Bridge Bearings

All elements have a temperature co-efficient, which specifies a change in a material when the temperature increases or decreases. Although very minimal, this change could be catastrophic for some situations if not properly addressed and compensated for.

Such is the case for iron, steel and concrete. They expand and compress as the temperature increases and decreases, respectively. A good way to remember which is which is by the expressions increase = expansion and decrease = compression.

For the mathematically inclined:

If 1 mile = 5,280 feet, then 1 mile equates to (12 inches x 5,280) = 63,360 inches.

1 inch of steel expands 0.00000645” for each 1 degree (Fahrenheit) increase; therefore,  a five degree increase will equate to 5 degrees x 0.00000645 inches = 0.00016125 inches of expansion.

Thus, as the weather gets warmer, steel will expand 0.00016125 inches for every one degree in temperature.

This doesn’t sound like a lot, but if a steel bridge abutment expands and compresses continuously with each increase/decrease in temperature, the steel will start to show signs of cracks, which is a dangerous sign that the integrity of the steel is being compromised.

In this Pipes and Tubes Temperature Expansion, you can see the minimal effect of stresses that are placed on certain metals; however, if not compensated for, the integrity of the elements will be compromised.

The question then is, what happens to bridges that have steel columns? If they expand and compress without any freedom of movement, the steel will crack and this can lead to a defective bridge that will buckle at any given time.

The answer to this are bridge bearings (also called rollers). They are placed in between strategic sections in order to allow for the steel to move as the temperature increases/decreases.

In the photo below, a cylindrical bearing is placed between between bridge superstructures (the vertical supports), called piers. This allows the bridge to expand and compress freely.

Bridge Bearings
Bridge Bearings

In summary, the function of a bridge bearing is to transmit and distribute superstructure loads to the substructure (bridge) and permit the substructure to undergo the necessary movements of stress, which can consist of compression, shear, and rotation, consequently, preventing overstress, which could compromise the structural integrity of the bridge.

There are several types of bridge bearings utilized and are dependent upon a number of different factors, including the length of the bridge span. The oldest bridge bearing involves just two plates resting on top of each other.

Here is a good illustration on bridge bearing plates.

It depicts how the plates are stacked on top of each other, as well as the limitations integrated on the sides, called the the guide block, which prevents the plates from sliding off. This plate scheme can be seen on many bridges, especially those that span highways and parkways.

Another form of modern bridge bearing is the elastomeric bridge bearing, which are more common today and besides allowing freedom of material stress, they also extend the life of bridges by reducing continuous wear and tear on bridge materials.

The Bay Bridge collapse after the 1989, 7.1 Richter magnitude Loma Prieta earthquake is a perfect example when using inadequate bearings for a structure within an earthquake zone. The use of elastomeric bridge bearingshould help to alleviate the engineering errors that caused this part of the road to collapse.

As you travel, you might want to locate the bearings are on the bridges you see and then have a heightened understanding of the importance of how meticulous the construction of various structures are.

Leave a Reply

Your email address will not be published. Required fields are marked *