Designing a beam as doubly reinforced though it could be done Single reinforced 18

[NewbieInSE]

Hello Engineers,
I'll elaborate it. Say i have a concrete beam having some dimensions, its moment capacity for singly reinforced criteria is 400 kips-ft, say. Moment induced from loads in that beam at the certain location is say 330 kips-ft. It means i can design it as singly reinforced, and say reinforcement requirement is 2.5 in^2 as singly reinforced section.

My question is can i design that section of 330 kip-ft moment requirement, as a doubly reinforced section requiring bottom reinf. say 2.3 in^2, and at top say from calculation .6 in^2 or anything. I think it is possible, considering bottom reinforcement is yielding, but want to know in depth.

I'm actually asking it for existing structural members, which contain less reinforcement (bottom) than required when considered singly reinforced, but contain some top reinforcement which maybe could help in forming doubly action.
Thanks.

 

[Agent666]

but I'd never allowed it to go above

So you just arbitrarily add tension reinforcement until you satisfy this provision you've invented?

So say to cover your design moment you need 2#6 bars, but this results in a 0.25" deep compression block. With your bar centreline being 2" deep at the compression face, you'd quite happily increase the reinforcement by a factor of 8 times to achieve a compression block depth of 2"?


Seems like an illogical approach to getting any efficiency in design? This is why people are calling BS on your insistence on having the compression block or NA depth lower than any reinforcement provided on the compression face.

Note substitute NA depth for compression block depth if that's how you roll, point is either way its a stupid criteria from a practical perspective.

 

[rapt]

hardbutmild

You are correct, it was the lever arm that was limited, not the neutral axis depth (memory must be fading) to .95d which is the same as a depth from compression face of .05d. So neutral axis is about .1d.

With top cover + transverse bar + half bar diameter = 30 + 16 + 32/2 = 62mm, the beam would have to be deeper than 600mm to achieve this to the centre of the bar, or 800mm deep minimum to get the neutral axis depth below the compression face bar.

 

[r13]

Agent66,

Don't get carried over, we are discussing a technical issue, and personal preference.

First we need to clear that my thinking does not involve seismic event, but typical loading conditions. Following satisfied code recommended span/depth ratio, there are still chances to have very small compressive block depth, for whatever reasons that causes. In the situation the NA is near compression steel, or above, if you are willing to take the chances, deflection check is a must. To me, I'll stop to waste my time and energy, and taking the easy route/sure way to get out of that situation, whether adding steel, adding beam depth, or add another beam. I think this will be may last word on this topic.

 

[hokie66]

Whew!

 

[r13]

Amen!

 

[Latexman]

Wow! After all that, I think it's time for an adult beverage in the Pub!

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[r13]

hardbutmild,

Sorry that I've missed your conservation with rapt. You've brought a very interesting criteria - z ≤ 0.95d. Follow up on the example in your discussion, given a 600 mm deep beam, beam top face to the center of the top #32 bar is 62 mm, the compression block depth, a = ß₁c, c is the depth of the neutral axis measured from top face, and ß₁ = 0.85, so a = 600*(1-0.95)*2 = 60mm, and c = a/0.85 = 72 mm > 62 mm -----> NA is located close to the bottom of the #32 bar. IMO, #32 seems too large for this beam size, #22, or #25 is more suitable.

 

[hardbutmild]

With top cover + transverse bar + half bar diameter = 30 + 16 + 32/2 = 62mm, the beam would have to be deeper than 600mm to achieve this to the centre of the bar, or 800mm deep minimum to get the neutral axis depth below the compression face bar.

You must be in bridge design because I've never seen such huge bars, it's usually (for me) 25+8+20 = 53mm, divide it by 0,12 (I actually took this number from a book that calculated a more precise lever arm with parabola + horizontal), that's larger than 440 mm, it's not THAT uncommon. I think you added something wrong with the 600 - 800 calculation.

Sorry that I've missed your conservation with rapt...

Hey, I never argued against your idea. I only drew what others were saying, trying to diffuse the situation. I guess it didn't work.
My point was actually something along the line of siding with you. I mentioned that something like you said existed (or still does, idk) and as visible in my post it happens in some practical applications.

I think it's pointless to discuss it, I made an analysis some time ago and (at least in eurocode) if you put the minimum reinforcement lever arm is around 0,97d (i can't remember exactly). When I tried putting 0,95d I rarely had to put one extra bar. But as someone mentioned, maybe it might be overly conservative in some cases, it depends on what you're dealing with I guess.

 

[r13]

hardandmild,

You explained well, thanks.

Yes, in practice I was the one always willing to broke the owner's bank when sensing future heartburns. The persistent in repeating my preference was trying to instill some sense in the younger ones head, that when encounter that situation, knowing there could be something wrong that had led to it, and there could be a trap ahead (serviceability), so either modify the design, or go ahead to do the necessary check. I wasn't arguing for the sake of "my way or highway", rather I think this is the forum about - a place to express our thoughts, and offer personal opinions. I am willing to learn, if I had mistake, or someone else has better idea, but not on this one, I believe you understand.