The portal and cantilever methods are two approximate methods for analyzing tall buildings for lateral (wind or seismic) loads - nowadays used for preliminary design purposes.
The portal method is suitable for buildings likely to deform in shear mode. It is based on the distribution of base shears proportional to influence areas.
Assumptions made in Portal Method:-
- Point of inflexion occurs at the mid-height of vertical member (i.e. column)
- Point of inflexion occurs at the mid-length of horizontal member (i.e. girder, beam)
- The Interior column carries twice the horizontal shear than that of the exterior column at each story.
The cantilever method is suitable for slender buildings likely to deform in flexure mode. It is based on the distribution of base moment as axial forces on columns proportional to their distance from the centroidal axis of the building.
An assumption made in Cantilever Method:-
An assumption made in Cantilever Method:-
- Point of inflexion occurs at the mid-height of vertical member (i.e. column)
- Point of inflexion occurs at the mid-length of horizontal member (i.e. girder, beam)
- The axial stress in a column is proportional to its distance from the centroid of the cross-sectional areas of the columns at a given floor level.
Since stress equals force per area, then in the special case of the columns having equal cross-sectional areas, the force in a column is also proportional to its distance from the centroid of the column areas.
