Reinforced Concrete beam advice required

[relentlessratt]

i am starting a new house project and would like some advise with regards to the design of a concrete beam.Tthe detail for the Reinforced concrete beam which sits above the garage is described below:

Beam size: 12" wide x 20" Depth
Top Reinforcement: 6x5/8" Rebar at 1/3 span to the ends of the beam. the Center 1/3 is reinforced with 3x5/8" rebar.
Bottom reinforcement: 6x5/8" Rebar for the center 1/3 span. 1/3 of the span on the ends is reinforced with 3x5/8" rebar.

The contractor has indicated that the quantity of of reinforcement may create an issue when pouring concrete and may cause voids. he has suggested that the rebar diameter be changed from 5/8" to 1" and reduce the quantity of rebar to 6 pieces (3 to the top and 3 below).

can someone with construction experience/knowledge provide some advise on this?

 

[relentlessratt]

i forgot to mention that the beam span is 20 ft

 

[BAretired]

I tend to agree with the contractor, but I do not feel comfortable in modifying a beam design without knowing a few more details. Who designed the beam? Why don't you ask him whether or not the contractor's suggestions could be employed?

BA

 

[TXStructural]

It depends on whether the beam is on its own, or fixed to a concrete frame. What you describe is really not the most economical solution. In general, the economical solution in a continuous frame is continuous bars top and bottom, and not more than 3 bars in something 12 inches wide. You will need a minimum of two bars top and bottom in all sections where there is shear reinforcement. Closer together than about 3-4 inches makes it difficult to get concrete into the space below the bottom bars. Design usually considers the total area of steel in critical sections. Larger bars do have longer development requirements, so use of non-continuous bars introduces unnecessary complexities on small jobs. The total cost of reinforcement will be about $250 if you use 3#8 T&B (not counting stirrups). If you reinforce it as you first describe, the steel cost will be about $200, and there could be problems.

 

[relentlessratt]

Thanks for your responses.

TX,
the beam is fixed to a concrete frame which supports the top concrete floor. If the 1" bars are used as i described (3 to the top and 3 on the bottom), the requirements of the original design will be satisfied (with respect to steel area in the different sections of the beam) and the bars will be continuous. The difference in cost is really negligible. I just wanted to know if my thinking on this subject was correct. I am obviously not a structural engineer.

 

[briancpotter]

The contractor is correct that concrete placement will be difficult with a spacing that tight, though as others have indicated the degree it can be changed depends on other details of the beam.

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[UcfSE]

If you change to a larger bar, you'll probably need a larger development length on the ends to get the top bars to work. Do you have that room, and that length, hooked or straight? The bottom may be ok as is.

Is it possible that the original design intended for the bars to be in two layers?

 

[TXStructural]

Sorry for the delay, I've been on the road.
Since you are not a structural, you do need to engage one before making changes (working outside of ones expertise is a good way to get sanctioned) It should be a very small or no fee decision, but the nuances are not easily explained here.

It may also be possible to simply bundle the #5 bars into pairs, with the same effect as changing bar sizes, but not changing the material requirements.