06-30-2010, 05:39 PM
Hello everyone!
Nice discussion...
The same issue has been bothering me for a long time as well. The answer here is that the reinforcement makes the final difference. As i recall EC8 (EN version) states that we need to the design the column in the matter to achieve:
McRd>or=1.3MbRd.
And reinforcement has a big deal here. But you are right regarding the beam height; the code should limit the depth of the beam ( i can't remember that the code actually does) in order to meet the requirements for hinge forming(just in case you can't change the column size, and you can't use endless amounts of reinforcement).
But, what bothers me more is this:
Yes, i know we reduce the stiffness in order to take account for cracking of concrete. But we already neglect concrete in tension(where cracking mainly appears) and leave it for the reinforcement, and now this.
I agree that we should reduce the gross I of the beam to account for hinge forming, but this concerns only the seismic design situation. This leads to reduced moments and thus to less reinforcement. But how does the gravity load case affect the hinge forming?
Here is what I mean:
Let's say we have this 2 load cases mentioned above, and at some beam-column joint in the structure we have the following:
(the numbers are just figurative)
1) Gravity load case - M=300 kNm => As=25cm2 (i assume here 100% I of the beam; please show me facts to do otherwise).
2) Seismic load case - M=150 kNm => As=13cm2 (assuming the gross section I is reduced by some amount).
Now, my questions are:
How do we meet the plastic hinge requirements now?
(we assume the beam is properly detailed to prevent shear failure)
I know i would design the beam for the first case.
So, how can we be sure(it's all relative :P) that the hinge will form in the beam designed with this reinforcement? Or it won't form at all? And, is it justified to modify the column reinforcement for this case of beam reinforcement?
Regards
Nice discussion...
Quote:To avoid catastrophic failure or collapse (brittle collapse), it is required that the hinges (at the plastic stage) form on the beams, rather than on the columns. For this reason, the codes required the designers to provide a margin of security on the strength of the column over that of the beam. Having this in mind, is there a situation in which it is justified to provide a beam of dimension greater than that of the column (say a beam of 500mm x 1200mm which is to be supported on columns of dimension 500mm x 500mm, assuming that the beam is continuous with the column)? Again, how do we meet-up with this requirement (I mean, what calculations do we have to carry out as to provide this margin of security between the beam and the column)?
Regards
Teddy
The same issue has been bothering me for a long time as well. The answer here is that the reinforcement makes the final difference. As i recall EC8 (EN version) states that we need to the design the column in the matter to achieve:
McRd>or=1.3MbRd.
And reinforcement has a big deal here. But you are right regarding the beam height; the code should limit the depth of the beam ( i can't remember that the code actually does) in order to meet the requirements for hinge forming(just in case you can't change the column size, and you can't use endless amounts of reinforcement).
But, what bothers me more is this:
Quote:For Capacity Design (weak beam strong column), in design you can take beam stiffness 50% of its full stiffness while still maintaining column stiffness to 100% of its full stiffness or :
Beam, I design = 50% I gross beam
Column, I design = 100% I gross beam
ACI uses :
Beam, I design = 35% I gross beam
Column, I design = 70% I gross column
or in general, I beam reduction factor is half of that I column reduction factor.
These apply to Ix, Iy & J (Torsional constant; - not polar moment of inertia).
Reinforcement detailing is also important provide this mechanism.
Yes, i know we reduce the stiffness in order to take account for cracking of concrete. But we already neglect concrete in tension(where cracking mainly appears) and leave it for the reinforcement, and now this.
I agree that we should reduce the gross I of the beam to account for hinge forming, but this concerns only the seismic design situation. This leads to reduced moments and thus to less reinforcement. But how does the gravity load case affect the hinge forming?
Here is what I mean:
Let's say we have this 2 load cases mentioned above, and at some beam-column joint in the structure we have the following:
(the numbers are just figurative)
1) Gravity load case - M=300 kNm => As=25cm2 (i assume here 100% I of the beam; please show me facts to do otherwise).
2) Seismic load case - M=150 kNm => As=13cm2 (assuming the gross section I is reduced by some amount).
Now, my questions are:
How do we meet the plastic hinge requirements now?
(we assume the beam is properly detailed to prevent shear failure)
I know i would design the beam for the first case.
So, how can we be sure(it's all relative :P) that the hinge will form in the beam designed with this reinforcement? Or it won't form at all? And, is it justified to modify the column reinforcement for this case of beam reinforcement?
Regards