What is Seismic Restraint?

Seismic Restraints are elements designed to keep nonstructural components from moving during earthquakes. Each seismic restraint includes 3 main components:

• Connection between the Restraint and the Nonstructural components
• The Seismic Restraint main element
• The connection between the Restraint and the Structural element.

Restraint design procedure

There are three stages for restraint design:

1. The structure of the equipment should be modeled. This model can be a SDOF system (Single Degree of Freedom) or a MDOF system (Multiple Degree of Freedom). For structurally similar non-structural equipment one can use commercially available finite element computer programs to model the structure – three-dimensionally where appropriate.

2. Quantify horizontal force and uplift applied to the OFC during an earthquake. In order to find the horizontal force (i.e. the base shear for structural analysis) the following parameters are required by most of the available codes:

• The seismic zone where the supporting structure is located, and attenuation parameters. The component’s elevation or its position with respect to the structure’s height.
• The flexibility of the equipment and existing attachments (i.e., in NBCC 2005 equipment with a natural time period less than 0.06 sec is considered rigid).
• The type of material the equipment contains (i.e. whether it is considered to be a toxic or explosive material).

3. After finding the applied horizontal and vertical forces the restraints and the required connections can be designed. In order to design the restraint system, sometimes one needs to use manufacturers directions (e.g. specification given for seismic snubbers in the manufacturer’s catalogue). On occasion, one needs to design a custom restraint. Wherever possible existing standards and guidelines are used for design.

Often one can find physical restrictions such as structural elements (e.g. pre stressed RC slab) or space limitations (e.g. not enough room to install the designed restraint) that result in modifications being made to the standard restraint design. In this case, a conceptual sketch is prepared at the preliminary phase of the project. For the best results, the design engineer should work closely with the fitting fabricator and installer to come up with the optimum plan. A good example of a typical situation is the use of a moment post as an alternative to a forty-five degree brace, due to lack of space or potential interference.