Seismic Load Generation per PIP STC01015 April 2017

Section 4.1.5 of PIP STC01015 April 2017 recommends that the guidelines of ASC 7-10 Minimum Design Loads for Buildings and Other Structures be followed for seismic loads. These have been implemented for vertical vessels on octagonal and square shaped soil supported foundations.

In Section 15.1.3 of ASCE 7-10, the code permits various methods of analysis for seismic ground motions: Equivalent lateral force procedure, modal analysis procedure, a linear/non-linear response history procedure, etc. In STAAD Foundation Advanced, only the equivalent lateral force procedure has been implemented.

The basic equation governing this procedure is mentioned in Section 12.8.1 of ASCE 7-2010.

V = C_s × W

Eqn. 12.8-1 ASCE 7-10

Where V is the lateral force (also known as base shear) which acts at the center of gravity (i.e., "CG") of the vessel. This force is assumed to be "rigidly" transferred to the foundation by the vessel, and hence causes an overturning moment on the foundation.

Since the point of connectivity between the vessel and foundation is the top of the pedestal, the transfer of the lateral force and the moment is assumed to occur at the top of the pedestal. Hence, the height of the CG of the vessel above the top of the pedestal also needs to be specified as an input by the user so that the moment can be calculated.

Note: STAAD Foundation Advanced can calculate only horizontal forces due to earthquakes. Any vertical forces resulting from vertical ground accelerations are currently not computed by STAAD Foundation Advanced.

There are two sets of input required for the calculation of the base shear and overturning moment.

Input required to calculate Cs
Input required to calculate W

Input required to calculate Cs

Cs is determined in accordance with section 12.8.11 with appropriate overrides or alterations described in section 15.4.1. In the Seismic load generation page, after selecting the code as ASCE 7-10, the following terms have to be specified by the user to enable the program to calculate Cs.

Setting	Description
Zip code	If the vessel happens to be located at a site in the USA, the user can select the zip code of that site from a list in the program’s GUI. The program will then fetch the mapped acceleration parameters Ss and S1 for 0.2 seconds and 1 second spectral response acceleration corresponding to that location from a database provided by the USGS and supplied with the program. If the ZIP code is not known, the user should use the Enter Value Manually option to enter the values of S_S and S1 corresponding to the site.
Site Class	see section 11.4.2 of ASCE 7-2010
Response Modification Factor, R	see Table 15.4-2 of ASCE 7-10
Importance factor, I_e	see Table 15.4.1.1 of ASCE 7-10
Long period Transition period, T_L	see section 11.4-5 and figures 22-12 through 22-16 of ASCE 7-2010

Input required to calculate W

W is defined in section 12.7.2 of the ASCE 7 document as the effective seismic weight. In reality, W comes from sources such as:

Empty Dead Load (De)
Operating Dead Load (Do)

The PIP STC01015 April 2017 definitions of Do and De are:

where

De	=	Empty dead load (empty weight of process equipment, vessels, tanks, piping, and cable trays)
Do	=	Operating dead load (empty weight of process equipment, vessels, tanks, piping, and cable trays (De) plus the maximum weight of contents during normal operation)

In other words, Do and De are input provided by the user to the program.

Note: All these are loads that are associated with the vessel and appurtenances (portions of the system above the pedestal).

Using Do and De, the program calculates two "base shear" terms Eo and Ee.

where

Eo	=	Earthquake load considering the unfactored operating dead load and the applicable portion of the unfactored structure dead load
Ee	=	Earthquake load considering the unfactored empty dead load and the applicable portion of the unfactored structure dead load

In order to calculate Eo, replace W with Do in the equation 12.8-1 of ASCE 7-10. Similarly, for calculating Ee, replace W with De. Thus:

Eo = Cs × Do

Ee = Cs × De

Eo and Ee are then used by the program in the load combinations. The example below shows two of the load combinations associated with earthquake that should be solved as per Table 4 of PIP STC01015 April 2017.

Operating Weight + Sustained Thermal + Earthquake
1.0Ds + 1.0Do + 1.0Ts + 0.7Eo

Empty Weight + Sustained Thermal + Earthquake

0.6 (Ds + De) + Ts + 0.7 Ee

where

Ds	=	Structure Dead load (weight of materials forming the foundation). This is auto calculated by the program from the volume of concrete in the foundation, density of concrete, volume of soil above the foundation, unit wt. of soil etc. The load factor by which this term should be multiplied for any given combination must be specified by the user in the column titled "Ds(structure dead load)" in the load combination table for the PIP STC01015-April 2017/ASCE 7-10 code.
Ts	=	Sustained Thermal or self-straining load

Output From the Program

Values of the various terms that constitute the W and other dead loads
Values of the various terms that go into calculating Cs
Calculation of Cs
Calculation of the base shear and moment terms (Eo and Ee)
Loads acting at the top of the pedestal from the various combinations

Several examples that illustrate the aforementioned steps for calculating the seismic force are available in the STAAD Foundation Advanced verification manual.