Vertical Input for Seismic Analysis of Offshore Structures

Abstract The vertical spectral shape suggested by the main offshore codes does not reflect actual earthquake behavior. The problem is exacerbated in time history analysis, as the incorrect spectral shape leads to unrealistic scaling of records. Vertical spectra from codes are compared to published measurements from offshore sites and the results of numerical modeling. The effects of the water column and the mismatch between code spectra and real time histories are illustrated. Results of the spectral matching show that the vertical spectral shape defined by offshore codes leads to over conservative vertical time histories at short periods. Often vertical response is expressed as a fraction of the horizontal spectral acceleration at the same frequency (V/H ratio). Current practice regarding V/H is reviewed, considering design codes and recent research. Conventional V/H ratios are compared to sea bottom measurements by Boore and Smith (1999) which highlight the peculiarities of offshore sites. A relatively simple numerical analysis of vertically propagating compressive waves is used to evaluate vertical amplification and the effect of the water column for a case history. Current practice shows a certain degree of variability among the codes/papers. For onshore sites, there is consensus that V/H>1 at low periods but that vertical is less than horizontal at long periods. Boore and Smith's seafloor recordings suggest that offshore motions have very low vertical motion compared to onshore conditions, especially at low periods. The numerical modeling shows that the vertical response is mainly controlled by the water layer, which induces a strong deamplification at the resonance period of P waves within the water layer. Such an effect is in agreement with the findings of Boore and Smith (1999). The paper highlights the poor definition of the vertical seismic spectra for offshore sites in seismic codes and makes a first step towards the development of more reliable vertical spectra. The overconservatism induced by the code vertical spectra on definition of time histories is presented to stimulate future developments.