Timber Frame Design

I need to attach posts to footings on an open air structure 16'x16', 32' tall, ideally with little or no hardware visible. Is there a "best" method? Thanks.

I've seen a metal rod embeded in the concrete sticking up into the bottom of the post, which was pre-drilled with a hole for this rod, but this requires careful planning or a larger hole than the rod, so that you can slide the post over a 1/4" if necessary.
I've also seen a black metal band put around the post and slid down half way over the same size concrete footing to hold it in place, like a collar, half over concrete-half over wood, so to speak. It looked good as it was painted high gloss black. And it secured the post to the concrete pad.
On other raisings I've seen just angle irons hooked to the concrete and then threw bolted to the bottom of the post. But I'm sure that's what you're trying to avoid.
Maybe others will post some of there observations or ideas.
Good luck, Jim

This may take even more precision and quality layout than Roger's idea, and also assumes you have not poured the footings yet, but... Have you considered burying wood splines into the footing such that they stick out like tenons at the post locations. Then mortise the end of the posts to receive the spline and peg them together. Those pegs "way down there" may stir some interesting conversation....

There are two connections here that should be addressed. First a common technique is to set threaded rod with a couple nuts and large washers into the concrete, the washers/nuts help hold the rod in the concrete. Alternatively, and much simpler although with limited uplift resistance is to drill the hole for the rod in already set concrete and epoxy grout into place. The benefit being that accuracy on site by you, rather than a concrete contractor, gives you the ability to be sure the rod is in the right place.

At the bottom of the post, simplest is a simple yoke (U shaped bracket with a hold in the bottom (for the rod) and opposing holes in the yokes vertical flanges for through bolts to hold the post in the yoke. Alternatively, there are also several "barrel nut" timber fasteners on the marketplace that could allow you to bury the connector and add "faux" pegs to hide the embedded hardware.... unfortunately the rod is fixed so this could be very awkward... ( if you can get the threads started then you have to "screw" the post onto the rod....)

If you have strong hold down requirements, variations on the threaded rod may resolve the uplift. For example the rod can have extra length, to allow you to bend the bottom 90 degrees and align and tied it into the concrete's reinforcing.

In the past, I read a book about installing fence posts. In there the author strongly cautioned about encasing wood in concrete, especially in an "open air" environment.
The reason being that, in the example of a fence post, encased in a concrete "cup", this cup would/could fill up with water and then the fence post would be sitting in a cup of water. Water creates rot. He suggested if you wanted to use concrete in your fence post installation process to just make it a cap over the fence post hole. Any water hitting the fence post that would run down the fence post and in between the fence post and the concrete cap would then run out the bottom into a bed of crushed stone, and hopefully not rot the bottom of the post.
Now, here, in this application, is there a possibility of rain water getting into the concrete mortise created if you were to use Shaun's idea?
It's an interesting idea but unless you can be sure that no water would run into this concrete cup created by the spline, it might not be the best solution.
Also there is the possibility of the wood wicking up moisture from the concrete. There was another discussion about what to put between the post bottoms and concrete to prevent this. I'm not sure if it was here at this forum or another timber framing forum. But I've seen barriers put between wood and concrete to prevent moisture from wicking into the wood. Encasing a spline tenon in concrete would have to be treated or prepared so that the moisture from the concrete didn't wick into the wood spline.
And what about the mentioned “hold down” issue?
Some more careful thought or other ideas may need to be researched. Good luck, Jim

Thanks for furthering the discussion on the embedded spline idea, Jim. I haven't tried it myself, but thought it up as I was reading Peter's query, wondering myself if it was a viable solution.

Greetings all:
Do not embed wood in concrete. I suggest an engineered connection for a building like Peter is discussing. Many pre-engineered solutions are available from the Simpson family of connectors.
I have seen many connections used succesfully but you must define what the connection must do before selecting what seems appropriate.
Let me know if I can help.

Curtis

I've used 1/4" (or bigger)steel kerf plates let into a mortise cut into the end grain of the post with a chain saw plunge cut, then through bolted & plugged with wood plugs. Also, I've seen, but not used, engineered connections using a threaded rod inserted into a slightly oversized hole in the post base, with the space around the rod then filled with epoxy though an angled access hole (1/4" +/-) drilled through the face of the post. Either way you'll want to engineer your connections. I often use black roofing cement to seal the bottom of the post, with a piece of pressure treated plywood for a bearing plate.

http://www.timberlinx.com/ makes a pretty cool connector for that application. You can get 1/2 pipes that can thread onto a 7/8" %%C all-thread that is embedded into your foundation.

During a recent telephone conversation with a timber framing friend, I mentioned the need for more information about attaching timber frames posts to concrete foundations. He suggested a web site where you can get connectors.
Here is a link to that site:
Simpson Strongties
This page has many connectors shown on it, some that are hidden, others that are not. And there are many more available from this company that you can use.
Good luck, Jim

All thanks to Ed L, the post bottom to concrete issue has been solved forever. Hinges with plates the same size as the post bottom, Moisture barrier material between the hinge/post AND the hinge/concrete. Net add to vertical height about 2" so set your concrete 2.5" below the subfloor and your will never see the seam. Was used for the Mountain School Barn just completed. Kinda makes kids work of raising the bents too. Lotsa labor and material savings. Conversationally the 8x8's used in Vershire, VT ran "about" $50.00 a copy. Added paranoia relief by welding them shut after raising.

As one would expect, Ed's solution is ingenious and simple. I'm compelled to add the caveat that this may not satisfy "hold-down" requirements. A hinge flap secured to the bottom of the post might simply be secured by screws into end grain...not very yielding against pull out. However perhaps used in conjuction with a product like those offered by www.timberlinx.com (similar concept to furniture barrel nuts) and a hinge would be satisfactory... Although the hinge and bolt head maybe a problem, perhaps timberlinx would offer a tapered bolt head, and the hinge might have countersunk holes.... In this case I would weld, in the prior case welding seems comforting, but I have little faith the hinge would be taxed before the screws pull out of the end grain.

I don't understand what the hinges are for. I think you can get all the uplift resistence you might need by ensuring proper embedment of the t-rod or sstb into the conc. and observing NDS specs for dolt dia's from end of piece. If you need more value, double up the timerlinx or barrel bolts.

Timberlinx look nice because they only use a 7/8" dia expansion thing which can be easily covered by a 1" dia peg. Also they can be tightened down for proper takeup of any "slop"

Most folks doing "timber style" (not timberframe) construction around here use a custom "embedment plate", basicly a simpson CB replacement made with 1/4" steel and a bolted, internal kerf plate. (2) 3/4" machine bolts usually attach the post. The kerfs are usually not seen on the sides and are cut freestyle with a chainsaw plunging into the end grain.

HTH

If you would like to see some photos of this "hinge" plate check out this post on the Forestry Forum/Timber framing section:
http://www.forestryforum.com/cgi-bin/board/YaBB.pl?board=tframe;action=display;num=1060266331
There are a couple of photos of this hinge there, which I took. Also check out the schools photos of the barn raising at:
http://www.mountainschool.org/program/barnchrono.htm
At the time I posted the photo of the "hinge" plate, I didn't know who designed or invented them. But Emmett was there and has told us that it was Ed Levin the frame's designer.
I'm not sure how deep the bolts go into the concrete but there were some drawings that showed "J" shaped bolts embedded in the concrete for hold-downs of other sill boards. Jim

Some clarification on the Vershire Mountain School hinged post bases. The panel screws from upper plate into post end grain are only locators. The real base-to-post connection is an inch and a half vertical pipe (1.9-in. o.d.) welded to the upper plate and inserted into a two inch diameter hole in post end grain. The pipe is, of course, invisible after installation.

In addition to horizontal restraint, this pipe can also provide hold-down capacity via a horizontal bolt/pin through posts and pipe. We did not bother with this in Vershire since structural analysis indicated that barn dead load exceeded predicted uplift by a safe margin (discounting any help from the vertical siding nailed to mudsills anchor bolted to the foundation).

Upper and lower hinge plates were welded together after the raising (the reason for the originally unpainted parts of the fixtures). Lower plates were bolted to the foundation with four three-eighth inch Hilti HVA adhesive anchors, two of which could easily handle side load or zero gravity uplift.

To my knowledge, there is no capacity for illustration on Ask the Experts, but I would be happy to supply PDFs of the shop drawings for the hinged bases. Or perhaps Jim Rogers could post them at his Forestry Forum Website? For future discussion there is a much more elegant prototype hinged post base on the drawing board waiting for an appropriate project.

Ed:
I'd be happy to post anything you'd wish me to post for you. Jim

Here is a drawing of Ed's hinge plate design used at the Mountain School barn project.
As mentioned in Ed's post the bolt threw the pipe was not needed.
Ed: Thanks for the drawing, hopefully this will help others to understand how it could be used. Jim

I've re-opened this thread because I am going to be building a very similar timber-frame structure. After reading the replies, I need to re-think how I am going to attach the posts to the concrete footers. I was just going to embed the post in the concrete, but I think I will stay away from that. Does anyone have any more to add on this subject?

Also, I will be using an 8x12 tie beam and king posts, 8x10 rafters, and 8x8 purlins. The discussion of "overkill" came up, and I am certain it applies here. I am more concerned with having too much weight from the rafters and purlins than would cause structural problems. I like the classic aspect of timber-framing "over-building. Being an open-frame design, I would like to end up seeing a "sturdy" structure complete with aspects of traditional joinery. But I certainly don't want to build something that is structurally unsound.

Does anyone have any thoughts on using 8x10 rafters with 8x8 purlins for a northern Wisconsin application?

Thanks, Tim

Tim:

8x8 purlins do sound like overkill. What is your roof pitch, rafter spacing, and proposed purlin spacing? CB (in Northern Minnesota)

Tim:

Well, first of all you are absolutely correct not to embed the posts into the concrete. Not only should untreated wood not be embedded, it simply should not come in direct contact at all with concrete as it will rot. As pointed out prior in this thread, a yoke or some other metal fastener is most appropriate in this situation.

Your second question should be answered when you have your frame engineered. It is not possible to determine whether it is overkill without establishing the loading of the timbers. Specifically the wind and snow loads, the pitch of the roof, and the spacing of the bents and purlins. In addition you will need to provide the species of the timbers. These questions will resolve timber sizes for bending, any point loads can further impact your size choices. Once you have the size established then you need to determine your joinery. It is not uncommon for the joinery to need a timber that is larger than that required to carry the load.

That said, if we assume Oak (midwest) and 40-50 psf snow load (CABO), 10 psf dead load (shingles on panels), 48" purlin spacing (panels), and a 15'6" bent spacing then your purlins would need to be 8x12 and you would still have close to a 1/2" deflection. So failure is the issue, not overkill! However, if you reduce the bent spacing to 11'6 then a fully dimensioned oak 8x8 would just barely work, nominal fails, that's how close it is. As long as the purlins pass over the principal rafters with minimal notching you might pull it off. Watch out for horizontal shear, shear kills more designs than bending. Unfortunately the 8x10 principals appear to be woefully underdesigned before beginning to address the joinery.

Don't forget bracing, wind loads, and other factors such as roof materials, and "low roof" conditions. There are also some signficant joint loads at the bottoms of the principal rafters, and possibly at the top and bottom of the king posts. This is only an opinion and is by no means an accurate engineering analysis. But as I said at the opening, your engineer will answer all of these questions for you.

These forums are wonderful for discussing these questions but they can not be used as a means of saving money on engineering fees. In the long run the costs resulting from a poorly designed frame greatly exceed the modest fee of an experienced timber frame engineer.

Yes, that was my point exactly. Upon re-reading my post, it wasn't clear that I was trying to point out that there are too many variables to give a simple answer. One must do the math, or hire someone that can.

All the wood I will be using will be treated pine, including the 8x8 posts. I know that treated wood doesn't necessarily preclude rot, but I was thinking of treating them with some additional chemical, for example, some paint/sealer type product. My dilemma is that the footers will be four feet down (frost-line), which makes assembling any type of post-bottom yoke quite difficult, let alone placing it in the correct spot on the footer.

I will be using a 7.5/12 pitch for the roof. There will two bents, the front side gable and the backside gable. The bent spacing is ten feet, on center. The 8x8 purlins will be spaced three feet apart, notched 1" into the rafter.

Please explain how an 8x10 rafter using an 8x12 tie beams, king posts and struts plan is underdesigned. I can do the math, I just don't know how. If you forward me to some resources where I could check some of these loads, I would appreciate it. Better yet, are there any 10'x20' timber frame pavilion plans out there?

Thanks for your help, Tim

Hi all

A very lively topic indeed. I have used a imbedded wrought 3\4" iron rod in the concrete footing which extended up for about 7 ". The top 4" was threaded. In the bottom of the timber pre drill a 1" hole as close to the location of the imbedded wrought iron rod, and about 9" in depth. drill from the side of the timber an access hole that intersects the vertical hole about 7.5" from the bottom, and then chisel out a flat shoulder on the horizontal acess hole, big enough to slide in a 3\4" nut and flat washer. After setting the post over the imbedded rod slide in the nut and washer and tighten down firmly. I suggest some type of moisture barrier between the end of the timber post and the concrete to stop moisture wicking, we used a copper sheet set over the imbedded rod.
best of luck NH

Tim: A good place to start learning "the math" is the Timber Frame Joinery and Design Workbook put out by the guild. It's actually a collection of articles from the journal.

Regarding securing wood posts in concrete, I spoke with a fellow who made the following recommendation:

Wrap the bottom of the with an adhesive water/ice barrier. A brand name that I found is made by a comapny called "Grace." This stuff is like a roll of very thick tar paper, or a shingle without the rough particles on the outside. Through the water barrier, place re-bar into the post that will secure the post in the concrete. Set the post in the hole, and pour in the concrete, enough so it doesn't go over the water barrier.

This is the simplest way to secure a post without having the concrete touch the wood. -- Tim

pouring the concrete after installing the wood is a great option in that you don't have to worry about precision in hardware placement, only getting the foundation forms as close as possible.
it does take extra time to do some temporary bracing and in the case of timberframing the temporary bracing may need to be substantial......

Gib, I would keep the post above ground and dry.

The quick and easy method for post attachment I use is:

1. Pour footing with above ground pier.
2. Bore post on center axis and embed with epoxy, 3/4 stainless all-thread.
3. Fab a p.t. wood base and clad with copper. Mount base to post, allow all-thread to extend 8 inches.
4. Bore pier on center line grid and embed rod with epoxy.

The copper has two roles, stop wicking and inhibit fungi.

Hey Mark, and yes, that is precisely what I want to do in getting the post placed in the four-foot deep hole, THEN moving it around to its exact location. Once it is where it is going to be, I will brace it plumb and pour in the concrete. I calculate that I will need to mix about 5 bags of ready-mix for my 2-foot diameter hole with an 8x8 post in the middle of it.

And Roger, thanks for your input, but there is nothing that sounds quick and easy about pouring a footer and four-foot concrete pier with it. I was really planning on just mixing the crete myself, and making a pier would probably be more than I care to mix by hand. But this would definitely be "PLAN B" if I wasn't so sold on wrapping the bottom with this Vycor stuff. I am going pack the rest of the hole (around the post) with 3/4" rock.

This is going to be an open-frame structure; so uplift because of heavy winds is part of the design consideration. IMO, embedding rebar in the bottom sides of a waterproofed post to secure it in the footer is a much more secure attachment than a "U" strap or a threaded rod.

I am interested in any other thoughts on this subject. -- Tim

In the interest of information, the epoxied rod scenario does have significant uplift values. See this

For a 5/8 rod, average ultimate uplift is 19835 lbs, and allowable design value is 4490 lbs. Seems to me the connection could hold to the point that the footing dislodges.

Tim...

My sense is that ALL of these suggestions are valid. If I were to put a wooden post in concrete, I definitely seal the bottom! But to be honest, I personally would not put wood into concrete. To me, a footing/pier/Strong-Tie plate or equivalent/wooden post would be the preferred approach.

I know many a person who has cut off and replaced the bottom of posts/studs...that were installed in a not-so-great manner and over time feel prey to Mother Nature.

Joel

I agree with Joel, as evidenced by my earlier opinions on this matter. Untreated wood should not come in contact with concrete whatsoever. Building Inspectors will not allow it. Also, burying a post in concrete creates an opportunity for concrete failure despite the longevity of the wood. Wood expands and contracts with moisture content. At some point there will be space for water and therefore concrete detoriation due to freeze/thaw cycles. The deeper the post is buried the more likely any flexure due to wind loads will incur significant pressure on the concrete at the surface due to the lever action of the bending in the post, which ultimately could cause spalling, especially if weakened from the freeze thaw cycle. Lastly I'm not sure that I have confidence in any product remaining water tight for the long term, and even if it does, there will be water inside the post due to absorption from the air that can only wick out through the exposed portion of the post, not to mention water that ultimately might make its way down the side of the post on the inside of the "post wrap". I am unfamiliar with any accepted practice allowing wood buried into concrete.

Paul brings up an excellent point about water being trapped in the wood when set in concrete. At the risk of encouraging this, I've read where IF you're going to do this, to have the bottom end of the post lower than the concrete to allow the water to drain. I'd venture a guess that some stone under the post would be a good idea.

Joel

Back to the solution by getting the concrete footers above grade. Cardboard forms (both cylindrical and pyramid style) are out there in the market place.

Since there's current activity on this, here's the solution we came up with for the original post:
We imbedded a 1/2 inch steel plate (8 inches x 48 inches - posts were 16x16) plate verticaly into a concrete pier, tied into the re-bar via holes in the plate. Concrete went from below frost line to ground level. As a platform for the post, we slid another piece of 1/2 inch plate (12 inches square)with a slit cut into it over the verticle steel. Each horizontal plate had four bolts to level it relative to the other posts and hold it approximately 3 inches above ground level. Once level, concrete was packed under to form a 3 inch pyramid capped with steel. A piece of EPDM rubber roofing topped this.
The post bottom was routed out so the bearing surface was up inside the post one inch. This provided a full drip edge around the perimeter. The slot in the post was plunge-cut freehand with a chainsaw. Bolt holes were drilled in the posts before raising, then through the steel plate once it was up. Bolts could have been countersunk and the holes plugged, but we knew a good blacksmith, so had some fancy bands made to go around the posts and cover the bolt heads.

Thanks Pete, for having the last reply on this (since you started this topic.) However, I am going ahead with the vycor wrap to prevent any concrete contact. This is the most economical/beneficial way to proceed.

Also, I want to point out that this is TREATED wood, .60 cca pine (most recommended for underground use,) structural 8x8 (nominal) treated pine that is going to support a widely used and recognized king post/strut truss design.

I realize that this forum is most familiar with the use of oak timber, thus untreated, and the only real test of a total design..........is time. I do not want to build a poor structure, however I feel that the comments presented are from professional viewpoint that doesn't represent any tolerance for change or ingenuity. Again, the only real test of a design is time.

My hope is that this simple structure will last for ages. Thank you all for your input.

Regards, Tim

Looks like we need a book on post bottom connections!

One I've used more than once is HDG "eye" bolts set in concrete wet (with nuts & washers) or dry with epoxy. Install a nut with a large square "washer" above it at about the desired height of the post bottom before you set the bolts. I like to leave some daylight under posts.

Next mortice the bottom of the posts for the eye bolts. Raise and adjust the "washers until the building is right and peg the posts through the "eye". Simple, but does require some care in layout. I would have to beleive a 1" dia. locust peg is pretty good for uplift too.

Rudy