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

PROGRAM
INFO, SALIENT FEATURES OF IS:18931984 & OTHER USEFUL INFO: 
This
program calculates time period & seismic horizontal base shear as
per IS:18931984. 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 = ß * I * ALPHA 0 OR
ALPHA H = ß * 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 ß

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 
25 
2 
STEEL
BOLTED 
47 
3 
CONCRETE 
510 
4 
PRESTRESSED
CONCRETE 
25 
5 
MASONRY
WORK 
510 
6 
TIMBER 
25 
7 
EARTHEN
STRUCTURES 
1030 
8 
TALL
VESSELS/PIPES 
23 
9 
SMALL
EQP/PIPES 
12 

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 infill 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.

