Hello Team,
I have modeled one of the RCC building,in that model I have applied vertical earthquake as per IS 1893-2016,
the problem is that when analyze the model I achieved base shear in x and z direction but in y direction I achieved zero value,
as per IS 1893-2016 cl.6.4.6the value of base shear in Y direction is 2/3 of base shear in X and Y direction, so that value not achieved.
Can you please share the file here?
MODEL-R1.STD
Hello Akash,
In your model there are a lot of small cantilever portions which are participating in the initial modes. As the seismic mass excited for those modes are very very less, you are not seeing any such base shear value. By default STAAD shows 6 modes and those modes are not sufficient to capture the full response. In this case, you need to increase the number of modes using "CUT OFF MODE SHAPE" command available under miscellaneous analysis commands.
With Cut off mode shape 150 for this file i am getting below result.
thanks sir for your reply, but correct me if i am wrong to interpret codal clause, what i understood that base shear value in y direction we get is 2/3 of base shear value in X or Z direction but here my base shear value in X direction 1157.88 kN but in Y direction i get 91.55 kN value. so is it ok ??
In order to compute base shear, you need to consider mass participation of a mode. In your case, mass participation in Y direction for first 6 modes are nearly zero and that is the reason why the base shear in Y direction is reported as zero. I have considered 320 number of modes in the analysis by using CUT OFF MODE SHAPE 320 command. Also direction factor in Y direction should be 0.036 and not 0.024. The 2/3 factor would be internally considered by program in Y direction.MODEL-R1_mod.STD
In the vertical direction, you can provide direction factor as (Z/2)*(I/R).
LOAD 3 LOADTYPE Seismic TITLE EQ_Y
*** Direction factor = ((Z/2)*(I/R) = (0.24*/2)*(1.5/5)
SPECTRUM CQC IS1893 2016 Y 0.036 DAMP 0.05
SOIL TYPE 2
For this load case, as per clause 6.4.6 of IS 1893 Part 1:2016, program will automatically multiply 2/3 factor to calculate Ak (Clause 6.4.2 of IS 1893 Part 1:2016) value and Sa/g will be considered as 2.5 for all modes
Design Seismic Coefficient in Y = (2/3)*Direction Factor*2.5 = (2/3)*0.036*2.5 = 0.06