Base Isolation Design Issues For Frame Structures Supported By Stepped Foundation

Frame structures supported by stepped foundation can take full advantage of the mountainous terrain. Seismic performance of these structures is quite different from general frame structures. It has been proved by engineering. practice that isolation technology can significantly reduced earthquake disaster. It is an effective way to reduce the earthquake disaster by combining the base isolation with the frame structure supported by stepped foundation.But systematic research of base isolation design issues for frame structures supported by stepped foundation has not been carried out.Plane frame structures supported by stepped foundation were treated as research objects,Key issues for base isolation frame structures supported by stepped foundation would be researched. This paper carried out mainly the following aspects of work:(1)Applicability of the existing base isolation analysis and design methods for frame structures supported by stepped foundation were discussed.It was found that forced uncoupling modal response spectrum method was a simple and fairly accurate analysis method, Both horizontal damping coefficient and base isolation design method were not applicable to frame structures supported by stepped foundation.(2)Damping coefficient based on the base shear force ratio was put forward. Upper and lower ground base shear force ratio of base isolation and non-isolation structures could be defined as an new horizontal damping coefficient and evaluated damping effect for frame structures supported by stepped foundation.By analyzing of base isolation single-story single-span and multi-story multi-span frame structures supported by stepped foundation, It was found that base shear force ratio of isolation and non-isolation structures could well reflect the effect of damping.(3)Simplified design method based on horizontal damping coefficient was put forward. Firstly, Theoretical formula of structure base shear force considering higher modes was derived.Then equivalent gravity load coefficient was used to reflect higher modes influence, Parameter analysis method was used to analyze reasonable ranges of equivalent gravity load coefficient of non-isolation frame structures supported by stepped foundation, Parameter analysis method was adopted to analyze base isolation frame structures supported by stepped foundation, It was found that base shear force can be simplified calculated utilized single-particle system. Finally, Simplified design method based on horizontal damping coefficient was put forward based on the above analysis.(4)Division of branches and stages isolation design method for frame structures supported by stepped foundation was posed. Division of branches base isolation design idea for frame structures supported by stepped foundation was posed, Grounded columns of first floor and parts of above floors(upper floors for short) were designed utilizing fixed model, Ungrounded columns of first floor and parts of off layers(lower floors for short) were designed utilizing upper ground movable model and under ground fixed model(movable model for short).The relationship between new horizontal damping coefficient and the maximum interlayer shear force ratio of upper floors and the relationship between new horizontal damping coefficient and the maximum interlayer shear force ratio of lower floors were researched in order to simplify design of superstructure. Then division of branches and stages isolation design method for frame structures supported by stepped foundation was posed based on the prior results and existing design methods. The design example showed that this method was simple and practical and it is suitable for popularization and application.