Step-by-step seismic load calculation (BNBC 2020 — Equivalent Static / ELF
procedure)
What is the most important thing you need to have or know
before you can proceed:
1.
Seismic zone coefficient 
for your site (from BNBC seismic zoning
map / Table 6.2.14– Table 6.2.15 or Figure 6.2.24).
2.
Importance factor 
(depends on occupancy/importance
category — BNBC Table 6.2.17).
3.
Site class / soil type (SA, SB, SC, SD, SE
— used to get soil factor 
, and spectral periods 
from BNBC Table 6.2.13
and Table 6.2.16). Also need soil properties (Shear Wave, SPT value,
Undrained shear strength) to classify the soil type.
4.
Structural system & response modification
factor 
(pick from BNBC Table 6.2.19 for
your lateral-force resisting system and use Table 6.2.18 for choosing seismic
force resisting system).
5.
Seismic (effective) weight of the
structure 
(BNBC defines how to compute W —
usually dead load + specified portions of other loads; see BNBC Sec. 2.5.7.3).
6.
Building height and per-floor weights
(for vertical distribution).
7.
Natural period 
from BNBC approximate
empirical formula 
with coefficients (BNBC Sec. 2.5.7.2
and Table 6.2.20).
Compute the normalized spectral factor(s) and design spectral acceleration 
BNBC gives the design spectral acceleration 
(design basis earthquake /
DBE level) by one of the equivalent normalized forms. The controlling
expression used in BNBC is:
[BNBC Eq. 6.2.37]where:
- = seismic zone
coefficient.
- = importance factor.
- = response reduction
factor for the seismic resisting system.
= normalized
acceleration response spectrum (BNBC gives equations for as functions of period 
and site parameters; see BNBC Eqns
6.2.35a–d).- = Soil factor which
depends on site class.
= a coefficient used
for the lower-bound expression (BNBC uses 
in the lower-bound
expression as given in the code notes).- The
factor
converts MCE→ DBE (BNBC uses DBE
level equal to two-thirds of MCE).
Notes on and period dependence: BNBC
lists normalized spectral forms (piecewise functions) for depending on relative to the Which are
the limits of the periods of the constant spectral acceleration branch You must
evaluate the correct branch of the piecewise expression (see BNBC Equations 6.2.35
(a–d) and Table 6.2.16 for TB, TC, TD
and site factors).
Compute the design base shear 
Once is determined (units of g), compute
base shear:
Where,
·
= is the total effective seismic weight
defined per BNBC (See Sec. 2.5.7.3).
·
Sa = Lateral seismic force
coefficient calculated above.
Distribute the base shear to story lateral forces
Use BNBC’s vertical distribution equation (Equivalent Static
distribution, BNBC sec 2.5.7.4). The typical BNBC form is:
Where,
- Fx
= Part of base shear force induced at level x
and 
= seismic weight at
level 
or x (floor weight).
and
= the height from the base to level i or x.- k = 1
For structure period ≤ 0.5s
- k = 1
For structure period ≥ 0.5s
- =
linear interpolation between 1 and 2 for other periods.
- n =
number of stories
Example:
Determine the design lateral forces due to earthquake on a
6-story concrete frame residential building using the equivalent lateral force
procedure. The structure is selected as a residential building to illustrate
the use of importance factors in calculating seismic design loads using the
procedures of BNBC 2020. The structure is located in sylhet, Bangladesh, Sa
= 0.244. The structure is located on soil determined to be site class D. Each
floor is 12 ft in height. The value of W for each floor is determined to
be 450 k, and for the roof, 200 k.
Solution:
Determine the total design lateral seismic force on the
structure
V = SaW = (0.244) (450 k) (5
floors) + (0.244) (200 k) (1) = 598 kips
The coefficient k is determined to be 1.05 by interpolation,
using a value of T = 0.6 sec.
The force at the top floor (roof level) is:
The force at the fifth-floor level is:
The remaining forces at other floor levels are calculated using the technique shown above for the roof and sixth levels, and the results are shown in Figure below-
Fig: Design
lateral seismic forces