Steel piles below high-rise buildings may carry larger varying axial force generated by the overturning moment than those below low- or medium-rise buildings. On top of that, the subgrade lateral stiffness reduces drastically due to the soil liquefaction during a significant earthquake. Steel pile’s horizontal strength may decrease due to the lower position of the inflection point. In this paper, centrifugal tests of the superstructure with high height-to-width aspect ratio, steel piles, and the liquefied soil system are presented. The collapse mechanism and the horizontal load bearing capacity of steel piles in the liquefied soil are clarified.
Fig. 1 shows the model and instruments. The specimen consists of a superstructure with mass, a footing beam with mass, two bending plates, four piles, and a saturated sand layer. Table 2 shows specimen parameters, which are the height-to-width aspect ratio of the superstructure and the relative density of the soil. The centrifugal tests were performed under the centrifugal acceleration of 40 g.
Figs. 5-18 show response time histories of Case 1-Case 6. The bending strain at the pile head gradually increases toward the one side after the soil liquefaction and reaches the maximum value, εb,max. After reaching εb,max, the piles of all specimens collapse, except for Case 4. As presented in Fig. 20, in the case of the same Dr values, the sum of pile head’s shear forces, ΣQph at εb,max are approximately equal regardless of the height-to-width aspect ratio. An additional shear force generated by pile’s P-Δ effect, QPΔ,pl, accounts for the largest part in ΣQph. On the other hand, in the case of high height-to-width aspect ratio, the value of QPΔ,pl is smaller issued from larger additional shear force generated by the superstructure’s P-Δ effect, QPΔ,s.
Fig. 21 shows the relationship between pile’s strength on the centrifugal tests and the M-N interaction curves of design criteria. Ratios of pile’s varying axial force to pile’s flexural buckling strength, Npl/Ncr0, are the same until at εb,max regardless of the height-to-width aspect ratio. For piles subjected to the dead load and the varying axial compression force of all specimens, the combinations of axial force and bending moment at εb,max are distributed roughly following the ultimate strength curve as presented in Ref. 6. On the other hand, the results of the pile subjected to the dead load and the varying axial tensile force in the case of Dr=30% are lower bound to the M-N interaction curve as shown in Ref. 9 and those of the case of Dr=60% approximately reach that curve.
As presented in Figs. 22-24 and table 3, steel piles’ collapse mechanism is concluded as follows. Pile’s bending moment reaches pile’s full plastic moment, vrMpc,cr0, at the top and the bottom of the pile subjected to the varying axial compression force. In the case of Dr=60%, pile’s bending moment also approximately reaches vrMpc,cr0 at those points of the pile subjected to the varying axial tensile force due to the increase of the distribution ratio of superstructure’s and footing beam’s shear force after at εyc.
As shown in Fig. 24, it is concluded that steel piles’ horizontal load bearing capacity can be evaluated easily using steel piles’ horizontal strength based on the location of the inflection point in the liquefied soil and pile’s plastic bending strength reduced by initial axial force.
