If steel pipes and steel boxes were the best shelter platform, the Military would be using them, and they are not. They are still using reinforced concrete. There has to be something to be said about that, and there is: Mass, Density, Thermal Resistance, Sound Attenuation and Cost, to name a few.
Steel shelters, which are typically 3/16- 1/4 of an inch thick, need to be buried DEEP, in order to provide the proper comparative level of protection against radiation. They are typically installed with 8-10 feet of earth covering the top and this presents a considerable number of challenges with regard to the costs for excavating, the need to hire a crane and other issues.
For example, a 10 foot Pipe that is 20 feet long will require an 18-20 feet deep hole and provides a gross interior area of 1,570 cubic feet. Keep in mind that the interior surface is curved ( similar to being in a submarine ), and therefore requires a floor to be installed, which reduces headroom. Simply put, a pipe does not lend itself to being space efficient and comfortable.