socoj2 Posted June 25, 2015 Report Share Posted June 25, 2015 I have an issue im trying to build a pergola with 2x12x24 the total unsupported distance is 18.5' but that is from post to post im thinking about doing some 4x4 bracing in as well to shorten the span with that it brings the unsupported distance down to 13 feet ish Thats the long side its 4 beams. one on either side of a 6x6 that are pocketed. The front to back unsupported distance is 11.5 feet with 18 stringers with a total length of 14.5 feet Quote Link to comment Share on other sites More sharing options...
Tpt life Posted June 25, 2015 Report Share Posted June 25, 2015 I don't see the question you have? Quote Link to comment Share on other sites More sharing options...
BobInAustin Posted June 25, 2015 Report Share Posted June 25, 2015 Seems like an engineering problem and since people will be under it, not one I'd be willing to guess at. Quote Link to comment Share on other sites More sharing options...
wtnhighlander Posted June 25, 2015 Report Share Posted June 25, 2015 The joists under my house are 2x12, max unsupported span is 12 feet. I think you will be OK to go 13' - 14' using them overhead, assuming you don't build above that. Your local building inspector can tell you what code calls for. Quote Link to comment Share on other sites More sharing options...
Tpt life Posted June 25, 2015 Report Share Posted June 25, 2015 Pergola are non supportive. This is why they can be made from cedar and span larger distances. Quote Link to comment Share on other sites More sharing options...
collinb Posted June 25, 2015 Report Share Posted June 25, 2015 Depending on municipality wind sheer might be an important consideration. Better safe than sorry. Quote Link to comment Share on other sites More sharing options...
Tee Cancelleri . Posted June 25, 2015 Report Share Posted June 25, 2015 If on doubt consult a structural engineer. Quote Link to comment Share on other sites More sharing options...
Tom King Posted June 25, 2015 Report Share Posted June 25, 2015 Or find a copy of Architectural Graphic Standards, which has all the span charts in it. Quote Link to comment Share on other sites More sharing options...
pghmyn Posted June 26, 2015 Report Share Posted June 26, 2015 If you're not parking your car on it, you're fine. In all seriousness, the lumber is strong enough. Quote Link to comment Share on other sites More sharing options...
wdwerker Posted June 26, 2015 Report Share Posted June 26, 2015 Snow load? Wind, like nasty tornado kinda wind ? Tie those crosspieces down so they won't go flying during a storm. Quote Link to comment Share on other sites More sharing options...
Beechwood Chip Posted June 26, 2015 Report Share Posted June 26, 2015 Check the Sagulator for basic "is this beam big enough for this span"? I'd find a pergola with similar dimensions and pay attention to how it was designed. Like Steve said, not just dimensions but also joints and fasteners. Quote Link to comment Share on other sites More sharing options...
sjeff70 Posted June 26, 2015 Report Share Posted June 26, 2015 I think they covered this in the latest 'Fine Homebuilding mag'? I think it's still on the stand, might be last issue. Quote Link to comment Share on other sites More sharing options...
socoj2 Posted June 26, 2015 Author Report Share Posted June 26, 2015 guess a picture is worth words Quote Link to comment Share on other sites More sharing options...
socoj2 Posted June 26, 2015 Author Report Share Posted June 26, 2015 My real issue is the side view. i can not guesset that side due to a doorway. so it will have an unsupported span of 18.5' Quote Link to comment Share on other sites More sharing options...
Tpt life Posted June 26, 2015 Report Share Posted June 26, 2015 http://www.awc.org/calculators/span/calc/timbercalcstyle.asp?species=Northern+White+Cedar&size=2x12&grade=Select+Structural&member=Rafters+%28Roof+Live-Load%29&deflectionlimit=L%2F360&spacing=24&wet=Yes&incised=No&liveload=20&snowload=-1&deadload=10&submit=Calculate+Maximum+Horizontal+Span#answer White Cedar live load and exterior rates at 16'. Any chance of moving your posts in 12" each? Quote Link to comment Share on other sites More sharing options...
wtnhighlander Posted June 26, 2015 Report Share Posted June 26, 2015 On the side view, can you add a brace on the section over the handrail? That would knock off a couple if feet. Quote Link to comment Share on other sites More sharing options...
Tpt life Posted June 26, 2015 Report Share Posted June 26, 2015 He has it^^ I misread your side view comment. Your design would benefit from a post at the end of that hand rail. Quote Link to comment Share on other sites More sharing options...
socoj2 Posted June 26, 2015 Author Report Share Posted June 26, 2015 He has it^^ I misread your side view comment. Your design would benefit from a post at the end of that hand rail. The section on the left would be the side, the one of the right would be the front elevation. the side elevation is only 11 feet unsupported. and all the stringers are running perpendicular to the side elevation. The horizontal beams depicted in the front elevation is what im mainly concerned with. I can brace the front and bring the unsupported distance down to 13 feet on the front. I can not brace the back due to a door and window, so its an unsupported 18 foot span. Granted there are TWO 2x12s per run that will be sistered together and put in on hips of the 6x6. I think with the double 2x12s i should be good with the 18 foot section on one side. Quote Link to comment Share on other sites More sharing options...
RPCV_Woodworker Posted June 27, 2015 Report Share Posted June 27, 2015 For the side against the wall, you could put a center post, and just build a shelving unit around it for cookouts etc. structural reinforcement becomes a feature! Quote Link to comment Share on other sites More sharing options...
Beechwood Chip Posted June 27, 2015 Report Share Posted June 27, 2015 Sistered 2x12s would probably be fine, but if it were me I'd want to reduce weight. Then again, I tend to over-think and over-engineer, so take what I say with a grain of salt. Sometimes, "just make it bigger" doesn't work for long beams, because the weight of the beam becomes significant. The strength is coming from the top (in compression) and the bottom (in tension) and the distance between them. The material in between adds weight (load) without adding much strength. You can cut holes in the beam to remove some unnecessary material. Circles or ellipses are good because they don't have corners, and corners are weak points. You can see this in aircraft design, where weight is a big factor. Or, build a truss into your design. Quote Link to comment Share on other sites More sharing options...
Beechwood Chip Posted June 27, 2015 Report Share Posted June 27, 2015 If the 18' span is against the wall, can you anchor it to the buildng? Quote Link to comment Share on other sites More sharing options...
Greenaqua Posted June 29, 2015 Report Share Posted June 29, 2015 The 2-2x12 spanning 18ft are capable of carrying a roof load of about 20lb/sq ft. This would give a deflection of about 0.82" in the middle. This is assuming you are using Southern Yellow Pine #2. Wind and earthquake loading will not be much of an issue for the 2x12's but will have an effect on your post and it's foundation. I don't recall you saying where you live but snow/ice loading would be a possible concern but I believe your roof is open framing so not too much snow would be able to accumulate. If you do decide to put the "knee" braces in on the front just know that the connections between the brace and the post/2x12 joists will have to be adequate to carry that reaction. Quote Link to comment Share on other sites More sharing options...
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