HOW TO CALCULATE WEAK STOREY (IRREGULARITY CHECK)

A weak storey is one in which the storey lateral strength is less than 80% of that in the storey above. The storey lateral strength is the total strength of all seismic force-resisting elements sharing the storey shear in the considered direction [Figure 6.2.28(e)]. An extremely weak storey is one where the storey lateral strength is less than 65% of that in the storey above.

The story strength is calculated in the load direction specified by the user, as well as perpendicular to the load direction. The following equation is used to calculate Story Shear Strength for vertical elements at a corresponding story.

Story shear strength = 2φ√f'c bd

Sample Calculation of shear strength:

Data:

φ =0.90, f'c=4000 psi,

bd at GF=19000.00 in²

bd at 1F=20000.00 in²

Here bd is the sum of lateral load resisting member cross-sectional area.

 Shear strength at GF = 2φ√f'c bd

=2*0.90*√4000*19000=2162998 lb

Shear strength at 1F = 2φ√f'c bd

=2*0.90*√4000*20000=2276840 lb

 Ratio= Concerned Floor /Immediate Top Floor = 2162998/2276840=0.95

Since the value is greater than 0.80, hence this is OK.

Concrete compressive strength for lateral load resisting member =					4,000.00
Storey	Floor Height(ft)	Total Area of column(bd) (in2)	Story Shear strength	Ratio (Floor/Top Floor)	Remarks 1>0.8
ROOF	11	24,000.00	2732208	1.00	OK
8F	11	24,000.00	2732208	1.00	OK
7F	11	19,000.00	2162998	0.79	NOT OK
6F	11	23,000.00	2618366	1.21	OK
5F	11	20,000.00	2276840	0.87	OK
4F	11	20,000.00	2276840	1.00	OK
3F	12	20,000.00	2276840	1.00	OK
2F	12	20,000.00	2276840	1.00	OK
1F	13	20,000.00	2276840	1.00	OK
GF	15	19,000.00	2162998	0.95	OK

If you want this excel, email me at mahmudul.ce@yahoo.com.

Resource Reference:

1. Bangladesh National Building Code 2020

2. Midas manual