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I am building a arch top pergola customer, 12 x 20.

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I am building a arch...
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: 3 years ago.Category: Structural Engineering
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Answered in 1 minute by:
6/30/2015
Structural Engineer: StructuralEng, Consultant replied 3 years ago
StructuralEng
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Category: Structural Engineering
Satisfied Customers: 7,756
Experience: Structural Engineer
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Hello. How are you doing? I'll be happy to assist you
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Customer reply replied 3 years ago
ok. please.
Customer reply replied 3 years ago
Did you receive my question?
Structural Engineer: StructuralEng, Consultant replied 3 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.
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Customer reply replied 3 years ago
Maple grove Mn. I will scan something ...
Structural Engineer: StructuralEng, Consultant replied 3 years ago
Thank you
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Customer reply replied 3 years ago
Hope this helps
Customer reply replied 3 years ago
The 2 x12s used to create the arch to be machined down to minimum 2 x 8 or as required.
Structural Engineer: StructuralEng, Consultant replied 3 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.
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Structural Engineer: StructuralEng, Consultant replied 3 years ago
The 2x6 perpendicular to the arches are fine
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Customer reply replied 3 years ago
Ok. What sort of load are you looking at? And what is the minimum size I need to span the 18'?
Customer reply replied 3 years ago
and, any ideas to alleviate the problem?
Structural Engineer: StructuralEng, Consultant replied 3 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.
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Customer reply replied 3 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?
Structural Engineer: StructuralEng, Consultant replied 3 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.
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Customer reply replied 3 years ago
That leaves the question on the size required to span the 16'6"? 2 x ?
Structural Engineer: StructuralEng, Consultant replied 3 years ago
As stated earlier, 3.125" x 7.5" glulam or 2x10 dimension lumber
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Structural Engineer: StructuralEng, Consultant replied 3 years ago
If you could rate my answer, I would appreciate it.
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