Seismic action on a combined foundation

[Deckard451]

I have two large stoves approx 30m tall, 7.7m diameter, 10m apart weighing about 1700tonnes each. Seismic forces will govern strength design of an (unfortunately) combined foundation.

Obtaining seismic forces is pretty straightforward but the question is ----

In the event of an earthquake, can the two stoves go "out of phase" of each other or will they always have forces acting on each of them in the same direction.

If they oscillate like a tuning fork then both can move in towards each other and bend the hell out of my foundation. Looking in the orthogonal direction, if one goes one way and the second goes the other way then the foundation suffers a huge torque or twisting action which will double my reinforcement.

Is it possible for the earthquake to induce oscillations in the stoves such that they move in different directions and effectively “fight” each other using my foundation.

I am inclined to think it is possible (mainly because I’m not sure and feel the need to design for it). However we are talking about a fairly sizable increase in build costs and there are some people trying to argue that it can’t happen and that earthquake forces from the two vibrating structures will always act in the same direction.

This alternative arguement is that it is the footing that moves under the stoves and the stoves are mearly trying to "catch up with" the footing. If this is so then I can see that the forces cannot oppose. But my arguement against this is pictures from Kobe which clearly showed that close buildings smashed into each other indicating opposing oscillations.

The two stoves are essentially the same.

Any views would be appreciated.

 

[Focht3]

I think that you have to assume that the stoves will "fight" to some degree. Since the earthquake loading can arrive at different times at each stove, they can (will?) be largely out of sync during a seismic event.

I understand that there are some fairly sophisticated modelling structural engineering computer programs that can use past earthquake events and finite element techniques to try and predict probable structural behavior. But I suspect that level of effort is beyond the scope - and needs - of your assignment.

Perhaps a structural engineer with relevant experience will respond - hope my message keeps yours "near the top."

Good luck -

 

[Sean2]

You could justify allowing for non-synchronous movements by the stoves, by addressing their behaviour (i) during construction, (ii) during operation, where one may be shut down for maintenance, (iii) during a major overhaul, where one stove continues in operation, and the other may be partly dismantled. This is likely to increase the phase shift of movement caused by the difference in arrival times of seismic forces.

Happy New Year!