Concrete Breakout strength on an anchor in tension

[EntryLevelEIT]

The goal is to calculate the breakout strength of two embedded studs. This is applying to a plate embedded to a concrete wall with two studs attached anchoring horizontally into the concrete wall. This is to support a stairwell landing.

1.) I want to confirm the correct equaction to use, which I believe is Ncbg from D.5.2.1 in ACI 318-05.

2.) Within that equation is a Nb value, here I'm not sure what equation to use to calculate Nb. I see the conc. breakout strength for a single anchor in tension which is not my case. I also see Nb for cast-in headed studs and headed bolts with Hef between 11 and 25. I am using cast in studs however my Hef is not between 11 and 25, the embedment is only 4". Any ideas?

 

[dcarr82775]

Use Eq <D-5> and get Nb from Eq <D-7>. The effect of multiple anchors is accounted for in the Anc factor. Remember that Anc/Anco can not exceed the # of anchors.

 

[msquared48]

The Nelson Stud Anchor Company has a lot of information on this.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[JAE]

Mike - I used to use the Nelson design guides but I think ACI 318, Appendix D may be the more current, applicable design criteria to use if the local jurisdiction has adopted the latest IBC code.

The Nelson "method" was never codified and was only used in the absense of a method provided by the UBC or IBC.

 

[Lion06]

Is it supporting the stair landing at the top or the bottom? If it's supporting the bottom, it likely won't see any tension. If it's supporting the top of the stringer on the face of the wall, you probably want the shear breakout, not the tension breakout.

Can you post a sketch?

 

[msquared48]

Thanks JAE...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[EntryLevelEIT]

Yes..attached is a sketch.

 

[ToadJones]

Looks like you'll just have tension due to moment caused by the eccentricity of the connection.

 

[Lion06]

You'll have shear on the anchors and tension on the top anchor as a result of the moment (as noted by ToadJones).

You shouldn't need to worry about shear breakout, but you'll still need to look at pryout and the shear strength of the steel.

 

[ToadJones]

I didn't mean to imply that there was only tension, just that the "only tension" would result from an eccentric moment.

 

[Ron]

Agree with SEIT that shear is there. Probably not critical to the concrete, but need to check for steel stress unity.

 

[EntryLevelEIT]

Even though the embedded plate/studs are cast in place, the connection between the studs and the plate should still be checked for shear?

Also, I agree that a moment will be produced putting the top stud in tension and bottom stud in compression(correct me if I'm wrong).Not sure how to calculate what exactly that moment would be in a continuous joint stair system.

Besides that, I plan to calculate the weld strength between the angle and the channel along with the weld strength between the channel and the plate.

StructuralEIT you also mentioned pryout. By this you mean the pryout strenght of the plate alone due to the tension at the top? Is this found in Appendix D also.

Thanks for the help.

 

[Lion06]

The moment would be the reaction times the distance from that reaction to the face of the wall.

You don't need to check the weld from the studs to the plate, these welds develop the stud so you only need to check the stud.

Pryout is a shear failure mechanism of headed studs embedded in concrete. It is in App. D (D6.3 in ACI 318-05), but as Ron notes, it likely will not be an issue. My guess is that the load would be less than (or pretty close to ) 20% of the capacity which allows you to ignore it for interaction purposes.

 

[EntryLevelEIT]

Thanks for your help. I attached a pic with calculations showing how I arrived to the concrete breakout strength of an anchor in shear. I'm not sure if everything makes sense.

 

[EntryLevelEIT]

Any comments on my previous calc? I'm just curious about the concrete strength, and if it was calculated properly. The breakout strength seems really high to me.

 

[PAstructural08]

The calculation looks correct. I believe your breakout strength is extremely high because your plate is in the 'middle' of the wall and not close to an edge. Usually, when you have a plate like this in the middle of the wall (far from an edge), your studs would fail before your concrete does because so much concrete is around the studs.

Since you said your embedment is only 4" I would check the concrete breakout strength of the stud in tension. Of course also check the pullout strength.

 

[EntryLevelEIT]

I agree! I appreciate the input.

 

[Lion06]

Just looking at your calc, I don't believe that failure mechanism exists for your situation. I am assuming that this wall is supported by a continuous wall footing. If that is true, the failure mode can't physically occur.

I haven't run through the numbers, because I don't have my ACI 318 in front of me, but the value seems unreasonably high. When I've gone through the calcs for this type of situation (admittedly, I've never done it for am 18" thick wall), the values are usually unreasonably low for a very large edge distance with a relatively thin member. The reason is that Avc/Avco gets extremely small very quickly.

For your case, I would look at the interaction of tension/shear using the worst case of Tension (for the steel strength, pull out, and concrete breakout (the concrete breakout will likely control)) and Shear (the steel strength and pryout).

 

[EntryLevelEIT]

Yes SEIT a continuous footing does exist. I attached relevant cals regarding the pull out, breackout ect. These values seem to make more sense. Of these numbers the steel strength in tension/shear governs due to a very low yield stress, chosen from the mechanical properties chart by Nelson. I purposely chose the lowest value from this chart to be safe. Otherwise as you mentioned the breakout strenght would have goverened.