Analysis of Shear Lag Anomaly in Box Girders

In box girders with wide flanges, an uneven flange stress distribution (the flexural normal stress is greater at the edges than that at the center of the flange), so-called shear lag (or positive shear lag), may occur following the characteristics of stress concentration problems. However, Foutch and Chang in 1982 discovered the shear lag anomaly called negative shear lag in which the stress distribution is opposite from that in positive shear lag. Several attempts have been made to explain it. However, existing studies lack a more general and physical explanation on the origin of negative shear lag. In this paper the origin of the anomalous phenomenon is explained from the physical point of view. It is shown that, at any given location, negative shear lag can take place whenever the portion of the shear flow acting along the flange edges, which produces the shear lag-aftereffect, is larger than the remaining portion of shear flow caused by positive shear lag.