The behavior of a structure subjected to horizontal forces depends on its height to width ratio. The deformation in low-rise structures, where the height is smaller than its width, is characterized predominantly by shear deformations. In high rise building, where height is several times greater than its lateral dimensions, is dominated by bending action.
To analyze the structures subjected to horizontal loading we have two methods.
Portal method and Cantilever method
1. PORTAL METHOD
The portal method is an approximate analysis used for analysing building frames subjected to lateral loads such as Wind loads/ seismic forces.
Since shear deformations are dominant in low rise structures, the method makes simplifying assumptions regarding horizontal shear in columns.
Each bay of a structure is treated as a portal frame, and horizontal force is distributed equally among them.
Assumptions in portal method
1. The points of inflection are located at the mid-height of each column above the first floor. If the base of the column is fixed, the point of inflection is assumed at mid height of the ground floor columns as well; otherwise it is assumed at the hinged column base.
2. Points of inflection occur at mid span of beams.
3. Total horizontal shear at any floor is distributed among the columns of that floor such that the exterior columns carry half the force carried by the inner columns. 
The basis for this third assumption is the frame is composed of individual portals having one bay only
2. CANTILEVER METHOD
The cantilever method is applicable to high rise structures. This method is very similar to the portal method. In this method we also assume hinges at the middles of the beams and columns. The only difference is that for the cantilever method, instead of finding the shears in the columns we using an assumption to find the axial force in the columns.
The assumption that is used to find the column axial force is that the entire frame will deform laterally like a single vertical cantilever.
Assumptions in cantilever method
1. The points of inflection are located at the mid-height of each column above the first floor. If the base of the column is fixed, the point of inflection is assumed at mid height of the ground floor columns as well; otherwise it is assumed at the hinged column base.
2. Points of inflection occur at mid span of beams.
3.The axial force in the column at any floor is linearly proportional to its distance from the centroid of all the columns at that level.
kindly upload solved examples of rc frames subjected to lateral loads for both methods
ReplyDeleteYeah! Follow the upper comment
ReplyDeleteIn cantilever method, is it Axial force or stress that varies linearly?
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