For the engineers doing the work.
Modeling parameters, testing protocols, glossary, FAQ. Everything you need to design and qualify a DIS isolation system.
Engineering properties tables.
Engineering property tables are available in US and metric units. Typical values across a wide range of DIS isolators are summarized, DIS will provide specific modeling parameters and fine-tune the isolation system throughout your design process.
Bilinear, hysteretic, ready for your solver.
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 short column may be used.
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 / Kd.
For response spectrum analysis, the effective stiffness Keff and equivalent viscous damping (derived from the isolator's EDC) are required. For nonlinear time-history analysis, the bilinear properties along with the vertical stiffness Kv are used.
An interesting characteristic of elastomeric isolators: their compression stiffness is about 100× their tensile stiffness. Care must be taken when modeling vertical stiffness to ensure the accuracy of the analytical results.
Qualified by code. Verified at scale.
U.S. codes require testing on each project. Prototype tests validate isolator properties over the project's load and displacement range; production tests check each isolator under project conditions. In Japan, JSSI pre-qualification eliminates per-project prototype testing, reducing cost and shortening schedules by up to three months.
Real-Time Testing
Over 30 isolators tested at actual earthquake velocities of up to 60 in/s at UCSD. More than 500 high-velocity tests over the past decade, on isolators up to 53.5 inches (1300 mm) in diameter.
Large Strain Testing
Isolators up to 53.5 inches in diameter tested to over 400% shear strain. The Berry Street Project: a 45.5" diameter isolator tested to 45" of displacement.
One-Mile Wear Test
Performed at the NEES Lab, SUNY Buffalo for the Woodrow Wilson Bridge. Simulates a lifetime of thermal expansion and contraction; the isolator's properties were unchanged.
Glossary for the spec sheet.
The force-displacement plot generated by shear testing of an isolator.
The initial stiffness, dominated by the lead core size, important for service-load response such as wind.
Secondary stiffness, a function of the rubber modulus, total rubber height and area.
Force-axis intercept of the hysteresis loop, relates to damping and service-load response.
Isolator force divided by displacement, a displacement-dependent quantity.
The point in the model where initial stiffness changes to secondary; used in analytical modeling.
Area of the hysteresis loop, a measure of isolator damping.
The vertical stiffness of the isolator.
Design Basis Earthquake, 10% chance of being exceeded in 50 years.
Maximum Considered Earthquake, 2% probability of being exceeded in 50 years.
Real answers from three decades of isolation projects.
What is the design life of the bearings?+
The normal design life 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 and a protective cover are expected to be more durable still.
What does shear strain refer to?+
Shear strain is the isolator lateral deformation divided by the rubber height. Design strains are up to 250%. DIS has tested isolators to more than 400%, each layer of rubber deformed laterally to four times its thickness.
What are typical design displacements?+
In high seismic zones (San Francisco, Tokyo, Istanbul) up to 30 inches (762 mm). For sites farther from faults or on better soil, up to 20 inches (500 mm). In low seismic zones, 2–6 inches. DIS has tested isolators to 47 inches of lateral displacement.
How is the structure's period shifted?+
Adding flexible isolators lengthens the fundamental period of the structure to >2 seconds. Because the dominant frequencies of an earthquake are in the 0.2–0.6 s range, the period shift takes the structure out of resonance.
Does the structure re-center after an earthquake?+
Yes, the rubber provides a restoring force and the motion's shaking characteristics oscillate the structure back. The Eel River Bridge re-centered to within ¼ inch of its original position after a magnitude 7.0 earthquake.
Can a tall building be isolated?+
Yes, the 18-story Oakland City Hall is one example. Tall buildings typically need an isolated period 2.5–3× the non-isolated period; many tall buildings in Japan have isolated periods of 4–6 seconds.
Can a bearing resist tension?+
Allowable tensile stress of up to 50 psi can be applied to an isolator. Actual allowable stress depends on displacement and rubber modulus. In general, tension is avoided in design.
Are isolators 'fire-rated'?+
Claims of 'fire-rated' isolators are not supported by any building code without testing to ASTM E119 by a Nationally Recognized Testing Laboratory. DIS engaged Holmes Fire LP to review code compliance. The code requires no rating in most contexts (e.g., basements without fire load); when needed, fire-rated board materials around the bearing are the most economical solution.
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