Are Concrete Masonry Units a Good Material Selection for a Storm Shelter?

Words: David Gillick
Photos: Mason Contractors Association of St. Louis

The answer to this question is: “Absolutely!”

Let’s get into it.

Controlling Documents (A Bit of History)
The first edition of Federal Emergency Management Agency (FEMA) P-361, released in 2000, set forth comprehensive design and construction criteria for tornado and hurricane shelters.

Using the first edition of FEMA P-361 as guidance, the International Code Council (ICC), in partnership with the National Storm Shelter Association (NSSA), formed a national committee that FEMA participated in to codify design and construction requirements for tornado and hurricane storm shelters. This consensus standard, ICC-500, was completed in the summer of 2008 and updated in 2014, 2020, and 2023. The ICC-500 is the code for the construction of “Storm Shelters”. It establishes minimum requirements for the design, construction, installation, and inspection of storm shelters.

The ICC-500 was first referenced in the 2009 and 2012 versions of the International Building Code (IBC). It essentially stated that if a storm shelter were to be constructed that it would be in accordance with the ICC-500. Then in the 2015 IBC, storm sheltering became mandatory for Class “E” occupancies (K-12 educational facilities) along with critical emergency operations (911 call stations, fire, rescue, ambulance, and police stations).

So, the IBC invokes the ICC-500; however, if you are working on a project that has FEMA funding involved, you will be held to the standards of the FEMA P-361.

FEMA P-361 provides guidance for “Safe Rooms”. Its stated goal is near absolute life safety. Its guidance is at times more stringent than the code minimum. If FEMA grant money is involved, these guidelines will be the controlling document.

Please note that each of these documents covers hurricanes as well; however, this article will cover only tornadoes.

Design and Construction Challenges
Unless you are a structural engineer seasoned in the design of safe rooms or storm shelters, it is very difficult to grasp the enormity of the loads applied to each of the building components.

For example, a windspeed of 250 mph can boil down to an out-of-plane load of 200 psf. That is to say that under these conditions, a wall system may experience a positive (pressure) wind load of greater than 200 psf, as well as a negative (suction) wind load of -200 psf. To put this in perspective, out-of-plane loads in a typical wall construction may be 25 to 30 psf. This 6 to 8 fold load increase can be difficult for most to wrap their heads around.

Take a look at Figure A2-1 from the P-361 document.

The graphic above shows the highest wind speed at 220 mph. Keep in mind that the ICC-500 and the P-361 specify wind speed design of a safe room or storm shelter based on location. Let’s take a look at ICC-500 Figure 304.2(1) below.

I am sitting in St. Louis, Missouri, which puts me in the bullseye of the 250 mph wind speed zone, which is also known as Zone IV.

Envelope Testing
Even though high wind speeds create extreme loading for the design of a shelter/safe room, the wind also creates other hazards, such as flying debris. You have probably heard stories of a drinking straw stuck in a tree, or an automobile on a roof, or I-beams found a mile from the impacted structure.

The building envelope of a storm shelter or safe room must pass the missile impact testing as laid out in Chapter 8 of the ICC-500. This means that walls, roof systems, doors, windows, and other openings must pass this test. Using Zone IV, walls are tested by shooting a 15-pound, sawn lumber 2 x 4 missile at 100 miles per hour at the vertical surfaces.

As we can see here in the P-361 document from tests run by Texas Tech University (TTU):

So, let’s talk Concrete Masonry Units (CMU):
I am unabashedly biased in favor of CMU. However, this does not change the facts that follow.

Tension and Compression
First, concrete products perform well under pressure. That means supporting loads, such as: the roof weight, equipment on the roof, the wall weight itself, live loads, dead loads…so carrying weight is no problem. Where concrete is sturdy and strong, it is not flexible. This is where rebar comes into play for a CMU wall system. Rebar provides for the tension needed for ductility. Designing the reinforcing schedule to place rebar on each face (EF) of the cell of the CMU allows the wall system to resist positive and negative loads as discussed earlier in this article. As a result, the tension of the rebar and compression of the CMU work together to create a beautiful wall system.

Impact Testing (“Missile Impact Testing”)
Secondly, as noted above, CMU passed the missile impact testing required for storm shelter / safe room construction.

Critical Connections
Each system is critical: footing, foundation, wall system, roof system, back-to-wall system, to the top of the foundation; footing. CMU construction allows for robust connections. Whether it is a specially designed anchor, embedded plates, or just rebar that is bent into the roof and floor diaphragm, the CMU wall system will easily accommodate your anchor provisions as a critical piece of wall stability.

Many CMU Sizes
Concrete Masonry Units come in many sizes: 4”, 6”, 8”, 10”, 12”, 14”, 16”. Why is this important? Based on the height of your wall, you may need a thicker CMU. Remember the discussion above about tension and compression? Well, the structural engineer may design a wall that requires a greater distance from the face of the wall (compression) to the rebar that is on the opposite face of the cell wall to reduce the tension as needed.

CMU is Modular
This means installing a masonry opening that is 7’ 4” tall requires no cutting of the CMU. It is nice to build your lintels over your openings right into the wall itself; no I-beams or angles required.

CMU Is Readily Available
The manufacturing of CMU is happening all over this country. The standardization of block production is through the ASTM C90 with a minimum compressive strength of 2,000 psi. In fact, if you are the structural engineer, it is easy to specify your f'm using the TMS 602 (only specify f’m, the rest will follow through the table and code).

CMU Meets or Exceeds Construction Schedules
Once the footings are installed, the CMU wall system can get started. I am fully aware of the panel system marketing. However, the panel industry faces its own challenges. For a tilt-up wall, all the mechanical systems are installed before the casting slab is poured. The stubs create a problem for the casting of the wall panel. In this area, the casting slab needs to be installed before frost gets in the ground, or you may have to wait until spring. And yes, I have heard complaints that bricklayers are onsite and in the way. It’s no surprise that people in construction complain; however, it’s not all bad. When panels are swinging, no other trades are working. With the brick crew on-site, all trades are banging away at the schedule. Taking it a step further, the way masonry is installed has changed over the years. We used to build structures and clad or veneer at the same time. Today, we have the advantage of mast climbing scaffolds that allow for the CMU construction and then drop back down to the veneer. This helps the project get under the roof much quicker. The process of storm shelter construction is essentially the same as traditional construction, with much higher attention to reinforcement.

What About the Cost?
In our office, when we help customers evaluate wall systems, we start with the price installed at the subcontractor’s level. I know for myself that I cannot follow along or give my full attention to a discussion of this type if I do not know the price associated. So, we give the installed price upfront before we discuss seismic design categories, staging, styles of brick, foundation depth, wind speeds, or any other factor needed to evaluate the best system to use for a particular wall system. And giving out square foot pricing is a terrible thing to do for all parties involved. What we find in our experience is that the upcharge for storm shelter pricing compared to standard construction usually falls between 30 to 40%. If you would like some assistance in evaluating your next CMU project, I will be happy to take your call. Even better, there is what is known as the Block Design Collective (BDC) that is just getting started and that is broken out into 5 regions across the country that can help assist in pricing, engineering, material selection, and installation. The BDC is an initiative of the CMU Check Off program. Check it out at https://concretemasonrycheckoff.org/

In conclusion, not only is CMU a good material selection for a storm shelter / safe room, I feel it is the best option for such a shelter. CMU provides the structural stability for high wind loads and safety from flying debris. Masonry walls can be sized and designed to meet nearly any size storm shelter or safe room, are readily accessible to begin construction and avoid project delays, and are very cost-effective.

Are Concrete Masonry Units a Good Material Selection for a Storm Shelter?

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