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Winds of Change: Storm Shelter Design Guidance
PHOTO © SOLARSEVEN
The Natinoal Oceanic and Atmospheric Administration (NOAA)
reports an average of 1,000 tornadoes
per year in the United States, mostly in the
Great Plains states. However, prior to 2000,
there was limited guidance for design and
construction to withstand the high winds
associated with tornadoes. The International
Code Council (ICC) and the National Storm
Shelter Association (NSSA) first published
ICC/NSSA 500 — Standard for the Design
and Construction of Storm Shelters in 2008,
with the latest edition published in 2014.
Prior to the ICC/NSSA 500, FEMA produced
two guideline documents, FEMA P-320 —
Taking Shelter from the Storm: Building a
Safe Room for Your Home or Small Business
and FEMA P-361 — Safe Rooms for Tornadoes and Hurricanes: Guidance for the
Community and Resident Safe Rooms, but
no standard had been established.
At first a voluntary or local requirement,
ICC 500 is now referenced in the 2015
International Building Code (IBC), which
mandates storm shelter construction in
K–12 schools where the shelter design wind
speed for tornadoes is 250 mph (Figure
1). There are 23 states that totally or
partially fall within the 250 mph tornado
wind speed zone and have state-wide,
or in some instances regionally within
the states, adopted the 2015 IBC. Other
design and construction that falls under
the jurisdiction of the 2015 IBC is located
in geographic regions where wind speeds
are less than 250 mph and independently
elects to incorporate storm shelters into the
building’s program must build the storm
shelter in accordance with ICC 500.
All rights reserved. www.ICCSAFE.org.
Figure 1: Shelter Design Wind Speed for Tornadoes (Source: Figure 423.3 — IBC 2015) Excerpted
from the 2015 International Building Code Commentary; Copyright 2015; Washington, DC: International
Code Council. Reproduced with permission.
ICC 500 is a comprehensive document
that addresses structural, architectural,
mechanical, egress, accessibility and siting
aspects of storm shelter design. Understanding
these requirements can influence design,
operational and construction cost expectations
associated with the storm shelter
design. ICC 500 applies to residential and
community storm shelters for hurricanes
and tornadoes; however, this article focuses
on community tornado storm shelters within
the 250 mph design wind speed region.
Storm Shelter Location
Storm shelters may be either separate,
detached buildings or rooms and areas
within the building. The selection process
should consider travel distance and accessibility,
occupant type and need for multiple
shelter locations. Desirable locations include
gymnasiums, multi-purpose rooms, music
rooms or similar spaces perceived as gathering
areas for large occupancies. These spaces
typically have tall walls and long-span roofs,
which will require a robust structure to resist
the tornado wind loads discussed in the
subsequent section. Alternative locations for
a more economic structure include multiple
classrooms and/or locker rooms.
PHOTO © JIM LAMBERT
Structural Criteria
The most drastic variations from conventional
school structural design are the
design wind pressures and missile impact
test requirements for the storm shelter
envelope components, which include the
foundations, walls, roof, doors, windows,
louvers and similar penetrations. For
storm shelters, design wind pressures are
primarily a function of the design wind
speed, and the extent and size of openings
in the building envelope. For most of the
Great Plains, the storm shelter design wind
speed is 250 mph, compared to 120 mph for
a typical school design.
A common misinterpretation of ICC 500
is to consider the storm shelter “enclosed”
when developing the wind pressures in
accordance with ASCE 7, Minimum Design
Loads for Buildings and Other Structures.
ICC 500 Section 304.6 requires the largest
door or window on a wall to be considered
an opening, which usually classifies the
building as “partially enclosed.” A partially
enclosed structure results in elevated
internal wind pressures which magnify
the storm shelter design wind pressures by
about 3x from wind pressures calculated
for an enclosed structure. Overall, the
elevated design wind speed and internal
wind pressure result in storm shelter
design wind pressures that are 5x to 6x that
of a typical school building design wind
pressure.
This article originally appeared in the School Planning & Management September 2019 issue of Spaces4Learning.
