Earthquake analysis

Is it possible and/or practical to do a earthquake analysis on a buried pipeline with Caesar?

Possible - yes. Practical - probably not.

Seismic loads enter a piping system through its supports. The load comes from the inertia of the piping system trying to follow the moving supports. Buried pipe is continuously supported so it will not “see” these inertial loads. The only flaw in this line of thinking is that if the pipe crosses a fault, where the soil shears, you're in trouble.

One other point to consider is that the buried lines will experience the pressure wave as it travels through the soil – and not only at fault lines. (An analogy would be a rope floating on the water where the water waves deflect the rope.) The soil waves are compression and extension (like sound waves) rather than the transverse waves of the water surface but the point is the same. Those deflections could pull the pipe apart. If you had the deflections, C2 might give it a go but this might not be a valid approach.

Richard,

One thing is not clear to me from your response.You have mentioned that the buried pipe will not be subjected to intertial loading as it is continuously supported.

My question is , from Newton's second law of motion, intertia force is equal to mass times absolute acceleration. In case of a buried pipe , which is continuously supported, the only way the intertia force can be zero is by the absolute acceleration to be zero. Why the absolute acceleration will be zero and how this absolute acceleration changes w.r.t whether the line is supported continuously like buried pipe or intermittently like above ground pipes?

With best regards

The reason the pipe won't see the typical inertial loads (typical compared to above ground piping) is because in a buried system, the pipe moves with the ground, just as the supports do.

Baburao,

You're right, Inertia force is mass times absolute accelaration.

Now what is this absolute acceleration? It is the sum of two components:

Acceleration relative to ground + Acceleration of ground.

You must be knowing that for engineering analysis, the Newtonian Frame of reference is the one fixed to earth ( although that was not the original Newtonian frame which was "fixed to sun" and not "earth"), hence the absolute acceleration is zero.

Regards

For further info...

http://www.iitk.ac.in/nicee/IITK-GSDMA/EQ28.pdf


Regards

Let's try to explain this from another viewpoint.

For above-ground piping systems, the supports are placed 20-50 pipe diameters apart. This gives a formula for the natural frequency of a straight pipe between two supports of the form f = K/(2 * pi) * sqrt (E*I*g/(w*L^4)). As the span between supports gets longer, the natural frequency decreases. As the natural frequency decreases, the amplitude of the displacement envelope increases. As the amplitude of the displacement envelope increases, the stresses induced by the vibration increase.

Agreed?

OK, now for buried pipe the distance between supports is infinitely small. What, exactly, do you think will happen to the displacements and stresses?

As Richard notes above, there are obviously direct interactions between the soil and buried pipe. However, these are on a scale where local FEA is necessary - C2 is not really valid for local effects.

So I would not recommend trying to solve this problem using C2. For the effects for which C2 is valid, the stresses are by definition very small. For effects which may cause significant stresses, C2 is not valid.