An emailer:

Many of our friends and loved ones were sitting at their desk or a conference table when McVeigh’s bomb went off.
They were doing what Americans do best, working when they were suddenly removed from this life.
I am not an architect nor have I seen the plan for the Pentagon memorial so I can’t comment on that; I am just a friend of some killed that April while working in the Alfred P. Murrah Federal Building.
When I first saw the chairs at the OKC bombing memorial, my first thought was the suddenness of my friends death, at work. The symbolism worked for me.