I'm still not seeing how ADDING compression reinforcement (which would Increase phi if you're below 0.9 because of strain consideration) could possibly decrease capacity. Throwing codes out the window for a second, I don't see how it's possible from a mechanics standpoint. I'm not saying it's IMpossible, but I would love to know the situation in which it is possible.
In a very lightly loaded beam, taking the compression reinforcement into account may actually reduce the calculated moment capacity (but not the actual capacity).
I'm just interested to see what code provisions, in which code, results in the calculated design bending capcity being reduced when you add compression reinforcement.
csd72 (Structural) 23 Nov 10 13:27
I cannot believe that there have been 35 replies to this!
Anyway, my 2 cents worth.
In a very lightly loaded beam, taking the compression reinforcement into account may actually reduce the calculated moment capacity (but not the actual capacity). I ignore it in these cases.
Where you have a highly loaded beam and the compression extends well below the level of the reinforcement then it does actually increase the moment capacity significantly.
I don't know how to make the pretty boxes show up when you're quoting someone's post, but this post suggests that the calculated moment capacity can be reduced by adding compression reinforcement.
I believe that the statement was a casual comment, not meant to be taken seriously. There is no code in the whole wide world which would suggest that the the addition of compression reinforcement would decrease the capacity of a beam.
BAretired - code provisions often have unintended consequences, and if some code provision results in the design bending capacity being reduced by including compression steel in the calculation I thought it would be interesting to have a look at it, that's all.
It is theoretically possible for a code capacity to be reduced in this case, if the "compression face" reinforcement is actually in tension as it may be in a lightly reinfroced beam. The deffective would reduce significantly, making the beam much less ductile and possibly reducing the capacity reduction factor and thus the calculated capacity. This would no longer happen for ACI code as itis basd on D to the extreme tension steel, but some other codes use deffective!
As someone said it is possibly an unintended consequence of a code rule..
As this thread seems to be mostly based on opinion then I suppose it is up to me to drag it back to the facts.
I have double checked my theory using a spreadsheet based on british standards and have confirmed that it is correct to that code. So I stand by my comments as valid.
As most of the people on this thread are based in the US then I will try and do my proof calcs to ACI code but I will have to wait until I am at home and can access my pdf copy.
what are the general conditions under which this is true? Extremely lightly reinforced? Extremely heavily reinforced? Compression steel that isn't really in the compression zone? Compression steel that is above the neutral axis, but below the compression block?
I'm not sure whose posts you are referring to, but I have just been asking for information.
I have double checked my theory using a spreadsheet based on british standards and have confirmed that it is correct to that code. So I stand by my comments as valid.
Great. Can you supply details please? i.e. cross section details, concrete grade, axial load, code, and bending capacity with and without compression reinforcement. I'd be very interested to see it.
As most of the people on this thread are based in the US then I will try and do my proof calcs to ACI code but I will have to wait until I am at home and can access my pdf copy
At least two of us are based in Australia. The Australian codes are close to US (at least for beam and column ULS design) but I'm very familiar with UK codes (5400 more than 8110 though, so if there is a significant difference in this context perhaps you could point it out).
Yes I am talking about extremely light reinforcing and extremely heavy reinforcing. I used the term lightly and heavily loaded which has caused some misinterpretations - hey that comes from doing it in spare 10 second slots between my paid work.
Anyway as I say in my second post these are extreme cases and not the norm.
My point was that you should not always make broad statements about these things.
IDS,
I bet your a Melbournite!
The differnces are mostly in the minimum reinforcement and the material factors although BS8110 has a specific formula for bending which is a bit different to the standard compression block formula.
This definately applies to the Australian method too.
I am working from home today so am not able to scan the calcs in.
