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 = 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 |
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.
|
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