A continuous RC beam chipped off at midspan 3

[IJR]

I have a continuous RC beam 2.5ft wide(75cm) and 1.5ft deep (roughly 50cm). At midspan, for architectural reason, I am required to chip off from this rectangular section a segment 1ft x 1ft (30cm x 30cm). This leaves me an L-shaped section at midspan with the horizontal segment at the bottom.


*
* leg "a" (vertical)
*
* * * *
leg "b" (horizontal)

Which one is healthier?

1) use the leg "a" all over the length for stiffness and carry out design using only this leg, treating the rest as simply an added wt(dummy block), with constructively selected reinforcement in this dummy block

2) Use full section at support zones and reduced section at midspan and detail accordingly.

I am worried about cracks in leg "b" due to strain compatibility and I think this post is worth its creation.

respects
IJR

 

[ishvaaag]

2 should be better in that it will give more accurate results both at ends and center. Otherwise you in particular in your model you will have at joints less stiffness and maybe negative than there must be. Since the in tension zone cracks are more than anything dependent of the amount of steel present, having the normal care with cracks should prevent any problem. Main problem I would feel in the reduced section be of scarce capacity, that from your statement seems not be the case here.

 

[Ron]

IJR...you can do either, but I would select Option 2, with your reinforcing cross section mimicking the "L" shape. I would also provide diagonals across the lower portion at 45 degrees to the inside of the "L".

*****
* *
* *
* \ *
* \*****
* \ *
* \ *
*********

 

[JAE]

Ron, you put AutoCADD to shame.

Agree with the diagonal. The beam cross section is very much like an edge beam with a brick ledge. We usually use typical stirrups (two vertical legs) in the "a" leg, and then add a similar "U" shaped stirrup, laid on its side, extending into the "b" leg.

Option 2 - agree that its best. With a continuous line of beam, the change in Ix will affect the resulting moments...but not by too much if the cut out is short.