Brace lowered into place with crane, choker at upper end, come-a-long at lower end. Bolt put in upper brace to beam connection. Crane took over brace's weight, come-a-long was removed, attached in the horizontal direction to pull brace in place. As brace was pulled, bolt in upper end of the beam failed. Brace swung down, hit a hand rail, then employee on left thigh.