Better Engineering for a Stormy World

Coreslab Structures (OKLA) Inc.

To meet the needs of today’s erratic weather patterns, construction must evolve. America’s Midwestern Tornado Alley region is especially at risk. But in Oklahoma City, Coreslab Structures (OKLA) Inc. is pushing the envelope of precast concrete to provide multi-story safety solutions.

Since its founding in 1975, Coreslab Structures and its 17 sister facilities across the United States and Canada have advanced the dual fields of precast and pre-stressed concrete in bridges and highway overpasses as well as large commercial and residential structures, including multi-story storm shelter structures.

We first spoke with Coreslab (OKLA) Inc.’s team last year, when the company was putting the finishing touches on a marquee project: a state-of-the-art headquarters for Oklahoma City’s KFOR news channel. The building not only provides highly efficient broadcasting capability but is also designed to withstand winds and debris from an EF-3 scale tornado.

The news headquarters reflects Coreslab’s growing mastery of precast tornado shelters. Thanks to the advantages of precast concrete, Coreslab’s engineering team is able to custom-build structures which incorporate tornado construction into their design. This places the company’s products far ahead of traditional custom-built tornado shelters, which largely sit unused or are used for storage.

Now, the company is placing its sights on even higher goals – literally.

School safety goes up
While the majority of tornado shelters have been high-capacity single-story, Coreslab is in the process of completing its second two-story structure designed to withstand EF-5 tornados. Two school buildings, both in the company’s home state of Oklahoma, are receiving new two-story building envelopes, the first such buildings in which Coreslab has participated in the design and construction.

Designed to keep the entire building envelope safe, as opposed to a single purpose-built shelter, the new projects herald a new age in precast concrete construction.

Engineering Manager Sean Morris relates how the idea grew out of a resistance to the complexity and expense of custom-building traditional, separate tornado shelters. “It turns out to be an expensive proposition, and it usually requires the client to make a lot of concessions,” he says. He adds that these concessions generally involve less usable space and considerably higher costs. More capacious structures require correspondingly larger shelters, which exacerbate the problem.

Precast concrete, by contrast, offers both more space at affordable prices and remarkably quicker construction. Project Consultant Coleman Harrison makes the point that construction can proceed in virtually any weather, since most components are manufactured in Coreslab’s factories under cover.

“We’re taking that front-end time that the other trades can’t utilize, because they have to wait for the site conditions to be right before they can begin,” he says, speaking of Coreslab’s typical gymnasium projects. “We already have our components fabricated, and when the contractor is ready for us on site, we have the structure erected typically in two or three days.”

More practically, tornado shelters for large structures such as high schools and office complexes must have higher occupancy standards to accommodate large student and worker populations. Two-story shelters, only possible thanks to the versatility of precast concrete, can double as classrooms or offices, helping clients recoup their investments and improve building efficiency. “These have been a very helpful way to go,” Morris says, “and they’re not a budget buster.”

Structurally, the second floor provides additional engineering support rather than vulnerabilities. Harrison points out that the additional concrete slabs form a stronger structure than previous single-story shelters that have often doubled as gymnasium structures. “Now we’ve got the second floor to help brace those walls, by cutting the clear span of the wall panels in half.”

Morris agrees. “All the second floor has to do is brace the mid-height of these wall panels that were already designed to clear-span 30 feet, so it does nothing but help our precast concrete wall panels resist the loads even more easily,” he says. “The only thing required at that second floor level is a concrete slab to provide the rigidity needed to resist those tornado-force winds.”

Living with safety
As a bonus, incorporating tornado survivability standards into building envelopes, rather than building separate shelters, helps normalize emergency drills and reduce stress during storm conditions.

Harrison tells of positive feedback from clients, who say they are able to carry on normal operations during severe storm events thanks to this new design. “There is a calming effect to walking into a building that you use every day, as opposed to using a shelter only when a storm is around,” he says.

Most significantly, Coreslab Structures enables clients to add tornado protection while staying within modest budgets. While it is true this might add some additional costs to a construction budget, Morris points out that the cost of the precast scope of work is minimally affected, compared to what would be required for normal building code design.

“Besides the precast/prestressed concrete components being slightly more robustly reinforced, there might be a little bit more cost in some dowels and additional connections around the perimeter of slabs, because of the tornado-force winds, but it’s fairly negligible.”

Not only do the two school projects provide new options in storm survivability, they continue to prove the viability and practicality of precast concrete.

“Precast concrete also automatically offers a tornado-debris resistant shell as well as a fire-resistant shell, and the pre-stressed members allow the long clear spans required of buildings as large as gymnasiums or theaters that typically double as tornado shelters for schools.” Comfort of occupants is another advantage; concrete provides thermal mass benefits as it reacts slowly to changes in outside temperature, thus reducing demands on heating and cooling.

Real advantages
The Coreslab team looks forward to these projects advancing the inclusion of precast and pre-stressed concrete in new construction. In an era of shrinking budgets and timetables, the value of precast – and Coreslab’s expertise in it – reflects the company’s ability to swiftly adapt to changing markets.

So as a rapidly constructed and cost-effective medium, precast brings real advantages to any modern construction project.

Additionally, the Coreslab team hopes the marquee success of these projects will provide new possibilities for larger precast buildings. The company is set on bringing this technology to schools and universities across the Midwest.

“We’re constantly looking for ways that the tornado shelter structures that we produce out of precast can accommodate the client as much as possible,” Morris says. “It won’t just be the gym – it can be media centers, classrooms, band rooms, auditoriums, libraries and so much more.”

Data centers and casinos also present strong opportunities. Coreslab already has experience working with casinos, having built a shelter to protect all in-house equipment for a major Oklahoma City-area casino.

The company’s own hard-won expertise in precast is possible thanks to its large number and wide variety of “casting” beds, with lines throughout to allow many pieces of the same dimensions to be produced simultaneously. While every Coreslab project is custom-engineered, the company’s close involvement with each project in its planning stages ensures it has the resources and tools to do the job.

“We know what our plant is capable of doing, and we also know how to produce it economically,” Morris says. “We’re trying to eke out every nook and cranny of economy, while at the same time having a safe and code-compliant custom-engineered product as the end result.”

Contractors and architects alike attest to the team’s professionalism and attitude; on one job, the company replaced embed plates at no cost when the originals were laid out of place, by another Trade.

Coreslab’s involvement in its projects, even in the planning stages, further reflects the company’s custom-design business model. Using Revit software and building information modeling (BIM), the team can present clear models to architects, including budgeting and exact estimates of materials. Once a design is approved, the company’s plant can begin fabricating concrete components to specifications.

Outwitting the weather
The ability to produce precast members off-site is a particular advantage in Oklahoma, which, like many Midwestern states is prone to frequent temperature fluctuations and changing weather patterns.

“We’re really in an area of the country where curing conditions for concrete are not ideal,” Morris notes, but Coreslab’s factory renders the Sooner State’s high winds, low humidity and temperature swings moot. While on-site jobs must be protected or halted, Coreslab’s 150,000 square foot manufacturing facility allows for 24/7 fabrication ability.

Although tornado shelters make up a sizable percentage of projects, Coreslab is equally experienced in sustainable building. Coreslab uses a proprietary ‘New-Tie’ system that utilizes wave-shaped fiberglass rebar for the construction of precast concrete insulated sandwich panels, reducing thermal bridging concerns.

“That allows the panels to be energy-efficient,” Morris explains, “while at the same time being tornado debris impact-resistant and able to resist the wind pressures associated with a tornado, which can be ten times the amount of wind pressure that a typical wall is designed for.”

Coreslab is able to integrate this sustainable technology into its tornado-resistant panels, thereby providing environmentally-friendly protection.

With precast concrete now part of two-story buildings, the company has set its sights on leading the way into this new construction era. “I think these two-story storm structures are something we’re going to be doing a lot more of,” Harrison says.

With the company’s long and innovative history in precast, Coreslab is the natural candidate to advance the science of precast into the future.

May 5, 2021, 9:18 PM EDT