Wednesday, 14 March 2012

PORTAL METHOD and CANTILEVER METHOD

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.
There are two methods-  namely PORTAL METHOD and  CANTILEVER METHODS to analyze the structures subjected to horizontal loading. 

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
This method is applicable to high rise structures. This is based on the simplifying assumptions regarding the Axial Force in columns.
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 basic assumption of the method can be stated as “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.

1 comment:

  1. kindly upload solved examples of rc frames subjected to lateral loads for both methods

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