From the METTLER TOLEDO Weigh Module Systems Handbook
Seismic Loading
Seismic forces, movement caused by earthquakes and other shifts of the earth, are
among the strongest external forces that can affect a tank scale. Figure 4-4 shows
seismic potential for the United States, with seismic zone 0 being the least likely
location for an earthquake and seismic zone 4 the most likely location for an
earthquake.
Seismic forces are analyzed in much the same way as wind forces. An equivalent
horizontal shear force (FEQ) is determined by using the appropriate formulas from the
governing building code. Formulas referenced in this section are from the 1988
Uniform Building Code (UBC) as adopted by the State of California, a state with a high
risk of seismic activity.
Figure 4-4: Seismic Zones in the United States
UBC Code Formulas
The following UBC Code formulas are used to determine horizontal shear force (FEQ) for
free-standing tanks and for tanks that are part of a structure:
V = (ZIC/Rw)W = FEQ for free-standing tanks
FP= (ZICP)W = FEQ for tanks that are part of a structure
where:
V = Base Shear
FP = Periodic Force
Z = Seismic Zone Factor
Zone 4: 0.40
Zone 3: 0.30
© 1999 Mettler-Toledo, Inc. (www.mt.com)
1
From the METTLER TOLEDO Weigh Module Systems Handbook
Zone 2B: 0.20
Zone 2A: 0.15
Zone 1: 0.10
I = Importance Factor
Nonhazardous materials: 1.00
Hazardous materials: 1.25 to 1.50
C = Lateral Force Coefficient: 2.75 for most conditions
CP = Lateral Force Coefficient (Tank as part of structure)
Nonhazardous materials: 0.75
Hazardous materials: 1.25
Vessels on roof of building: 2.00
RW = Numerical Coefficient from Tables 23-O and 23-Q of UBC
Bins & Hoppers: 4.00
Tanks: 3.00
FEQ Factors Based on UBC
Code
Table 4-1 provides a simpler way to determine horizontal shear force (FEQ). The factors
listed in the table are based on the UBC Code formulas presented above.
Nonhazardous
Hazardous
Conservative
Nonconservative
Free-Standing Bin/Hopper
Zone 4
Zone 3
Zone 2B
Zone 2A
Zone 1
0.28
0.21
0.14
0.10
0.07
0.41
0.31
0.21
0.15
0.10
0.34
0.26
0.17
0.13
0.09
Zone 4
Zone 3
Zone 2B
Zone 2A
Zone 1
Structural Bin/Hopper/Tank
Zone 4
Zone 3
Zone 2B
Zone 2A
Zone 1
Roof-Mounted Bin/Hopper/Tank
Zone 4
Zone 3
Zone 2B
Zone 2A
Zone 1
0.37
0.28
0.18
0.14
0.09
0.55
0.41
0.28
0.21
0.14
0.46
0.34
0.23
0.17
0.11
0.30
0.23
0.15
0.11
0.08
0.75
0.56
0.38
0.28
0.19
0.63
0.47
0.31
0.23
0.16
0.80
0.60
0.40
0.30
0.20
1.20
0.90
0.60
0.45
0.30
1.00
0.75
0.50
0.38
0.25
Free-Standing Tank
Table 4-1: Horizontal Shear Force Factors (FEQ) Based on UBC Code
2
©1999 Mettler-Toledo, Inc. (www.mt.com)
From the METTLER TOLEDO Weigh Module Systems Handbook
Find your application in the table, based on tank location, tank contents, and seismic
zone. Multiply the corresponding factor by the gross weight of the tank or vessel. The
resulting value will equal the horizontal shear force (FEQ) applied at the tank’s center of
gravity (see Figure 4-5).
V == FFEQe q
V
W gross
Figure 4-5: Horizontal Shear Force Applied to Tank
Reaction forces at the weigh modules are determined using Statics (see Appendix 4)
based on the shear force (FEQ) applied at the tank’s center of gravity. Compare the
reaction forces with the allowable loads for the weigh modules (see Appendix 5). The
weigh modules can then be sized to accommodate the resulting seismic loads, or
external checking can be added as needed to counter seismic loads.
CAUTION
CALCULATIONS ARE PROVIDED AS GUIDELINES ONLY. THEY SHOULD NOT
REPLACE A STRUCTURAL ENGINEERING EVALUATION OF THE APPLICATION BY A
REGISTERED PROFESSIONAL ENGINEER WHO IS FAMILIAR WITH LOCAL
BUILDING CODES.
© 1999 Mettler-Toledo, Inc. (www.mt.com)
3