CALCULATION OF TIME PERIOD & SEISMIC BASE SHEAR
( Actual program is available in Super Civil CD )

PROGRAM INFO, SALIENT FEATURES OF IS:1893-1984 & OTHER USEFUL INFO:
This program calculates time period & seismic horizontal base shear as per IS:1893-1984. Up to 15 Storey can be analyzed.
The members of RC structures shall be designed as under reinforced section so as to cause a tensile failure.
For ductility requirements of various members refer IS:13920.
Whenever earthquake forces are considered along with other normal design forces, the permissible stress in materials may be increased by 33.3 %.
Permissible increase in allowable bearing capacity on soils.
SOIL TYPE FOUNDATION TYPE
PILE ON GOOD SOIL PILE ON POOR SOIL RAFT FOOTING
+ TB
FOOTING WITHOUT TB
ROCK/GOOD SOIL :N>30 50 %   50 % 50 % 50 %
MEDIUM SOIL :N>10 50 % 25 % 25 % 25 % 25 %
SOFT SOIL  N < 10 50 % 25 % 25 % 25 % - NIL-
Pile should be designed for lateral loads neglecting lateral resistance of soil layer liable to liquefy { N < 10 }.
Design horizontal seismic coefficient = ALPHA H = * I * ALPHA 0 OR
ALPHA H = * I * F0 * sa/g .
The vertical seismic coefficient where applicable may be taken as 0.50 * horizontal seismic coefficient.
The horizontal earth quake force shall be calculated for the full Dead Load & % of Live Load as given below.
LL up to 300 kg/m2 ............... 25 %
LL > 300 kg/m2 .................... 50 %
LL on roof to be neglected.
Values of
SOIL TYPE FOUNDATION TYPE
PILE ON GOOD SOIL PILE ON POOR SOIL RAFT FOOTING
+ TB
FOOTING WITHOUT TB
ROCK/GOOD SOIL :N>30 1.0   1.0 1.0 1.0
MEDIUM SOIL :N>10 1.0 1.0 1.0 1.0 1.2
SOFT SOIL  N < 10 1.0 1.2 1.0 1.2 1.5
Importance Factor { I } shall be taken as follows :
Residential Buildings ................................................... 1.0
Containers of inflammable & poisonous gases ............... 2.0
Community center, Hospitals, Water Towers, Schools,
    Bridges, Power Houses, Industrial Buildings .................. 1.50
In buildings having Shear Walls together with Frames, the frames shall be designed atleast for 25 % of seismic forces.
The following methods are recommended for various categories of buildings in various zones.
HEIGHT IN M. ZONE METHOD
< 40 ALL ANY 
40<H<90 I. II & III ANY
40<H<90 IV & V #
> 90 I & II #
> 90 III, IV & V DYNAMIC ANALYSIS
ANY -> SEISMIC COEFFICIENT OR RESPONSE SPECTRUM.
# -> RESPONSE SPECTRUM WITH MODAL ANALYSIS.
Check for Drift & Torsion for buildings > 40.0 M.
For buildings having irregular shape & / or irregular distribution of mass & stiffness in horizontal or vertical plane it is desirable to carry out Modal analysis using Response Spectrum method.
In case of buildings with floor capable of providing rigid diaphragm action, shall be analyzes as a whole for seismic forces.
In case of buildings where floors are incapable of providing rigid diaphragm action, shall be analyzed as frame by frame with tributary masses for seismic forces.
Base Shear  { VB } = K * C *ALPHA H * TOTAL WEIGHT
Values of performance Factor of System { K } are as follows :
SR_NO. STRUCTURAL SYSTEM K
1 DUCTILE MRF IN STEEL OR CONCRETE 1.0
2 DUCTILE SHEAR WALL OR BRACED FRAMES 1.0
3 AS IN 1, BUT WITH CONCRETE IN FILL PANELS 1.3
4 AS IN 1, BUT WITH MASONRY IN FILL PANELS 1.6
5 ANY OTHER 1.6
MRF -> MOMENT RESISTING FRAME.
C -> Coefficient defining the Flexibility of structure, depending up on fundamental time period T as follows :
T C T C T C
0 1.0 0.1 1.0 0.2 1.0
0.3 1.0 0.4 0.9 0.5 0.8
0.6 0.75 0.7 0.65 0.8 0.625
0.9 0.55 1.0 0.55 1.1 0.5
1.2 0.45 1.3 0.45 1.4 0.425
1.5 0.4 1.6 0.375 1.7 0.35
1.8 0.325 1.9 0.325 2.0 0.3
2.1 0.275 2.2 0.25 2.3 0.25
2.4 0.25 2.5 0.225 2.6 0.225
2.7 0.225 =>2.8 0.20  
TOTAL WEIGHT = DEAD LOAD + PART OF LIVE LOAD.
Fundamental Time Period " T " is calculated as follows, when Storey heights are between 2.70 M & 3.60 M.
T = 0.10 * NBR OF STOREY ................ FOR MRF WITHOUT MASONRY INFILL.
T = 0.09 * TOTAL HEIGHT (BASE WIDTH)^ 0.50
Distribution of seismic forces along the height of building is given by 
Qi = VB * Wi * Hi * Hi (SUM OF Wi * Hi * Hi)
The maximum horizontal relative displacement (DRIFT) due to earthquake forces between 2 successive forces shall not exceed H 250.
Please note that IS 456:2000 specifies total maximum lateral sway at the top of the structure as H
500 for Wind Loads.
Horizontal twisting [Torsion] takes place in the building when center of mass & center of rigidity (EI/L) do not coincide. The design eccentricity for Torsion shall be taken as 1.5 times the computed eccentricity between the center of mass & center of center of rigidity. -VE TORSIONAL shears shall be neglected.
Damping in structures in % critical shall be as follows:
NO ITEM DAMPING
1 STEEL WELDED 2-5 
2 STEEL BOLTED 4-7
3 CONCRETE 5-10
4 PRE-STRESSED CONCRETE 2-5
5 MASONRY WORK 5-10
6 TIMBER 2-5
7 EARTHEN STRUCTURES 10-30
8 TALL VESSELS/PIPES 2-3
9 SMALL EQP/PIPES 1-2
Towers, tanks, parapets, stacks & other cantilever projections attached to the buildings & projecting above the roof shall be designed along with their connections for 5 times the horizontal seismic coefficient.
All horizontal projections like canopies, balconies & their connections shall be designed to resist a vertical force equal to 5 times the vertical seismic coefficient * weight.
It may be noted that values obtained by seismic coefficient method are the maximum values as obtained by the response spectrum method for 5 % damping & period = 0.2 second.
Higher the Damping Lesser the Seismic Force.
Higher the Time Period Lesser the Seismic Force.
Ignoring the effect of masonry in-fill panels in 3D modeling can lead to larger value of " T " & low value of seismic force.
Difference between Wind {WL} & Seismic Forces {EL} :
WL is proportional to exposed area, EL is proportional to total floor loads.
WL is constant throughout, EL is parabolic in nature, maximum @ top & minimum @ bottom.
For design of equipment foundations resting on ground  can be treated as rigid body, & seismic coefficient method can be used or response spectrum method can be used considering time period T  as zero.
In order to arrive at correct value of base shear using STAAD or other 3D software a user has to calculate all natural period above 0.04 seconds.