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 friction damper can be treated as an ideal elastoplastic element, this allows the application of Wen’s model. While a simplification of the damper’s behavior, the Wen model simplifies the analysis.

A yielding brace or a 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.

In contrast, the inline seismic friction damper slips instead of yielding and by means of the elasticity of the primary structural elements return to its original position. Studies have shown that a moment frame capable of even just 25% of the slip load elastically is sufficient to recenter the damper. The damper’s slip load is symmetric in tension and compression (hence the name Ten-Co) and is independent of displacement. This important characteristic simplifies modeling and maximizes energy dissipation.

The damper’s hysteretic curve allows the damper 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.

Slip Load 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 will vary depending on the slip load 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.

Model parameters for Friction damper

Hysteretic Loop

Some software allows for the direct input of the hysteretic loop. In the case  that the engineer would like to perform the analysis using these features, the quasi-rectangular hysteretic loop can be used

 

Since the damper stiffness is equal to the brace stiffness up until it slips, the effective and brace stiffness are equal.
The damper slips at approximately constant load and therefore the post-yield stiffness ratio can be estimated at near zero, 0.0001.

 

 

hysteretic loop of friction damper

Finding the Optimal Slip Load

The optimum slip 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 slip load(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 dampers in your project.

response vs slip load

Connections and Installation

Can be installed in

  • Steel Frames
  • Reinforced Concrete frames
  • Concrete or Steel Shear wall
  • Timber frames
Can be used in any lateral force resisting system as a damping system or as a reusable safety “fuse” to protect structures from inelastic deformation or failure.

While there are many possible methods of installing the friction damper the single diagonal tension compression configuration is commonly used. The next most common installation method tends to be in chevron where the damper is installed in the diagonals or sometimes installed at the interface between the beam and the braces. Friction Dampers can be customized to fit almost any building and the link below has commonly used slip loads and strokes.

Friction Damper Catalog

The Catalog will provide you with some examples of standard dampers, their dimensions, and masses. Although small strokes are shown with small dampers the inverse is common: we can also provide large strokes on small dampers and small strokes on large dampers.

When you complete your analysis provide us with the required slip load and stroke and Quaketek will custom design the damper you require.

download quaketek catalog