Thousands of large diameter drilled shaft foundations are constructed every year to support electric transmission line steel structures. The design of these relatively short, laterally loaded shafts is quite often controlled by performance criteria, i.e. top of shaft deflection and rotation. Traditional design considers possible impacts to overhead conductor and structure performance. Yet, little research has been done to determine how these criteria relate to foundation performance. A study of historic full scale drilled shaft load test results shows a strong relationship between top of shaft rotation and normalized top of shaft deflection (deflection divided by shaft diameter). Test data indicate that lateral loads cannot be maintained without rapid progression to geotechnical failure when top of shaft rotation exceeds about 1 degree. Shaft movement less than this threshold most often exhibits linearly elastic load-deflection and load-rotation behavior, while more plastic movement is generally observed above this level. Evaluating top of shaft rotation/deflection and below ground line deflection results from full scale tests shows short, large diameter reinforced concrete shafts are most accurately modeled assuming rigid body motion with uniform flexure stiffness of the shaft. The paper presents a simplified state-of-practice approach for establishing appropriate performance criteria in relation to drilled shaft diameter.
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