New Millennium Building Systems

Multi-use facilities like gymnasiums, community centers, and commercial spaces can double as storm shelters

Storm shelters are costly necessities that can account for 20 percent of a project’s budget. Composite joist and deck solutions can turn these structures into cost-effective multipurpose facilities.

Storm shelters are the superheroes every community needs – they’re there when you need them most, yet you hope you never have to sound that alarm.

But with great power comes great responsibility – and significant cost. A storm shelter must be able to withstand 250 mph winds and 100 mph projectiles. Its design and construction also cannot break the budget – a difficult task when a storm shelter can account for 20 percent of a project’s cost.

Engineers and architects throughout the United States have developed creative solutions to this problem. The growing trend is to design storm shelters as fully functional structures like gymnasiums, community centers, and commercial spaces.

Composite joists and deck solutions from New Millennium can help create storm shelters with year-round functionality.

New Millennium collaborated on this solution with Ponce-Fuess Engineering, LLC, for an addition to North Dallas High School, in Dallas, Texas. The new gymnasium doubles as a storm shelter, thanks to its innovative design and use of composite joists and deck. The structure is ICC 500 compliant, which means it’s been engineered to withstand wind speeds of up to 250 miles per hour, and 100 psf of live load.

Weathering the storms

The United States is experiencing its worst tornado season in 14 years. As of the end of June 2025, the U.S. was 386 tornado reports above historical average for this time of year. Texas, Illinois, Missouri, and Mississippi have been hit the hardest, accounting for 493 of the 1,296 reports through June 29.

The call for storm shelters in the United States intensified after historically destructive tornadoes touched down in Alabama in 2011 and in Moore, Oklahoma, in 2013. In 2015, the International Building Code (IBC) responded by mandating all new structures in Tornado Alley with 50 or more occupants – as well as emergency operation facilities like 911 call stations and fire, rescue, ambulance, and police stations – have standalone storm shelters in compliance with ICC 500.

And FEMA will reimburse up to 75 percent of the eligible costs of storm shelter projects.

Evolution of storm shelters

As long as there have been tornadoes, there has been a need for storm shelters. According to Alex Therien, Market Development Manager at New Millennium, there are three generations of storm shelter design:

• First, they were built as concrete pillboxes – simple structures with four walls and a lid.
• Next came structures with hollow core walls and precast lids.
• Today, composite joists and deck tied into solid precast concrete walls or concrete block walls are getting the call.

Gymnasiums like North Dallas High School are a perfect “disguise” for storm shelters since they are large spaces that can hold more than 50 people at any given time. Taking the additional steps to make them sturdier to comply with ICC 500 just makes sense.

“Given their size, gymnasiums are a natural place to take what’s already a large area and then reinforce it to make it stouter than what it was going to be,” said Joe Voigt, Marketing and Business Development Manager at New Millennium.

Role of composite joists

Composite joists play a huge role in the construction of storm shelters. These joists create a stronger, stiffer, lighter, and less expensive steel structure compared to other structural steel frame approaches. Considering all the benefits, a composite joists and deck solution from New Millennium is ideal for storm shelters.

Composite joists are designed to work together with the overlying concrete slab as a single load-bearing unit. The two components are connected using field-installed shear studs that are welded through the composite steel deck to the joist’s top chord. Once the concrete is poured and cured, the studs become embedded in the slab, creating full composite action between the joist and the slab.

Because the concrete slab contributes to the structural capacity, the joist top chord can be smaller than that of a comparable non-composite joist. This reduces the amount of steel required while still meeting design loads. Composite joists can also be spaced farther apart, further reducing steel tonnage, manufacturing effort, shipping weight, and erection time.

In the case of North Dallas High School, massive 82-inch-deep composite joists span an impressive 116 feet and are topped with a composite cellular acoustical deck.

An 8-inch-thick concrete slab covers the 2-inch composite deck. This thickness prevents uplift from tornado winds and protects against debris damage. This combination creates a room that can withstand a tornado.

New Millennium offers multiple composite deck profiles available in 1.5, 2.0, and 3.0 inches with cellular and cellular acoustical options. Cover widths range from 24 to 37.5 inches, depending on the profile selected.

The roofing system is just one component of a storm shelter that must meet ICC 500 standards. FEMA lists all the standards and codes, guidance, mandates, and regulations storm shelters must adhere to:

• Windows must be able to withstand impacts from windborne debris, specifically a 15-pound missile traveling at 100 mph at vertical surfaces (67 mph at horizontal surfaces).
• The walls must be either cast-in-place or precast concrete reinforced. They must be at least 6 inches thick tilt-up wall panels or insulated concrete reinforced form with no discontinuities. Concrete block walls with a thickness of at least 8 inches are also acceptable.

For the North Dallas High School gymnasium, these walls are fitted with L-shaped rebar so it can tie into the 8-inch-thick roof slab. The structure also has egress through an alcove that allows occupants to get outside for some natural ventilation in case the doors are blocked off by storm-related debris.

Benefits of flush-frame connections

Although not used on the North Dallas addition, Therien said composite joists paired with Flush-Frame Connections can be a better solution for storm shelters. Instead of relying on tilt-up walls, Flush-Frame Connections can be used with concrete block walls with structural steel within them – a more aesthetically appealing option.

Steel joists with flush-frame end connections offer significant weight savings compared to wide-flange beams while providing equivalent stiffness and vibration performance. This Steel Joist Institute-compliant innovative design from the engineers at New Millennium, our flush-frame connections feature a joist reaction point designed to occur at the center line of the wide-flange girder. This better-performing design eliminates girder torsion concerns during erection and/or due to final design loading of a perimeter wide-flange girder.

“Simplifying the flush-frame specification process, our published set of standardized flush-frame designs and details improve coordination between the specifier, joist manufacturer, steel fabricator, and erector,” said Therien. “Our standards for flush-frame end connections reduce design time, accelerate joist and girder fabrication, and streamline erection.”

Unheralded protectors

Storm shelters aren’t the superheroes communities want, but they are the ones communities need. When designed as multipurpose structures, they keep citizens safe, project budgets intact, and provide functionality year-round.