Whether it is PTFE (Teflon®) Slide Bearings, Elastomeric Vibration Absorbing Bearings, AASHTO grade Neoprene Bearings or any other bearing, it is imperative that the substrates are accounted for if the bearing is expected to function properly and serve to the longevity it is capable of.
The most common substrates, steel and concrete, present two different situations. The outsides of steel I-Beam Flanges tend to be relatively flat and provide adequate substrate surfaces. In comparison, concrete is often uneven and inconsistent. Normally, where a bearing can often rest directly on an I-Beam flange, if the substrate is concrete, the plans will frequently call for an “adjustment pad,” known as an UNDERLAYMENT.
Below, Fig. 1 illustrates a typical Elastomeric Bridge Bearing designed to sit on a concrete pier. The thin pads located to the left of the bearing are the underlayments.
Its required lifespan and constant pressure require the underlayment to be very durable. Frequently, the product is made from a Viblon or Sorbtex material. These materials are fabric infused rubber pads, built by adding multiple layers of fine cotton mesh to increase the lateral and compressive strength of the rubber.
Fig. 2 illustrates a side view close up of a ¼ inch thick Viblon pad that may well serve as an Underlayment.
The function of this entire bearing assembly is to dampen the vibration caused by traffic passing over the bridge. This benefits the bridge structure, spans and the concrete piers on which the bridge rests. The black Elastomeric Bearing (Fig. 1) directly absorbs almost all of that vibration. Any residual energy passing through the Elastomeric Bearing will transfer into the steel base plate and cancel in the Underlayment.
As previously mentioned, the underlayment serves as an additional purpose. On uneven surfaces, as will be the case with concrete, the underlayment pad provides significant relief for the high spots that would otherwise support 100% of the pressure. This protects the concrete from unnecessary wear, cracking and premature failure.
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