slab punshing shear and steel cap plate
slab punshing shear and steel cap plate
(OP)
will the punching shear in a concrete slab supported by a steel cap plate be reduced if the cap plate edges were chamfered 45 degress?
the chamfering will provide a transition in the slab thickness from say 7.25 above the plate to a full slab thickness of 8.0 in. the cap plate thickness being 0.75".
Thanks
the chamfering will provide a transition in the slab thickness from say 7.25 above the plate to a full slab thickness of 8.0 in. the cap plate thickness being 0.75".
Thanks






RE: slab punshing shear and steel cap plate
RE: slab punshing shear and steel cap plate
RE: slab punshing shear and steel cap plate
RE: slab punshing shear and steel cap plate
Now, the question is: What "d" value should be used in the punshing shear equation?
1. should "d" be based on slab thickness - plate thickness
or
2. "d" based on total slab thickness?
thanks.
RE: slab punshing shear and steel cap plate
RE: slab punshing shear and steel cap plate
RE: slab punshing shear and steel cap plate
"d" is be based on the diagonal crack due to shear.
1. if the cap plate edges are not chamfered then the diagonal crack will initiate from the top corner of the plate edge. in this case "d" should be based on the slab thickness minus the plate thickness.
2. if the plate edges are chamfered then the diagonal crack is forced to initiate from the bottom of the slab. hence, in this case "d" is based on the total slab thickness.
to me this make sense since. As an extreme example, it is not logical to base "d" on the total slab thickness for an 8" slab with a 5" thick cap plate. in this unrealistic but acadamically viable case the crack line will initiate at 3" below the top of the slab. so in general I think that a cap plate does reduces the effective slab thickness in ressisting punshing shear.
RE: slab punshing shear and steel cap plate
The theory behind taking the critical section at d/2 from the support face comes from the assumption that the stress between the support face and the distance d/2 will go directly into the support through arching action in the concrete instead of through shear stress as does the rest of the load. SlideRule pointed out that is has withstood the test of time and is therefore a good assumption.
With these things in mind, if you need more shear capacity you need to reduce the shear stress in the concrete, by having say a larger base plate to increase the perimeter distance bo, a drop panel or steel reinforcing with the slab itself. The base plate will not help you with punching shear by taking the shear stress or adding it's thickness to the structural depth.
Punching shear failure is sudden and brittle by nature. If you are close enough to capacity that you are trying to justify the addition of 0.75" worth of plate then you need to just add shear reinforcing or preferably a drop panel and be safe. Messing around with punching shear and doing it wrong is a good way to get people hurt or worse.
RE: slab punshing shear and steel cap plate
RE: slab punshing shear and steel cap plate
RE: slab punshing shear and steel cap plate
What exactly is backward?
In a previous post you referenced ACI318 section 15.4.2c, the footing section telling you where the critical section is for moment in footings under a steel base plate. In your first post you ask us about a concrete slab supported by a steel base plate. To me that means you are designing an elevated structural floor slab. Are you asking us about footing design or about floor slabs? You should not look in the code section for the location of the critical section for moment in footings and try to apply that to punching shear in slabs, or punching shear period.
Shear in slabs and footings is covered in section 11.12. One way shear (beam shear) in 11.12.1.1 and two way shear (punching shear) in 11.12.1.2 where the critical section is defined as d/2 from 1) edges or corners or 2) changes in slab thickness. "d" is defined on page 318R-140, in the definitions in the beginning of chapter 11. You can add reinforcing steel if needed, as allowed by code, including stirrups, shear heads, bent-up bars, or embedded structural steel shapes. I'm sure there are others, but I don't think this includes steel bearing plates, as in plates under the slab. When you say slab supported by steel cap plate that to me suggests a bearing plate. Your other option besides adding reinforcing steel is to increase the concrete capacity by adding drop panels or in some way increasing the perimter "bo" used in the referenced concrete equations so that Vc increases. Anyway, review these sections in the code, and related equations and see if you still say I'm backwards. I hope some of this helps you understand your original question and actually gets you where you were trying to go.
RE: slab punshing shear and steel cap plate
As I understand it the proposed steel plate is located within the slab depth, ie. underside of slab and plate coincides. That being the case, the argument that chamfering the plate will force any crack to occur within the slab thickness adjacent to the plate has merit, but I would follow the conservative approach unless adequately confirmed otherwise eg. by testing.
UcfSE
It appears from your response on Nov 24 that you didn't realise the location of the plate within the slab, and therefore the reason for proposing use of different effective depths was unclear.
I do not have access to American codes but the principles are universal. To me the bearing plate is a form of shear head, or a steel drop-panel if you like, a method of increasing the shear perimeter as you pointed out. I can not see a rational reason for not using at a slab support.
I believe the original question should have read " will the punching shear STRESS in a concrete slab supported by a steel cap plate be reduced" Obviously the shear force will not reduce.
RE: slab punshing shear and steel cap plate
RE: slab punshing shear and steel cap plate