Nuclear bombs (both fission and fusion) direct energy symmetrically in all directions. So any bomb detonated at or above ground level will direct half of its energy towards space. The other half eventually produces damage on the ground, but there are 2 models: surface burst, and air burst. The air burst produces a shock wave that hits the ground and is reflected, and then interferes constructively with the front of the wave that comes at an angle. Generally strikes are designed to maximize the effect of this constructive interference, this was the case with Hiroshima and Nagasaki. However, even this "doubled up" shock wave front is not really that destructive, so any half backed blast resistant shelter can survive it (it doesn't even need to be a concrete bunker, a well designed dug-out shelter will still do it). A surface burst will vaporize a semi-sphere of ground, and no bunker will survive, including even a missile silo, but then the blast effects will be much more localized while the radioactive fall-out will be increased maybe one thousand fold.