Launch Pads 39A and 39B were initially built for either the [Saturn C-8][1] or the [Nova][2] rockets, each of which has a lift off thrust of about 7 tons, about twice that of the Saturn V. These were alternative designs to allow for a return directly from the Moon to Earth, but ultimately scrapped in favor of the Saturn V Lunar Rendezvous design.

Note also that the Space Shuttle actually has a higher thrust on lift off, namely about 6000 tons, then the Saturn V.

I suspect that when the pad was designed, it was designed to support 7000 tons of thrust at liftoff, with a fair bit of margin. I'm not sure that the design can fully support the BFR, but it should be close. 


  [1]: https://en.wikipedia.org/wiki/Saturn_C-8
  [2]: https://en.wikipedia.org/wiki/Nova_(rocket)