اثر مولفه قايم زلزله بر تغييرات ضريب اصطكاك در پي لغزشي

Influence of Vertical Earthquake on Coefficient of Friction in Sliding Bases

چكيده نيروِي اصطكاك يكي از پركاربردترين پديده هاي طبيعت است كه در مهندسي زلزله نيز كاربردهاي فراواني دارد. بر خلاف تصور، مقدار ضريب اصطكاك در طول زمان زلزله ثابت نيست و علاوه بر سرعت نسبي دو سطح به فركانس و دامنه نوسان قايم نيز بستگي دارد. براي محاسبه مقدار اين ضريب بايد فركانس و دامنه نوسان قايم را در هر لحظه محاسبه كرد كه يكي از بهترين راههاي محاسبه اين كميات استفاده از تبديل موجك مي باشد. يكي از كاربردهاي اصطكاك در مهندسي زلزله، پايه هاي اصطكاكي مي باشد كه در اين تحقيق نيز مورد توجه قرار گرفته و تغييرات ايجاد شده در ضريب اصطكاك در سه ركورد مختلف زلزله محاسبه و نشان داده شده است. لازم به ذكر است مولفه قايم زلزله علاوه بر اينكه ضريب اصطكاك را متاثر مي سازد، مقدار نيروي قايم بين پي و ساختمان را نيز تحت تاثير قرار مي دهد. بررسيهاي اين تحقيق نشان مي دهد كه در لحظات لغزش مقدار ضريب اصطكاك با در نظرگيري اثر مولفه قايم همواره كوچكتر از زماني است كه از زلزله قايم صرفنظر مي گردد.

Abstract Friction is one of the most natural phenomena which is applied vastly in earthquake engineering. Despite the imagination, recent researches have shown that the coefficient of friction is not constant and varies by instantaneous frequency and amplitude of movement in vertical direction. Chawdury and Helali [7, 10] have shown that the friction coefficient decreases by increasing the frequency (f) and amplitude (A) of the vertical direction, however it rises by relative velocity (V) of the contacting surfaces. They also showed that the presence of vibration affects the friction force considerably. The values of friction coefficient for the considered materials of above-mentioned research decreased with the increase of amplitude of vibration at different frequencies. The percentage reductions of friction coefficient under vibrating and non-vibrating conditions increased almost linearly with the increase of amplitude of vibration at different frequencies. But the percentage reductions of friction coefficient were different for different materials. The reduction of friction coefficient is also related with sliding velocity, surface roughness and normal load under vibration condition. In other words, the new finding of their research is that the friction coefficient depends also on V/A.f. They tested some material and determined their governing formula for calculating their coefficient of friction, relating to the frequency and amplitude. Despite the mentioned experimental tests, in which the frequency and amplitude of the vertical vibration was constant, these quantities are not constant during a real earthquake and vary time by time. Therefore, instantaneous frequency and amplitude of vertical component of the earthquake should be used to calculate the friction coefficient of the moment. Wavelet transform is applied here to calculate these instantaneous quantities. MATLAB was applied for such a purpose. It is worth noting that the coefficient of friction is constant and equal to static one, when the relative velocity of the contacting surface is zero. However, for other cases it should be calculated for every moment. The instantaneous quantities are determined by wavelet transform for the displacement function of the vertical component of the earthquake. The relative velocity is considered as the one in the previous iteration. It is worth mentioning that the vertical earthquake not only change the coefficient of friction, but also vary the normal load, and therefore it changes both governing parameters of the friction. In pure sliding bases, which are focused in this paper, variation of the friction coefficient is investigated in three earthquake records, including Bam, Tabas and Gazli earthquakes. The real friction coefficient, regarding influences of the vertical component of earthquakes is shown for the considered earthquake records in comparison with the constant value in Figures 13 to 15. As shown, the real friction coefficient is always less than the considered constant values. This relies that the maximum acceleration, applied to the superstructure, is overestimated when the variation of the friction coefficient is ignored.