Hooked Bar Anchorage

[Galambos]

Is there any reason that the attached connection cannot develop the full capacity (ACI) of the deformed bar anchors located at the edge of the slab?

I have met the hook development length and spacing requirements. My slab has the flexural and shear capacity from a slab analysis standpoint.

I will be adding hair pin ties around the hooks in accordance with 12.5.3.

Could there be something I am not thinking of that would limit the capacity of the DBA's to take the full tension?

Thanks ahead of time.

 

[JedClampett]

You might be able to develop the bars, but that won't stop you from having a shear issue.

 

[Galambos]

jed, can you elaborate?

Im thinking I should cantilever a wide flange out from the bottom of the slab such that the load is set back a distance d from the slab edge.

 

[hokie66]

A lot of load to hang from the edge of a slab. You mentioned 'wide flange' in your latest post, so if this is a steel structure, hang it from a beam.

 

[Galambos]

No, not its a mild reinforced transfer slab. Concept tells me that the shear and flexure of the slab is adequate.

I cant help but feel like i am leaving a check out of the analysis, to account for the local effects. Is a punching check appropriate here?

The wide flange, say 4-5 ft long, would act as a propped cantilever, and run perpendicular to the slab edge, underneath. Connected in two locations, one near the slab edge and one further back. The connection nearest the opening would be set in, say 18", but allow the hanger to be connected near the slab edge.

 

[dhengr]

Galambos:
Could you use a larger and longer plate on the underside of the slab, for the HSS hanger, and a like top plate on the slab; with through bolts btwn. them. This would distribute the hanger load to more of the slab edge, as you wished and need.

 

[hokie66]

Yes, punching shear certainly needs to be checked. Your idea of a cantilevered, underslung steel beam, sounds appropriate. As does dhengr's plate top and bottom. I have often used plates top and bottom, joined by bolts, welded studs, or sections, to distribute uplift loads to a slab, and that would be a similar situation to your hanger.

 

[a2mfk]

This is an odd detail- what is this for?

I immediately thought top plate also, like Dhengr and Hokie. Then you can use punching shear values of the concrete rather than connection pull-out values. Then it becomes a concrete shear and bending problem and not a connection problem.

 

[Galambos]

The detail is to suspend floor framing below.

The top plate sounds good, but now you are venturing into Appendix D territory and relying on that shear cone. There are no pullout values beyond meeting the requirements of ACI 12.5 (?) for standard hooks, including development length.

This is the part that has me confused. If I develop the hook into the slab, why isnt that enough? Haven't I avoided the App D requirements?

Thoughts?

 

[a2mfk]

The top plate keeps you OUT of Appendix D. It is simple punching shear of the plate through the slab, no different than calculating for a base plate on a standard footing. Just the axial load is coming from below rather than above the concrete, but the concrete doesn't know that.

 

[dhengr]

Galambos:
Your current detail is an Appendix D detail, in a cracked conc. region, a pretty difficult detail. There is another thread here right now, thread507-359891, and TXStructural pretty well covers the current thinking on the issue. The rebar bearing forces on the inside radius of the hooked rebar crush the conc. and actually allow some relaxation of the bar length, rather than development of its tensile strength. Furthermore, your welded studs and the forces associated with them and their failure cone, tend to weaken the slab right where you would like max. slab moment cap’y. to support your hooked bars. At the least, the hooked bar may not be fully developed right where you need it, it’s not well supported because of the stud cones, and the total cap’y. is not the sum of the rebars and the studs under normal conditions. The top pl. detail causes a punching shear problem like that btwn. a slab and an exterior conc. column, a three sided truncated cone.

 

[dcarr82775]

I agree with dhengr, it is an App D situation. Tricky decision on what the breakout surface will look like.

 

[slickdeals]

Agree with others. Provide a top plate (top of plate flush with top of slab), probably a 12"x12" plate with a knife plate extending through the slab. I would provide holes in the knife plate that will allow a couple of #5 "U" to pass through. In addition, I would detail #5 bars welded to the top plate and anchored into the slab (like a precast embedded plate).

In a similar situation before, I provided some stud rails (upside down) butt up against top plate. Analyze it for punching shear like a 12" x 12" column with a 50 kip load (edge condition).

 

[KootK]

I always get antsy about the fire rating issues with suspended framing. The post probably has to be fire proofed. Does the embed plate? Does the upper side plate if you use one? Seriously, if anybody knows, I'd like to hear about it.

I think that what you've shown probably would work if you can deal with the shear in a convincing manner. This is a pretty high stakes connection though so I can see why you're concerned. For some reason, I feel bothered by how close the DBA is to the slab edge in your connection.

I've attached a suggestion to this post. It should be doable if your sketch was approximately to scale. My preference would be for a connection that possesses some ductility. If desired, you could also run an RC beam perpendicular to the slab edge. Then, it would basically be punching shear but handled with stirrups rather than diagonal concrete tension.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[haynewp]

If you replace the headed studs with more DBAs and still have enough length for development, I think that would exclude the Appendix D requirements. You have eccentricity on that plate to check too. A top plate is the most safe.