Question

What can we summarize about simply supported beam bridge?

Answer 1

A)When a bridge is made up of beams spanning between only 2, supports it iscalled  simply supported beam bridge.

If 2 or more beams are joined rigidly together over supports,then bridge becomes continuous.

Simply supported beam bridge

• Short span bridges.
• Simplest form of bridges.
• The bridge beam is supported by piers or abutment at ends .
• There is no transfer of moment throughout the support.
• Requires low design effort.
• Can be made out of Prestressed concrete, Reinforced Cement Concrete , Iron , Steel and Timber.
• They are usually not made longer than 250 feet or 80 metres.

Forces acting on the beam bridges due to movement of loads are Tension(pulling) and Compression(pushing) forces.

On the Top there is compression and on the bottom, tension. The forces are carried down to the ground via the supports.

Also, as opposed to fixed support bridges as SS (simply supported) bridges have non-rigid connections. Next time when you are driving on a bridge, notice the bridge moving little. You can feel it swaying left and right. This happens because the bridge is simply supported on both of its sides. Hence connections are not rigid which makes the movement free. This helps in the durability of the bridges. Otherwise, fixed connections could lead to cracks and cause the failure of the bridge.

Now let us o through some technical aspects.

This is BMD and SFD of a fixed beam with UDL

This is BMD SFD for SS beam::

As we can see:

In a fixed beam, BM at span is lower than SSB. So for this length of span lesser concrete and steel will be required. This helps in material savings.

However in fixed beams at supports moment is generated.

You will find zero BM at support in SSB while in a fixed beam it has some value. In frame structure of beam and columns, this moment is transferred to foundation through columns. Hence foundations have to be designed for additional moment which results in increase in the quantity of steel and concrete required.

For the simply supported span, if the support sinks there is no consequence to the spanning beams. If there is a continuous beam over this sinking pier(support) additional moments are introduced resulting in additonal stresses.

Another aspect is the expansion/contraction due to temperature variations. On a simply supported span, these have no effects.

So, most of the bridges ( where the depth is not too much and there are many design constraints )are constructed as SS Beam bridge.