Assumptions of Multi-stored and Multi-bay Frames

Portal Method

1. Point of contra flexure is assumed to be at mid-height of each column.
2.  Point of contra flexure assumed to be at mid-span of each girder or beam. 
3. Horizontal shear on each story is divided between columns in that story such that each interior column carries twice as much shear as exterior Column caries.

Cantilevers Method

1. Point of contra flexure is assumed to be at mid-height of each column. 
2. Point of contra flexure is assumed to be at mid-span of each girder or beam. 
3. Intensity of axial stress in each column of a story is proportional to the horizontal distance from the c.g of all the columns of that story.