tension controlled section question
tension controlled section question
(OP)
I was reading some of the topics and thinking of an elementary topic that would like to share with you.
Regarding the flexure in beams or slabs, we usually design our section to be tension controlled (strain in steel should be higher than 0.005) which will guarantee a ductile behavior since reinforcement will yield and have excessive deformations before concrete will break in compression.
However, many engineers like the fact of being conservative so they will always have tendency to increase the required steel area and here comes my question.
Tension controlled means that fs ≥ fy. When As (steel area) will increase, the neutral axis will do the same and fs will decrease. Let us suppose that the section, for a reason or another, has been reduced and the acting moment is requiring a certain amount of steel and is tension controlled (strain in steel is slightly above the required 0.005).
In this particular case, being conservative and adding more than required (or for some deflection purpose) will result in the fact that steel will not yield and we will not get the required ductile behavior since the strain in the steel will be less than 0.5%.
Is there something I am missing ?
thank u
Regarding the flexure in beams or slabs, we usually design our section to be tension controlled (strain in steel should be higher than 0.005) which will guarantee a ductile behavior since reinforcement will yield and have excessive deformations before concrete will break in compression.
However, many engineers like the fact of being conservative so they will always have tendency to increase the required steel area and here comes my question.
Tension controlled means that fs ≥ fy. When As (steel area) will increase, the neutral axis will do the same and fs will decrease. Let us suppose that the section, for a reason or another, has been reduced and the acting moment is requiring a certain amount of steel and is tension controlled (strain in steel is slightly above the required 0.005).
In this particular case, being conservative and adding more than required (or for some deflection purpose) will result in the fact that steel will not yield and we will not get the required ductile behavior since the strain in the steel will be less than 0.5%.
Is there something I am missing ?
thank u






RE: tension controlled section question
RE: tension controlled section question
RE: tension controlled section question
RE: tension controlled section question
RE: tension controlled section question
RE: tension controlled section question
RE: tension controlled section question
RE: tension controlled section question
Where's Koot when needed to lend a good blend of CSA and ACI knowledge? Is this one of the few times the CSA and ACI codes differ slightly?
RE: tension controlled section question
it slides to use a phi more 'appropriate' for a more brittle failure.
RE: tension controlled section question
RE: tension controlled section question
So the Compression force is already reduced by its factor in the calculation of the section strength and the reinforcement is reduced by its factor. Then you get C and T to be equal.
Because the reduction factor on the concrete is much higher than the one on the reinforcement, this automatically increases the neutral axis depth and reduces the design lever arm in the strength calculations depending on the level of compression/ductility.
And all comparisons are then done with Balanced Condition.
Because the neutral axis depth etc in ACI code is calculated from forces based on the full material strength, the zone between .75Balanced and Balanced has to be allowed for and this is done by fudging the capacity reduction factor as mentioned above by others.
RE: tension controlled section question