Tag Archives: masonry tornado shelter

Tornado Shelters-The “3 ½” Concrete Slab Issue

The 3 ½” concrete slab issue is one that I have attempted to distance myself from because I spoke against it in the initial ICC-500 code committee and got resoundingly shot down by the rest of the group. Please allow me to explain the issue as I originally saw it and now that I know more, how it stands now.

Section 309.1.2.1 of the 2008 edition of the ICC-500 states”

Slabs on grade shall be designed for applicable loads in accordance with Section 301. Where a slab on grade is being used to resist loads, the minimum thickness shall be 3 ½ inches (88.9 mm) and the minimum steel reinforcement for slabs on grade resisting forces on the storm shelter shall be 6×6-W1.4 by W1.4 or No. 4 bars, 18 inches on center in either direction.

I was opposed to this language for one reason and one reason only. I knew when people (i.e. designers, engineers, manufacturers) read this, they believe that a 3 ½” slab will work for any type of shelter as long as it has the reinforcing indicated without the need to have it engineered. Low and behold, we have small residential shelters made of steel, wood, and other composite materials that are light weight being bolted to existing concrete slabs without doing any design work to confirm that the 3 ½” slab will work as required by the first sentence of the paragraph noted above! I knew it would happen!! I knew it!

I know of four competent, practicing structural engineers that have looked at this very issue and not a one of them could engineer a 3 ½” slab to work with these light weight shelters that experience the full wind load; NOT ONE!!! Why? BECAUSE IT DOESN’T WORK!!! The wind forces attempt to overturn these light weight shelters but doesn’t have enough “lead in its ass” to resist it so it depends on the dead weight of the concrete slab to resist the force. Problem is, the 3 ½” concrete slab doesn’t have enough “lead in ITS ass” to resist the force either!

With that said, there are exception to every rule. If you have one of the heavier shelters, i.e. fully reinforced masonry, and/or concrete, there may be enough dead weight in the walls and roof of the shelter to resist the overturning force. There are stipulations to this in the ICC-500, and only engineering will determine this. Shelters that are located in basements and do not see the full wind load may also be an exception, but again, this needs to be verified!

The latest version of the ICC-500 which has yet to be released has reworded this paragraph to hopefully emphasize the need for engineering these slabs. The problem is going to be, will pre-fab shelter manufacturers pick up on this and comply? Do you think Home Depot which sells shelters on the internet is going to make sure that you have an engineered slab on which to install your newly purchased shelter? I hardly doubt it!

So when you see manufactured shelters that say they can be bolted to an existing slab without any type of engineered verification, STAY AWAY! It is a must that there is enough dead load to keep the shelter from overturning, where ever that dead load comes from. It could be footings/foundations, thick floor slab (like 2’ thick), heavy walls, and/or roof structure. But bolting to an existing slab that is 3 ½” thick, or 4” thick is not going to cut it! You probably did not purchase a tornado shelter, you purchased a bad carnival ride! And one that could kill or serious injure you and your family.

Be careful out there!

Post by Corey Schultz, AIA, LEED BD+C

Tornado Shelters – Arkansas Tornado Shelter Door Failure

On April 27, 2014, in the state of Arkansas, an EF-4 tornado ravaged the communities of Vilonia and Mayflower killing a total of 12 individuals. One of those deaths, a woman, was killed and her husband seriously injured as they sought refuge in an above ground shelter that the husband had built for the couple’s home. It was a small masonry shelter with a hollow metal door and frame. Unfortunately, the door failed, the woman was killed and her husband seriously injured while the two of them were bracing the door all the while, the rest of the shelter performed the way it was intended. A true tragedy. My heart goes out to the woman and her family. This is not the outcome the couple had planned.

The door and frame were analyzed at Texas Tech University Wind Science Lab in Lubbock, Texas. The goal was to find out why the door failed and the study found just what the problems were with the door system. This is what the study found:

The door and frame were hollow metal with (3) residential grade dead bolts and (3) standard duty hinges.
The frame was not properly anchored to the masonry walls, as a matter of fact, there was no anchorage at all, the cavity of the frame was filled with mortar and the adhesion between the mortar and the masonry was the only thing holding the door in the opening. That is why the couple was attempting to brace the door because it was shaking in the opening.
The door, frame, and hardware were not designed or constructed to withstand tornadic wind forces. The door was apparently struck by a piece of OSB or plywood in the center of the door, bending it and destroying the center dead bolt and the lock set. Further, the top dead bolt failed and the door hinges and hinge screws were also damaged. This all happened almost simultaneously while the couple was pushing against the door.

These three issues lead to this tragedy. Unfortunately, from my understanding, the gentlemen THOUGHT he was being sold a tornado resistant door but in the end that was not the case. Along with the fact that the door was not anchored properly.

As I have said before, the regulations on tornado shelters to date has been minimal. It is like the Wild West out there and the buyer MUST BEWARE!!! Regardless if you’re buying a pre-manufactured shelter/safe room, doing a DIY shelter/safe room, or hiring a design team to design/engineer your shelter/safe room, DO YOUR HOMEWORK! Make sure the proper materials are being purchased/utilized and installed properly. Make sure your designer knows what they are doing. THIS IS AN EXTREMELY COMPLICATED DESIGN/CONSTRUCTION PROBLEM! Not everyone knows how to deal with them and what pitfalls to avoid.

I can tell you right now that one should expect to pay quite a bit more for a tornado resistant engineered/constructed door system than your run of the mill metal door. How much more? Maybe four to six times as much! Why are they so much? Because they are specially designed and constructed for two purposes, 1) to let you in and out of the shelter/safe room, and 2) keep you from perishing in a tornadic even! If you are paying $400 for the door, YOU’VE BOUGHT THE WRONG DOOR! These are not items that you typically find at your local lumber yard. These are special order items. You have to make sure that you are getting and paying for what you need to protect yourself, your family, and friends, whatever the case may be. Because if you stop at the door, you may have just wasted all the rest of the money you put into the shelter/safe room.

The shelter/safe room is only going to be as good as its weakest link. The problem is that weakest link is going to expose itself at the worst possible moment just as it did for the couple in Arkansas. Don’t let the same happen to you.

Be careful out there!

Post by Corey Schultz, AIA, LEED AP BD+C

Tornado Shelter Walls versus Kinetic Energy and Deformation

Growing up, my Dad had sitting on his office desk a little device called Newton’s Cradle. It had 5 suspended steel balls hung from a frame. You raised one of the end balls, let it go and when it hit the others, the one on the opposite end would fly up. This device represents Kinetic Energy. I didn’t understand it at the time but for a kid, it was fun to play with!

Now, using that same principal, think about one’s head leaning up against the exterior wall of a concrete or masonry tornado shelter. On the opposite side of the wall, a 15 pound 2×4 traveling at 100 mph strikes right where one’s head is against the wall. Get the picture?

Along those same lines, the ICC-500 states that a maximum of 3” of permanent deformation of an interior surface is allowed. 3”! So, you are sitting in one of these steel shelters where the only thing between you and the tornado debris is a sheet of steel that can deflect up to 3” and the shelter manufacturer has so conveniently put a built-in bench for you, on the exterior wall of the shelter with the interior surface of the shelter as you back rest. A backrest that can have a permanent deformation of 3”! How do you think one’s skull, spine, and/or shoulder blades are going to react to that potential 3” deformation?

The long and short of this is regardless of the shelter or the shelter material, during a tornadic event, shelter occupants should stay at least 3” away from the exterior walls of the shelter.

Be careful out there!