How should twist moment due to seismic load be calculated for tank ringwall foundation?

[tmgczb]

As per Chapter 5.6.4, PIP STE03020, there should be MT value, torsion due to seismic load, to calculate the equivalent bending moment.
For stability check, we consider ring wall as an isolated foundation.
So maximum pressure is equal to MsE/W. W is section modulus.
Than MT could be obtained.
But nobody calculates it in this way.

 

[HTURKAK]

there should be MT value, torsion due to seismic load, to calculate the equivalent bending moment.

MT could develop for vertical and wind loads also. It is not necessary that the shell axis will coincide with CL of FTG. You may shift the shell CL from CL of the ftg. to avoid development of MT. ( That is , choose x_bar 1 on the left of CL so MT = WP(L)(x-bar 2)-PT(x_bar 1) this is common practice )
There is a worked example at Page (Example 1 – El Segundo, California )
If you choose x_bar 1 = -0.23 ft instead of 0.58 ft , MT would not develop.

If this is a real question , provide more info. to get better responds.

 

[tmgczb]

Dead load ,wind and seismic load all will result in eccentric load.
Wind is not combined with seismic load and smaller than seismic load(for the studied tank).
So consider dead load and seismic load.
In Example 1, MT due to seismic is provided by 'OTM', but nobody provides it to us.

 

[HTURKAK]

Dead load ,wind and seismic load all will result in eccentric load.

This is not true .. You can shift the foundation so the shell axis and CL of ftg will not coincide to avoid torsion.

In Example 1, MT due to seismic is provided by 'OTM', but nobody provides it to us.

Pls look App. E E.6.2.2 for shell axial load due to seismic . There is a worked example at App. EC