Understood, I see your point.
"cold war era nukes would easily wipe out a city thousands of 10+ story smaller buildings, they are about 30 megatons of explosive power." didn't need my multiplying explosive power theory to know that would happen. Grade school understanding?, Whatever pleases yourself, I was just posting a simple theory. I am aware that a nuclear blast and atomic blast are different and aware that structures of 1949 Nagasaki and Hiroshima were much weaker than modern day skyscrapers but that still won't stop a 30 MT bomb from leveling the city besides the fact that the city is NOT 100 percent made of skyscrapers. I think you need to go back to grade school, they way you think and act is inappropriate. But to be fair, nobody really knows what would happen if a 30mt nuke was dropped on a modern city so, it really is a challenge to say what would happen.
I'll repeat - WE HAVE NEVER TESTED A NUCLEAR WEAPON IN A LARGE CITY WITH SKYSCRAPERS AND THOUSANDS OF LARGE BUILDINGS. You can repeat all day "That's a lotta megatons, it would of course wipe out all those buildings" but there is no evidence or reasoning (beyond "bigger numbers are better") behind that assertion.
In nuclear weapon tests, we've shown that large concrete and steel constructed buildings can withstand the blast wave. On a flat plain with no obstructions a 30MT (friggin huge, only a few existed, and 3 times more powerful than the biggest in use today) nuke's overpressure will have reduced to 5 PSI in less than 5 miles. At this distance from ground zero, damage would be "Complete destruction of ordinary houses, and moderate to severe damage to reinforced concrete structures". Now, if this blast wave only does damage to a concrete structure, that means this concrete structure has absorbed the energy from the blast, and things behind that structure are going to receive even less blast energy. This absorption of energy at ground level is going to be cumulative, and if there are thousands of large buildings, this is going to logically reduce the level of damage from what it would be if there were no obstructions to the blast wave.
Now, on that hypothetical flat plain (the one without a bunch of concrete buildings to disperse and absorb the blast), your antique supernuke's blast is going to be reduced to 2 PSI of overpressure at a little under 9 miles. This level of damage is "Severe damage to ordinary houses, and light to moderate damage to reinforced concrete structures". Military planners calculate an app. 20% immediate casualty rate at this distance (this does not include later deaths due to radiation, of course). Even with no buildings in the way, most people would survive the blast of this supernuke at 8.7 miles, and probably most houses would be still standing though severely damaged.
I live within the Dallas, TX city limits. I also live over 9 miles from downtown. If one of these supernukes was dropped on the middle of Dallas, I would probably survive, as would most of the people in North Dallas, and nearly everybody in Richardson, Plano, Addison, and the other suburbs north of where I live. A single 30MT nuke would not wipe out the entire city.