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StructuralEng, Consultant
Category: Structural Engineering
Satisfied Customers: 7320
Experience:  Structural Engineer
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I am building a arch top pergola customer, 12 x 20.

Customer Question

I am building a arch top pergola for a customer, 12 x 20. On the left side, we are attaching to the existing right side of the deck (hangers). The right side has three 6x6 posts and 2x10(2) beams. The clear span is approximately 18'. The beams (5) running from deck to posts are 2x12x12 cedar, but planning to create an 'arch' down to 2x8 with a split in the center and 1/4" x 6" x 16" steel plates to make the connection. There is a 2' decorative cantilever on the right. There are seven (7) 2x6x 12 purlins perpendicular to the arch.
I have looked at many span tables, but cannot locate anything for this application as the load is approximately only 250-300 lbs total.
Please review
Submitted: 2 years ago.
Category: Structural Engineering
Expert:  StructuralEng replied 2 years ago.
Hello. How are you doing? I'll be happy to assist you
Customer: replied 2 years ago.
ok. please.
Customer: replied 2 years ago.
Did you receive my question?
Expert:  StructuralEng replied 2 years ago.
Do you have a sketch you can post? What is the city and state so I can look at snow and ice loading.
Customer: replied 2 years ago.
Maple grove Mn. I will scan something ...
Expert:  StructuralEng replied 2 years ago.
Thank you
Customer: replied 2 years ago.
Hope this helps
Customer: replied 2 years ago.
The 2 x12s used to create the arch to be machined down to minimum 2 x 8 or as required.
Expert:  StructuralEng replied 2 years ago.
Thank you for the sketch. That is helpful. The 2x10 beams with a 5’ span are fine. The 2x8s spanning 18’ are not. Even as full length 2x8, the strength just barely works, but they will deflect much more than you want to see. When you add in a splice at the worst location, it’s not a good idea.
Expert:  StructuralEng replied 2 years ago.
The 2x6 perpendicular to the arches are fine
Customer: replied 2 years ago.
Ok. What sort of load are you looking at? And what is the minimum size I need to span the 18'?
Customer: replied 2 years ago.
and, any ideas to alleviate the problem?
Expert:  StructuralEng replied 2 years ago.
I used the most minimal loads – only 5 psf for dead load and 5 psf for live load. To keep deflection to span/360, you will need 2x10s for the arches and can’t have the splice in the middle, it just will not support the load. The best option I can think of are curved glulam beams. They will be more expensive, but they will achieve the look you want with a more shallow depth. 3.125” x 7.5” pressure treated glulams 24F-V4 will hold span/360 deflection for the 18’ span. You only have 5 of them, so the cost shouldn’t be a huge factor, but they are more expensive than dimension lumber. Although, they are probably less expensive than what the final product will be with what you’re trying to do with dimension lumber.
Customer: replied 2 years ago.
Last thoughts, one error, the span is 16'6". It was 18' overall with the decorative end. Does this change anything?AND, losing the arch and changing to a solid board would require what size 2x8, 2x10, 2x12?
Expert:  StructuralEng replied 2 years ago.
Losing the arch will have no impact. It must be designed as a curved beam, not an arch, because the support conditions are not consistent with an arch.
Going from an 18’ span to a 16’-6” span helps, but not enough to get it to a 2x8.
Customer: replied 2 years ago.
That leaves the question on the size required to span the 16'6"? 2 x ?
Expert:  StructuralEng replied 2 years ago.
As stated earlier, 3.125" x 7.5" glulam or 2x10 dimension lumber
Expert:  StructuralEng replied 2 years ago.
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