MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
(OP)
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.
Just for additional info, the subject matter is for singly reinforced concrete beams.






RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
Can you clarify?
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
How come there is no option to edit posts here...(out of topic)
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
The only thing I can come up with is an attempt to ensure ductility at supports to allow for greater moment redistribution.
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
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.
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
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.
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
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.
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
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
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
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.
RE: MINIMUM REBAR AREA FOR COMPRESSION SIDE OF RC BEAMS
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.