Experimental Series 1

The role of nonlinear foundation response on the performance of a structure–footing system.

An extended experimental investigation, which involved static push-over tests, cyclic loading tests, and dynamic shaking table tests, was carried out to study the role of nonlinear foundation response on the performance of a structure–footing system in light of the effect of the design safety factors (FSV for pure vertical loading and FSE for seismic loading).

A bridge pier of moderate height was selected as a typical high-rise structural system. The pier was founded on shallow foundation on dense sand. Keeping the structural weight and the soil properties constant, FSV was varied by changing the foundation size. Three different foundation sizes were considered designated as “large”, “medium” and “small”, representing a conservatively designed foundation, a less conservative, and an under-designed foundation respectively.

The most important outcome of this study is the verification of the seismic isolation potential related to nonlinear foundation response, which gives encouraging evidence in favour of the DARE idea for moving foundation design towards a less conservative, even unconventional, treatment. The results show that mobilization of the foundation ultimate capacity drastically restrains the inertial loading transmitted to the superstructure in such way that the maximum acceleration possibly developed at the super-structural mass reduces consistently with reducing the foundation size/capacity.

On the downside, an under-designed foundation presumably experiences increased permanent displacements. Settlements are guaranteed to increase proportionally to the reduction of FSV. The response is less predictable when permanent rotation is considered, as this depends primarily on the detailed sequence of the seismic motion and its (a)symmetry. Although this increased liability to permanent displacements is an important drawback, to our opinion the only one, of the concept for allowing nonlinear foundation response in design, the problem practically reduces in defining the acceptable displacements of the structure in relation to required performance.

Relevant Publication: A5 [Drosos et al., 2010].

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