VezQc
Civil/Environmental
- Jun 19, 2014
- 1
thread507-122089
after reading the thread above, I was not quite sure how to understand the behavior of interface shear in the presence of a Moment. I was also not quite sure how to understand the commentary R11.6.7 of ACI 318-08 mentionning that it was "not necessary to provide additional reinforcement to resist the flexural tension stresses, unless the required flexural tension reinforcement exceeds the amount of shear transfer reinforcement provided in the flexural tension zone". I am pretty sure this was the sentence DaveAtkins was refering to when saying "The Code is clear -- you don't have to add shear friction reinforcing to the reinforcing required for bending. Because the bending causes a compression force that will not allow the shear planes to slide, the flexural reinforcing in effect can resist bending and shear friction at the same time. "
I would like to express my understanding through my design problem, which is not as affirmative as DaveAtkins's :
Here is my problem : I have to rebuild concrete beam to its columns that are conserved. Since I am not monothical anymore with the columns, I have an interface shear transfer at the face of the columns. I am conserving the existing tension steel required by flexure on 5ft from the column face, so there will be a significant negative moment at the interface new beam/existing column.
lets say the required steel resisting the interface shear transfer is 2-20M at top and bottom of the beam.
Then, if the steel required to resist the flexion at the face of the column is less than 2-20M, let's say 2-15M (very little moment), then it would not be required to add additionnal steel reinforcement to resist the additionnal tension brought at the face of the column by the Moment. I would therefore design my interface using 2-20M at top and bottom of the beam, corresponding to the steel reinforcement required to resist the shear.
Nevertheless, if the required steel reinforcement to resist the moment is 2-25M at top of the beam, and the required uniform steel reinforcement required to resist the interface shear transfer is 2-20M at top and bottom of the beam section, then the 2-25M are required at top since they bring more tension in the flexural tension zone than Avffy, where Avf is my 2-20M at top required in the tension zone. My design would therefore consist of 2-25M at top and 2-20M at bottom.
So, I believe 2 checks shall be made when designing an interface shear transfer surface involving a moment :
1) what would be the uniform steel reinforcement required to resist the shear stress (eq. 11-25 of ACI 318-08)
2) a. determine what part of that uniform steel reinforcement required to resist the shear transfer is located in the tension area resulting from the flexural effort
b. compare that steel reinforcement area determined in a. with the area of steel reinforcement required to resist the flexural stress (Asflex). If it is smaller, than Asflex is definitely required to be placed in the tension area. However, if it is larger than Asflex, then the steel reinforcement determined in point 1 is sufficient to resist both the flexurale and shear stress at the interface.
As I understand it, the tension in the top reinforcement (in the tension zone) due to the interface shear transfer would be "transfered" to the shear resistance provided by the additionnal compression in the compression zone. It's like the compression due to the bending transfer more shear and therefore release globaly the tension in the interface shear transfer steel reinforcement(Avf), therefore allowing it to be stressed by the flexural effort. The top reinforcement would therefore be stressed only by the tension due to the flexure, and not the tension due to the shear. (I doubt that in reality it totally goes that way, the top steel reinforcement must certainly sustained a part of the tension required to resist the shear in addition to the tension du to the flexure, but it seems that it has been experimentally demonstrated, as mentionned in the commentary R11.6.7 of ACI 318-08)
in the end, the flexural reinforcement can be used to calculate the interface shear transfer in addition to any specific shear transfer reinforcement that would be required at the interface. If I have 2-25M at top of the beam section, 2-20M in the middle and 2-20M at the bottom, all that steel can be considered to be resisting to the interface shear tranfer (Avf). Only, the 2-25M are actually in tension to resist flexural tension and that exact quantity of steel calculated to resist the shear is actually resisted by the additionnal compression brought by the flexural effort. So at the end of the day, all the steel crossing the shear interface, including the flexural steel, can be incluced in the steel area resisting the interface shear transfer (Avf).
I would also like to mention two points that I think are important if you ever have a similar design to do to mine :
1) even if 2-30M at top and bottom are theoretically sufficient to resist the interface shear transfer, I would not design my interface shear transfer steel area (Avf) like this, especially for a large interface area. The steel must be uniformely distributed through the interface for the interface shear transfer to be effective. even for a 2ft high beam, I would not totally be confident without using additionnal steel reinforcement at mid section of the beam at the interface.
2) In the commentaries of ACI 318, a dowel action is mentionned to be effective when the interface is not monolithical or intentional roughened. Therefore, I would be very cautious in those cases to have more than only the minimum recoverment on Avf, even if this steel is principaly stressed in tension, therefore not particularly dependant of the recoverment (ex : retaining wall). Or it could also be inside stirups, as in the case of a beam or a pier.
Do you agree with my understanding? It seems to me a little different than the understanding agreed in the thread mentioned above....
thanks a lot!
VezQc
after reading the thread above, I was not quite sure how to understand the behavior of interface shear in the presence of a Moment. I was also not quite sure how to understand the commentary R11.6.7 of ACI 318-08 mentionning that it was "not necessary to provide additional reinforcement to resist the flexural tension stresses, unless the required flexural tension reinforcement exceeds the amount of shear transfer reinforcement provided in the flexural tension zone". I am pretty sure this was the sentence DaveAtkins was refering to when saying "The Code is clear -- you don't have to add shear friction reinforcing to the reinforcing required for bending. Because the bending causes a compression force that will not allow the shear planes to slide, the flexural reinforcing in effect can resist bending and shear friction at the same time. "
I would like to express my understanding through my design problem, which is not as affirmative as DaveAtkins's :
Here is my problem : I have to rebuild concrete beam to its columns that are conserved. Since I am not monothical anymore with the columns, I have an interface shear transfer at the face of the columns. I am conserving the existing tension steel required by flexure on 5ft from the column face, so there will be a significant negative moment at the interface new beam/existing column.
lets say the required steel resisting the interface shear transfer is 2-20M at top and bottom of the beam.
Then, if the steel required to resist the flexion at the face of the column is less than 2-20M, let's say 2-15M (very little moment), then it would not be required to add additionnal steel reinforcement to resist the additionnal tension brought at the face of the column by the Moment. I would therefore design my interface using 2-20M at top and bottom of the beam, corresponding to the steel reinforcement required to resist the shear.
Nevertheless, if the required steel reinforcement to resist the moment is 2-25M at top of the beam, and the required uniform steel reinforcement required to resist the interface shear transfer is 2-20M at top and bottom of the beam section, then the 2-25M are required at top since they bring more tension in the flexural tension zone than Avffy, where Avf is my 2-20M at top required in the tension zone. My design would therefore consist of 2-25M at top and 2-20M at bottom.
So, I believe 2 checks shall be made when designing an interface shear transfer surface involving a moment :
1) what would be the uniform steel reinforcement required to resist the shear stress (eq. 11-25 of ACI 318-08)
2) a. determine what part of that uniform steel reinforcement required to resist the shear transfer is located in the tension area resulting from the flexural effort
b. compare that steel reinforcement area determined in a. with the area of steel reinforcement required to resist the flexural stress (Asflex). If it is smaller, than Asflex is definitely required to be placed in the tension area. However, if it is larger than Asflex, then the steel reinforcement determined in point 1 is sufficient to resist both the flexurale and shear stress at the interface.
As I understand it, the tension in the top reinforcement (in the tension zone) due to the interface shear transfer would be "transfered" to the shear resistance provided by the additionnal compression in the compression zone. It's like the compression due to the bending transfer more shear and therefore release globaly the tension in the interface shear transfer steel reinforcement(Avf), therefore allowing it to be stressed by the flexural effort. The top reinforcement would therefore be stressed only by the tension due to the flexure, and not the tension due to the shear. (I doubt that in reality it totally goes that way, the top steel reinforcement must certainly sustained a part of the tension required to resist the shear in addition to the tension du to the flexure, but it seems that it has been experimentally demonstrated, as mentionned in the commentary R11.6.7 of ACI 318-08)
in the end, the flexural reinforcement can be used to calculate the interface shear transfer in addition to any specific shear transfer reinforcement that would be required at the interface. If I have 2-25M at top of the beam section, 2-20M in the middle and 2-20M at the bottom, all that steel can be considered to be resisting to the interface shear tranfer (Avf). Only, the 2-25M are actually in tension to resist flexural tension and that exact quantity of steel calculated to resist the shear is actually resisted by the additionnal compression brought by the flexural effort. So at the end of the day, all the steel crossing the shear interface, including the flexural steel, can be incluced in the steel area resisting the interface shear transfer (Avf).
I would also like to mention two points that I think are important if you ever have a similar design to do to mine :
1) even if 2-30M at top and bottom are theoretically sufficient to resist the interface shear transfer, I would not design my interface shear transfer steel area (Avf) like this, especially for a large interface area. The steel must be uniformely distributed through the interface for the interface shear transfer to be effective. even for a 2ft high beam, I would not totally be confident without using additionnal steel reinforcement at mid section of the beam at the interface.
2) In the commentaries of ACI 318, a dowel action is mentionned to be effective when the interface is not monolithical or intentional roughened. Therefore, I would be very cautious in those cases to have more than only the minimum recoverment on Avf, even if this steel is principaly stressed in tension, therefore not particularly dependant of the recoverment (ex : retaining wall). Or it could also be inside stirups, as in the case of a beam or a pier.
Do you agree with my understanding? It seems to me a little different than the understanding agreed in the thread mentioned above....
thanks a lot!
VezQc
