I working a on Pergola (simple rectangle). I have 6X6 #1 grade Western Red Cedar. My Beams need to span 12'. There will be 30" knee braces on them and the total weight of the rafters will be 414lbs at 20% MC. The beams themselves will be 138lbs. Total weight is under 600lbs. No snow in my area, but I worry more about people doing chin ups etc on it.
One beam should be able to handle a 1200lbs by my calculations (two beams double that).
Is my beam big enough? Finding a 6x8 will be tough and I don't want to add a middle post.
I got a pass using the uniformly loaded simple beam calc for heavy timbers in the toolbox at the lower left of the page and inputting your loads and spans in #1 WRCedar. Long term deflection might be an issue, or not. If there is a milled log home supplier in your area a 6x8 is a common dimension if you want to go deeper. Typically the knee braces are not considered in beam calcs.
It would be a good idea to remove the pic in your post, although it was informative for what you are proposing I'm sure that is a copywritten pic ;). Click the modify button and pull the image code.
Good luck with the project :)
Thanks I dropped the picture. I did not think it was an issue as the builder was credited in it.
Glad to hear you got the same numbers as I did. I should be able to find 6x8, but I already have 6x6's, so it would be an extra cost that I only want to take on if I have to.
Are you spanning 12' post-post or between the knee braces? The knee braces cut your span down. A good general rule of thumb is 1" of header height for every 1' of width opening. So if your knee braces come out 30" from the posts(42 7/16" long) that means you've cut 5' from your span, so would be close enough for a pergola. If your knee braces are 30" long point-long point then I would go up a beam size as you only cut the span down about 3' 6".
Yes the 12' is post to post, so between knee braces will be 7'.
I had an engineer look at it and he said though it will be strong enough, he figured deflection over time would be more than I would like.
Short version, I am going to to hunt down some 6x8. I think it will look better visually anyway and I would rather overbuild it.
It isn't really a good idea to use the knee braces to try to decrease span. As the posts shrink in width the knee braces are no longer tight. If there is any slop in the joinery this contributes to the gapping. If the brace material is green the angle at the end of the brace becomes more acute as it shrinks forcing the load onto a sharp corner which will crush a bit under load. Add all that up and by the time the brace takes load the beam has probably already deflected considerably. They are normally ignored in beam calcs.
Thanks guys - this makes perfect sense to me.
Good luck on the build. All else being equal, bigger is better! That being said there is nothing wrong with using properly installed knee braces to decrease the span. I spent 15 years contracting in a left coast state that has frequent and sometimes large earthquakes, and therefore has some of the toughest building codes in the nation. On more than one occasion to achieve desired head height a downsize beam had to be used. Everything we did had to be approved by the planning department and was inspected. If its good enough for an structural engineer to stamp its good enough for me. Unless you live a charmed life, using green wood and/or sloppy construction practices you are asking for and likely to get headaches and heartaches that come from such practice. A knee brace and truss web perform similar functions, one of which is transferring a portion of the load from point a to point b. You wouldn't tell someone that it isn't a good idea to use webs in a structural truss would you?
Logsmith, you are using some hard words about construction practice. Green wood construction has it's own lists of concerns but likely headaches and heartaches do not make my list.
As to common brace schemes to be equivalent to trusses, that's hogwash. Theory on truss work is to produce a fully resolved framework by arrangement of members that distribute the load axially and without placing members in to bending. The common brace is not in a fully resolved system, the force transfer from the beam to the post has two components, vert. and horz. The horz. force will bear against the post and impart an outward flex to the post and an outward force at the joints of the post at top and bottom. The thrust at joints depends on the load from the beam, the angle of the brace and the ratio of post length to brace leg (that's right a leverage system). In order to make the common brace more truss like would be to have a tie with tension joinery from post to post at the brace bottoms.
Something may sort of look like a truss but will not qualify if members are missing and the joints are not up to the task.
Not hard words, just my experience's of 30 years in the trade. Maybe your a structural engineer, I'm not so defer to them when I'm doing something non standard. It seems we're in a circular firing squad here, as we keep coming back to sloppy construction practices and the pitfalls associated. Me, I don't use green wood, once bitten twice shy. If it sounded like I'm the final authority that wasn't intended. And my mistake since we are talking timber framing, tenoning joints at 45 degree angles was assumed. I didn't say webs and knee braces performed equivalent functions, just trying to make the point one of the things they do is transfer part of force to another member.(maybe similar was the wrong word to use). I do not hesitate to use a PROPERLY installed knee brace on a PROPERLY sized post to utilize a downsize header if required, and put my name and reputation on every job I do. That being said, don't believe everything you read on the internet including what I write, due your due diligence and research. Most of all have fun making sawdust !
Robert