Seismic Load Generation per PIP STC01015 April 2017
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 = Cs × 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.
There are two sets of input required for the calculation of the base shear and overturning moment.
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 SS 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, Ie | see Table 15.4.1.1 of ASCE 7-10 |
Long period Transition period, TL | 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:
The PIP STC01015 April 2017 definitions of Do and De are:
where= | ||
= |
In other words, Do and De are input provided by the user to the program.
Using Do and De, the program calculates two "base shear" terms Eo and Ee.
where= | ||
= |
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
where0.6 (Ds + De) + Ts + 0.7 Ee - 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