seismic analysis , how to match base shear

1. MATCHING BASE SHEAR IN ETABS 2016
Base shear is an estimate of the maximum expected lateral force that will occur due to seismic
ground motion at the base of a structure. Calculations of base shear (V) depend on:
 soil conditions at the site
 proximity to potential sources of seismic activity (such as geological faults)
 probability of significant seismic ground motion
 the level of ductility and overstrength associated with various structural configurations
and the total weight of the structure
 the fundamental (natural) period of vibration of the structure when subjected to dynamic
loading
Matching base shear, After analysis, users should review the base shear due to all modes,
reported in the Response Spectrum Base Reaction Table. If the dynamic base shear reported is
more than 85% of the static base shear, no further action is required. However, if dynamic base
shear is less than 85% of the static base shear, then the scale factor should be adjusted such that
the response-spectrum base shear matches 85% of the static base shear. In this case, the new
scale factor would be (I g / R) * (0.85 * static base shear / response-spectrum base shear).
Analysis should then be rerun with this scale factor specified in the response-spectrum case.
Note :- look up clause 7.8.2 of IS-1893-2002 page-25
Matching Base Shear in Etabs 2016
Step1
Before analyzing (Run) the model for the first time ensure the value in scale factor in
spec(x) and spec(y) be “1” or the value obtained by resolving the (I g / R). However these value
will be altered after the analysis if dynamic base shear (i.e spectrum (x,y) base shear) is less than
85% of the static base shear(i.e eqx, eqy base shear).

2. Define Load cases spec x Modify/show cases
Fig-1
Spec y Modify/show cases
Fig-2

3.  After modifying the scale factor now run the model
Fig-3
 After run analysis go for
DISPLAY
SHOW TABLES
STRUCTURE RESULTS
STOREY FORCES
EXPORT TO EXCEL

4. Fig-4
Fig-5

5.  If dynamic base shear is 85% of the static base shear there no need of doing any
alteration.
 If dynamic base shear is less than 85% of the static base shear then there is need
to modify as per IS 1893-2002, clause 7.8.2 as shown below.
Base Shear in x direction =
Base Shear in y direction =
 Reference See Base Shear Values Obtained In Fig 5
STEP 2
 After obtaining scale factor that is to be modified to match base shear from first time
analysis ie 23.637 & 234.61 in x and y direction of dynamic load cases. Then release the
analysis performed for first time and as shown in fig 6.

6. Fig-6

7.  After inserting the obtained scale factor run analysis again such that base shear will
be matched.
 To cross check whether base shear is matched or not see the values obtained after
second time analysis in fig 7
Fig -7
Hence dynamic base shear reported 85% of the static base shear, no further action is required.
Thank You
Vamsi Krishna reddy (VKR)
Vamsi.vasipalli@gmail.com
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