Goes to show changing a detail by a small amount can change the performance by a large amount.
How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban
The latest structural journal from aci was completly dedicated to punching shear. I hope the manufacturers of these proprietry items actually look into the research a little further.
I think have never used such kind of reinforcement in shear. "Something" indicates it lacks integration enough with longitudinal rebar to form the required properly anchored ties for required strut and tie schemes. I find also interesting the research finding the strap holding the studs being flimsy, I also thought that when seeing the 3D representations in the literature. Whilst the matter is cleared, better keep clear of the detail.
Another interesting case in which following "the code" means not necessarily the sound and safe approach, and a reminder of the individual responsability of structural professionals to actively pursue structural safety.
Concrete shear is based mostly on empirical equations making it hard for an engineer to assess the results and thus are left having to rely on manufacturers information.
How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban
Thanks, hokie. I'm including stud rails in a project I'm working on now. I think I might just throw some radial rails at the corners to help me sleep a little better!
The original research on shear studrails seems to have been done at the University of Calgary. I wonder how their tests compared to the Ann Arbor tests.
There are at least two types of studrails currently available: single headed studs welded to a rail which serves as the bottom anchorage, and double headed studs with a rail which just spaces the studs. I couldn't see where the report said which type was tested.
I haven't used studrail reinforcement, but have thought about doing so. This report has major ramifications, and will be a worry to lots of people.
How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban
This is likely just the beginning of what is going to be a very long and contentious discussion on the subject. The original punching tests run in Calgary that you speak of had some serious deficiencies. For one, Dilger and Ghali always used an overly conservative amount of flexural reinforcement to "force" a punching shear failure. For a while now tests with more realistic reinforcement ratios have reported lower capacities than from calgary. The current code equations don't take into account flexural steel at all, and if you ask me, this is the major downfall.
This article has been refuted point by point by one of the studrail manufacturers who, in my opinion, makes a clearer case and has much more data to back themselves up than the writer of the original ENR article. Letter attached.
I don't know. The Decon letter is just an argument justifying the product. Until further studies are done which are not paid for by the manufacturers, I will be dubious. But then, I have not used studrails.
I do note int eh return of serve letter, that the decon rep states "Thus, it is not reasonable to expect an increased capacity by adding shear reinforcement, but not doing anything to increase the flexural strength."
This statement would support the theory that to much reinforcement has been used to justify the punching values. Punching shear is effected by the amount of reinforcement, this isn't stated in this particular code, but is covered in a few other codes, and punching shear articles.
Slick - that article shows both sides fairly well.
On side says that:
1. Thousands of buildings have the ACI design integrated into the structure without any failures to date.
2. The tests that imply a problem were not designed with enough flexural reinforcement.
3. The code requires the stud rails to extend from column corners while the tests may have had rails not aligned with column corners.
4. The research doesn't directly show that there is a strength problem.
The other side says:
1. There may be lots of buildings with this implemented, but perhaps none have experienced a major seismic event where the problem would arise.
2. The code only shows rails from the corners in the commentary - the code itself does not require it.
Apparently a special committee has been established to look into the matter and advise ACI on how to proceed.
The initial testing was done by calgary and also Leonhardt design group in Germany.
I saw some of the original German test results and discussed them with the Leonhardt suppervisor of the tests. They were baesd on a rail at the bottom and it was not a thin spacer. It was about 15-20mm deep. I am not sure of the actual layouts of studs that were tested but had alwaays understood that the concept of stud reinforcement was to be able to achieve a radial pattern. The early specifications I saw used this radial pattern. Obviously they have decided it is too hard to achieve on site so have gone to orthogonal patterns but that does not mean it is the best option!
They even did tests with continuous rails versus rails that stopped at the column edge. This had little effect on the basic punching shear strength but Collapse capacity increased by about a factor of 2 or more with continuous rails as they presumably acted as shear dowels. Interestingly the Canadian code now requires continuous bottom reinforcement through the column zone in the bottom of the slab in both directions, presumably as a response to this effect!
