Design & Modeling of LRB’s
Isolators can be modeled explicitly in analysis software such as ETABS, SAP2000 and LARSA. When software does not support an explicit isolator element, a spring element or a short column may be used to simulate the isolator.
The behavior of a lead rubber bearing is modeled as a bilinear hysteretic element, with an initial stiffness (Ke), yield force (Fy) and secondary stiffness (K2 or Kd).
For response spectrum analysis the effective stiffness (Keff), and the equivalent viscous damping which is derived from the isolator’s EDC (Energy Dissipated per Cycle) are required.
For nonlinear time-history analysis, the bilinear properties of the isolator (initial stiffness Ke, yield force Fy and the secondary stiffness K2) are used. The vertical stiffness of the isolators is also required as part of the element description. An interesting characteristic of elastomeric isolators is that the compression stiffness is about 100 times the tensile stiffness. Care must be taken in modeling the vertical stiffness to ensure the accuracy of the analytical results.
The normal design life of the bearing is over 50 years. Elastomeric pads in highway bridges have been in use for over four decades exhibiting good durability. Isolators with modern rubber formulations surrounded by a protective cover rubber are expected to be more durable and stable in their long-term performance.
Stairways and access points are detailed to be fixed to the superstructure and be “simply supported” on the structure below the isolators. Small sliders are sometimes used to support stairs and accommodate lateral movements.
The bottom section of the elevator is suspended from the superstructure of the building. The framing cantilevers down and is not supported by the substructure. Alternately the plane of isolation can be lowered several feet locally to allow the elevator pit to be isolated as part of the superstructure
Fire protection is dictated by the requirements for the fire-space, not by the materials from which the isolator is constructed. When isolators are located in areas of the structure with no fire load, fire protection is often not required. When fire protection has been required, then sprinklers, spray-on mineral fiber, fire blankets and fire board enclosures have been used.
DIS can provide specific modeling parameters and assist with the fine tuning of the isolation system throughout the design process.