Introduction

The Atlantic hurricane season runs from June 1 to November 30, and the tornado season in the United States is March through June.  But tornadoes, including violent tornadoes and major tornado outbreaks, have been documented in the United States during every month of the year.  The United States sees more tornado and hurricane activity than any other place on earth.  In fact, the United States could see as many as 1,200 to 1,500 tornados on any given year.  To add to this, the number of tornado events in the US is generally increasing from one year to the next.  2004 was the highest number in recent history with over 1,800 tornados in the US and as recent as 2008 and 2011, we saw almost 1700 tornadoes in the US (1691 both years). This, with the fact the population is increasing substantially, causes issues we need addressed through storm shelter design, documentation, and construction.

National Centers for Environmental Information

Storm season each year creates severe weather in the form of storms ripping through our states and along our coasts leaving destruction of our buildings and infrastructure, as well as many lost lives.  The heaviest of these storm (high wind) events, is the tornado.  If you are someone that has experienced the power of a tornado, you are likely to agree the event is much more than simple wind intensification. The National Centers for Environmental Information, a division of National Oceanic and Atmospheric Administration, charts a trendline increase in frequency of tornados since 1950, with the occasional spikes like 2004 where the US saw 1,819 tornados.

This frequency increase has caused a heightened need to regulate the protection of the public during these extreme events.  The International Code Council (ICC) 500-2014 Standard and Commentary addresses this and requires special considerations from designers mandating storm shelter design and construction for emergency operation centers, fire, rescue, ambulance stations, police stations, and K-12 school education buildings with a capacity of fifty (50) or more occupants.[1]  (There are two types of educational occupancies where the construction of a storm shelter is not required – Group E daycare facilities and Group E occupancies accessory to places of religious worship).  ICC 500 applies to the design, construction, installation, and inspection of both residential and community storm shelters.  By definition, a community shelter is one that is NOT associated with a dwelling unit and / or has an occupant load of less than 16.  These shelters do not have to be open to the public and they may be designed for the host building occupants only.

The FEMA P-361 (which ICC 500 is based) publication provides guidance from the Federal Emergency Management Agency (FEMA) about the planning, design, construction, and operation of safe rooms (storm shelters). It presents important information about the design and construction of residential and community safe rooms that will protect people during extreme-wind events such as tornadoes and hurricanes.

Studies show, every year tornados events in the United States do about $400 million in damage to buildings and infrastructure and kill about 70 people on average.  Every U.S. state has experienced twisters, but Texas holds the record: an annual average of 120. Tornadoes have been reported in Great Britain, India, Argentina, and other countries, but they are most often seen in the United States.  On average, 800 tornadoes hit the United States each year. Since 2008, that average has increased causing more than 1,300 injuries each year. The most violent tornadoes have wind speeds of more than 250 mph and leave a damage path a mile wide and 50 miles long.  The need for proper design, documentation, and construction is of great importance today, and likely tomorrow of an even greater significance.

Questions:

• How many storm carrying tornados can we expect to see in the US in any given year?
• What sets community storm shelters apart from residential storm shelters?
• What document does FEMA produce for storm shelter design and what is the International Code Council’s ‘equivalent’ document?

The Need for Shelter

Tornado wind loading on a structure has unique performance requirements beyond the ordinary high winds experienced by a structure.  The tornado wind speeds are faster that of the typical hurricane or other high wind event, with higher loading pressures and an increased need to protect the occupants from flying debris.

The old scale of tornado wind speeds for F-5 tornados had wind speeds into the 300 mph (ultimate) range (F-5 – 261-318 mph), while the new scale lists an EF5 as a tornado with wind speeds above 200 mph and causing sufficient damage previously ascribed to the F5 range of wind speeds.  The EF Scale was revised from the original Fujita Scale to reflect better examinations of tornado damage surveys so as to align wind speeds more closely with associated storm damage.[2]

Tornadoes and hurricanes are among the most destructive forces of nature, and tornados more so due to the higher winds and flying debris associated with these storms. Unfortunately, these types of windstorms continue to cause injury and death to people who are unable to safely evacuate or find shelter from these events. FEMA has long supported the development of hazard-resistant codes and standards by assessing how structures respond in a disaster, namely tornados and hurricanes.[3]

Storm Shelter Signage

Shelters are intended to provide protection against both wind forces and the impact of windborne debris. The level of occupant protection provided by a space specifically designed as a shelter is intended to be much greater than the protection provided by buildings that comply with the minimum requirements of building codes. The model building codes do not provide design and construction criteria for life safety for sheltering during high wind events, nor do they provide design criteria for tornadoes.[4]

We will briefly review the ICC 500-2014 (FEMA P-361), the current code for storm shelter design for community and residential shelters, and look over the architectural considerations, the structural engineering considerations, and finally the documentation requirements for proper Strom Shelter Performance Requirements as addressed by the architect and the structural engineer.  This article will be specifically addressing Community Storm Shelter design and documentation requirements, and how the codes are used to mitigate risks from these storm events.  In closing, we will briefly look to the future, the future being this year, and the release of the new ICC 500-2020 and what we might expect to see.

Questions:

• Has the tendency of storm events in recent history increased, decreased, or remained relatively constant?
• What are the unique performance requirements required from the high wind events and what codes are used to mitigate the risks from these events?
• How do the shelters’ protection from high wind events differ from the protection provided by normal business office buildings?

Architectural Considerations – The ‘Art’ of the Storm Shelter

The foremost concern of the architectural design of community shelters is the safety of the occupants.  From that point, the elements concerning the necessary occupant floor area space, accessibility, and short-term utilization of the shelter’s features are factors in the design. This occupant safety comes in several forms.  First, the community shelter must provide access, in terms of the requirement for accessibility of the mobility challenged and the distance any potential occupant must travel to arrive in the refuge.  Second, the shelter must have the area and volume necessary to comfortably contain those being sheltered, as well as the accommodations including seating and restroom facilities necessary to be operational for the occupants. Third, addressed by the architect or other design professional, the shelter design is to have a path of travel from all building spaces being served by the shelter, to the shelter, while meeting code and ADA requirements along this path (1000 ft max).  This makes the ‘Art’ of the storm shelter start with the ‘Safety’ of the occupants while meeting the ‘design challenges’ of the project.

The ‘Art’ of the Shelter

Siting the community shelter above the floodplain is a minimum requirement. The finished floor elevation must be two feet (610 mm) above the flood elevation having one percent (1%) annual chance of being equaled or exceeded in any given year, or simply two feet above the one-hundred-year floodplain.  This requirement accounts for continued development surrounding the shelter and allows for coverage of the one-hundred-year flood event into the future.  This recognizes that as development continues, the storm level events’ flood elevation also rises due to the increase of impervious cover in the areas surrounding the shelter.

Other architectural criteria for siting Community Shelters follows with setting floor elevations above the flood event with respect to:^[5]

1. The flood elevation, including coastal wave effects, having an 0.2 percent annual chance of being exceeded in any given year; or
2. The flood elevation corresponding to the highest flood elevation if a hazard flood study has not been conducted for the area; or
3. The maximum flood elevation associated with any model hurricane category, including coastal wave effects; or
4. The minimum elevation of the lowest floor required by the AHJ for the location where the shelter is installed.

Additionally, siting of Community Shelters must deal with the proximity of hazardous materials.  When a community shelter is located within a precautionary zone that includes facilities which manufacture, use, or store hazardous materials, they are to be provided with protection from the hazardous material releases as deemed necessary by the Local Emergency Planning Committee (LEPC) and the Authority Having Jurisdiction (AHJ).^[6]

Building Occupant Load served by the shelter is handled by the shelter area allotment and is guided by the International Building Code to meet a design occupant demand.  The Occupant Load determines the area needs of the Storm Shelter serving the spaces per Table 501.1.1 below.^[7]

Table 501.1.1
Occupant Density – Community Shelters 

Because of the larger area requirements, community shelters are almost always designed for multiple purposes including community centers used in gymnasiums, cafeterias, assembly halls, or music rooms in public schools.

Critical support systems, structures, equipment, and components required to ensure the health, safety, and well-being of occupants, are also required. A tornado shelter requires critical support systems for two hours including functioning bathrooms, fire extinguishers, first-aid kits, lights on back-up or battery power, and ventilation. A loss of power will need to be mitigated with a battery backup system or a generator to maintain emergency lighting, any plumbing supply/waste systems or sump pumps, and mechanical ventilation. Natural ventilation is preferred since mechanical equipment is typically exterior and difficult to protect against high-speed winds. In education facilities, the most efficient shelters function as classrooms or gymnasiums when not in use as a shelter. But when parking garages are used for storm shelter use; the mechanical venting requirements are more easily met with the openness and natural ventilation provided.  With this, when you start providing more enclosure in the design, mechanical ventilation and lighting become of a higher need and invoke additional requirements in the design.

Parking Garage as Storm Shelter

As the shelter design becomes enclosed, i.e. in a gymnasium, or cafeteria, etc., a mechanical ventilation system must be connected to an emergency power system (installed in accordance with NFPA 110 or 111) and the ventilation rates must be provided in accordance with the applicable building and mechanical code provisions for the normal use of the space.  In tornado shelters, the emergency power must be capable of providing power for a minimum of two (2) hours.^[8]  In addition, the mechanical exhaust or intakes shall be protected from debris by the provisions of ICC 500-2014 Section 306.3 for exterior wall and roof impact protective systems. The ‘Art’ of the storm shelter starts with ‘the safety’ of the occupants and continues through comfort while meeting the ‘design challenges’ of the project.

Questions:

• What is unique about the siting of a storm shelter?
• Who is the public official/s most knowledgeable of storm shelter requirements?
• What are some primary uses of storm shelters when not being used for safe harboring during storm events?
• How long must the tornado shelter be relied on to support critical systems?
• What is the preferred mechanical system for a shelter?
• What are the standards used for designing emergency power and ventilation to the shelter?

Structural Engineering Considerations in the ‘Art’ of the Shelter

Performance Issues are the center of engineering design work.  As in all structural engineering designs, following the load path is of utmost importance in storm shelter designs so to ensure the higher loading is carried to the foundation and accounted for in all connection detailing.  Finding the site-specific loadings on a structure can be established by https://hazards.atcouncil.org/.  This internet site covers wind, snow, tornado, and seismic loading for any specific address in the United States.

When designing for elevated loading conditions to be carried to the foundation adequately, a review of detailing strength requirements has increased importance.  In ordinary loading of a structure, the connections tend to carry an ‘over capacity’ due to the normal allowable strengths of the materials and the ‘buildability’ needed of the typical connection.  But in extreme conditions, the connections may become the ‘weak link’ in the design due to the concentration of loads in these points.  This is where it becomes more important to follow the load path to make sure the extreme demands on the structure are still being met with the design of the connections and the possible adjoining of several load paths on these points.

Unlike code requirements earlier than 2010, design based on Ultimate Load has become the predominant format provided in the codes for wind loading.  As such, wind load provisions of the ICC 500-2014 are provided in an Ultimate Load format and should be equated against a factored Limit State resistance.  Allowable Stress Design can still be used but will require additional factoring performed.  The Design Wind speed shall be determined in accordance with Figure 304.2(1) of the ICC 500-2014 for tornado events in the US.

Along with Wind Loading Requirements are the Impact Loading Requirements of flying debris from these high wind speed events.  Often this will govern the design of vertical and horizontal surfaces of the shelter.  The debris impact test missile for all components of the shelter envelope of tornado shelters shall be a 15-lb sawn lumber 2×4 traveling at the below-noted speeds.[10]

Table 305.1.1
Speeds for 15-lb Sawn Lumber 2×4 Missile
For Tornado Shelters

The angle of the surfaces (doors, walls, and other shelter surfaces) 30° or more from the horizontal will be considered a vertical surface and less than 30° will be considered a horizontal surface.  Numerous structural systems have been tested and documented for impact in the Texas Tech National Wind Institute (NWI) and the Wind Engineering Research Field Laboratory (WERFL).  This allows the designer the ability to use specific structural systems in his / her design by simple selection of details meeting certain pretested load requirements to ensure the load is adequately carried to the foundation.

Storm Protective Shutters

Roof live loads are set for a minimum of 100 psf for tornado loadings and 50 psf for hurricane loadings due to the added impact of wind loads carrying debris hazards.  Following the load path through the structure, the foundation must be adequately designed and constructed to distribute the loads to the subgrade in normal operation but also under the heightened loadings of the storm event.  This begins with a site-specific soil report from a reputable Geological Engineering firm qualifying the capacity of the subgrade and continues through the proper foundation design to distribute these loads adequately to the subgrade.  The ‘Art’ of the shelter continues through the sufficient and efficient design of the structure.

Questions:

• In following the load path through the structure, where is likely the weak link in the design to account for the higher loadings of the storm event?
• What is the predominate design format used in the codes since 2010?
• Besides high winds, what is another loading requirement for storm shelters?
• Where might a designer find prequalified data for structural systems used in storm shelter designs?

Documentation Requirements – Instructions for the ‘Art’s’ Assembly

To ensure that the shelter is properly designed and constructed, the design team produces construction documents thoroughly illustrating to the contractor the completed assemblies, as well as documentation of the properly distributed loading to the subgrade for 3^rd party, and other shelter reviewers to validate.  All professions involved in the design of the storm shelter will be concerned with identifying the following information within the construction documents for Community Shelters, usually on a single page in the front of the document package [11]:

1. Type of Shelter: residential of community tornado, hurricane, or a combination of both.
2. A statement the wind design conforms to the provisions of the ICC/NSSA Standard for the Design and Construction of Storm Shelters, with the edition year specified.
3. The shelter design wind speed, mph.
4. The wind exposure category.
5. The internal pressure coefficient, GC_pi.
6. The topographic factor, K_zi.
7. The directionality factor, K_d.
8. A statement the shelter has/has not been constructed within an area susceptible to flooding in accordance with Chapter 4 of the Standard.
9. The Design Flood Elevation and Base Flood Elevation for the site.
10. Documentation showing the components of the shelter envelope will meet the pressure and the missile impact test requirements identified in Chapter 3 and Chapter 8 of the Standard.
11. A floor plan drawing or image indicating location of the storm shelter on a site or within a building or facility; including a drawing or image indicating the entire facility.
12. A storm shelter section or elevation indicating the height of the storm shelter relative to the finished grade, finished floor, and the host building.
13. The lowest shelter floor elevation and the corresponding datum.
14. The Occupant Load of the storm shelter.
15. The usable area of the storm shelter.
16. Venting area (sq.in.) provided and locations in the shelter.
17. Calculations for the number of sanitation facilities.
18. Minimum foundation capacity requirements.
19. Shelter installation requirements, including anchor location and minimum required capacity for each anchor.
20. For hurricane shelters, the rainfall rate of the roof primary drainage system.
21. For hurricane shelters, the rainfall rate of the roof secondary (overflow) drainage system.
22. For hurricane shelters, the rainwater drainage design rainfall rate for facilities subject to rainwater impoundment.

This list addresses multiple professionals, and the architect should be prepared to compile this list together on a single reference sheet for review by LEPC, AHJ, 3^rd party reviewers, and others.  Providing this sheet in the front of the document set will ensure easier reference.  The architect, and other professionals that have the overall design responsibility, will be required to ensure the documents adequately show the load path is thoroughly represented in the detailing of the Storm Shelter assembly from roof, to walls, to floor diagrams, into the Main Wind Force Resisting System (MWFRS), through MWFRS connections, and into the foundation.

Typically falling to the structural engineer is the requirement of a Quality Assurance Plan for each MWFRS and each wind resisting component.  This Quality Assurance Plan list will include[12]:

1. The MWFRS and wind resisting components.
2. The special inspections and testing to be required.
3. The type and frequency of testing required.
4. The type and frequency of special inspections required.
5. The structural observations to be performed.
6. The required distribution, type and frequency of reports of test, inspections and structural observations.

The ‘Art’ of the shelter carries into the preparation and creation of a comprehensive set of contract documents by the project design professionals for the contractor to execute from.

Questions:

• What professions are involved in storm shelter design?
• What is the primary focus of the structural system loading for the storm shelter?
• Where is the best place to locate the design parameters used in the storm shelter design?
• Who is the professional usually responsible for the Quality Assurance Plan for the MWFRS?

Conclusion

We have been reviewing the current ICC 500-2014 but with a new year comes new information.  2020 is set to introduce the ICC 500-2020 with updated guidance to storm shelter design and documentation requirements.  The goal of publishing this new code edition is to make the ICC 500-2020 available for a 30-day public review in November so to have the 2020 edition published by December 2020.  This would allow it to be included in the 2021 edition of the International Building Code.  But currently, the criteria established is under review by the ICC Consensus Committee.  Some of the possible changes / updates include the testing of storm shelter components – flying debris test impact locations on the components, speed for the flying debris, minimum number of impacts under the test, etc. all while remaining attached during the testing.

Simultaneously with the increased in tornado frequency, is the growth in the population.  With this increase in population density, is the growing need to protect this population more evident than ever.  See the chart below from ‘Our World Data’ internet site showing the relative populations of the six most populated countries in the world.  We can see the similarity of this graph with the graph of the tornado frequency in the US and question the correlation between the two.

Country Population Growth, 1800 to 2019[13]

Is there a correlation between storm frequency and population growth?  In fact, is there a correlation between any of earth’s catastrophic events and population growth?  Is the correlation solely due to population growth and the fact the human population is covering more of the earth’s surface?  Is there a correlation, or is there just an increased need to protect the occupants?  How do we mitigate this higher frequency risk of elevated storm events to the public?

The frequency and intensity of storms are increasing, as well as the population. As the population increases so does the likelihood that this elevated weather event will affect greater numbers of occupants and structures, both old, young, new, and used.   The guidelines and codes will require us to take a harder look at occupant safety, structural performance, and the communication of these factors from the design team to the contractors.

Addressing these selected issues of concern in the architectural and engineering (structural) design of storm shelters does not fully encapsulate all the possible issues that are being addressed in Storm Shelter design.  There are numerous additional architectural and engineering requirements of which the designer should be fully aware and further reference should be made to the ICC 500-2014 (ICC 500-2020) for a more comprehensive understanding of these additional requirements.

Comparison of Frequency of F3 or Larger Tornadoes

The briefly reviewed architectural and engineering concerns stated here simply begin with the Performance Issues in the design, and documentation requirements of Community Storm Shelters.  Heightened performance requirements of shelters are of great importance due to the vulnerability of the occupants and the importance for the continuation of facilities during these elevated events.  We hope you better understand some of these tools provided for Storm Shelter design, and the documentation requirements to be prepared to help control these risks.

The ‘Art’ of the shelter begins with, and addresses the Risk Management inherent in the design of the architecture and the engineering, through the proper documentation and final construction – when the ‘Art’ and ‘Safety’ collide.  As Walter Wriston is noted as saying, “All of life is the management of risk, not its elimination.”

In design and in life, ‘Always be curious!’ and seek to mitigate your risks in your storm shelter design and documentation!

Sources:

• [1]ICC 500-2014 Standard & Commentary
• [2] https://en.m.wikipedia.org/wiki/Enhanced_Fujita_scale
• [3] Safe Rooms for Tornados and Hurricanes, FEMA P-361, 3^rd Edition, March 2015
• [4] Storm Shelter: Selective Design Criteria, FEMA DR 1679
• [5] ICC 500-2014 Section 401
• [6] ICC 500-2014 Section 402
• [7] ICC 500-2014 Section 501.1.1
• [8] ICC 500-2014 Section 702.1.2
• [9] ICC 500-2014 Figure 304.2(1)
• [10] ICC 500-2014 Table 305.1.1
• [11] ICC 500-2014 107.2.1 Design Information
• [12] ICC 500-2014 107.3.2 Quality Assurance Plan Preparation
• [13] https://ourworldindata.org/world-population-growt