Newbie question on joining beam to post

[nick_m]

Hello forum members,

I've been browsing the forum for the past few evenings and I've found a wealth of knowledge, particularly in Jim Rogers' posts on General Rules and Brace Layout. You all seem to be generous with your knowledge and accepting of newcomers, so I hope you'll go easy on me for asking some basic questions.

I'm building a gazebo on a 12x16 patio in the timber frame "style", but I'm using hardware to join the members (gasp!). Here's the basic design of the major structural members, in SketchUp:



 



 

It's rather brute force, but it pencils correctly for the expected loads. I feel confident building it because it's easy to visualize and there's minimal cutting, since the timbers are all used at their nominal lengths.

The intersection of the beams over the posts is a bit clunky, and I was thinking of changing the design based on some of the techniques I learned here:



 

If I have a 1" bearing surface notched into the post and the tenon is 1/4 the width of the beam (gleaned from Jim's posts), will the beam retain its original load ratings, and can carriage bolts through the angle plates serve as a substitute for pins?

In my original design, the braces serve only to keep the joints square; any additional support they provide the beams is incidental. I'm not sure if that is true in timber frame design, since the bearing surfaces of the braces appear to be critically important. If I tenon the beam, do I have to redesign the braces to carry some of the beam load?

Thanks in advance for any critical feedback you can provide!

Nick

[witterbound]

Is it going to have a roof or be exposed to the weather?

[nick_m]

Hi witterbound,

It will have a roof that uses traditional framing with rafters on 2' spacing and sheathing (or T&G pine) and asphalt shingles. I have not finalized the pitch, but that should have little bearing (pardon the pun) on the design. It's designed for a 15 lb/sf snow load. Beams are 6x10, posts are 6x6.

Nick

[shane77m]

Hello Nick. Welcome to the forum. I just, as of a few hours ago, bought off of Ebay a slick from someone in Gig Harbour. It was made by Swan and Company. I hope it is a good one. If it is, I think I got a deal on it.

[witterbound]

Looks to me like it should work.  If it were me, I wouldn't take the tenon all the way through, but that might be easier if you don't have the tools to cut the mortise.  Seems like the only concern is whether the bolts through the tenon will hold.  You could add another l plate on top.  Probably overkill, but if it won't show, I'd do it.  If you've got to satisfy an inspector, you need to ask him before you do it, and he'll give you the "right" answer.

[nick_m]

witterbound,

Yes - I chose a through tenon because I felt I could handle the cuts; a proper mortise is more than I want to attempt.

I thought the idea of running bolts through the tenon would be overkill, so I don't understand your concern. In a traditional approach, the tenon would be held in the mortise using two pegs. If I connect the opposing angle plates using 4 carriage bolts, they'd serve a similar function, plus they'd provide additional hold through friction due to compression. Is your concern due to the fact that the bolts would be smaller than the holes they pierced, whereas pegs would be tight to both the post and the tenon? If so, could I address that with pegs though the post and tenon below the angle plate?

Also, you suggest adding "another I plate on top". Are you referring to something like this, or is an I plate something that would go on top of the post, under the long beam?



 

Again, thanks for the advice.

Nick

[witterbound]

Yes, that's the new plate I would add, although Im not positive it's needed.  The second set of carriage bolts toward the outside won't be holding that much as it doesn't look like those bolts will catch much of the tenon.  I don't think you should count on the squeeze factor to hold things together, as wood expands and contracts.  Let's see what other folks think.

[Brad_bb]

You have your tie beam(horizontal) resting on a shoulder in your proposal, which is good.  You don't need a thru tenon, nor do you need a tenon the full height of the beam.  Cutting that thru mortise, in my opinion would be harder and take longer than a traditional blind mortise for a regular tenon.  The connection of your post to the tie beam, if done with a shoulder as you proposed, means all of the weight will be transferred from the tie to the post through the 1 inch shoulder.  This joint will be in compression which is good.  The regular tenon I propose with a peg, would serve only to prevent the two ends from spreading.  In other words the pegs will not see much load other than a little bit of racking.   
   Similarly, the top tie going the other direction, can easily be retained with a regular tenon on the top of the post, and a blind mortise in that tip tie with a peg.  You don't need to use any brackets or fasteners.  Same goes for the braces.  Once you cut a few mortises and tenons, I think you'll see that it's just as easy to cut those as it is to procure and make brackets.  Plus doing it all with mortise and tenon will be cleaner, and the metal won't stain and rot the wood over time.  Metal in contact with wood will rot the wood over time.  Epoxy coated hardware reduces that, but it's still not preferable for something exposed to the weather.  What wood are you planning so use.  White Oak would be a good choice.
  If your timber is 6 inches, you can use a 1.5 inch tenon, if 8 inches, step up to 2 inches.  If you want the LOOK of a thru tenon- like a Dutch tenon with a wedge to hold it instead of a peg, that's another story and you can do that, but then you'd have to figure on lowering that tie, to give yourself enough relish to put a tenon on top for the other tie.


 

[Brad_bb]

Disregard the white oak, I see you are in Washington State, so you will likely be using Doug fir.

[nick_m]

Brad_bb,

Your explanation is very clear and makes perfect sense. You also raise many of the issues that concern me, particularly appearance and the metal/wood interface. I was planning to use Simpson HL55 angle plates and was thinking of having them power coated. The cost of the hardware - 24 plates and 96 each of 1/2" lag screws and carriage bolts - is not insignificant. I have to weigh that against my lack of the necessary tools and experience for timber frame construction.

I was at first skeptical that a 1" shoulder could support the necessary load, but it works out to less than 200 psi (in my case). Knowing that, and seeing that the mortise and tenon is only to prevent separation, it all begins to make sense. Would it be correct to say that braces are designed only to prevent racking of the structure and not primarily to carry some of the beam load? Also, if I mortise the tie to join the post (king post?) that supports the ridge beam, how much bending strength does the tie lose?

There are many recommended books on the topic, but is there a seminal volume - a bible of timber frame, so to speak - that I should consider buying? They're predicting a long, warm fall in Western Washington, and I'm thinking of doing this without hardware.

Thanks for the help,
Nick

[Jim_Rogers]

You don't need a thru tenon, nor do you need a tenon the full height of the beam. 

I would never make a statement like that without knowing all the facts. And those facts have not been posted. How do you know he doesn't need to through tenon?

The pictures don't have labels on them and I'm not completely sure which timber has been moved down to the elevation where the through tenon is now shown.

Without understanding everything which includes type of wood, exact snow loads, total load on the beam, and actual dimensions saying that he doesn't need a through tenon is not good advice.

The reason for through a through tenon is that you need some wood beyond the "hole" whether it is a peg hole or a bolt hole is important. Codes say how many "hole diameters" are needed beyond the hole to the end of the tenon. Short tenons as was hand sketched in, have no "relish" beyond the peg hole and should not be considered without understanding all the facts.

It maybe possible in this design to not have a full height through tenon as the bolts holding the upper bracket may indeed hold "UP" the tenon when the load is placed on it. A timber shrinking could lift the beam off the 1" step in the post. If this were to start happening with the bolts at the top then the tenon could split the tenon and loose it's holding power as the relish could blow out the end of the tenon.
In a blind mortise you'd never see this or know it has happened. not good.

Today's engineering recommendations are to peg or bolt tenons low so that when the timber shrinks the timber will not lift off the step in the post but that the top shrinks down. Again securing the top causes the tenon to split.

Braces are for stiffness of the frame. Most engineers do not consider any load to be carried down from a beam to a post by a brace. The beams have to be large enough to carry the load from the beam down to the post without relying on braces.

When dealing with plates and through bolts there are rules for bolt sizing and placement that need to be understood and followed. Placing plates and bolts without following these can weaken the joint and cause the timbers to split and the joint to fail.
I don't know these rules and I rely on my engineer to design and specify the size and location of the bolts for the specific joint for the specific frame.

I may sound like a broken record here when I say that you should have your frame design reviewed by a professional "timber framing" experienced engineer. He/she can advise you about bolt sizing and placement so that you don't weaken your timbers.

I have posted, in on this forum, pictures of failed and failing joints other have made using plates and bolts when they didn't use an engineer and didn't understand the correct size and placement importance.

Post and beam construction with plates and bolts is an area where I don't have a lot of experience and I rely on others who have that experience and they advise me what to do.

Jim Rogers

[Brad_bb]

  Jim is right, you do need to know the loads, and figure out how much relish you need on your tenons.  When the structure racks under load, pressure will by applied to the tenon on one or both shear planes of the peg,  You need to have enough surface area to resist this with a margin of safety. 
   You also have to take into account shrinkage if working with green timbers.  Or you can season them and it will become less of a possible issue.
   Don't let this intimidate you as the possible issues are similar with bolts and brackets if working with green timbers.
Oh, and yes the braces are to resist racking, but that doesn't mean you don't have some taking some load over time.  Things will settle.

[nick_m]

Jim, Brad;

Thanks for the clarifications on the beams/braces. I asked what I thought was a theoretical question; now I see how much the answer depends on the specifics of my project.

I based my design on #2 or better Douglas fir dimensional lumber from ProBuild: 6x6x8 posts, 6x10x12 ties and 6x10x20 plates/ridge. The bents would be 10.5' wide, center-center, and the plates would bear on posts 14.5' apart, center-center. I had Simpson post bases set to those dimensions when the patio was poured. Under a material + 20 lb/sf snow load, the shear, moment and deflections are all within limits. Permits are not required for structures of this sort under 200 sq ft, so there isn't a big market here for designers in this space.

My inspiration came from a design I saw on Timer Frame HQ, which I adapted to my dimensions using metal fasteners instead of timber frame joinery. I have a call in to them to see if their plan could be adapted to my dimensions, and to learn how they design for snow load. Please feel free to weigh in on their designs (if you feel that's appropriate and the forum permits it). Or, feel free to point me in the direction of plans you think would be appropriate for my project.

Thanks again for the guidance.

Nick