bridge bearings explained for the layman

For almost the entirety of the past decade, I’ve been associated professionally with the design and development of bridge bearings in some way or the other for my current employer in the United Arab Emirates. I’ve worked on other product ranges as well, but my expertise and professional acumen are persistently pinned to this particular product owing prominently to a large amount of time I have spent on the design of bridge bearings over the past several years. Many of the people whom I meet, not necessarily from an engineering or technical background often get curious about what I do and when I start talking about bridge components, they’re bewildered. So I decided to write about bridge bearings for the layman in sort of a lucid non-technical parlance. As a prelude, let me enunciate that bridge bearings, as such is a broad discipline and I’ve worked only with a few types of bearings and is still on the path of learning for expanding horizons and filling knowledge gaps. With that sphere of humbleness, let’s begin.

Next time, when you travel through a bridge or any underpasses nearby, just before entering a bridge, pay close attention to the piers of the bridge and how those segments are held together. If you notice closely, you’ll find some devices at the interface of the top and bottom structures. These special devices are bridge bearings. Chances are that you wouldn’t have even noticed that these devices are in place. In some cases, these devices may not be visible directly as they’re mostly covered by Aluminium or steel cover plates to prevent any potential corrosion risks. But now you know that there are devices like this around.

The top portion is generally called “superstructure” and the bottom portion is called “substructure”. “Bridge Bearings” are devices used for gradually transferring loads from superstructure to substructure. These loads would include self-weight of the structure, traffic loads, wind loads, earthquake loads, etc. They also facilitate movement induced by expansion/contraction of concrete. These movements include thermal movements as well as creep and shrinkage of concrete. To some extent, they can also take rotations as well. Rotations are generally due to traffic loading, construction tolerances and general settlement of foundations, Based on the load capacities, configurations, etc, there are several types of bridge bearings used worldwide in civil installations. These include pin bearings, roller type bearings, rocker type bearings, sliding bearings, knuckle pinned bearings, pot bearings, elastomeric bearings, disk bearings, spherical bearings to name a few of them. Different configurations are designed and manufactured depending on the specific set of applications in which they are being used. Bearings can also be used for restraining movements in one or both directions as well as allowing movements in any one direction. Accordingly, different types of accessories are used on the bridge bearing assemblies to achieve the required purpose.

Some of the components which are used in the bearing include elastomers (rubbers and other engineered polymers), structural steel plates, bolts and sockets, round bars, PTFE (commonly known as Teflon), stainless steel sheets, etc. Depending on the type, a bearing can contain one or many of these components. Bearings are used in bridges, buildings, steel frames, offshore structures and a lot of other places. Many international codes are used by international manufacturers for designing these components.

To give some slice of history, until the end of the 18th century, all major structures were built using stone, bricks or mixed masonry. These structures are least affected by normal environmental changes and any slight movements that might occur are normally compensated by minor support displacements or deformation of materials involved. In the 19th century, cast iron and steel structures were increasingly used which were flexed to larger span lengths and they eventually turned out to be more slender and flexible and were required to be fitted at their support points through special devices. These were the first “bearings”. These could withstand movements induced by expansion/contraction due to temperature changes. Initial bearings consisted of simple metal plates sliding over each other or steel rollers or both of these combined. To provide rotational capabilities, swivel arrangements were extensively used. As technology progressed to the 20th century, Reinforced Cement Concrete (RCC) structures took steam. After the second world war, the situations demanded and favoured the extensive rapid development of reinforced concrete and pre-stressed concrete. Slender structures started to be reliant more and more on these bearing devices to accommodate rotations and movements. Subsequent years lead to the development of elastomeric bearings made of plain rubber. Later, steel plates were used inside these rubber pads and this specially designed stack became capable of sustaining larger loads and magnitudes. Subsequently, other modern bearings like pot and spherical bearings were developed which includes a combined usage of elastomeric, polymeric and structural systems that work together to be used for achieving higher load capacities, movements and rotations. Elastomeric bearings and its variants (with support plates and accessories) still continue to be the most flexible, robust, cost-effective and maintenance-free bearing solution.

Now, if we delve in more, we’d digress away from the layman terminology we’re using and would enter the techno geeky world. So for now, that’s some basic layman understanding on bridge bearings.

Bearings are an integral part of large infrastructures such as buildings,  heavy buildings and high rise buildings serving critical functions and ensuring the safety and stability of structures.