punshing shear and unbalanced moment 1

[CHAKKOUR]

hi guys.
An alternative for not taking into account the punshing shear is to consider the structure as a pinned one. If potentially, there will some moment, a crack will occur and then a plastic hing which will stop the transmission of the moment. What do u think ??

 

[rapt]

Ash060,

As long as there is no moment connection, that is perfectly ok. It is much harder to develop a pin support with an RC column!

JedClampett
So you either have to define 3 data items, cover, lig diameter, bar diameter or 1 that combines all 3 as cover + lig diameter + bar diameter/2.
RAPT used to require this single data input of depth because it did not know the bar size the user wanted to use and I used to get complaints. Now the user has to define it. So now I get complaints the other way. You cannot please everyone. We have gone the cover + bar size option lately more because people used to change bar size without changing depth for user defined bars.
If that is your biggest worry with software, there is not much wrong.

RE,
Same with your punching shear problem. Cannot please everyone. I too do not like punching shear reinforcement. But most designers are using it these days.
RAPT used to give this punching shear warning automatically, until we started getting complaints that there was a warning in the output about punching shear reinforcement so checking engineers were rejecting the design because there was a warning. But it was not really a warning about a problem in the design, just a comment on what had happened in one area, So we stopped calling it a warning and just made it a comment in the punching shear output.

I suppose we could offer an input option allowing the designer to specify if they want a warning message if punching shear reinforcemernt is added. But, then, what other areas should we give similar user options in to control warnings.
As I said, cannot please everyone, unless we create more input options, then we get even more complaints.

 

[rowingengineer]

RAPT,
RAPT isn't that bad for hiding the punching shear reo, other programs that I have to deal with hide it so only those with x-ray vision can find it.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[rscassar]

There is a concurrent thread running regarding column projection and it's effect on punching shear. thread507-280391. Why not just add an extra 10mm to the covers, have a little more reserve punching shear capacity and then not worry about it.

 

[hokie66]

Why would adding 10 mm help, when columns can be cast too high by any amount? 10 mm extra concrete in the floors of a large building costs a lot of money. There is no excuse or reason to cast columns too high.

 

[JedClampett]

rapt, I'm not familiar with RAPT software. My criticism is related to Enercalc, where you have to enter the bar size (or area). I'd much rather input cover and stirrup size, which are unlikely to change, and adjust bar sizes. As I said, it's a tedious little hand calc, subject to error, that needs to be documented in the margin.

 

[rscassar]

Silly question, but what is the correct unbalanced moment to be transferred into the column? Is it the moment from a frame analysis (i.e. a structure modelled as wireframe elements) or is it the difference in column strip moments taken at the critical section.

By the way, I now use 100% column stiffness when calculating unbalanced moment for punching shear.

 

[rowingengineer]

Hokie,
I assume you’re replying to Kikflip, however I must admit I try to ensure drop panels are not governed by punching shear at the column face for this exact reason. I don’t know what the local practice is in your part of town, but the guys that I have been dealing with tend to like to cast the column a little bit high (sure I believe them when they say they didn’t mean too). Reason I think is so you don’t see the joint, yep you think they would have learnt after the first time they had to jack hammer the columns down, but alas each different job, same bloody problem, and they just don’t o a good job of jackhammering down once the steel is in place for some reason.


An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[hokie66]

RE,
Part of the problem is that we have too many engineers who aren't inspecting their own work and are delegating it to "inspectors" who don't know the difference because they are trained to look for the silly things rather than the important ones. I despair sometimes.

 

[rapt]

kikflip,

It is based on the total unbalanced moment from the frame analysis, not just the portion of the unbalanced moment in the column strip. The full unbalanced moment is caused by the column stiffness, a lot of it through torsion in the slab.
Part of that moment is transfered directly to the column and part in torsion on the side of the column. AS you are designing to AS3600, these portions are allowed for in the formulae. ACI code gives a way to calculate the relative proportions.