Column vertical reinforcement resist cone pull out

[kynto3]

Hi there,

I have a chemset connection into a compression column with dense vertical reinforcement. The connection is a shear connection however tension arises from a small moment on the connection due to the stiffness of the column v the chemsets. The chemists will be developed 300mm into the centre of the column.

Just trying to convince myself (or otherwise) that concrete pull out failure mechanism can be resisted by the vertical steel reinforcement.

Unfortunately the existing structural drawings are insufficient to determine the presence of ligatures that could be used in tension to resist this tension pull out failure mode.

If the vertical bars are sufficiently developed in this region, am I able to take the tension resistance of the concrete cone as equal to the shear capacity of this vertical column steel that is encased in the concrete cone (1.5hef from the anchor centreline)?

Thanks.

 

[Ron]

Chemical anchors do not typically fail with a conical failure. They generally fail in shear along the bond interface. For that reason, the reinforcing steel has little effect on the chemical anchor failure mode.

If you were using wedge or other mechanical anchors that fail in a shear cone, then wherever the rebar intersects the shear cone, there is positive influence on the failure.

 

[kynto3]

Ron,

Thank you for your reply.

I am using the Hilti Profis software and concrete cone failure governs when I alter my edge distance, hence my identification that this is the critical failure mode. I don't think a 200mm edge distance with 4 36mm diameter bars hurting through it is really 200mm of concrete in terms of concrete cone failure in tension.

Splitting is not governing.

Thanks.

 

[Ron]

If you have rebar intersecting the cone at any point, and in your case it appears that you have lot of rebar in the cone area, there will not be a true cone failure. The stress path gets convoluted around rebar, but your capacity is probably more than stated.

I have not used the Hilti software much, but I'm still not convinced you should be getting a cone failure mode....even Hilti shows that adhesive anchors do not fail with cone shear.

 

[KootK]

This sounds like a straight up anchor bolt anchorage scenario to me. The only special condition is the use of post installed anchors. I think that concrete tension breakout must be adressed somehow. The options for that include:

1) Satisfying appendix D provisions (Hilti Profis).
2) Using manufacturer's test data as evidence of performance. This should include criteria for edge distances etc.
3) Using reinforced concrete theory. Stuff like AISC design guide #1 or Widianto's strut and tie method.

If you're going to make use of the existing vertical steel, 300 mm sounds a little short to me. If you went with the AISC method, for example, you'd be looking to develop the vertical steel for your tension force on both sides of the tension breakout cone.

Do you have plenty of capacity with regard to shear breakout? If you post a sketch showing the particulars of your situation, I'd be happy to take a closer look.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[kynto3]

Hi Kootk,

Sketch attached. Excuse how rubbish it is - I am off work with a hand injury currently!

Top shows plan of column: vertical reo is max 75mm from centreline of the anchors.

Bottom shows elevation of column.

The profits software is giving me sufficient capacity for shear break out with the 230mm edge distance shown, and the edge distance above and below the anchor is infinite as its a column part way up a building.

 

[KootK]

Thanks Kynto. I was originally thinking that you were fastening to the top of the column. Appendix D gives you a bump for rebar crossing the pryout plane but it's nothing close to the shear capacity of the bars. I expect that the compression in the concrete improves matters as well but I don't know of an accepted way to quantify that.

Could you rationalize a lower moment somehow? It seems extreme but, if you had to, you might be able to install through bolts and an anchor plate for the top bolts to mobilize the shear capacity of the section.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[kynto3]

Kootk,

What is the expression for the additional capacity for the rebar crossing the tryout plane? And Appendix D of what resource? Sorry I am not familiar.

Thanks.

 

[kynto3]

Should read pryout! Computer autocorrected.

 

[KootK]

It's in ACI 318 and is expressed as a flat rate increase I believe.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Leftwow]

In your drawing, I see the lateral reinforcement but no vertical. You specified it as dense vertical reinforcement, just confused. From what I've learned from previous engineers and APPX D, the ties affect the side face blowout but not the normal breakout and only vertical reinforcement affects the breakout. Please correct me if I'm wrong

 

[hokie66]

Playswow,
You need to look again. His plan and elevation both show vertical column bars, but no ties.

kynto3,
May be just me, but I would say your concern is unfounded. If it is a shear connection, why worry about a bit of relaxation in the fasteners?

 

[KootK]

Similar to Hokie's point, I've known more than one engineer to make the argument that a connection that fails in self limiting pryout still retains it's shear capacity.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.