Negative Reinforcement in RC beam

[ravreyes1]

I am constructing a 6 span continuous 500mm x 500mm RC beam with a clear span of 3.6m between RC column supports. I noticed that the top bars at the support(negative moment) is only 3- 20mm diameter deformed bars while the bottom bars is 5 20mm diameter. Is it possible to have more bars in the compression side than the tension side of the RC beam? The beam is to be constructed in a place with no history of earthquake.

 

[JAE]

There is the ability to re-distribute negative moments over to positive moments...check out the code you are under - in the ACI 318 I believe its in Chapter 9.

Also, negative moments can be taken at the face of the column/support instead of the centerline of the support.

The negative moments are also affected by the stiffness of the columns or walls...if they were analyzed together. If this is someone else's design, perhaps you should call them if you are concerned about the reinforcing.

 

[jike]

Although redistribution is possible as JAE mentioned, I would expect to still see more negative reinforcement that positive for a normal loading condition.

Contact the design engineer for a re-evaluation.

 

[dawn836]

i think that may be the connection between beam and column is designed to be rigid connection(moment transfer between beam and column)

But i suggest there may be something drawing in the drawing
refer to the Structural Engineer.

i do not think that moment distruction is done for this beam.

 

[dawn836]

sorry for spelling mistakes in last Post

i meant there is something wrong in the drawing.

and i do not think that moment Redistribution is done for that beam.

 

[flamby]

I suspect the design will result in formation of some hinge at the column joint and may act as pinned joints at best. Does not look like a good design and you call the designer for an explanation.

Ciao.

 

[J1D]

for a 20”x20” continuous beam, regardless of the connection regidity to column, 3-20M at top while with 5-20M at bottom seems a bit unusual. Using 5-20M top and bottom makes more sense.

 

[JedClampett]

I agree with the above posts that this distribution of reinforcing is highly irregular. However, U.S. code ACI 318 section 8.7.3, allows negative moment to be designed at the face of supports. Might this reduce the moments to a point where less negative reinforcing is required than positive?

 

[J1D]

This shouldn't change the fact that the negative bending moment is higher (or much higher) than the positive one even if the the continuous beam is in a rigid frame system and lateral load plays a big role. If 5-20M is required for the positive bending, at least the same amount should be specified for the negative bending.

 

[Tomfh]

Whats the beam going to support?