by: Stan Fuller, P.E., Principal

A Summary:  The Basics of Storm Shelter Design, Construction, and Operation

Introduction

Put on your seat belts! The new Ohio Building Code goes into effect on November 1. One of the most impactful sections of this code can be found in section 423, titled “Storm Shelters.” Occupying less than one page of text, this section invokes the ICC-500 code and now requires these bunker-like structures in most new critical facilities and K-12 buildings throughout Ohio. In trying to understand the magnitude of this change, we recently invited an expert in storm shelter design, Cory Schultz, AIA, to visit us from Wichita, Kansas to educate our clients on these new provisions. After an enlightening and sometimes sobering program, the following are some key takeaways.

Code History and Standards

Back in 2000, FEMA published the first edition of FEMA P-361 to identify provisions for designing storm-resistant structures. This effort later resulted in the creation of ICC-500. The ICC-500 code laid-out the provisions for designing shelters that provided near absolute protection from high intensity wind events. The 2009 and 2012 International Building Codes first incorporated ICC-500 by making it an elective requirement for owners who choose to build a storm shelter. This changed drastically in the 2015 IBC where storm shelters now became required in all new emergency operations centers, 911 call centers, police and fire stations, and most use group “E” occupancies (i.e. K-12 schools). Ohio has now adopted the 2015 IBC and retained this storm shelter verbiage in its entirety.

The Storm Shelter Burden

It would be short-sighted to say that the design of a storm shelter is solely a structural engineering problem. The fact is that this burden is shared among several partners. Certainly, the structural engineer must design for greatly enhanced wind and impact loads. Architects, however, need to play a critical role in choosing the proper space for the storm shelter, and specifying the proper door and window protection. Mechanical, electrical, and plumbing engineers must also be involved in storm shelter design to provide the proper ventilation and emergency backup resources. Construction entities must be held accountable for proper execution through quality assurance and inspection plans. A final burden then lies in the hands of building owners to properly maintain the shelter, craft emergency operations plans, and to diligently drill their personnel on its use.

Structural Design

Storm shelters are designed to withstand a massive amount of load when compared to typical environment forces. For instance, the typical wind loads on the walls of a building are around 25 pounds per square foot (psf). The wind loads resulting from a 250 mph tornado event, on the other hand, may generate wall loads in excess of 125 psf (a five-fold increase). In another example, a typical roof will be designed for a transitory live/snow load of about 20 to 25 psf. A storm shelter roof, however, must be designed for a minimum of 100 psf.

These high loads and impacts result in unusually heavy structures. Most storm shelter walls are comprised of 8” thick solid reinforced concrete or 10 to 12-inch thick solid-grouted CMU with vertical reinforcing bars in every cell. Storm shelter roofs are typically thick reinforced concrete slabs on composite metal deck and steel beams, solid precast plank, or reinforced topping slabs on precast concrete tees. The hallmark of a good structural storm shelter design is the interconnectedness between elements. Like the links in a chain, every structural component of the shelter must be connected to one another from the smallest roof element down to the final foundation anchorage.

Architectural Considerations

Outside of structural considerations, there are several issues that architects must address in storm shelter design. The first is where to put the area, and whether it will serve a dual role. Shelters that are completely separate from the host building are uncommon because they require occupants to travel outside the building during potentially hazardous conditions. On the other hand, storm shelters that are completely enveloped by the host building have issues as well. The shelter must be structurally separated from the host building, and all MEP penetrations through the shelter’s envelope must be kept to a minimum. Furthermore, interior spaces run the risk of being inaccessible after an event due to surrounding collapse debris. The best location for a storm shelter is typically one that is abutted to the host building. A good example in a typical school would be a series of classrooms at the end of an academic wing.

Most storm shelters serve more than one purpose, and combine their space with other programmed functions. Some caution needs to be noted here. The available floor area of a shelter is affected by the use of the space. A storage room, for instance, would not make a good shelter since it takes time to first empty its contents. Classrooms and multi-purpose rooms on the other hand strike a good balance between usable space and the ability to use the shelter for more than one function. Other good spaces include art rooms, music rooms, wrestling rooms, locker rooms, conference rooms, and break rooms.

There are several areas that are typically a poor choice to use as a storm shelter. Corridors, used alone, are poor since they have numerous door openings to protect. Mechanical rooms, science rooms, and kitchens are not great choices either since they contain dangerous elements and have many utility penetrations to protect. Stairs are difficult due to the lack of usable space, structural inefficiencies, and door swing issues. Basements tend to have handicap access issues, and difficulty with structural separation.

Protection of Openings

A storm shelter is meant to be a sealed envelope from even the smallest debris. Therefore, space selection must take into account the number of doors, windows, and mechanical openings that must be protected. The vault-like leaves of a storm shelter door are both expensive (approx. $6,000 each) and prone to breakage if cycled too often on a daily basis. All doors and shutters must be tested by the manufacturer for the impact of a 15 pound 2×4 traveling at 100 miles per hour. Please note that this is far in excess of the impact testing for doors and windows in hurricane-resistant construction. For mechanical openings, ICC-500 requires special protection for all openings exceeding about 2 inches square. This protection typically consists of 1/4” thick steel plate shrouds or baffles surrounding all openings and penetrations.

Atmospheric Pressure Change and Occupant Ventilation

Remember when you were told to crack the windows of your house during a tornado lest it “blow up?” ICC-500 allows two options to deal with this atmospheric pressure change (APC). The first is to allow for enough free ventilation to account for the change. The amount of area required, though, is extremely large. There are almost no storm shelter designers who use this free ventilation option. The second option is to account for APC in the structural loads on the shelter envelope. Although this results in higher structural loads, it seems to be the approach that most design teams utilize.

The ICC-500 identifies two different approaches to occupant ventilation. Unlike the APC decision, storm shelter designers appear to be split on the best method. In the first approach, natural free area is provided at strategic locations and heights to circulate air throughout the storm shelter space. This method has the advantage of being simple and inexpensive. Providing this ventilation, and protecting the openings, can be a bit of a challenge. Other storm shelter designers utilize mechanical, or powered, ventilation. This requires more up-front cost and thought put into protection of backup power and systems.

Permitting and Construction

It is important to review the proposed shelter design early in the process with the local authority having jurisdiction. These are new provisions within Ohio, and practitioners are still working-through best practices and other code-related issues. Also, permit applications must include a signed and sealed letter from a third-party peer reviewer. This is not the time to get your buddy from a partner firm to look over your design. This peer reviewer is attesting to the fact that this shelter conforms to the requirements of ICC-500 and will provide near absolute protection of its occupants.

A good starting point for peer review services is the National Storm Shelter Association. Their web site lists about a dozen peer reviewers, including the speaker for our event, Corey Schultz, AIA. If you are new to storm shelter design, it is advisable to get this peer reviewer on board early. Have them review your design while there is still time to modify it. There are a number of “gotchas” in storm shelter design that only a seasoned professional can help guide you through.

Storm shelters require an enhanced amount of quality control, assurance, and inspection compared to the host building. The plans for this will be prepared by the structural engineer for the project and must be adhered to throughout construction of the shelter. The old adage of “measure twice and cut once” is especially applicable to storm shelters. These structures are not designed to come apart easily. When in doubt, contact the structural engineer. Construction errors will be costly to fix. Finally, at the end of the project, the finished storm shelter must have a permanent plaque that bears the name of the lead contractor who built it. This, perhaps more than anything else, will keep contractors accountable when building the shelter.

Operating and Maintaining a Shelter

When the keys of a storm shelter are handed over to the building owner, the brunt of the responsibility falls squarely in their lap. The best designed and built shelter is utterly useless to its occupants if not used properly. Emergency operations plans must be drafted by building owners and drilled regularly to ensure that the proper personnel are buttoning-up the shelter components. Proper maintenance cannot be overlooked in storm shelter operations. Door hardware is especially prone to breakage, especially if cycled on a regular daily basis. Remember, components of a storm shelter are not intended to come apart easily. Therefore, a simple door replacement can easily turn into an expensive demolition and replacement project. Finally, owners must be sure to provide proper, regular maintenance and testing of the emergency power and ventilation systems that serve the shelter.

Another sticky issue with operating a storm shelter is whether or not the space will be available for members of the community at large. For a school, it almost goes without saying that strangers should not be allowed to share cramped quarters with a room full of students. The issue gets less clear, however, when considering shelters in police and fire stations. Should the storm shelter in these types of buildings be oversized to allow for additional community occupants? If so, how many? How do you turn away excess folks? Truly, the issue of community-use shelters brings up issues that are far beyond the scope of this article.

Conclusion

We’ve only scratched the surface of what constitutes a successful storm shelter design. There are many questions and challenges that face us as we embark on this journey together. Only through close communication and collaboration can we find the best solution and successfully implement these new provisions. We at Jezerinac Geers are up to it. Are you?