Portal Method For Rough Moment Frame Design

The Portal Method for rough moment frame design is based on these assumptions:

•  Lateral forces resisted by frame action
•  Inflection points at mid-height of columns
•  Inflection points at mid-span of beams
•  Column shear is based on tributary area
•  Overturn is resisted by exterior columns only

1.    Single moment frame (portal)
2.    Multistory moment frame
3.  Column shear is total shear V distributed proportional to tributary area:

4.   Column moment = column shear x height to inflection point

5.  Exterior columns resist most overturn, the portal method assumes they resist all
6.  Overturn moments per level are the sum of forces above the level times lever arm of each force to the column inflection point at the respective level:

7.  Beam shear = column axial force below beam minus column axial force above beam Level 1 beam shear: