The gable walls only see 6’ of wind load while the interior bends see 12’ of wind load. That’s my definition of a free standing TF. Those interior bends do not benefit from the gable walls additional shear capacity and must carry their full 12’ share of the wind load acting as a ‘standalone frame’.

Well that depends on the design and use of the building.

I feel like I'm missing something here. I've always understood that braces are ALWAYS compression members. Preferably used in pairs, depending on the direction of force, one is doing the work (compression) and the other is on holiday. As has been mentioned often in this forum, it is not at all uncommon to see peg-less braces in old barns and other traditional frames.

EHC- You state that in your 3-post example, the lateral load would put two braces in tension and do nothing to the other two. I would argue that the load would do something to the other two- compress them! Eliminate the pegs to eliminate tension. The 3-post bent should be designed so that the compression of two of the braces is adequate for the lateral load from one direction and the other two are there for load from the opposite direction. This just takes advantage of wood's property of being stronger in compression than in tension.
A great example is a multiple queen post truss covered bridge. The braces/struts are all in compression, that is they run diagonally from the top of each post to the bottom of the next going toward the abutments so the two in the middle form an inverted 'V'. If you look at a steel truss you usually see the opposite since steel (for the sake of this discussion, let's assume flat stock, riveted or welded) performs better in tension than compression.

I'm curious to see exactly why braces in tension are required. I don't want to assume too much and I hope I'm not stepping on any toes!
Cheers

Dan F

did not check my post until now. My mistake. Tension was supposed to be Compression.

I'm with Dan.

what do you suppose this was for? Medieval English

Derek,

I have seen and worked on a lot of roofs that were not sheathed. They had a tyvek type mebrane across the rafters and furring strips on top, laid perpendicularly to the rafters. The purpose of the furring was to provide cross ventilation and be the installation layer for shingles/slate/reed. The furring cetainly did not add to the structual integretity of the roof or frame. So the stand alone frame is no myth.

Thanks for the replies.

I’ve tried to add some pictures of a few recent projects, but my brain has limited capacity. How do I do it??

The pictures are of engineering analysis results for two frames that were engineered for 110mph wind loads. The largest problem in each frame was brace tension. Releasing the tension in the braces, compression only, causes other problems within the frame such as very high post bending.

Thanks,
Pete

MTF,

if the picture is on your computer you would have to upload it onto say Photobucket or any other online site of your choice. The picture then gets a URL that you can copy and paste when you post or you can just add a link to the picture. To add the URL just click on the image or URL button below ( when you write a post you can select below the frame you are writing in. A window opens up and you can type in the URL code.
Hope this causes enough confusion for now.

Thanks for the thread MTF!

I think it was mentioned in the other thread on braces, but the back-issues (#62, 63, 64 I think) of the tfg newsletter have excellent articles on braces working in tension and compression. A person built (for his thesis?) real timber frames (one and two story, oak and pine) and instrumented them with load cells, and pushed on them with hydraulics to obtain stiffness #'s, and ultimately pushed them to failure. It's a very scientific analysis. The whole set of back issues is available on DVD here on this web site. I am not associated with the tfg in any way, but I was happy to buy these back issues for the wealth of information they contained, and I figured some of that $ might help support this discussion forum.

Don't know if it's already been mentioned in this thread, but the post-to-beam joint that completes the triangle formed by the "brace, post, and beam" is one that needs to be carefully looked at too. The smaller the brace, the more stress this joint will see under wind loads, etc.

These images might be an interesting sidenote to this discussion, the thought to post them was sparked by Mo’s graphic.

These are from a smallish Dutch barn ( though not a classic NWDB ) built in the Highlands of New Jersey when it was still part of New Netherland.

These are tension braces meant to augment the tieing joint. The system was meant to overcome the thrust of a simple rafter type roof , but the design was insufficient and likely began to go into failure almost immediately. The iron rods were placed as the roof continued to spread over time, no two seemed to have been forged by the same blacksmith, some are much later and have turnbuckles.

The gable bents have the drop tie, and the main ties, from there it gets progressively worse…

Only the first bay has this odd “upper loft” and only bent II has this secondary tie lapped through the standing tension brace.



The bents from thereon only have the brace to overcome the massive thrust and bending moment being imparted into the posts and the four plus feet of “kneewall” above the tie, with this resulting



An interesting aspect of this frame is that the carpenters connecting the scribe points were curiosly free to choose joinery semi-randomly, sometimes cutting a lap dovetail, sometimes useing this tensile cog. Every inboard bent with the cogs on the laps has this shear failure, which runs from the shoulder which supports the plate to the tie, made all the worse in that a shear plane was created in cutting the joinery, which runs from inside the plate tenon , through the cog and on through the peg holes in the tieing joint. They all popped relativly identically.



How it’s weathered as many years as it has is almost miraculous, that having far more to do with the succesion of stubbornly frugal farmers that maintained it than it does with good design.