Employee was stepping up from the floor to beam to torque bolts on vib. conv. Heavy rains made work area wet & muddy, the floor had been swept though still wet, along with beams and soles of employees shoes. Employee's fingers slipped off beam, fell approx. 3.5-4' back onto diamond plate floor. Contusion/bruise left elbow. X-rays negative; ER requires visit to Orothopod.