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walkereng, Consultant
Category: Structural Engineering
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Can I build a 3 ply dimensional lumber beam to span 24 feet

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Can I build a 3 ply dimensional lumber beam to span 24 feet? If not, what should be used for the application. It is for a 20 foot, by 24 foot pergola. The potential plan would call for two 20 foot outside beams.  Supported by 3 (6x6) posts each.  To span the gap, would be 3 parallel beams 10 foot on center, at 24 feet in length.  All Material: Pressure Treated pine.

walkereng :

Hi, I can help

walkereng :

Can I get back to you a little later this morning when I get to my office? Thanks

walkereng :

Is there any way you draw a rough sketch with some dimensions, I want to make sure I have your configuration correct.

walkereng :

You can attach the sketch to the chat window with the icon in the toolbar. If you can not scan the sketch, you could take a close-up picture of the sketch with your phone and attach that.

walkereng :

What type of roof will be on this structure?

walkereng :

What city and state do you live in so I can look at possible snow load?

walkereng :

I am going to switch over to the Q&A format and we can continue with the e-mail process. When you come back online, just reply to this last e-mail (use the e-mail account that you gave to JustAnswers) to tell me you are back. You do not need to rate any answer yet until we are complete and you are satisfied. Thanks.

I am here to help when you come back online.
Let me know if you still need help
Customer: replied 4 years ago.

Attachment: 2012-11-03_080722_scan_doc0003.pdf

Yes, I still do request an answer. I've attached a scanned image (overhead view with posts at corners, and posts in center of the two short sides (each grid square equals 8 inch x 8 inch) ). The roof would be one of two likely designs... with rafters parallel to the long dimension of the frame depicted in the scetch, and 16inch on center.

Roof option # XXXXX: arches built by halves of 2x10 or 2x12 lumber with a ridge plank in the center, tied together by full length 2x4 perpendicular to rafters. (no sheathing, no cover)

Roof option # XXXXX: Gable design roof with same orientation as the the arches of 2x8 or 2x10 lumber., tied together by full length 2x4 perpendicular to rafters. (no sheathing, no cover)



This would be built in Washington County, MD..

If you roofing is basically going to be open, I did not use the 30 psf snow load form the code, I assumed a 10 psf Live Load and a 10 psf dead load. Is my assumption correct that the roof will be open and not hold any snow? For this condition, you would need to use triple 2x14 Southern Pine (#1 grade) beams for the two outside 24' long beams. Or you could use a quadruple 2x12 Southern Pine (#1 grade beam)for the two outside 24' long beams.

If you are going to have a ridge board with arches, or a gable roof, why do you need the middle 24' long beam?

The sketch you attached was great, thanks!

I am not sure I follow the description for Roof Option #1, arches built by halves of 2x10 or 2x12 lumber?

Could you possible provide a rough elevation sketch of the roof section for your two options?

I want to make sure I have your configuration 100% correct before I size your members.

Before we get started I’d like to point out that a Professional Engineer’s standard of care typically includes a site visit to assess field conditions and get an overall understanding of the structure. This can obviously not be accomplished through the internet. The information provided here is meant for planning purposes only (general sizing and budgeting) and is based on the information provided by you. All loading cases considered are for vertical loads only, no lateral analysis has been completed. The information should be verified by a professional engineer who can visit the site to ensure that potentially important information has not been overlooked or omitted.
Customer: replied 4 years ago.

Attachment: 2012-11-03_165008_arch_roof_pergola.pdf

Attachment: 2012-11-03_165104_gable_roof_pergola.pdf

Attached are both rough scetches. The shaded portion is the pergola roof design, and the unshaded roof is a nearby existing house roof. I'm only comparing the existing house roof for asthetics, and no other purpose. Grid Squares are still the same @ 8in x 8in. This span is the 24foot dimension. The arch halves would be hand cut from 2x lumber and combined at the center with a ridge plank. Thank you for your assistance.

Customer: replied 4 years ago.

As for center 24 foot beam. That is only something I like to do to add stability for potential future use (not necessarily for current structural need).

For your three 24' span beams, you would need to use triple 2x14 Southern Pine (#1 grade) beams.

I looked at your gable roof attachment, that shows the gable roof is supported on the 20' long walls (two 10' spans)on each side with a ridge board. The 20' long walls (two 10' spans) can be triple 2x10 Southern Pine (#1 grade) beams. The rafters spanning 12' between the ridge board and 20' walls can be 2x8 Southern Pine (#1 grade) members and they can be spaced at 24" o.c. You will need to provide rafter ties on each roof rafter at the level just above the 20' support wall level, so that they act like trusses.

I will not be able to advise you on the arch roof pergola members cut from 2x10's or 2x12's. Analysis of arched members is beyond the capabilities of this forum.

You will need to have a local engineer detail your connections, check your lateral stability and determine foundation requirements.
walkereng and other Structural Engineering Specialists are ready to help you
Customer: replied 4 years ago.

Thank you for your assistance. This conversation does provide confidence in putting the whole concept together.

Good Luck with your project!

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