This section explains the implementation of response
spectra according to IS 1893 (Part 1): 2016, 6th Revision.
For further information, refer to
IS 1893-1 (2016): Criteria for Earthquake Resistant
Design of Structures, Part 1: General Provisions and Buildings (Sixth
Revision).
Design Seismic
Base Shear: Section 7.2.1
The total lateral force
(or design seismic base shear) along principal directions is determined from:
where
W | = |
seismic weight of the building (Section 7.4.2) |
Ah | = | design horizontal acceleration spectrum value (Section 6.4.2)
using the fundamental natural period Ta (Section 7.6) in the considered direction
of vibration. |
Seismic Weight
(W): Section 7.4
Total seismic weight of building is summation of seismic
weight of all floors. In calculation of floor seismic weight, Section 7.4.1
states that full dead load and approximate amount of imposed load (&.3.1
and 7.3.2) should be included in this calculation. This is not implemented in
the program. Note that the program calculates seismic weight from building mass
information. For this reason, engineer needs to modify mass information to
account for live loads. On the other hand, modifying building mass also alter
dynamic characteristics of structure (i.e., periods and modes).
Design Horizontal
Acceleration Spectrum Value (Ah):
Section 6.4.2
The term Ah is
defined in Section 6.4.2 as follows:
The term Z is zone factor
and its defined as follows:
Table 1. Zone Factor
Z
Seismic Zone
|
II
|
III
|
IV
|
V
|
Seismic Intensity
|
Low
|
Moderate
|
Severe
|
Very Severe
|
Z
|
0.10
|
0.16
|
0.24
|
0.36
|
where
| = | for converting MCE (maximum considered earthquake) to DBE (design
basis earthquake) |
I | = |
importance factor (Table 6) |
R | = |
response reduction factor (Table 7). It is defined separately for each
direction.
|
The term
is average response acceleration coefficient and it is defined as follows
(Section 6.4.2)
- For rocky or hard soil sites
- For medium soil sites
- For soft soil sites
Table 2. Multiplying
Factors for Obtaining Values for Other Damping (Clause 6.4.2)
Damping, percent
|
0
|
2
|
5
|
7
|
10
|
15
|
20
|
25
|
30
|
Factors
|
3.20
|
1.40
|
1.0
|
0.90
|
0.80
|
0.70
|
0.60
|
0.55
|
0.50
|
Noted that values for given above is valid for 5% damping.
Fundamental Natural
Period Calculation (Ta): Section
7.6.2
7.6.2.a. Bare MRF building (without any masonry infills):
7.6.2.b. Building with RC structural walls:
where
Aw | = | the total effective area (m2) of walls in the first
story of the building given by:
|
h | = | height of buildings in meters |
Awi | = | effective cross-sectional area of wall
"i" in first
story (m2) |
Lwi | = | length of
structural was
"i" in the first
story in the considered direction of lateral forces (in meters)
|
The value of Lwi/h to be
used in the above equation shall not exceed 0.9. This implementation does not
enforce this condition.
7.6.2.c. All other buildings:
where
h | = | height of buildings in meters |
d | = | base
dimension of building along considered direction of lateral force (in
meters) |
All conditions defined in 7.6.2a, 7.6.2b, and 7.6.2c are
implemented. Also, note that fundamental natural period can be directly
obtained from an Eigenvalue analysis.
Distribution of
Design Forces: Section 7.6.3
Calculated base shear is
distributed to floors (vertical distribution) in accordance with the following
equation:
where
Qi | = | design lateral force at floor
"i" |
W | = | seismic weight at floor
"i" |
hi | = | height of floor
"i" measured
from base |
n | = | number of stories |
Orthogonal Load
Cases: Section 6.3.2.2
Orthogonal cases (combining 100% of load case in one
direction and 30% in other) is supported.
Design Vertical
Earthquake Load: Section 6.3.3
Vertical earthquake loads are not calculated in
the program.
Torsion (or
Accidental Torsion): Section 7.8
The 5% eccentricity rule is implemented for response
spectra eccentric load case (eccentricity percentage can be changed in
Diaphragm Mass dialog). The following design eccentricity is enforced for
floors
where
edi | = | dynamic eccentricity |
esi | = | static eccentricity at floor
"i" defined s
distance between center of mass and center of rigidity |
bi | = | floor plan dimension of the floor
"i"
perpendicular to the direction of force |