Seismic Isolators - Tech Notes

MECHANICAL, ELECTRICAL AND PLUMBING (MEP), SEISMIC RESTRAINT SELECTION GUIDELINES:

With the continuous push to ever more stringent life safety-related building codes, it has become commonplace for historically non-seismic regions of the country, to require seismic restraint of MEP equipment. VMC as being members of many of the councils and committees on the subject, and as being a founding member of VISCMA, (Vibration Isolation and Seismic Controls Manufacturers Association), has the expertise necessary to assist you in the proper selection of seismic restraints. There are many places to receive information on the application of seismic restraints. Two such references are ASHRAE Applications, Chapter 58 and FEMA 412, 413 & 414, Seismic Installation Guides for MEP Equipment. 

With the local adoptions of the latest model building codes based upon ASCE-7, installations of non-structural components must now be viewed as part of the seismic system in a building. Examples of the new model building codes are IBC? 2000 through IBC? 2018. This condition requires the manufacturers of components within that system to be structurally and/or operationally compliant with the loads specified in the building codes. Depending on the calculated ‘Seismic Design Category’, nonstructural components may be required to fully function after a seismic event. The manufacturer will now be required by the local building official to have a ‘Certificate of Compliance’ attached to its equipment. An in-depth study of IBC? Chapter 17 is required to fully understand the scope of this requirement. You may also consult www.ibcapproval.com for further information on compliance.

In the latest version of the code, all calculations are performed per ASCE7.

The predominant force from a seismic event is a lateral or ‘shear’ load which is introduced at the base of the building or structure.

This load is transmitted through the building and into the equipment to which it is attached. After this, the load is assumed to act at the center of gravity of the equipment as an inertial load measured in “g’s”. The initial design work will be to determine the adequacy of the supports that attach the equipment to the building structure. The attachment points where the equipment attaches to the building structure need to be designed to ensure that the equipment stays in place after an earthquake. These points are also called reaction points.

To calculate the loads at these reaction points, a free body diagram must be made of the equipment, showing the CG (center of gravity) location and the reaction locations. The direction of the applied seismic load is to be such that it creates the highest loads at the reaction points. These reactions will have horizontal and vertical components. The vertical component will be induced by the vertical cg height of the equipment. The angle of the force that creates this worst case is called the ‘principal angle’.

Once the highest reaction load is determined, the support is analyzed for adequacy. Generally, this load will be applied to the top of a seismic restraint and carried into the building structure through the restraint. The calculated load shall be compared to quoted allowables for the particular isolator being used. Throughout this catalog, VMC shows curves of allowables for our seismic mounts.

After this load is carried through the isolator, it must now be carried into the building structure by means of bolting or welding. VMC provides bolt holes in all of its floor isolators for anchoring to concrete or bolting to steel. A calculation needs to be performed to calculate the induced tension on the bolts and compare these with quoted allowables for the particular bolt being used. 

Typically, ICC-ES publishes load ratings for most common concrete anchors. Where the installation is into building steel, the use of ASD allowables is acceptable.

What follows is a typical approach taken when determining what seismic restraint is required for a particular application.

When beginning a seismic installation analysis, there are a few important questions that must be answered.

1. What is your location in the country?

a. This will determine the un-factored local seismic force.

2. On what type of soil is the structure standing?

a. This will determine the soil factor to be applied to the seismic force.

3. In what type of structure is the equipment being installed?

a. This will determine the seismic use group of the building.

4. What type of component is being installed?

a. This will determine the amplification and importance factors to be applied to the seismic force.

5. Where within the structure is the equipment being installed?

a. This will determine the amplification due to the installed height within the building. As the unit’s installation point within the structure gets higher, the seismic load increases on the equipment due to the ‘pendulum’ effect.

6. How am I attaching the equipment to the structure?

a. This will determine the attachment amplification factor.

These questions each have a specific piece of information necessary to determine:

1. Does the equipment need to have seismic restraints?

2. For what level of seismic force do I need to design?

Using the most prevalent codes currently available, we will go through a few examples. Pages and tables are referenced to ASCE7-10.

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