Options for Vibration Isolation with a Seismic Mass

When it comes to designing a seismic mass to isolate vibrations from your test system, there are several options. The mass can be suspended on springs or airbags, on vibration isolation blocks, on piles or piers, or it can be mounted directly on the soil. Let’s explore the various options:

Airbags

Probably the most deterministic approach to isolation is to suspend a concrete mass on elements with predictable stiffness and damping characteristics. Airbags provide an excellent suspension system with low stiffness, and relatively high damping. An external air volume can be added to lower their natural frequency, and intermediate plates with orifices can be added inside the airbags to increase their damping. It is now possible to build airbag-mounted seismic mass systems with natural frequencies below 1Hz, with up to 10% damping (up to 25% damping is possible with higher natural frequencies). 

The issue with airbags is that they require a large expensive pit to allow access for maintenance. The foundation system and related hardware is also very expensive. The structure requires a lot of concrete pours, which adds significant construction time, and the system requires compressed air. Another issue for free-body automotive testing is that while airbag natural frequencies are low, they are still very close to the wheel-to-body natural frequency of a vehicle; a very critical test frequency. Airbags rob the test system of valuable low frequency displacement performance that is taken up with moving the mass.

Isolation pads and blocks

For smaller masses, it is possible to use a grid of rubber isolation blocks. Again, these blocks have a well defined natural frequency, and work well if the test system or equipment does not operate at or near that frequency. Their damping is low, which can be good and bad. Bad if you happen to excite them at their natural frequency, but good if you are above it. On an underdamped system, the transmissibility drops rapidly above resonance. The natural frequency of these blocks (3~6Hz) is higher than the natural frequency that can me achieved with airbags.

Piles/Piers

If the soil properties are poor, it is possible to support the mass on piles or drilled piers. If the pier engages with bedrock below, the piers themselves become springs, with high natural frequencies and low damping. Piles engage a column of soil, and have more damping, but again, their stiffness is relatively high. It may be possible to design a pile-based system where the natural frequency is higher than the bandwidth of the intended usage. Piles and piers can reduce the mass required, and are still less expensive than airbag-based systems.

Soil coupled

By far the majority of masses are simple blocks resting on soil. Most soils can support a pressure of 3000 psi or more (144 kPa), which is enough to support the mass, and the soil provides excellent stiffness and damping properties. What makes soil unique is that it’s stiffness drops and its damping increases as a function of shear strain. In other words, as the mass moves on the soil, the damping improves and its natural frequency goes down. This is a perfect scenario, because when you are operating above the natural frequency, it is better to have lower damping, so that you get better isolation. This is also the time when the shear strain is low. Also, if you happen to operate at the natural frequency, it becomes self-attenuating. Soil coupled masses are the least expensive to build, and require no maintenance. The only disadvantage is they are difficult to design, and require more rigorous soils analysis than is typically performed by a geotechnical company. However, the cost of this analysis and design is very small compared with the added expense associated with using the active element solutions described above. 

Our Services

Our extensive experience with working in over a hundred test labs around the world since the early 1980's places us in an excellent position to provide you with the solution that meets your needs, budget and timeline.

Beware that most companies that specialize in vibration isolation will tend to use methods that sell more of their products. Re:Test, Inc is completely unbiased, we source isolation products from multiple vendors, and have designed masses using all the above approaches. We use the most appropriate method given the site, budget, schedule, and intended usage.

Re:Test, Inc. also designs the bedplate and anchoring system for the flexibility you need, while ensuring the maximum loads caused by events such as a runaway actuator rod do not shear the plate, concrete or anchors. We also have access to registered Professional Engineers in all 50 States, so that the drawings we provide are stamped Ready for Construction.

We can also help you design the rest of your lab, including trenching, control rooms, cableways, utilities and hydraulic power.

For more information visit https://re-test.com/lab-design