Design Equation for Wave-Induced Uplift Forces on Elevated Structures

Abstract

This paper introduces a conservative design equation for wave-induced uplift forces on an elevated structure. The equation is an extension of previously developed design equations to estimate the horizontal wave–induced pressure distribution on an elevated structure. The uplift equation relates the pressure at the lowest element on the face of the structure to the pressure at the leading edge on the bottom of the structure via an empirical uplift coefficient, Cu. The uplift coefficient was determined experimentally from a large (1:10) scale experimental dataset of random wave pressures and forces on an elevated specimen. The pressure distribution along the bottom of the elevated structure was assumed to be triangular, with zero pressure at the trailing edge. The proposed design equation was shown to be conservative in predicting large vertical forces (F1/250) with safety factors (SF) between 0.96 and 3.08 and characteristic vertical forces (F1/3) with a mean SF of 5.06 across a range of significant wave heights, peak wave periods, and air gaps. The proposed equation can be considered more applicable for engineering design compared with existing equations for vertical uplift, which are shown to be either unconservative (SF < 1.0) or highly conservative (SF > 10.0) in predicting wave-induced uplift forces. ASCE 7-22 Chapter 5: Flood Loads was revised in ASCE 7-22 Supplement 2 to require designers to consider lateral and vertical wave–induced forces for onshore, elevated structures. The lateral force equations are based on previous work that modified well-established equations for caissons. Although the vertical (uplift) force is a required consideration for design, Supplement 2 does not provide explicit design equations, instead providing commentary for guidance, leaning upon earlier work with applications to offshore platforms and coastal bridge decks. This paper presents a conservative design methodology consistent with well-accepted wave pressure formulae to predict wave-induced uplift forces on elevated coastal structures. The design equation is compared with large-scale laboratory data, and the safety factors of the design equation are considered with respect to extreme (F1/250) and characteristic (F1/3) wave forces. The design equation is compared with alternative equations, and restrictions on the equation’s applicability are given. Amid the modern context of more frequent, more intense, and more rapidly developing hurricanes and extratropical storms, a conservative design of elevated coastal structures against combined surge and wave forcing is becoming increasingly critical, especially as larger proportions of populations move toward coastlines, shorelines retreat, and relative sea levels increase.