Seismice Force Computation for Equipment
Complete details are available in the specific code documentation. We will discuss the two typical seismic force equation formats.
1) First Typical Seismic Force Equation: Used by all codes prior to 1994 as well as by the latest versions of the BOCA and SBC. It is also used to establish a maximum and minimum boundary condition for the 1997 UBC and 2000 IBC:
Where Fp is the Seismic design force, Av is the ground acceleration coefficient for the equipment site, C1, C2, etc., are coefficients drawn from various tables based on building importance, type of equipment and type of attachment and/or other factors, and W is the equipment weight.
2) Second Typical Seismic Force Equation:Used by the 1995 NBC, and is the basic formula used in the 1997 UBC and the 2000 IBC. The format used is:
Where Fp is the Seismic design force, Ap is the ground acceleration coefficient for the equipment site tailored for soil or fault proximity conditions. C1, C2, etc., are coefficients drawn from various tables based on building importance, type of equipment and type of attachment and/or other factors. Rp is a ductility factor representing the ability of the anchorage to survive repeated impacts (it is again drawn from a table). D1 is a constant and varies from code to code (For the NBC = 1, for 1997 UBC = 3 and for the IBC = 2). Hx and Hr are the height above ground of the equipment and of the structure's roof, respectively, and W is the equipment weight.
While the general application of these equations is relatively straightforward, it is important to review all of the footnotes in the tables to ensure that the appropriate factors are being applied.
3) Sample Computation: Using the 1997 UBC, 2000 or 2003 IBC would look something like this:
- Where:
- 0.38 is the ground acceleration coefficient;
- 1.0 is the component amplification factor;
- 1.5 is the importance factor;
- 2.0 is the component response modification factor;
- 45 ft. is the elevation of the equipment;
- 60 ft. is the roof elevation;
- 2500 lbs. is the weight of the equipment.