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-   -   Pergola span question (http://www.diychatroom.com/f14/pergola-span-question-18323/)

boondox_saint 03-10-2008 09:33 PM

Pergola span question
 
I am planning out the pergola I plan on doing this spring. I so far am planning on it being approximately 14' X 14'. For material I plan to use pressure treated lumber. I will be going with 4 4X6 columns with two 2X10X16 lentils on each side (not sandwiched but bolted to either side of the two 4X6's on each side). There will be 12 2X8X16' rafters laid across the lentils. My question is will the two 2X10X16' lentils on each side be enough for that 14' span? In addition I was hoping to hang a porch swing from the middle of one of those two 2X10X16' spans. Would this sag with the swing?

Ron6519 03-12-2008 12:08 AM

To use 2x8's you would need to reduce the span or they will sag. I would put another pair of 2x10's in the middle and split the span.
I don't think the swing would cause deflection.
Ron

boondox_saint 03-12-2008 10:41 AM

So the 2X8's over a ~14' span will sag unwieghted, but the 2X10's will not sag weighted? I would rather not have another set of columns if possible, is there another option? would I be better off using 2X12's with 2X10 rafters? This seems it would be pretty bulky though....

Ron6519 03-12-2008 12:12 PM

I would probably unitize the 2x10's with the swing on it so the load is on both framing members. If you have alot of overweight friends who like swings, upgrade to 2x12's or move the swing closer to one of the supports. Maybe someone else will have a solution to the 2x8's and deflection issue. But I don't think even using 2x10's will eliminate all deflection without a middle support.
Ron

boondox_saint 03-12-2008 12:21 PM

I think we are not on the same page.. This is going to be a square pergola, the 2X10's with the swing will have the same span 14' as the rafters.. So the rafters if anything would deflect less than the supporting 2X10's.

Ron6519 03-12-2008 04:35 PM

The 2x8's will deflect due to the smaller size. I don't know how much the 2x10's will deflect with the load of the 2x8's on them. You can reduce the deflection rate of the 2x10's by redesigning their configuration. You could box them out which would increase their rigidity. You could add flitch plates to the inside of the box to further increase rigidity.
The current design criteria puts a strain on the proposed framing members is the bottom line. I think you will not be happy with the result, over time, with the proposed setup.
I know you wanted the openness of only 4 posts, but I think the addition of a middle pair solves the problem of the 2x8's deflecting and the box cures the 2x10's deflecting.
I do have a tendency to overbuild.
Ron

boondox_saint 04-22-2008 10:31 PM

progress
 
1 Attachment(s)
Well I decided to go with a 12' span instead of 16', but here is the progress.


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