MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS 5

[engrbon]

Been working abroad for quite some time now and I was kinda surprised when my friend told me that he attended a seminar in our country and the speaker told them that the required reinforcing area for the positive moment of a reinforced concrete beam is 50% of what is required for the negative moment at the same cross section. Our country's code is almost an adaptation of ACI for concrete structures. But I'm not well aware of such from the ACI code, if there is, can anyone please guide me and show me the section of the code stating what I have mentioned. Thanks.

Just for additional info, the subject matter is for singly reinforced concrete beams.

 

[engrbon]

Thanks Sir msquared48 .... another lesson learned.

 

[engrbon]

Thanks Sir steellion.

 

[TXStructural]

Nothing in ACI would require this in a general case.

In a continuous beam, or a fixed end, moment frame beam, ignoring lateral moments, you can determine strength required for a single reinforced beam (bottom, midspan, using WL^2/8), and place half the bar bottom midspan and half top at the ends to redistribute the moment.

You would do this to control deflection and cracking in certain situations, or to reduce congestion. But you would not need to put full single reinforcement in bottom, mid-span, AND 1/2 more at the ends.

 

[engrbon]

^ WL^2/8 Isn't this the formula for the maximum moment of a simply supported beam? Why not just use the actual moment as per analysis whether it be simple, continuous, fixed end, etc. and from there you start your design and adjust your rebar area & curtailments as per code requirements.

 

[engrbon]

WL^2/8 - I mean the formula for the maximum moment of a simply supported beam with uniform load.

 

[msquared48]

Depending on thew fixity or release conditions, you may wind up using a lot more reinforcing than necessary. In fact, you coulod end up with an over-reinforced beam for the conditions if you are not careful.

Best to either use the appropriate equations for the actual conditions, or revert to those listed in ACI.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[engrbon]

^That's right sir we might end up having a steel area more than 75% of the balanced steel ratio.

 

[TXStructural]

Obviously, you would use appropriate resistance factors when determining the appropriate design. And as you know, whether or not a beam fails in compression is not a function of the nominal strength, but rather the overall design. Having a beam which is stronger than required is the whole point of using: phi*Mn>=Mu

My point is that taking a maximum moment (as I point to with the example equation for uniform loading), you could end up designing for redistributed moments, with 1/2 of the steel in top (ends) and 1/2 in bottom (mid span).

As to your original question, there is good reason to use compression steel, but not an IBC or ACI 318 code requirement to do so when designed for simple span, gravity moments.

 

[JAE]

TXStructural - I'm not sure it is appropriate to just assume a re-distributed moment as you seem to be describing (from bottom positive to negative top steel).

ACI is pretty explicit in how you redistribute moments - usually taking 90% of the calculated negative moment and adding it to the positive - but only if you meet particular rules regarding reinforcement ratios.

WillisV provides the correct answer, in my view, to the original post in refering to the seismic portions of ACI to explain perhaps where the source of the 1/2As came from.