Shearhead detail/cruciform for slabs (ACI or other)
Shearhead detail/cruciform for slabs (ACI or other)
(OP)
(Re-post from Concrete discussio)
Hi,
I would appreciate any hints for shearhead detail usign I steel beams for support in flat slabs. I am looking at a detail with holes all around column and slab supported off a crucirfom. I am after any design guidance/examples-there are few available but to my understanding they dont address how the encased steel is actually providing support to the slab. ACI deals with shear and punching, but how about supporting the slab. You would normally be looking at bearing on bottom flange? welded bars on steel beam etc?.. Top bars running on top of steel beams dont provide a great deal (dowel action? very tnin layer).
This detail has been apparently proven in pratise, but I would like any comments or any relative docs. Some proprietary stuff dont give any explanation, just tables.
Thanks
GC
Hi,
I would appreciate any hints for shearhead detail usign I steel beams for support in flat slabs. I am looking at a detail with holes all around column and slab supported off a crucirfom. I am after any design guidance/examples-there are few available but to my understanding they dont address how the encased steel is actually providing support to the slab. ACI deals with shear and punching, but how about supporting the slab. You would normally be looking at bearing on bottom flange? welded bars on steel beam etc?.. Top bars running on top of steel beams dont provide a great deal (dowel action? very tnin layer).
This detail has been apparently proven in pratise, but I would like any comments or any relative docs. Some proprietary stuff dont give any explanation, just tables.
Thanks
GC






RE: Shearhead detail/cruciform for slabs (ACI or other)
If you are trying to increase the punching shear capacity without thickening the slab or providing column capitals try using a propriety stud-rail product. Try some searches for Dencon, Nelsons, Reids and Ancon.
He is a post that I started a late last year regarding the use of stud-rails.
thread592-260551: Studrails...how much can they increase punching shear capacity
It sounds like what you are describing is a steel collar to help with punching shear. These are installed below the slab.
RE: Shearhead detail/cruciform for slabs (ACI or other)
I don't think the intent is to use a cruciform shape as a reinforcing beam. I believe the cruciform shape is the column. A plan view of the column with typical holes in the slab would be helpful.
Channels and WF shapes can be used as shearheads. The shearhead picks up the load of the slab on the bottom flange. Also, holes may be provided in the web through which reinforcement is threaded, or top bars may be placed over the shearhead and bent down at 45 degrees each side and extended down to the bottom of the slab.
BA
RE: Shearhead detail/cruciform for slabs (ACI or other)
A column with holes all around is a nightmare for the structure. We often provide guidance at the very beginning of a project to other consultants as to where holes can be located.
RE: Shearhead detail/cruciform for slabs (ACI or other)
With any shearhead, consolidation of concrete has to be done very carefully.
BA
RE: Shearhead detail/cruciform for slabs (ACI or other)
RE: Shearhead detail/cruciform for slabs (ACI or other)
BA
RE: Shearhead detail/cruciform for slabs (ACI or other)
RE: Shearhead detail/cruciform for slabs (ACI or other)
Respect details themselves, single crosses of WF shapes or crosses of 2 Cs giving one the back to the other can be used. Even if the inserted arms themselves use to be tolerant in terms of stresses (a FEM analysis uses to prove) with big spans and loading at some distance of the column support point this will be going worse and worse and then a extremely strong structural shearhead may be needed.
To enhance the shear transfer to the shapes, I have used with success half dozen of times in commercial buildings studs welded to the webs of the shearheads. Older designs used also small rebar wound around the shearheads' arms to the same purpose.
Apart from caring for the transfer in the ACI recommended way, modeling the shearhead arms going into the slab, with nodes connecting the slab and the structural steel shearhead will give you complementary appraisal of the validity of the arms, and help to decide on a proper length for them. Of course rebar must enclose the shearhead, and if you do not produce a design with an overstressed shearhead, support will appear automatically, not having any problems of spalling or ejected concrete, nor need of any bottom of the slab plate; with steel columns, however, it is not uncommon to provide a bottom of the slab plate that helps to transfer the slab solicitations to the column, but this is nothing of use when like in your case you seem to want only to have the arms to pass the forces to the column. In this case the shearhead to column connection will have to care for this.
RE: Shearhead detail/cruciform for slabs (ACI or other)
1st line
Even if this is an undesirable detail...
5th line
then spat three floors ...
RE: Shearhead detail/cruciform for slabs (ACI or other)
Ishvaaag, just picking what you said "support will appear automatically", how would you quantify that e.g. which mechanism you assume gives you the support: bearing on bottom flange in the 4 directions? I can see how that should be working in principle but putting number in for someone to check it's a bit more complex. And also, would you assume continuity of slab over the beams? I couldn't see no reason for not doing so as long as top bars are in tension and bottom section (steel beam included) is in compression (ACI 11.12.4.4).
RE: Shearhead detail/cruciform for slabs (ACI or other)
As you see, by inspection, between other things you have several areas of interest to check
1. A embedded arm able to pass the slab forces to steel arms.
2. Design competent steel arms to take the solicitations created by the overall behaviour of the structure.
3. Study the connection between the shearhead and the column to ensure the standing solicitations will be properly covered.
All in the process, the shearhead shape is understood to be the same.
As you see only a FEM design modeling both arm and slab can give you proper feeling of what required for points 1 and 2. Normally, since cantilevers, the strength of the shape as an arm of structural steel will govern the selection of its size. This is the first point to check after analysis, for its size may be incompatible with the wanted thickness of the slab (assuming an embedded arm). Then if you have found a size of arm that can be embedded in the slab (and sometimes even just 50% of the total of the slab is difficult to compatibilize with standing rebar and covers) you will deal with force transfer to the exposed part of the arm.
Then you have the connection. If the unbalanced moments are big it is unlikely a detail as sized will work properly as a reinforced concrete column, it might, maybe, as a structural steel or composite column. The thing is as always to pass the forces standing in the relevant hypotheses (all are, just eliminate those that do not control some - any- aspect of the connection design).
There are a variety of ways of tackling with the embedment issue. The model will show that adding more length than required won't serve to any purpose, hence, use fem as a guide; it will be saying you that for the standing solicitations a satisfactory arm won't be overstressed at such embedment. FEM also gives stresses at the slabs, and from them one can reinforce the concrete. It also gives the shear stresses in the arm, both in the exposed and the embedded part. By examination of the shear forces in the embedded part of the arm, you can derive what forces are being passed to the slab, since they equilibrate. These you can take either by direct support on the appropriate flange and, by shear friction through studs perpendicular to the web, or trough strut-and-tie action towards a surrounding closed steel cage surrounding the arms (and from there to a shear surface), or any combination of. Here to be attent to excessive compression in struts and high shear stresses.
For the case where the shearhead crosses are under the slab, normally composite action would be used to ensure a stable structure. Hence studs would appear, and there are structural analysis programs able to represent all beam, slab and studs at proper relative position, and hence get then structural requirements for them. In particular wil get this way the required amount of studs that doesn't cause the studs be overstressed.