I think that's pretty difficult. During the firebombings in WW2 people asphyxiated in the shelters.
It's very doable. If you have a masonry building with no plants within 20 feet, you're pretty safe
These fires burn hot enough to melt any metal in the construction, consume all the oxygen and create an atmosphere equivalent to the surface of Venus. What you're proposing is essential a giant oven.
I highly doubt that these fires create an atmosphere of ~90bar with wind speeds in excess of 300km/hr, and temperatures averaging about 450C. The surface of Venus is a type of brutal that we don't experience on Earth, luckily.
Except I live neara wildfire zone and there is a nearby college where their plan is to shelter in place, and this plan worked for a wildfire that burned down several buildings on the campus.
Though i was wrong about the no vegetation zone; it's 200 feet, not 20
During the "Black Saturday" bush fires in Victoria in 2009 they estimated the temperature of the fires to be around 1200 degrees celsius (2190 F). My father-in-law is a volunteer in the rural fire service in Australia. He says some fires he's fought have been so hot they hurt your face to look at them for more than a few seconds, even when they are hundreds of meters away.
https://en.wikipedia.org/wiki/Black_Saturday_bushfires