Seismic Loading

[silotank1]

How do I go about designing for seismic loading? Our normal calculation for designing a tank uses the 4 x the bending moment x 1000 divided by pi x diameter^2 is this correct way to go about this? We state that this is designed to 0.1g, is this correct? if so how does changing the g change the calcs? also how does designing to UBC 94 zone 1 change things? I would be greatful for any assistance thanks

 

[JStephen]

What kind of tank? How is it supported?

If it's a flat-bottom vertical steel tank, you could design to the seismic provisions of API-650 or AWWA D100.

If it's not a steel tank, or not in the US, you may find other applicable tank standards.

Otherwise, check into ASCE 7. It may be kind of a mess to extract an actual load out of it, but it does include tanks as "non-building structures".

As to the "4 x the bending moment", where does the bending moment come from?

If you're actually required to use UBC-94 or IBC, check very carefully to see if they just refer back to ASCE 7. And note that for certain tanks, ASCE 7 refers to tank standards.

 

[silotank1]

Thank you for your reply

It is a flat Based GRP tank and it is not in the US. The bending moment comes from Wieght of the full tank/10 times the Height of the tank/2

We normally design to BS4994 which doesn't really have anything on Seismic Loading. in the UBC code there are some formulae but I am not sure where they fit into our calculations.

 

[MiketheEngineer]

You might want to get a structural engineer involved that knows and understands seismic codes and forces.

 

[silotank1]

I have been given this equation to work to
R(T) = aN*RD(T)*P*t
Where aN = 2.0 m/s-2 (nominal acceleration for zone 1b and class C building)
RD(T) = Spectrum for Site S2
P = Damping
t = topographic Coefficient equal to 1

There are 2 graphs for RD(T)

Top graph(The graph is in French and says Composantes Horizontales)
y axis is RD
x axis is T
with 4 Lines (S0D, S1D, S2d, S3D)
with respective equations R=1.12/T*(2/3); R=1.36/T*(2/3); R=1.60/T*(2/3); and R= 1.86/T*(2/3)
So I take it that for site S2 I would use the line S2D (R=1.60/T*(2/3))


Bottom Graph (Says Composante Verticale)
y axis is RD
x axis is T
with 2 Lines bottom line (S0D eq. R=1.12/T*(2/3)
Top line (S1D, S2d, S3d eq. R=1.36/T(2/3))

So I am getting there now what I need help with is
1) how do I work out the damping
2) What value should I take for T? from this value I could read of the value of RD from the graphs
3) Would I then take the 2 graphs seperately and add this to my longitudinal and circumferential loadings I have already calculated? Or would I combine these graphs to get a single value for RD(T)?
4) Would this calculation be sufficient? Or would I need to examine any further calculations?

 

[swearingen]

API 650 deals with this directly (including sloshing in the tank) and it refers at times to ASCE 7. The last few ASCE 7's (since 1998, I think) don't use zones any more. It's based off of ground acceleration maps. Unfortunately for you, the only maps included are for the U.S. and its territories.


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