Punching Shear Design | Transfers slabs and two way flat plate slabs
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- เผยแพร่เมื่อ 24 ก.ค. 2024
- Punching shear or deflection are the governing factors for the design two-way slabs or transfer, so it is critical that all structural engineers understand punching shear design.
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Punching shear stresses are induced by concentrated loads, typically from columns either over or under a slab. These stresses are due to a combination of moments and shear forces and will form a perimeter around the concentrated slab forming punching shear stress field in the slab. This is a brittle failure so a slab can fail in punching shear with little to no warning signs so it is critical that the slab has been designed for punching shear correctly
I will be covering the structural mechanics of punching shear and some area in your design you require careful considerations as they may affect your punching shear stresses.
There are many factors that can affect punching shear so it is important that all structural engineers know what the critical design factor are for punching shear design.
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Thank you Brendan for this piece of knowledge
Sir lots of respect from heart. Thank you all the way from Canada
Hi Ayedit, Thanks for watching and the support.
Thanks for the explanation
Glad you found the video enjoyable.
I like your videos, thank you!
Hi FE, thanks for the support.
Thanks Brendan, most cases I've seen is by providing drop panels
Hi Anas, Drop panels have become less popular in Australia as it increases the complexity of the formwork, building prefer flat plate slab and the additional reinforcement. the additional labour on the formwork in most cases is more expenive then the additional reinforcement.
@@BrendanHasty i too have seen drop panels more often, but it definitely makes sense that the cost for the extra reinforcement would generally be less than the additional complexity in cutting, installing, bracing and removing formwork. also i think architecturally, no drop plate would be advantageous, since thin floor plates would allow for more head room in the living area
Hi Brendan, This is one the topic I was eagerly waiting for. As you have mentioned Aus Standard does not cover stud rail design we have to refer it either euro code/British Standard. I think Ram Concept has incorporated ANCON in punching shear design (I think its similar to Shear Fix from Ancon). Even though we are designing in Australian Standards, Ram Concept does show the design of studs, and I think it is based on BS. What do you suggest in this ? It is relevant to rely on Ram Concept for the stud design in Australia even though we are designing it in Aus Standard. Please correct me if i have misunderstood.
Hi Prabin, you are correct Ram design using Ancon, it doesnt provide the torsion lig design. so this strickly isn't code compliant, so you need to address this with the certifier but is common practice. as for using Ram for the punching shear checks i would reccomend you check the punching shear by hand as sometimes the punching shear checking in Ram can be incorrect, generally conservative. If design using stud rails also check using there software. and thanks for your comments
Hi Brendan, you mentioned in video about adding column cap to increase punching shear capacity.
I have a case with drop cap where the section within drop, at 2d from column face is not required shear reinforcement, but the section at the drop cap face and slab needed to be reinforced.
I want to ask you in this case, do i have to provide shear reinforcement within the drop cap or I just need shear reinforcement in slab. Thank you in advance
Hi Nguyen, thanks for watching. As for the shear check, if the shear past the drop cap requires shear reinforcement this area should be reinforced as required. The slab can still have a shear failure. Maybe increased the size of the drop cap to cover shear failure if not too far.
@@BrendanHasty thank for your answer. In my case, because of architectural design, i can't change the structural layout.
In EC2 code section 9.4
Quote:"Where punching shear reinforcement is required (see 6.4) it should be placed between the
>>>loaded area/column
hi Brendan,
In the case where we have a set down do you think the avg Dom formula should be =(u1 x dom1 +u2 x dom2)/(u1+u2)?
hallo Brendan, i encountered such a case where the slab supposed to have these studs reinforcement in it, our job was to search for them and to find out if they are really built in or not, it all started as the slab starts to crack and taking the shape of rays going out of the columns, the cracks follows exactly the pattern that the claimed built in studs should be taking (the studs built as sun rays out of column). if the punching shear would form around the column, what makes the cracks then starts from the column and extend in different directions?
Hi Rawaa, thanks for watching. Punching shear is formed from two mechanisms, flexure and shear, then this can cause two types of cracks the shear cracking and the flexure cracking.
The shear cracking the classic ring around the column.
and Flexure/tension cracking, these are the ones that radiate out like sun rays, as the punching shear forms a ring there is a perpendicular tension force, and this is why they form this way.
The failure for punching shear is caused by the stress ring around the column and can be an issue in locations of high shear and a low moment or medium shear and high moment. This is due to the moment causing increased stress on the perimeter, the classic failures where issues on the edges and corners of slabs that had high torsion/moment.
So there are two approaches to address this in design
Strengthen the shear capacity as pre stud rails
Or strengthen the torsion/moment capacity similar to AS/NZ or ACI.
Not sure if the above clarification helped you, if you need more details let me know and good luck
@@BrendanHasty oh thanks for the clarification, i think i need more details if u have.
Some codes require a certain minimum amount of PT strands to go through the column itself so this would give you some PT compression across the failure perimeter. You would have to lay this out to determine what part of the perimeter might have compression and what part of the perimeter might not.
Hi Bill sometimes it is hard to get PT through the column due to congestion of the column reinforcement, even tendons adject to the perimeter adds compression to the interface and can be used to in the punching shear assessment. So typically you assess the punching shear in two directions and take the corresponding P on A.
@@BrendanHasty I agree with the congestion. In the US we use individual unbonded strands so they can be easier to fit through, but fitting in the anchor castings is the bigger problem. However the ACI code requires a minimum of 2 strands of 1/2" diameter each to go through the columns.
Hi Brendan, I hope you are well. Big fan of yours. Much appreciated for your guidance. I am working on a project, where we had to re-design a job from PT to a conventional slab for childcare. PT designers managed 200 deep slab to work for punching shear above columns. However, I could only make 400 deep drop panels above columns to work. My question is, "does punching shear behave differently in PT and conventional slabs"?
Hi Aslan, The compression from the PT helps greatly, it compresses the failure plain
Hi Brendan, is there a video on Site Inspection, could you please upload, thank you.
Hi Krishnan, this is on the books just need to workout how to do it.
Hi brendah! Thanks for your advice it was really amazing .I would like to ask you on flat slab.which column has high punching shear failure among middle column and edge column. In project ,I used flat slab of 250mm in SAFE 20, BS 8110 code drop panels (250mm) for resisting punching but it stills fail ,when I did hand calculation my flat slab didn't punch at all. What can I do ! I'm looking forward for your answer.
Champlain towers bought me here. Those balcony slabs sure looked thin!
Hi Klam, the slabs at Champlain Towers are thin. But I don't think this was the cause of the failure. I am about to post a video on the Champlain tower collapse you may find interesting.
@@BrendanHasty looking forwards!