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October 2013
Be Prepared to Weather the Storm
New Scoping in the 2015 IBC for Storm Shelters Using the ICC 500 ICC/NSSA Standard
for the Design and Construction of Storm Shelters
By Kimberly Paarlberg, RA, Senior Staff Architect and Dave Bowman, Manager of Codes, ICC
The Storm Shelter standard, ICC 500, provides design and construction requirements
for “safe rooms” offering refuge from high winds and debris generated by hurricanes or
tornadoes. Two significant code changes, G94-12 and G95-12, will result in storm shelters
being required in some buildings located in areas where tornado design wind speeds
are 250 mph, per ICC 500, Figure 304.2(1) (shown below). This is basically the Midwest
area of the United States, otherwise known as “tornado alley.” This includes all of Illinois,
Indiana, Iowa, Missouri, Arkansas and Ohio, and part of 14 bordering states.
FOR LARGER VERSION OF THIS MAP, CLICK HERE.
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Be Prepared to Weather the Storm continued
In tornado alley, storm shelters will be required in:
necessarily an indication if its intensity. The Fujita scale
1.Group E with 50 or more occupants, including all
(F-Scale), is a scale for rating tornado intensity, based
grade schools and high schools, but excluding day
on the damage tornadoes inflict on human-built struc-
care facilities and Sunday schools.
tures and vegetation.
2.911 call stations, emergency operation centers,
lance stations and police stations; basically, the
other parts of the country, or in other types of facilities,
Scale
Estimated
wind
Speed
(mph)
will remain optional. However, where storm shelters are
F0
40–72
F1
buildings for emergency responders.
Providing storm shelters in these same facilities in
provided, they must comply with ICC 500. The ICC 500
addressed requirements for both hurricane and tornado
shelters. While the scoping criteria in G94-12 and G9512 states “storm shelters,” given the referenced figure,
this is requiring tornado shelters, not hurricane shelters.
This distinction will be explained later in this article. This
article will be focused on tornado shelter requirements.
What’s the Hazard?
Tornadoes can move with phenomenal speeds,
Average
Damage
Path
Width
(meters)
used for hurricanes or tornadoes. Wind is wind, so why
make the distinction?
The primary difference is the duration of the storm. Tornadoes are very short-lived storms; hurricanes can maintain
their strength in one area for many hours. The base dura-
Potential damage
tions for shelters designed in accordance with this standard
10–50
Minor damage.
are 2 hours for tornadoes and 24 hours for hurricanes.
73–112
30–150
Moderate damage.
F2
113–157
110–250
Considerable
damage.
F3
158–206
200–500
Critical damage.
F4
207–260
400–900
Severe damage.
F5
261–318
1,100 ~
Total Destruction.
F6
319+
Theoretical category.
Immeasurable
damage.
sometimes exceeding 300 mph. Tornadoes last from a
few minutes to a few hours. Most tornado deaths are
Hurricane or Tornado Shelters
The standard makes a distinction between shelters
The Fujita Scale
fire stations, emergency rescue stations, ambu-
Types of Shelters in ICC 500
Hurricane and tornado shelter requirements differ for:
• Occupant density
•Sanitation
• Water supply
•Ventilation
• Emergency power
• Size and speed of flying debris
Community or Residential Shelters
This standard makes a distinction between shelters used in the community and shelters used with a
Approximately 74 percent of the tornadoes in the
residence or small group of residences. The definition
from flying debris. The National Weather Service docu-
United States are category F0 or F1. Twenty five percent
for “Residential Shelters” limits the occupant load to a
ments about 1,000 tornado touchdowns per year. A key
are strong tornadoes of category F2 and F3. Only 1 per-
maximum of 16. Shelters used for larger residential com-
point to remember is that the size of the tornado is not
cent are the violent tornadoes of categories F4 and F5.
plexes—such as hotels, larger apartment buildings or
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October 2013
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Be Prepared to Weather the Storm continued
dormitories—would be considered community shelters.
Essential Features for Commercial Tornado
Shelters
rooms, the library, etc. These rooms are hardened to resist
on the number of anticipated occupants. If mechani-
the wind loads and flying debris at the levels required. For
cal ventilation is provided, it must be connected to an
these types of shelters, the number of exits is based upon
emergency power system.
the number of exits required for the primary occupancy
during normal use, not the occupant load of the shelter.
Occupant Density
The occupant load used for sizing the shelter is
based on the needs of the shelter. When calculating the
Egress doors must be capable of being opened from the
inside without special keys or knowledge.
Example: A classroom in a school has a maximum
Potable Water and Sanitation Facilities
The requirements for sanitation are minimal (Table
702.2). Toilets and lavatories in the shelters can be part
of the required sanitation if they can be converted for
usable floor area, reductions must be made for furni-
occupant load of 40. The number of exit access door-
emergency mode. Other sanitation facilities can be tem-
ture or equipment that will remain in the shelter when
ways required from that classroom would be one.
porary facilities or chemical toilets.
occupied. For example, if a classroom is used for a storm
Therefore, regardless of the amount of occupants using
shelter, the usable floor area is reduced by the area
the classroom as a shelter, the required number of exit
occupied by permanent counters, as well as desks in
access doorways remains at one.
the room. If an alcove or baffle entry system is used, this
area must also be subtracted (804.9.7).
The occupant density for community tornado shel-
Emergency Features
Tornado shelters must include fire extinguishers and
first-aid kits. Community tornado shelters must have
Emergency Escape
emergency lighting.
When a community shelter is permitted to have a single
Location and Accessibility
ters is 5 square feet. per person (Table 501.1.1). Larger
doorway, a second emergency escape route is required via
areas are required for shelters that will contain persons
a stairway or ladder (501.4.1). The minimum dimensions of
in wheelchairs or persons who are bedridden. At least
the emergency escape opening are the same as those for
route in accordance with ICC A117.1 (501.6). The assumption
one wheelchair space is required for every 200 occu-
emergency escape windows given in the IBC. The purpose
is that building systems are operational before the storm
pants the shelter is designed for.
is to prevent the occupants from being trapped by debris
hits. Therefore, that route can be via an elevator. Standby
that may pile up in front of a single door.
power for the elevator is not required by this standard, but
Number of Doorways
Community shelters are usually “safe rooms” designed
for other uses during day-to-day operations. For example,
a school “safe room” could be a classroom, the bath-
Community shelters are required to have an accessible
may be required for other reasons. At least one entrance
Ventilation
Ventilation can be via natural or mechanical means.
The size of the natural ventilation openings depends
to the storm shelter shall be an accessible entrance. The
entrance to the shelter must have signs. A layout of the
shelter should be provided within the shelter.
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October 2013
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When storm shelters are located within areas subject to floods, coastal wave effects
or storm surge, the floor of the shelter may be required to be raised to ensure the
occupants will not be dealing with rising water, as well as high winds and debris (401).
If storm shelters are located within precautionary zones for facilities containing
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Committee to also shelter occupants from possible exposure (402).
STRUCTURAL ISSUES
The issues the storm shelter addresses include: the wind loads that should be used;
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and the size and velocity of flying debris.
Wind Loads
Wind loads for storm shelters will be based upon wind speed contour maps developed
specially for this standard. For tornado wind forces, the best available data is the design
wind speed map contained in FEMA 361, Design and Construction Guidance for Community
Shelters. The wind speeds for tornado shelters [Figure 304.2(1)] is an adaptation of this map.
The wind pressure will be determined based upon Method 2, Analytical Procedure, of
Chapter 6 of ASCE 7, with several modifications to the factors used in the formulas (304).
Debris
The typical “missile” used for simulating possible flying objects that could breach the
shelter envelope is a 2 x 4 of nominal sawn lumber. The speed of these missiles is dependent upon the wind zone in the tornado map. For the tornado alley, with tornado wind
speeds of 250 mph, the horizontal speed of a 15 pound (7 kg) 2 x 4 is to be set at 100 mph
(44 m/s) and the vertical speed at an astounding 67 mph (29 m/s). Chapter 8 of the stan-
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>>
13-08490
October 2013
resistance to impact from flying debris.
Other Structural Issues
Chapter 3, Structural Design Criteria, addresses
weather protection; enclosure within a “host” building; connection of the shelter to foundations or slabs;
and penetration of the shelter envelope by systems
and utilities.
Evacuation to a storm
shelter should be part of
a building’s safety and
evacuation plan.
Because fires could break out during or after a
storm, the shelter must be protected by fire barriers
and horizontal assemblies with a fire-resistance-rating of at least two hours (601).
Conclusion
Schools in tornado alley should include movement to tornado shelters in their
safety plans and practice drills. Having tornado shelters in the buildings with emergency responders and services will increase the chance these facilities will remain up
and running after the tornado has passed.
Home or building owners should assess their risk and determine the best type of
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shelter for their needs. A safe room is the preferred method of wind protection in highrisk areas. Having a shelter can greatly reduce injury or death caused by the extreme
winds and storms associated with tornadoes and hurricanes. If the decision is to build a
shelter, make sure evacuation plans are made known and are practiced.
The current edition of the standard, ICC 500-2008, is undergoing a development cycle.
The work plan is for the 2014 edition to be completed in time to be referenced in the 2015
IBC. The proposed revisions will not significantly affect the technical requirements discussed
in this article. Additional information can be found on the ICC website under Standards
Development.
13-08548