Results show that all these methods are able to display roughly the same direction for water transfer but with remarkable difference in the amount,Comparison indicatesthat the instantaneous method is most accurate and layer-to-layer mean method next to it butwith rather a big interannual fluctuiation in errors.

To study the dynamic effect resulted from the initial damage to the structure during progressive collapse,the lumped plastic hinge model and the instantaneously applied load method are adopted to perform the elasto-plastic dynamic response analysis of a multi-story plane steel frame and a multi-story spacial steel frame corresponding to various demandcapacity ratios(DCR) respectively.

With discrete components amplifier and integrated operational amplifier circuit,this text introduces a method combined sampling point,comparison point with instantaneous polarity method to judge feedback configuration and feedback type fast and accurately.

Through the conclusions of the principles of judging feedback questions by adopting instantaneous polarity method , the article points out some simple but applicable methods ,which will help new learners master the feedback polariry quickly.