Base Shear

 

Base Shear

In the IS code, the horizontal base shear V_B is given by

 

V_B = KC.α₀.I.β W                                           ....(1)

 where K is a performance factor depending upon the structural system and ductility of the construction, as given in Table 1, C is a coefficient defining the flexibility of structure with the increase in number of storeys depending on 'T’ where T is the fundamental period of the building in seconds, α₀ is the basic seismic coefficient corresponding to seismic zone. β is a coefficient which depends on the soil-foundation system, I. is the importance factor and W is the effective weight. The variation of C with T is showing in Fig.1 The trend corresponds to that of spectral acceleration diagram.

Fig. 1: Flexibility Factor ‘C’ as a function of Period

Table 1:VALUES OF PERFORMANCE FACTOR K

Sr. No.	Structural Framing System	Values of Performance Factor, K	Remarks
1.	a)     Moment Resistant Frame with appropriate ductility deatils as given in IS: 4326-1976, in reinforced concrete or stell	1.0	--
	b)     Frame as a above with R.C. shear walls or stell bracing members designed for duxtility	1.0
2.	a)     Frame as in 1 a) with either stell bracing members. or plain or normally reinforced concrete infill panels.                       b)     Frame as in 1 a) in combination with masonry infills.	1.3                               1.6	These factors will apply only if the steel bracing members and the infill panels are taken into consideration in stiffness as well lateral strength calculations provided that the frame acting along will be able to resist at least 25 percent of the design seismic forces. -        do    -
3.	Reinforced concrete Framed Building (not convered by 1 & 2 above)	1.6	- -