They have the 4 inch channel skeleton which makes them stronger and also adds protection from debris which could fall and/or be driven into the ground. Even below ground isn't completely safe, unless it has some protection from debris entering the unit in this manner. During installations, we have driven a bull dozer and other large equipment over them, as the one picture in our web site. You can be sure that whatever does fall up there above you will remain up there, rather than inside of the unit with you.
Many people feel uncomfortable going underground, even though they know this is the safest place to be in a storm or tornado. Most people are surprised when they go into our shelters, especially our Below Ground with Stair entry unit. The underground with stair is designed with a large entry (30" X 5'0). The stairway has 8 steps into the unit. The body of the Below Ground model and the Hillside shelter are made the same and both models can be viewed at our store. The entrance way is the only difference in the two models.
The exterior of both units are covered with tar mastic which protects them from rust. The underground with stair is embedded in concrete for protection of the steel and to make sure they stay where you put them. This helps prevent a 'float up' due to ground water. It can also be built with an extended floor plate that excavated dirt sets upon to weigh down the unit.
The unit design is engineer stamped for either option.