Shearhead detail/cruciform for slabs (ACI or other)

[GioC]

(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

 

[asixth]

I haven't seen a cruciform beam used as punching shear reinforcement and I image it would create issues with compacting the concrete around the beam. This is not ideal because it would be located in a peak stress region where good quality placement is required.

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

It sounds like what you are describing is a steel collar to help with punching shear. These are installed below the slab.

 

[BAretired]

asixth,

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

 

[hokie66]

I did read, as did asixth, that the OP was investigating using structural steel shapes in a welded cruciform shape (in plan, not in section) as shear reinforcement. This approach is included in ACI 11.11.4. I have never used this method, and agree that it would create lots of problems with consolidation of the concrete. The approach of using crossed reinforcing cages as discussed in 11.11.3 or studrails as discussed in 11.11.5 makes more sense to me.

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.

 

[BAretired]

Fig. 6 in the attachment illustrates a suggested method of handling openings near the column. A cruciform shape in plan view may not be adequate if the openings are large.

With any shearhead, consolidation of concrete has to be done very carefully.

BA

 

[hokie66]

Interesting reference, BA. It seems strange to me that of the 6 photos, Fig 6 with the prefabricated shear head is the only one without column bars projecting.

 

[BAretired]

I agree, hokie. It is strange, but it was the only illustration I could find of a shear head intended to accommodate slab openings near the column.

BA

 

[asixth]

It's better they actually reinforce the opening than what they did here (attachment). The holes were never approved, I was walking thorough the structure during fit-out and noticed them (Post-Tensioned Multi-Storey Parking).

 

[ishvaaag]

The approach to use is M-V force transfer along ACI 318 lines. Even it this is an undesirable detail, and I NEVER would recommend it for places where earthquake has a say, you can almost anywhere else make work without problems. The warn about not using it in sites subject to significant dynamic behaviour comes from the fact that then the concrete near the steel members gets overstressed, cracked, and then bond lost, losing hence support. In the first attack in the basement to the WTC, a van full of explosives was placed besides foundation walls under the façade. The explosion found such walls difficult to harm and the spit three floors of solid slabs towards the surface. You can see photos of then with the steel columns with their welded shearheadd neatly standing in the air with no concrete at all, just the crater created by the explosion. Hence, bond to shapes, that attains values that allow practical use, is not, anyway, something to be messed with.

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.

 

[ishvaaag]

errata...

1st line

Even if this is an undesirable detail...

5th line

then spat three floors ...

 

[GioC]

Thanks for the comments guys, and ishvaaag for extended comment. The intent is attached, is not for shear enhancement, but the arch requirement. It’s not an earthquake region otherwise I wouldn’t be considering it.
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).

 

[ishvaaag]

A shearhead of the sizes in your detail is really cutting the column, except that the column is made of structural steel. This is I think the more worrying aspect of the intent in this particular design.

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.