Seismic Mass Isolation

[OlafTheBlack]

I have a project to refit 25 tons of bells in a bell tower, which is to reframe a bell carriage. This steel framed tower was built in the 1930’s before seismic loading was considered a requirement in this locality. I am looking to improve the seismic performance of the bell to tower interaction. In this location seismic forces are about the same as hurricane forces, so the local practice is to design for both using the same lateral system. For this retrofit, I suspect some sort of damping system is the way to go, but in this arena I have no experience.

Where is a good place to start to learn about the analysis and design of seismic isolation of large masses? Is there a good resource to look through for product devices?

 

[Qshake]

First let me say that isolation and damping are two different though not exclusive items.

Isolation would require that the largest contributing mass be independant (or nearly so) of the mass-spring-damper system. Accomplishing that and keeping the bells functioning is the tricky part. Since we don't know the details of the connection and the mechanism for making the bells work we can't help a lot.

As for damping, this requires that we dissipate energy through a velocity dependent system. Presumably would be the angular velocity of the bells as they are excited by either ground/tower vibration, sinusodial wind surges, or the tolling of the bells.

Braces which act to dampen seismic induced forces in hanging utilities are available but are not practical to use on the bells since they would impede the tolling.

You can start with the bracing company and expect to some research with their help.

Good luck

Regards,
Qshake

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[OlafTheBlack]

Thanks for the response.

The bells will be hung from a new carriage, a steel frame. This frame is a substructure within the building primary frame at the belfry level. The bells don't swing. They are struck with whackers put into motion manually by bell ringers. Some of these bells are big. One is about nine feet in diameter. The biggest bell is struck with a whacker that weighs over 800 pounds, so this provides some lateral force (probably not that large) that needs to be resolved in the carriage.

My first analysis of the primary frame of the tower is that it is very stiff. Wind deflects it a fraction of an inch. Seismic on the other hand from a static loading approach deflects the tower 8 inches. I haven't yet determined how much of that is from the mass of the bells.

I'm trying to sort out my design approach. The code does not require me to make this building designed for earthquake loading, but since we are buidling a new carriage, I want to take the opportunity to accomodate earthquake mitigation if it is economically viable.

I see the project ranging from building a new carriage ignoring earthquake loading (our state code permits this for existing old buildings), to a mitigation of seismic damage perhaps by isolating the base of the new carriage or adding dampers to knee bracing or a combination of both.

I think I first need to determine the dynamic deflections, velocity, and accleration of the belfry with and without the bell weight. This should give me an idea of the magnitude of displacements and relative contribution between the massive bells and the primary frame to displacements.

To get an image of this, the bell carriage would be about 24 feet square with a column at each corner. I may include a column in the middle to help spread the loading over the floor framing. I expect I might have two rings of girts. One at the tops of the bells say about ten feet up and another ring about 15 feet up. I can connect these two rings together and to the posts with knee and corner bracing for lateral stability. I'm thinking that dampers could be part of the bracing. If the improvement from decoupling shows promise, maybe I could add base isolation to the carriage.

Seismic isolation and damping a new problem area for me. Any insight is welcome.