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.

 

[Lion06]

I'm guessing you mean negative moment of the same beam. I guess in a frame situation you could have negative and positive moment at the same section, but that's really the only condition I can think of.

Can you clarify?

 

[ToadJones]

This doesnt seem to make sense.....

 

[engrbon]

Sorry been a li'l confused let's disregard the positive moment, my mistake. What they are saying is that in a section of a beam the required reinforcement for a singly reinforced beam on its compression side must be 50% of what is required for tension.

 

[engrbon]

Sorry Sir Toad. Just a li'l mixed up please ignore anything that doesn't make sense or you can at least ask me to clarify things thanks.

How come there is no option to edit posts here...(out of topic)

 

[ToadJones]

singly reinforced beams don't have compression steel...hence the name "singly".

 

[engrbon]

^ Yeah you're right but in actual construction you still put rebars at the compression side. Please correct me if I'm wrong please so in my understanding on what my friend told me is that the rebars required for opposite side (I'm not calling it compression anymore to satisfy your point) directly below the tension side is 50% of what is required for tension. I'm looking for a provision in the ACI that states such 'coz as far as I can remember I haven't encounter anything like that.

 

[Lion06]

I don't know why there would even be a requirement for a minimum of steel on the compression face of a beam experiencing negative moment if you're not accounting for it in design.

The only thing I can come up with is an attempt to ensure ductility at supports to allow for greater moment redistribution.

 

[engrbon]

^We only used to consider them as stirrup holders that's why I was a li'l bit curious by such statement...so what I'm asking is that is there a provision in the ACI that states what I have mentioned above if none, maybe the seminar speaker have his own resources or maybe they have updated our country's code to which I'm not aware of. Thanks a lot sir.

 

[ToadJones]

I was thinking that the only compression steel would be that which is placed so that stirrups can be hooked around steel on the compression side

 

[engrbon]

^Yes sir you are 100% right that is what we used to do during actual construction of singly reinforced beam, we use compression steel as stirrup holders. That is the reason why I want to verify the idea that was relayed to me. Thanks.

 

[mijowe]

Continuous top bars are put in to support stirrups, these can be nominal. Top bars can also be added to help control defelction, and reduce creep.

My boss tends to add a similar number of bars that are on the bottom, but with one bar size less. This of course can be overly conservative.

 

[WillisV]

The speaker was probably referring to Special Moment Frames which require positive moment strength at the joint face to be at least one-half of hte nagative moment strength at the joint face (ACI 318-05 31.3.2.2)

 

[engrbon]

^I think that's really conservative as compared to the 50% I mentioned. Thanks.

 

[WillisV]

wow..fingers got all out of wack on that one - try "...the negative moment strength at the joint face (ACI 318-05 21.3.2.2)

 

[engrbon]

Nice one Sir WillisV I'll check that one right away!!!

 

[ToadJones]

You may want to look into ACI 318 under the provisions for shear reinforcement and see if there is anything in that portion of the code. Also for the development of hooked bars in tension.

 

[msquared48]

I'm not surprisedd at the requirement as for many years, basement walls 10" thick or greater have had the requirement that the soil or compression face have at least half of the rebar required for the tension face.

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

 

[steellion]

I use top steel = bottom steel near the supports, then at 1/3 span drop the top steel down to 2 beams to act as stirrup supports.

Going back to the original question, the only thing that I can think of that applies is that a fixed-fixed beam has negative moment = WL^2/12 and positive moment = WL^2/24. Therefore, positive moment is 1/2 of negative moment. But this is only applicable in "true" fixed-fixed conditions. I'll usually design top and bottom steel for WL^2/12 for continuous beams.

 

[steellion]

"2 beams" should be "2 bars", of course. We really need an edit feature...