Decoupling Lateral Spread Response from Structural Inertial Loading: A Design Case Study

Thomas McCormack1 and Rick Thrall2

1Portland, OR, USA.

2PBS Engineering and Environmental, Vancouver, WA, USA..


This paper presents the case history of a new 8-story building on Oregon’s seismically active coast, sited next to a 40-foot deep dredged ship channel and overlying highly liquefiable soils. A steel pipe pile foundation was designed to support gravity loads, vertical and lateral earthquake forces, and to stabilize the underlying soils against lateral spread by “pinning.” The pile foundation was designed in accordance with recently published liquefaction design guidelines that recommend “decoupling” the effects of lateral spread from the effects of the structural inertial loading, along with the use of different performance criteria for these two seismic effects. Separating these two seismic effects is a rational technique because the peak vibration response is likely to occur in advance of maximum lateral spread displacement. Significant economy was achieved by using this approach and design procedure. In accordance with the published design recommendations, the development of a plastic mechanism under the action of the spreading forces was allowed. Approximately ninety 24-inch diameter concrete-filled pipe piles were drilled into underlying sandstone to achieve base fixity and uplift resistance, while the pile headswere fixed by embedment in four-foot thick pile caps. In addition, pile analysis considered the effect of inelastic deformation on lateral deflections, and the effect of P-delta moment on pile stress and pinning capacity.

Keywords: Liquefaction, Lateral spread, Pile pinning, Deep foundations, Earthquake.

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