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walkereng
walkereng, Consultant
Category: Structural Engineering
Satisfied Customers: 2596
Experience:  Over 30 years of Structural Engineering experience.
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Looking guidance on mounting a cantilevered two post pergola

Customer Question

Looking for some guidance on mounting a cantilevered two post pergola to an existing 3' raised concrete patio. Rough pergola dimensions are 16' wide, 10' tall, and 6' deep. A 6x6 post would be used at each end. There would be two 6x10 beams at 16' long with the overall span at 14'. One beam would be 1' behind the post the other would be extended 3' out with a 2x8 and braced against the post. The entire structure is to be mounted near the edge of a raised patio. The posts would be approximately 3.5" from the patio edge. The cantilever section would overhang the patio. What is the best method for attaching this to the existing patio? Initial thought was to use 4x8x16 cinderblock to make some small columns around the post at 32" high, and placing 0.5" rebar (drilled into the patio) at the corners and fill all cells with concrete. It just not clear if this has enough mass to support the unbalanced structure. The rebar would also be ~1" from the edge of the patio and I am concerned that it could break out as most of the force would be on that side.
Submitted: 6 months ago.
Category: Structural Engineering
Expert:  walkereng replied 6 months ago.

I can help

Expert:  walkereng replied 6 months ago.

Let me take a look at your drawing.

Can I get back with you later this evening when I get home from the office?

Customer: replied 6 months ago.
Sure, no problem. Thanks
Customer: replied 6 months ago.
Have you had a chance to look at the drawing?
Expert:  walkereng replied 6 months ago.

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.

You really need to cut an 18" square hole in your concrete slab, for each of your 6"x6" posts and dig down at least 36" and then set you pressure treated posts in the the holes and wrap the posts with 15 lb felt paper and pour the holes full of concrete. You need to drill two holes perpendicular to each other (the 1st one 6" up from the bottom and the 2nd one 12" up from the bottom) and place 16" long steel rebar through each hole before you pour your concrete. This will help with uplift resistance and also lock your 6x6 posts into your concrete and brick masses.

You still need to build your cinder block and concrete pilasters around your posts up to 32" or 36" above the patio slab. You need to also wrap roofing felt around any part of the posts that will be in long term contact with concrete.

You will need to have a local engineer detail all of your connections to resist the loads you’re your 6’ cantilevers in your roof system.

If you feel you have received a satisfactory answer to your question, click the Rating button that is appropriate. Experts are credited for each adequately Rated answer they provide. If you have additional questions, please let me know. Thanks

Customer: replied 6 months ago.
What if the pergola doesn't have a cantilever? In other words, build a similar structure with two balanced equally braced beams. Could this be mounted to the top of the raised patio?
Expert:  walkereng replied 6 months ago.

So instead of approx. 6' off of one side, do you propose approx. equal 3' to 4' cantilevers off of each side of the center posts?

Customer: replied 6 months ago.
Correct.
Expert:  walkereng replied 6 months ago.

Even with this revised configuration, you would need some embedment into the ground with your posts

Expert:  walkereng replied 5 months ago.

If you feel you have received a satisfactory answer to your question, click the Rating button that is appropriate. Experts are credited for each adequately Rated answer they provide. If you have additional questions, please let me know. Thanks

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