In the current issue of The New Yorker magazine there is a very
interesting article on the design of tall buildings, with special
emphasis on The World Trade Center towers, including an interview
with Leslie Robertson a lead structural engineer when they were built.
"The Tower Builder," by John Seabrook, The New Yorker, November 19,
2001, p. 64ff. And the article is online (temporarily) at:
http://www.newyorker.com/FACT/

In the article the following mention is made of slide rules:

Charlie Thornton, of the Manhattan-based structural-engineering firm
of Thornton-Tomasetti Engineers, a leading designer of the structures
of modern high-rises, said to me recently, "A building like the
Empire State Building is way overdesigned and overbuilt. The building
didn't need all that support. Those engineers didn't understand loads
the way we understand them—they used slide rules to work them out,
whereas we have computers—and so they erred on the side of caution."

- Jim Cerny