Fixed and simply supported RC beam behaviour

[cn12dn]

Hi all,

Typically, if we had a reinforced concrete beam spanning between two RC columns, cast monolithic, we could treat the beam as fixed-fixed and provide mid-span bottom reinforcement and top reinforcement near the supports.

However, if we decide to design the beam conservatively as simply supported, will the concrete behave as simply support as it is detailed in this way? Initially if it behaves as fully fixed until the top of the concrete cracks in flexure at the supports, would it then shed load to behave as simply support as that is how the reinforcement is detailed (bottom only)? If we are designing a beam as simply supported, how much reinforcement should be specified at the supports to prevent flexural cracking?

Thanks!

 

[BridgeSmith]

It will behave as a partially restrained connection. Where it actually is on the continuum between free and fully restrained, depends on how it's reinforced. A typical connection, utilizing a fair amount of reinforcement on the tension side, will be closer to the fully restrained end of things. Getting a close approximation of restraint condition would require a fairly detailed finite element model.

The way we typically handle it is to do an 'envelope' design, where we check the critical reactions on each member and connection for the fully restrained and fully free assumptions. There are some situations where the stiffness of one member is significantly greater than the connected member, where we assume the stiffer member to be rigid, in order to simplify the analysis.