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I want to install a fence along the top of a retaining wall. The wall is made of standard 8x8x16 CMUs and is filled with 3500psi concrete and #4 rebar in every cell. The posts have 4"x4" surface mount plates on the bottom:

Fence post surface mount plate

The manufacturer calls for 3/8" fasteners @ 4" deep. I think my choices are sleeve anchors, wedge anchors, Tapcons / Titens, or 3/8" galvanized threaded rod cut in 5" segments and epoxied into the holes.

I'm concerned about the edge distance with wedge anchors even though they're the easiest to use. All roto-hammer drilled holes will only be about 2.125" from the edge. This should get me into the concrete-filled cells, but some of the angles may put me closer to the block's walls. Either way, I'm close to the stated minimum edge distance of 5x the hole's diameter.

Would a chemical anchor (epoxy) be less likely to have issues being this close to the edges of the blocks? What about Tapcons?

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  • 2
    I suggest replacing the block and regrout with embedded anchors.
    – r13
    Jul 22 at 0:41
  • How tall is the existing retaining wall? How far is the existing retaining wall anchored into the ground? Is the existing retaining wall holding anything (dirt) back? If so, how high is it placed against the wall? How high is the new wall going to be on top of the existing retaining wall?
    – Lee Sam
    Jul 22 at 2:23
  • The wall is roughly 3 feet tall and holds level dirt behind it. There's about a 3 inch lip from the top of the wall to the dirt on the backside. The wall's footer is 2.25 feet deep and regularly ties into ground granite boulders with drilled holes and epoxied rebar. It's quite substantial anticipating this 6 foot fence.
    – pennstump
    Jul 22 at 3:25
  • does it have a bond beam, or are the cells all separate?
    – Jasen
    Jul 22 at 4:07
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There are many factors that affect the stability of the wall. I’ll assume all those issues are correct: 1) grade of cmu, 2) grade of mortar, 3) grade and placement of grout, 4) grade and placement of masonry reinforcement, 5) adequate design and correct placement (edge distance) of reinforcement for seismic activity for your area, (and you have high stresses in your area), 6) correct corrosion protection for your area, (and you have high corrosion in your area), 7) lateral stability, (including adequate soil resistance in your area).

Instead, I’ll concentrate on the single issue of supporting a 6’ high fence from rotating and causing the anchors to fail.

Assuming the fence posts are about 6’ on center and the fence is 6’ high and wind resistance is about 30 psf in your area, the load is 1,080 ft.-lbs. Rotation is taken 3’ from the base for a load of 3240 ft.-lbs. Assuming there are two anchors resisting this force, so each anchor will need to resist about 1620 ft.-lbs. Those anchors you’re suggesting to use are inadequate.

The Code (see ICC Section 2109.7) outlines steel connection requirements. The only anchors that will provide adequate withdrawal resistance are wedge anchors or anchor bolts with a 2” hook. Wedge anchors will provide about 3000 lbs. pullout strength (which has a safety factor of about 2 with adequate edge distance). See

https://www.google.com/search?q=wedge+anchor+vs+sleeve+anchor&rlz=1CDGOYI_enUS900US900&oq=wedge+anchor&aqs=chrome.2.69i57j0l5.13383j0j7&hl=en-US&sourceid=chrome-mobile&ie=UTF-8

Using a 1/2” wedge anchor, you’ll need about 2 1/2” edge distance. This is impossible with your 4” steel plate and about 1 1/4” side shell on the cmu. The Code requires an edge distance based on the size of the bolt:

  1. 1/4” = 1 1/2”
  2. 1/2” = 2 1/2”
  3. 5/8” = 3” This is based on standard grade concrete, (i.e.: 2500, 3000, etc.) If you’d use a “high-strength” concrete it can be reduced. (See ICC Chapter 19, Table 1908.2)

Perhaps the reason the previous owner never installed the fence is because they knew it would not be adequate.

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Consider:

  • wedge anchors require striking, which could be a concern if you are close to the edge
  • sleeve anchors require a larger hole, which would be undesirable if near the edge
  • Tapcons are lame (IMO)
  • epoxied threaded rod would also require a larger hole than the 3/8" rod (but perhaps not as large as a 3/8" sleeve anchor) and no striking necessary... this would be my pick.
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  • Epoxy anchor has a limit on edge distance as well, watchout.
    – r13
    Jul 22 at 0:37

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