Reinforced Concrete Beam End Restraints

[haynewp]

I have a RC beam that is bearing on a wall at one end and integral with a column at the other. I could design the beam as partial fixed to the column and pinned at the wall. To make reinforcement placement simple, I was thinking instead to design the beam as pin-pin so it will mainly just have bottom bars. I don't have to worry about the integrity of the column to beam joint either.

I know the beam will work this way, however I am a little concerned with the cracking that will occur at the top of the beam at the column end as this end rotates to achieve a "pin" condition. Anyone else have experience with this situation? Should I add a few top bars at this end anyway to control the crack width?

 

[dbuzz]

Cracking at the beam-column interface, would also be my concern. Why not design the beam and beam-column connection for the pinned-fixed support conditions? The calculations are certainly simple enough.

 

[Taro]

In general, you can not ignore end fixity and design beams with negative end moments as simply supported. You can redistribute some of the end moment to the midspan if the section at the support is properly reinforced and ductile enough to withstand the rotation required for redistribution. If ACI 318 is the governing code, see Section 8.4 for specifics.

 

[haynewp]

Dbuzz:

It is a T beam that I designing on the ramp of a parking deck. In the negative moment state, it doesn't have the wide compression flange like it does in the positive region.
The amount of negative reinforcing required to achieve a fixed end is larger than the maximum allowable amount of tension reinforcing (due to the decrease in section).

Taro:

It is ACI that I am using. This is a single span, (simple span if pin-pin), is 8.4 not addressing continuous members? Do you feel these provisions should apply to my case as well?

 

[JAE]

I'd think then that you'd have to increase either your web width, b, or deepen the member to get under the max. steel limit.

I agree with the above that you cannot just ignore the top moment. The cracking that would result could significantly affect your shear capacity in the beam at the column.

 

[haynewp]

Thanks guys

 

[AlohaBob]

Or you could be creative with design of your bearing condition. Suppose you designed the bearing surface of the connection to allow rotation and yet mechanically tie the members together? The precasters use masonite pads in their connections effectively for this.