1. Department of Civil, Environmental, and Architectural Engineering, 428 UCB, University of Colorado, Boulder, Colorado 80309-0428, The National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention, Public Health Services, U. S. Department of Health and Human Services, 4676 Columbia Parkway, MS R-3, Cincinnati, Ohio 45226-1099, and Department of Mechanical Engineering, 427 UCB, University of Colorado, Boulder, Colorado 80309-0427