Construction Methods

Winds of Change: Storm Shelter Design Guidance

• By Matthew H. Johnson, Andrea La Greca, Connor J. Bruns
• 09/01/19

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.

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.

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.

ICC 500 Section 304.9 requires that where an element of the host building is connected to a storm shelter, the storm shelter shall be designed to resist the “ultimate failure strength” of the element being connected, in the event of the host building pulling away from the shelter. Where there is no expansion joint around the shelter, this provision results in substantial out-of-plane forces on perimeter walls concurrent with the tornado wind loads. Preferably, storm shelters should be isolated structures intended to stand alone after a tornado and resulting failure of adjacent construction.

The storm shelter roof structure is also subject to substantial wind uplift pressures. This may require increasing roof framing sizes/strength, additional bracing for roof framing elements not commonly provided in conventional building design and/or adding a weight (e.g. a concrete slab) over the roof framing to overcome wind uplift forces.

It is important that the design team clearly convey a basis of design and the non-standard loads and connections requirements for deferred design items such as tilt-up precast wall and roof panels, steel bar joists and similar items.

ICC 500 Section 305.3 requires the structural design consider other debris hazards such as wind-borne debris from adjacent, taller structures that could fall on the structure, laydown of towers or large trees or rollover of vehicles or temporary classrooms.

Non-Structural Criteria

ICC 500 Chapter 5 and Chapter 6 provide criteria for non-structural components of storm shelters including occupancy, egress, access and accessibility and fire safety. Where storm shelters are appendages to a host building, the area designated as the storm shelter is required to be large enough to accommodate all occupants of the host building and storm shelter, per 2015 IBC, and be accessible from every part of the host building. Natural or mechanical ventilation and toilet and hand wash stations must be included and appropriately sized. In most instances, these systems are increased relative to the typical use of the space. Refer to the 2018 IBC, if necessary, for revised requirements for storm shelter occupant capacity and accessibility.

Shelter essential features and critical support systems required by the ICC 500 Chapter 7, such as ventilation, emergency lighting and plumbing, must remain functional for a minimum of two hours following a tornado event. To ensure critical support systems meet this requirement, all systems not within the storm shelter envelope are required to be housed within independent structures that meet ICC 500 criteria for tornado wind loads and missile impact testing. For example, air handling units that service the storm shelter should not be located on the roof of the host building since the host building is assumed to have collapsed during a tornado event. Since tornados are often associated with heavy rain events, it is advisable to consider water mitigation strategies, particularly at below grade levels where this equipment is often maintained.

In accordance with ICC 500 Table 305.1.1, openings in the storm shelter envelope for windows, doors and louvers are required to be protected by products tested to resist a 15-pound, 2-by-4 sawn lumber missile traveling at 100 mph for vertical surfaces and 67 mph for horizontal surfaces. When verifying manufacturer’s testing, consider the necessary surrounding construction for door, window, and other penetration anchorage design at the storm shelter compares to that of the tested specimen. Penetrations through the envelope for mechanical, electrical, and plumbing systems are generally considered openings, and require structural steel plates and grating to protect the opening in the event that the system component fails during the tornado. Ideally, these penetrations are minimized in the storm shelter envelope. For example, roof scuppers can be utilized in lieu of roof drains, mechanical systems are independent from the host building and housed within the storm shelter, and plumbing systems connect to central lines below grade.

It is important to understand that the ICC 500 Section 106.1.1.1 requires an independent peer review and sealed peer review report. Engaging the peer reviewer early in the design can help streamline agreement around key decisions in the storm shelter design.

Best Practices

The following list provides recommendations for consideration during the design and planning phase:

• Engage a storm shelter peer reviewer during the design phase. Plan for additional review for deferred construction submittals.
• Isolate the storm shelter from the host building with an expansion joint.
• Design the shelter as a partially enclosed structure.
• Minimize mechanical, electrical and plumbing penetrations through the storm shelter envelope.
• Locate support systems (e.g.; mechanical, electrical, and plumbing) within the storm shelter’s envelope or below grade.
• Confirm FEMA P-320 and FEMA-361 compliant building envelope components also meet ICC 500 impact testing criteria.

An updated ICC 500 is anticipated for 2020 and will be referenced in the 2021 IBC.

This article originally appeared in the School Planning & Management September 2019 issue of Spaces4Learning.