Pier Anchor Bolts - ACI 318, Appendix D ?

[vmirat]

So, I'm designing a canopy similar to a gas station canopy. The columns are supported on concrete piers which are 6'-10" tall from the footer (top of footer is 2' below grade). The columns are designed such that they are moment resisting at the base and the canopy is pinned at the top.

I initially used ACI 318, Appendix D, for design of the column anchor bolts, using hf = 12". But I've heard others use a different method where the length of the anchor (hf) is based on the development length of the column rebar. I was going to use 8 -#8 rebar for the column, but that would give me about a 48" ld, which seems a bit too long for an anchor bolt.

I'm wondering who out there uses this method of anchor bolt design and how they implement it?

 

[3doorsdwn]

What I have always done is first check it by App. D to see if it will work regardless of any help from reinforcement. And if that doesn't work [and typically it doesn't because of edge distances], I then fully develop some rebar into the failure zone and then fully develop it outside of this zone (so at minimum its full length will be Ld x2). The arrangement varies based number of bolts and edge distances and so forth.

Some years back I attended a seminar by S. K. Ghosh Associates on this. They provided an excellent guide for anchoring bolt groups where the loads exceed what is in appendix D.

 

[Lion06]

I have a paper on developing anchor rods with pier reinforcement, but it's at the office, I can post it tomorrow morning.

A couple comments -

1) In order to use the pier reinforcing they want to be within a few inches of the anchors (plan dimension offset).

2) I would use the Asreq'd/Asprovided to reduce the developement length. If you're using (8) #8 for the 1%, that's likely going to be way more than you need for a tension member - take advantage of it.

 

[TXStructural]

The issue is developing to what the anchor rods connect. You only need to develop the required strength. If you hook the rebar, use Ldh. Use washers and nuts on the anchor bolts, which effectively develops the anchors. If you do not need the full strength of #8 bars, pick a bar size which meets the strength requirement for the count of bars - use development length for that size. This will be reasonable since development relies on the deformations. Make sure you have ties around the bars in the development zone to mitigate wedging. Use App D D5.2.9 (see the commentary).

 

[msquared48]

If there is some reason to cut the development length short, you could always full pen weld the #8 bars to a common embeded plate and let the pedistal steel take the transferred tension to the foundation.

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

 

[ToadJones]

Tell me if my thinking is off base, but for anchors with high tension loads, I always lap the anchors along side the pier stem steel for a full development length so that the rebar is developed fully to transfer the tension load.
I also like to use anchor plates and nuts or just nuts on the anchor.
Like EIT said, if the pier steel is not close to the anchor, I don't rely on it as being effective in transferring the forces.
Attached is a decent paper on the matter by some guys at Bechtel, I think...

 

[slickdeals]

See the sketches in Appendix D of ACI 318-08. They have recommendations on how close the bar has to be the anchor for it to be effective.

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[slickdeals]

Oh, and I recommend to confine the anchors with closely spaced ties (like the top of a column cage)

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[SeanMD]

I agree with above, i would lap the anchor rods with the vert reinf. AISC design guide #7 has an example.

 

[KootK]

@3Doors: Can you share the name of that Gosh publication? I'd like to pick up a copy if they're still available.

 

[Lion06]

Here is the article

 

[slickdeals]

@3doors:
Is it this publication from SK Ghosh?

http://secure.skghoshassociates.com/onlineorder/show_book.php?isbn=200810030006

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Go HOKIES

 

[ToadJones]

EIT...same as I posted, I think.

 

[KootK]

Thanks slick

 

[EntryLevelEIT]

From that article, in example problem number 5 a factored load of 80 kips in tension was given. Can someone explain how the anchors are in tension. If the tension comes from a lateral load (wind or seismic) it produces a moment at the end of the column. How is this moment converted to a pure tension in kips. Also the example assumed equal tension among the 4 anchors. In actuality would the force be distributed equally amongst the anchors?

 

[slickdeals]

It could be a pure tension load if there is negligible dead load and very high wind uplift.

We are Virginia Tech
Go HOKIES

 

[slickdeals]

The 0.9D + 1.6W can sometimes wreak havoc.

We are Virginia Tech
Go HOKIES

 

[3doorsdwn]

@3doors:Is it this publication from SK Ghosh?

http://secure.skghoshassociates.com/onlineorder/show_book.php?isbn=200810030006

That looks like it. What I have is the pdf from the seminar, and that drawing on the cover is one example from the pdf.

 

[Lion06]

Toad-

You're right, I posted the wrong one. Thanks for catching that. Here is the article I meant to post.

 

[ToadJones]

Entry-
I wouldn't read too much into how they came up with the loads.
In order to actually determine the tension in the rods due to an overturning moment you need to analyze the column-baseplate-anchor rod assembly and take into account the stiffness of the plate, bearing of the plate on concrete, and the anchor rod layout.
For a simple approach your intuition is right. A moment on the bolt group, M, would result in tension in two bolts and compression in the others. M/d(the distance between bolts) would give you a rough number for tension & compression.