concrete breakout strength in shear 2

[wrxsti]

hi i have a pedestal which is surrounded by 4 No. tie beams each 12inches by 12inches
pedestal is 18x18
bolts are 4 number and offset 3 inches both way from centerline (form a 6x6 square about centerline of pedestal)
embedment about 20 inches

the top of the tie beam is about 6 inches down from top of pedestal

see rough sketch attached

can the tie beam be considered to help with concrete breakout in shear?

also can the tie beam reinforcement be considered as shear reinforcement to help the anchors?
the top steel in the tie beam would be around 9 inches down from the pedestal
i know commonly 3 layers can be used at 2-3-3 from the top of the pedestal


thanks

 

[KootK]

can the tie beam be considered to help with concrete breakout in shear?
also can the tie beam reinforcement be considered as shear reinforcement to help the anchors?

I believe that the answer to both of those questions is, rationally, yes. The trick then becomes how to account for those benefits in a way that you feel is code compliant. Obviously, that would be easier if the beams were as wide as the piers and the tops of the beams were aligned with the tops of the piers.

 

[Agent666]

If 6" below, I'd ignore it. While it likely contributes, the beams are not protecting/confining the most critical region of the breakout, that being the top concrete surface where the load is being applied from your anchors.

If you want to enhance the shear breakout, then you'll need some appropriately detailed anchor reinforcement in the top of the pedestal itself. Some ties around the vertical reinforcment, and terminate the vertical reinforcement as close as possible to the top of the pedestal.

i.e.

from this paper

https://engineervsheep.com

 

[wrxsti]

shear reinforcement and concrete breakout strength from concrete alone are additive right?

but friction and above are not?

 

[KootK]

shear reinforcement and concrete breakout strength from concrete alone are additive right?

No, I don't believe that most design methods do consider them additive. That's a departure from how we hand shear reinforcement in beams etc.

but friction and above are not?

I'm not clear on what you mean by that. Can you elaborate?

 

[Agent666]

Not additive, the reasoning being the concrete effectively has to fail to engage the anchor reinforcement to retain the chunk that has failed. ACI chapter 17 goes into this, it's one or the other, not both once you're reliant on anchor reinforcement. Supplementary reinforcement is another matter, this is/can be taken into consideration in the concrete shear capacity.

Supplementary reinforcement on the other hand is reinforcement which is provided, but not necessarily accounted for in the calculation except to allow for a beneficial factor to increase the concrete shear breakout.

Whereas anchor reinforcement is specifically taken into account. ACI provides a better definition so have a read on how the difference between the two types is formally defined.

https://engineervsheep.com

 

[wrxsti]

frction due to column axial

 

[wrxsti]

yea

The use of supplementary reinforcement is similar to the anchor reinforcement, but it isn't specifically designed to transfer loads. If supplementary reinforcement is used, the concrete strength reduction factor f is increase 7% from 0.70 to 0.75, which is not that significant in terms of increasing concrete breakout strength.

 

[KootK]

friction due to column axial

You need both of these things for a valid design"

1) Friction capacity at the horizontal interfaces at the underside of the base plate and underside of the grout. I believe that reliable axial compression can be used for this in most, non-seismic cases.

2) Break out shear capacity along the usual, diagonal plane through the concrete. Axial compression probably does help with this but, to my knowledge, there's no established way to account for that.

 

[wrxsti]

does this
"If anchor bolts are placed in the top of a column
or pedestal, the bolts shall be enclosed by transverse reinforcement that also surrounds at least four longitudinal bars
within the column or pedestal. The transverse reinforcement
shall be distributed within 5 in. of the top of the column or
pedestal and shall consist of at least two No. 4 or three No.
3 bars."

really mean that the anchor shear reinforcement have to be within top 5 inches

or just good detailing practice to have transverse reinforcement around anchor bolts

 

[wrxsti]

 

[wrxsti]

 

[Agent666]

They need to be sufficiently high enough to the top to be effective if that is what you are asking. Put them too low and the concrete breaks out above the ties and you do not develop the anchor reinforcement strength.

https://engineervsheep.com

 

[wrxsti]

but where is the sufficiently high enough

is it 5 inches

is it between 8do

is it 6.5 inches

 

[bones206]

It's a judgment call which bars you choose as being part of the STM model. I generally stick to the precedent that Widianto set by saying only the top two ties within 5" of the top of the pier are effective in the STM model. That STM modelling decision is separate from the ACI detailing requirements. But it's conservative to ignore the ties below the 5" and it's logical that most of the shear force will go into the uppermost ties.

Also keep in mind that the lower your shear reinforcement, the higher the force in the reinforcement due to the eccentricity.

 

[wrxsti]

yea and the widianto top 5" is adopted in ASCE anchor design .... publication

 

[KootK]

1) My read of the ACI 7.10.5.6 provision is that it is a "good detailing" thing rather than an explicit statement of where lateral shear would be dealt with. The ACI 318-19 version is reproduced below.

2) I agree with the other fellows in that you almost can't have the lateral reinforcing placed too high in the pier. I've always felt that Widianto's [8 x d_o] was probably a bit optimistic, particularly given that many would not consider the bolt to be rotationally fixed at the top. But, then, Widianto has given us a path that was lacking. In the land of the blind, the one eyed man is king.

 

[Agent666]

Judgement would also suggest if your reinforcement is fully developed each side of the concrete failure plane and the reinforcement is close enough to the anchors, then its effective. This is the principals ACI works to, there are some pictures that explain it quite well with detailing rules. From memory in Eurocodes there are means of determining how effective partially developed bars are as well if you struggle.

https://engineervsheep.com

 

[wrxsti]

well they more lean towards either
(a) hair pin
or
(b) fully developed both sides with edge bar



the widianto example uses a ratio of actual length to developed length to come up with effective stress

they also address the position of hooks

which in the case of not knowing where they are you can only use this value
in my case for #3 was like 2. something kips
so at 2.something x lets hope atleast 4 legs of the stirrups in top 5 inches
will only be say 10 kips *insert crying face*

 

[bones206]

You should check out the CivilBay calculator for anchor reinforcement. It's pretty well done and covers a lot of the little nuances:

http://asp.civilbay.com/03-index/12-conc-anchor.aspx

Example calc:

http://asp.civilbay.com/04-help/02-sample/02-print-sample.pdf