Quasi-static and pseudo-dynamic testing of rocking spread footings for bridges

From some of the actual engineering practices used in Taiwan, the size of spread footings was found to be exaggeratedly large due to the strict design provisions related to footing uplift. According to previous design code in Taiwan, the footing uplift involving separation of footing from subsoil is permitted only up to onehalf of the foundation base area as the applied moment reaches the value of plastic moment capacity of the column. The reason for this provision is that rocking of spread footings is still not a favorable mechanism. However, resent researches have indicated that rocking itself may not be detrimental to seismic performance and in fact can act as a form of seismic isolation mechanism. In order to gain a better understanding of the problem of rocking, a series of rocking experiments were performed. Experimental data of six circular RC columns subjected to quasi-static and pseudo dynamic loadings were presented. During the tests, columns were rested on a rubber pad to allow rocking to take place. Experimental variables included dimension of footings, strength and ductility capacity of columns, and level of the earthquake intensity applied. Results of each cyclic loading test were also compared with the benchmark test with fixed base condition. By comparing the experimental responses of specimen with different design details, the beneficial effect of rocking in reducing ductility and strength demand of columns was verified. The energy dissipation from the inelastic rocking mechanism of the footing was also observed.