and you are
designing for an event that is way out on the probability scale. The homebuilders
would fight that tooth and nail.”
“Impact-resistant windows are part of a whole structural system designed
to protect the home against hurricanes,” says Inks. “If they are to be
installed or required for homes outside of hurricane regions, than all
of the other structural requirements for homes in hurricane prone regions
would also need to be met in order to provide the same level of protection.
“The increased cost for all of those requirements is significant and because
winds in stronger tornadoes can exceed those of hurricanes, they may be
of limited benefit,” he adds. “Imposing them on all homes constructed
in the vast areas of the United States where tornadoes can occur is not
considered necessary or practical.”
When it comes to the cost argument, Block also points to what happened
after Hurricane Andrew.
“People talked about the expense of impact-resistant products 20 years
ago in Florida,” she says. “To some extent that is true—until the building
codes required it people didn’t consider the need as seriously as they
do today. But now we have 20 years of missile impact solutions that are
an option for builders and homeowners.”
“You can’t pick and choose certain building components,” cautions Inks.
“It is so much greater cost. If a builder were to offer to build a home
in Joplin in compliance with Miami Dade and showed them the price I bet
most homeowners would opt out of that. You can’t make claims as to what
this home will protect against.”
Block believes, however, that homeowners who want added protection will
be more vocal than perhaps homeowners were in Florida.
“I think you are going to see more people raising their hands and asking,
‘What can I do?’” says Block. “That’s a little different than what we
saw 20 years ago from homeowners who did not understand their options
for impact protection.”
She adds that the industry offers many more impact-resistant door and
window solutions than were in place 20 years ago and that today’s products
deliver better performance.
“There is not a lot of product development at this point for just tornadoes
and I think that will start to change,” says Block. “It will be led by
people who have concerns for safety. We are at the beginning of this conversation,
and I believe companies will develop tornado products. They realize this
is a new opportunity for them.”
But aside from tornado-specific products, Lukens says the same formulas
manufacturers use to build doors and windows for a bomb-blast setting
can play a role in a tornado impact situation.
“We started doing research into how we test for Miami-Dade and found that
the DP ratings that are required for testing can far exceed the wind speed
for an EF5 tornado, which has wind speeds in excess of 200 mph and we
have products that can handle DPs up to 200 mph,” he says. “So if we look
at the DP requirements for windows and doors in hurricane zones, that
exceeds the wind speeds for an EF5.”
And no matter what happens with the codes, there may be an interest on
the part of some companies to build units to higher voluntary standards.
“We’ve been working on this AAMA standard for a few years as our members
determined this was a gap in the market,” says Brenden. “If manufacturers
will test to these standards it is up to them on how they market these
products. I would expect that, at least initially, you will see commercial
companies adding this to their offering. Whether it gets to residential
or not I think that is driven by code.”
While there is a divide as to whether fenestration codes should change,
and whether consumers will pay a premium for impact-resistant windows,
most agree that installing impact-resistant windows is not enough.
“There needs to be a holistic approach,” says Brenden. “It doesn’t pay
to have a window that will withstand the elements when the wall itself
can’t withstand the extreme storm. The overall safety and protection of
the occupants is key.”
“The biggest problem I see is the entire building envelope,” adds Lukens.
“There are products out there today that will exceed the impact requirements
and DP ratings but when it is attached to the building in which it is
designed it does little or no good. Those are the hard parts and the model
used for building envelope construction in hurricane areas should be the
same model used in tornado alley. The walls need to be designed to meet
the pressure requirements.”
Reinhold adds that in lesser tornadoes such as 1, 2 or 3, impact-resistant
doors and windows can provide more protection but it “shouldn’t be your
only form of protection.” “It puts a stronger surface between you and
outside world. If people are willing to spend money, and some will, then
these [doors and windows] will provide additional protection,” he says.
Lukens agrees, saying impact-resistant products can go a long way in offering
protection from flying debris in an EF1 or EF2.
“If you can minimize the damage in smaller-scale storms, then you are
getting off to a very good start. You can take minimal impacts on frame
walls and the product won’t disassemble,” he says.
“Even if the optimal home is designed, if the windows don’t hold up that
system can fail,” adds Forrest Masters, assistant professor, Department
of Civil and Coastal Engineering, University of Florida. “If the windows
break you will have a dominate breech in the building envelope. As a result
you will have extreme pressures in the building.”
He adds that it is crucial that the different trades work together.
“The fenestration industry has done a good job at innovating solutions
for storm resistance,” says Masters. “The next frontier is to look at
tornadoes, but it will take a group effort. It will take all the different
trades to work together. In an event like this, if one system doesn’t
work it doesn’t matter. The system is only as good as the weakest link.”
Prevatt discovered this first-hand when he visited the tornado-devastated
“Sadly this is not new knowledge,” he says. “This has been said by me
and other engineers in many previous past-tornado surveys. The issue of
why structural systems fail where they do is the same. This has developed
over time through decisions to build non-engineered homes in a particular
way that lack an integral vertical load path.”
Still, Masters and Prevatt say that many in the building industry believe
that buildings simply can’t survive an EF4 or EF5 tornado—no matter to
what standards they are built.
“Someone has to shake it up and see what buildings can withstand. If someone
comes up with a structural design that can withstand extreme limits, then
it is incumbent upon the industry to withstand the loads and the debris,”
Many questions about future construction methods remain. When rebuilding
occurs, will building professionals rebuild differently? Prevatt will
do his part to see that they do.
“I think the issue has to be driven home by people who know the loads
and structural behavior of houses, like myself and other engineers, working
with builders and code officials,” he says. “Previously there has been
the sense that the building codes are responsible primarily for loss of
life and life safety. As a result we have been focusing on storm shelters
as the major design element to protect from a tornado. But after witnessing
the personal tragedies unfolded when a family has lost everything they
possess, there is a compelling argument that we need to also consider
building homes somewhat more robust—particularly to protect life and reduce
property damage from smaller (less intense) tornadoes.”
He has seen some positive signs of people willing to listen.
“It is already happening,” says Prevatt. “I have had positive feedback
from structural engineers and building inspectors in Joplin. They are
receptive to some change and waiting for the opportunity to get in front
But continued education of building professionals is crucial.
“I would like to work with organizations to spread the word on how to
build better by developing research knowledge with practical applications.
Further, I would like to work with high school students from areas affected
by these tornadoes, from whom we can find the next generation of engineers
working to solve the issues and develop tornado-resistant housing. Let’s
see what we can do in terms of learning and then improving designs,” says
"One of the reasons
why the codes don’t require impact-resistant windows outside of hurricane-prone
areas is because the probability that a home will face a storm of this
severity is very low compared with the probability a home in certain hurricane-prone
regions will experience a hurricane.”
—Jeff Inks, WDMA
Until then, Prevatt, Masters and other researchers, including teams at
the University of Iowa and Texas Tech University, will continue looking
into these storms for items that can give insights into the building industry,
including door and window manufacturers, when designing products to withstand
extreme weather events.
Until now research by Prevatt and his colleagues at the University of
Florida has focused mainly on hurricanes but that is now changing. And
the hurricane research will go a long way in helping researchers as they
study tornadoes more closely.
“Given what we have learned through the past 20 to 30 years of hurricane
research we can apply some of those research results to structures in
tornado-prone regions,” says Prevatt. “The question is how far can we
go? It really depends on the community’s tolerance for risk and willingness
to pay for a safer community.”
Prevatt’s tornado research will be a major focus, and his first report
from Tuscaloosa and Joplin was due to be published in August at press
Many agree that the tornado conversation is just beginning. The industry
needs to start thinking about increased protection, says Masters.
“We are really looking five years out and ten years out and we need to
get the door and window industry to start thinking this way and coming
up with cost-effective solutions,” says Masters. “The research community
is working on developing the fundamental science and hopefully the fenestration
industry is thinking about this and there will be an intersection between
science and technology.”
For more on this investigative report about tornadoes as it relates
to the door and window industry, see upcoming issues of DWM/Shelter magazine
Tara Taffera is the editor/publisher of DWM/Shelter magazine.
© Copyright 2011 Key Communications Inc. All rights reserved.
No reproduction of any type without expressed written permission.