Seismic Design with Friction Dampers

Structural Analysis of Seismic Friction Dampers

The friction damped brace or yielding restrained brace (YRB) is modeled as a link element in most software. In other words, it is modeled as a fictitious yielding brace.  Because the Ten-Co seismic brake can be treated as an ideal elastoplastic element, this allows the application of Wen’s model. The assembly behaves as a friction damper. While a simplification of the damper’s behavior, the Wen model simplifies the analysis.

A yielding brace or Buckling restrained brace (BRB) would yield and begin to deform allowing the building to absorb and dissipate the earthquake’s energy.  However, the brace would need to be replaced after the earthquake and the capacity of the fuse element changes as the steel strain hardens.

In contrast, a brace equipped with a Ten-Co seismic brake (usually idealized as an inline seismic friction damper mechanism) is a damage free approach that slips instead of yielding, and, by means of the elasticity of the primary structural elements returns to its original position. This is an important distinction as the technology reduces displacements through energy dissipation allowing the primary elements to remain within their elastic range (e.g. 1%of storey height or less in some structures). Studies have shown that a moment frame capable of even just 25% of the design base shear elastically is sufficient to re-center the damper mechanism. Other studies have shown that the moment resistance in shear tabs can also be sufficient in many cases. Please contact us for detailed references.

The inline friction damper assembly’s ultimate response force remains symmetric in tension and compression (hence the name Ten-Co), and has a high initial stiffness and is independent of displacement. This important characteristic simplifies modeling and maximizes energy dissipation as large displacements are not required in order to dissipate significant amounts of energy. For very large spans with low loads or tension only bracing please see our X-brace rotational friction damper.

The Ten-co Seismic brake’s hysteretic curve allows the damper assembly (Ten-Co and brace) to be modeled as a link in static, dynamic and non-linear analysis. The only information needed is the properties of the link which in this case is a fictitious yielding brace with its own linear and non-linear properties. This can be modeled in popular software such as ETABS or SAP2000 using the parameters below. The hysteretic loop is characteristically rectangular and based on rapidly converting seismic energy to thermal energy: which maximizes the energy dissipation.

Response force should be equal to 75% of the actual brace’s yield strength and 130% of the service loads (e.g.wind shear). The mass of the damper assembly will vary depending on the dimensions of the seismic brake and travel required.

This technology allows new ways to answer architectural and customer constraints. We encourage you to contact us at any time with any questions you may have.

Finding the Optimal Response Force

The optimum response force maximizes energy absorption for a given frame configuration and a given lateral force. It has been found that this force is below 50% of the story shear but different forces are often selected by the structural designer depending on his/her constraints and objectives.

Once found, small changes to the response force (e.g. +/-20%) have minimal effect on the structure’s response.

For quick calculations use 1/3 of the story shear, ensuring that the ratio of lateral brace stiffness to total lateral story stiffness (frame + braces) is strictly greater than 0.5 and constant throughout the building height. Please communicate with our Engineering Department for further advice on how to integrate friction damper idealizations and other concepts in your project.