Experimental Investigation Of SSDI On A Steel Frame-piled Raft Foundation

Buildings are few directly located on the bedrock, most are based on the soil.Soil, foundation and upper-structure have much to do with each other. As soil’smovement will influence the stress and displacement of foundation and upper-structure, the vibration of upper-structure will also make foundation and soil moveand change the stress of soil and foundation, which in turn influence the stress anddisplacement of upper-structure. Soil-structure dynamic interaction (SSDI) is widelyexisted in the seismic design of structures. How to effectively evaluate the effect ofSSDI in earthquake (especially great earthquake) in seismic design, making thestructure safer, more economic and reasonable, is an important task of seismicresearch of our country. At present, the domestic seismic codes consider the effect ofSSDI cursorily. Simply use cycle reduction method to calculate the dynamic responseof structures in earthquake only when the design intensity is8or9degrees and site isthe third or fourth class kind, but for the second class kind site, codes don’t consider.Factually, when the relative period ratio of upper-structure and foundation soil issmall, the effect of SSDI will greatly influence the stress of structures. If simplyaccording to rigid foundation assumption, structures will be too conservative orunsafe.Based on the study of SSDI, a steel frame-piled (raft) foundation model wasmade in soil bin indoor. By increasing braces and human quality, the stiffness andweight of the upper structure were changed and18different test conditions wereformed. A controllable frequency vibrator excited the steel frame by inputting whiteGaussian noise, El Centro wave, Taft wave and Shanghai artificial wave. Dataacquisition instruments, type891amplifiers and type941vibration pickups were usedto test the dynamic response of the steel frame. After the tests in the soil bin, the steelframe was then fixed on the rigid foundation and excitation tests were done the sameas in the soil bin. To research: structures under different relative period ratios, whosebasic natural vibration period, acceleration, displacement and bottom shear willchange how much when SSDI is considered. The experimental results showed:First, as for the natural vibration period, when relative period ratio T is lessthan0.80, the period elongation increases with the reduction of. The effect ofSSDI is increasingly obvious. When is greater than0.80, is basically near1.0, the effect of SSDI is not obvious. The paper has fit a simplified calculation formula ofthe natural vibration period for structures built in the second sites when SSDI isconsidered.Second, as for the bottom shear, when relative period ratio T is less than0.70,bottom shear of structures in soil foundation is smaller than on rigid foundation; shearreduction rate increases with the decreasing of, so the bottom shear of upper-structure can be properly reduced. When is greater than0.80, shear in soilfoundation may be larger than on rigid foundation, structures may be unsafe. Thepaper has fit a calculation formula of bottom shear reduction coefficient forstructures built in the second sites considering SSDI and is less than0.70.Last, as for the displacement of the top floor, the experiment shows that the peakdisplacement in soil foundation is generally larger than on rigid foundation. Whenis when relative period ratio is less than0.80, the displacement magnification increases with the reduction of; when is greater than0.80, the magnificationbecomes small. The paper has fit a calculation formula of displacement magnificationof the top floor for structures built in the second sites when SSDI is considered.