If you are planning on building a classic style blast shelter as recommended by Cresson Kearney and Oak Ridge Labs in Tennessee (as we have done at Vault-Co) then you must read everything on this page in detail. You could spend hours studying abstracts from Oak Ridge to glean what you can get in five minutes reading this website.
Personally I felt their old website had better information, but this one still gives extensive directions in construction of these kinds of corrugated galvanized blast shelters that you won't find consolidated anywhere else. Don't neglect a detail they have mentioned - these people build several of these shelters a year and have learned from their mistakes many times over.
For construction of other kinds of shelters you can seek other sources of expertise but for this particular kind of shelter (tested under real nuclear weapons) there is no substitute.
This shelter is rated to survive a one megaton blast directly overhead or on the lip of a five megaton ground detonation. It is also specifically engineered to provide high survivability against atomic energy weapons aimed at maximizing human casualties like the neutron bomb. Buried at suitable depth with crushed rock it will provide shielding against radiation far in excess of almost all conventional shelter designs.