Steel Interchange

Drift Index - H/400

Answer

Engineers that have been practicing structural engineering for up to half a century probably remember that the drift index criterion of H/400 (0.0025) was a commonly accepted practice when they first started. I remember reading that in the 1960s, when light curtain wall systems were beginning to be used in high-rise framed buildings in lieu of the heavier masonry infill wall systems, a significant reduction in lateral stiffness of the structure was noticed.

Doing a little more research to refresh my memory, I found the subject discussed in the “classic” 1960s steel design textbook Structural Steel Design, which often is referred to as “We the People” in reference to the long list of distinguished authors from Lehigh University at the time. The discussion in this text on wind drift stated:

“The results on measurements of wind deflection of the Empire State Building and a model study of that building show that the masonry walls increased the rigidity of the building about 350 percent above the rigidity of the steel frame. With the trend toward lightweight construction and cladding, much of this restraint may be reduced in the future. An ASCE Committee(1) study of wind bracing in tall buildings recommended limiting deflections to 0.002 times the height of a building although buildings with drift indexes of upwards of 0.004 or 0.005 have behaved satisfactorily.”

(1) ASCE Subcommittee No. 31, “Wind Bracing in Steel Buildings (Final Report), Transactions American Society Civil Engineers, 105, pp. 1713-1738 (1940).

In discussions with Bob Disque, he indicated that “when I started out as a designer in 1950, the H/400 limit was common. I had the feeling at the time that there was no analytical basis, but that it was a result of the experience of engineers who designed high-rise buildings.” A review of the referenced ASCE Subcommittee No. 31 Report from 1940 reflects this same concept of being primarily based on experience.

Kurt Gustafson, S.E., P.E.
With consultation from:
Robert O. Disque, AISC alumni emeritus
Ralph M. Richard, Ph.D., Professor Emeritus, University of Arizona