fence posts set in a cast retaining wall
We are going to be building a bunch of cast concrete retaining walls at our place. Atop the walls, which will have an average thickness of 6.5″, we will be setting fence posts, and in some cases posts for trellis (up to 8′ tall)
For lack of any better ideas, I have decided to use CB44’s to anchor the posts, but I am nervous about the alignment of the anchor in the wall. Running the straps of the post bracket perpindicular to the axis of the wall seems to offer better resistance for post movement in the bracket (assuming that the fence offers resistance on the wall axis) The problem is that this orientation disallows me to run rebar through the embedded section of the bracket. I am considering burning a hole through the embedded legs to allow the bar to run through. The local building inspector (my wife) thinks I’m nuts, so I’m turning to you, my brethren. The job is unpermitted, but it still must be done right.
Your thoughts?
Replies
I don't do this for a living so I don't really know how concerned to be, or whether this is really an issue, but this comment about the CB44 & similar always got my attention:
"Post bases do not provide adequate resistance to prevent members from rotating about the base and therefore are not recommended for non top-supported installations (such as fences or unbraced carports). "
Don't know what your alternatives are; food for thought. Oh yea, the ZMAX version for sure if you're using PT.
CB44's are wonderful, but there are better ways to anchor your fence posts so they resist overturning. Wind loads are the primary consideration in most places. High winds can snap embedded 4x4's off at the ground, so it all depends on what you need to resist.
When I lived in Colorado a very rich person hired a fence company to erect a 6' cedar fence about 1400 feet long, next to a road I drove daily. Posts were 4x4's set in concrete. Not long after it was completed, we had a big blow, so-called Chinook winds. All but a few sections of that 1/4 mile had blown down. The rails blew off the posts. Rails were bolted to posts. Next big blow snapped all the 4x4's, one every 8 feet for a quarter mile. Unbelievable. It was never properly repaired, and eventually all pulled and tossed.
A neighbor in those same mountains built himself a 6' privacy fence and used 6x6 posts. Still there after 20 years of windy winters.
For your situation I would consider using 2" schedule 40 galvanized pipe for posts and detailing the job to eliminate standing water inside or out.
If the posts just HAVE TO be wood, then I have had success fixing wood posts to concrete by epoxying 4 stainless all-threads into the concrete and into the post, near the corners. I glue the rods into holes drilled at a shallow angle toward the center from the corners, then bend them straight after the epoxy cures. Then tape off and drill/clean/epoxy posts to concrete.
This is very strong; I've never had a failure. 8" of embedment into the wood and 5" into the concrete is sufficient for 4x4's with 1/2" all thread. I've gone a bit deeper (7" concrete and 12" wood) with 6x6's so the epoxy joint did not become the weak link. Getting solid epoxy all under the post end is helpful for strength (gotta mask the post and the wall really well, and pull tape before epoxy cures), and also stops water from wicking up the end grain.
I once needed to demo such a post, 4x4 @ 4' tall. Sledgehammering the post just caused big bounce until I hit it lower down and shattered the wood around the all-thread. Much of the epoxy stayed clinging to the stainless rods. Post had to be reduced to splinters to get it off of the rods.
Another idea is to embed short lengths of the 2" galvanized pipe and glue it into the wood after the wall cures. Fill the pipe with concrete or mortar so you don't waste a ton of epoxy. Fill 'em with foam if you plan on cross-bolting. Use a Hawg with a self-feeding bit to bore the endgrain. I have not done this, but a friend did and reported success. The pipe will need to be drilled for your rebar, but the 4 rods method nicely misses it.
Bill
Your details sound way more elegant than the CB44's I had imagined, but for 40-odd posts probably more love than I'm willing to give this one. More importantly for me, I'm trying to imagine how the threaded rod solution offers better resistance for post-breakage, uplift or twisting than the Simpson option.
I also noticed the verbage in the catalog disclaiming their suitability for fence posts. Wind loads in my neck of the woods are exceptional, and the fence we will be putting up will be wire in most cases, with a top and bottom rail. The trellis will be a bit more of a concern, but I am hoping to argue succesfully with the landscape designer (again, the wife) for a "three-dimensional" trellis with secondary posts set back from the wall for stability.
Assuming CB44's any thoughts on the reinforcement within the concrete wall?
Assuming CB44's, the top rebar should capture the loop on the post base. Drill or burn holes if you have to to get the desired geometry.The 4 rods method is more strong in all those ways than the CB44 because the epoxy evenly distributes the forces over a wider amount of wood fiber than the purely mechanical connection of straps with bolts can.With a wire fence, wind loads should be no problem. Best of luck on your project.Bill
Much thanks Bill
Bill,
I've read of a similar approach to yours at the boards here.
If I recall correctly, the method was to run two galvanized lag screws into the end of a 4x4 post. The two lags were placed in opposing corners of the post (axis of the lags parallel with long axis of post).
Then, the head end of the lags was inserted and expoxied into rotohammered holes in the concrete.
I would guess that the lags would be about 12" long, with about half the length in the post, and the other half in the concrete.
Do you have any comments or experience with this approach?
Ragnar,I do not have any experience with that approach, but I have considered it. Here are my comments:It ought to work, but not as well as epoxied all-thread. The lags will tend to split the posts when there is significant overturning moment. Proper pilot holes will minimize this problem. The holes in the concrete will have to be bigger to receive the hex heads, but that's not a major expense. The concrete portion will be way stronger than the wood portion. I'd suggest doing only 4" embedment there and 8" of lag in the post itself, if using 12" lags..Aiming the lag at a shallow angle from near the corner toward the post's centerline, then bending the lag straight, will improve strength by encasing the threads deeper in the post. Running the steel diagonally across the grain also reduces the likelihood of splitting, versus running up the post parallel to the grain. Finally, the holding power of wood screw threads is least in end grain, so diagonal boring will help that aspect also.Bill
Thanks for the reply, Bill. I like your approach and think I will try it next time I am faced with that problem.
To be honest, my kneejerk reaction is that it would take a significant amount of force to bend a 1/2"-dia stick of allthread -- I'd be concerned about splitting out the end of the post. (But obviously, your experience has proven otherwise.)
So how exactly do you go about "straightening" the allthread once it's been inserted slightly askew into the end of the posts?
Also, have you ever experimented with using only two sticks per post (as opposed to four)? What are your thoughts on that?
Thanks for your interest, Ragnar.I straighten the all-thread with a few hits from a lump hammer. Remember, I am hitting towards the centerline of the post, so the rod transmits the force to the thickest section of the post, and the force is reacted by the part buried deep in the wood. The epoxy holds the all-thread so firmly that it bends easily, just at and past the base of the post. It is angled about 10-15 degrees into the wood, so I don't need to bend it much.Doing this with a lag could be problematic, since the metal is smaller where the threads are, and the root tapers. A 3/8" lag should work okay. I typically use 1/2" all-thread, stainless for corrosion resistance and for toughness. Low grade mild steel all-thread is much more prone to stress fractures originating at the root of the threads when the rod is bent. Stainless is much less prone to this sort of failure, very tough material. Luckily, the bending is not much. 3/8" rebar also works well, but prone to corrosion.I have never tried using only 2 sticks per post. That would result in an installation much stronger in one diagonal axis than the other. It will work fine where ultimate strength is not essential. It just seems lke false economy to skimp on such a small part of the total job.I first dreamed this detail up in the early 90's when a customer wanted a post growing out of an existing stemwall to support the upper end of a grab rail for some outside basement access stairs. They really wanted redwood to match the rest of the project, it needed to be very stiff and strong in all directions, and I had the stainless all-thread handy, plus WEST epoxy. It is much easier to do this with the Simpson anchoring epoxy in the siamesed cartridges, as the mixing is automatic, the gel time is fast, and the holes can be filled from the bottom up. It's what I usually use now. Because of gravity, and the angled holes, I do all the posts at one time, with one nozzle, straighten 'em, then prep my concrete with cleaned holes, masking, bracing at the ready, etc. Then I set all the posts at one time, using a second nozzle. On long runs this requires a helper. To repeat, it is important to completely bond the bottom of the post to the concrete below it for the strongest, most durable installation. One thing I have never tried that would add strength is to encircle the post with a compression band, an inch or two above the concrete. This would hold the wood fiber together longer under extreme strain. Picture a worm-gear clamp on steroids. This would be ugly, and the method is awesome without it. It is just my nature to picture how a thing will fail when tested, and to discover means for postponing said failure.I hope you get a chance to try it sometime. My customers have been suitably impressed by the invisible and awesome strength.Bill
Thank you again for the help, Bill. It sounds like a great method, and I appreciate you taking the time to explain it more thoroughly.
Since you repeated the part about bonding the bottom of the post to the concrete, I should ask for some more clarification there. What I'm envisioning is that you tape off the bottom of the post and the concrete area immediately adjacent to where the post will be set. (Is regular blue painter's tape adequate for this task, by the way?) This is just to protect the finish surfaces from getting all gunked up, right? How long do you need to wait before you can pull the tape off?
I'm assuming that you install the post bottom pretty much flush with the surface of the concrete. That is, you don't intentionally hold it up a 1/4" off the concrete, etc. (I'm assuming that the epoxy should just about completely seal the end grain, so there may be no need for a stand off.) Is it worthwhile to apply a bit of epoxy directly to the end grain before setting the post down, or is the excess epoxy being displaced by the allthread in the concrete holes more than enough?
Thanks again!
Ragnar
Ragnar,I butter the end grain with a shim (disposable!) before setting the post. As you surmise, the tape is just to keep glue off decorative surfaces. Blue tape works on smooth wood and smooth concrete, but duct tape often works better on the concrete. Red duct tape is better than gray. It lets you see the gray epoxy more clearly.Set the post and brace it plumb, then wipe excess spooge from the base joint with gloved finger, wipe on paper towel. Pull tape shortly after wiping excess glue. The best time is after the initial stickiness abates, but the epoxy is still soft. Pulling tape too soon is not a big deal, but can pull strings of glue that get on stuff you wanted to keep clean.I like to set the post as close to the concrete as I can, so long as it is plumb. A standoff detail would be more work, and not look as elegant, IMO. There are locations where it would be a good method to keep the wood dry. A guy could butter the endgrain with epoxy to seal it, set the all-thread with nuts and washers for standoffs, then pack nonshrinking grout under the post after the epoxy cures. The rods would need to be longer by the standoff distance.This technique for setting posts makes it look and feel like they "grew there."Bill
why not just leave 12-18" of 3/4" rebar sticking out of the concrete center drill a hole in the bottom of your post and slide the post over the rebar?
my second option and this is what we have always done to set hand rairs and small fence sections... cast a long neck beer bottle or a slim wine bottle into the concrete... break it off when the concrete sets... use left over hole to set your post or rail into...... usually have to fill the bottle with water so it doesn't try to float out...
p
Pony,
Could you elaborate a bit on the bottle method? How would you connect a 4x4 or 6x6 post, for example?
when the rail wasn't there... or yet to be fabricated... we've used the bottle in the concrete method... gives you some room to adjust postion before setting the post in the resulting hole... in your case could just be a matter of setting a steel pin or pipe into the hole...
depending to the height of the wood post... which if over 4ft I don't like the idea period...
I'd much prefer to see a steel fence post set into...... or on the wall with a 4 bolt mount...
i don't know how your wall will be set or the weight but even a 6ft post/fence will have alot of leverage to move heavy concrete
p