pergola posts in the ground vs simpson ties to support a hammock

[tom coleman]

Should pergola posts be buried in the cement 1/4 of their length or set on top of footings using Simson ties? I am a first time pergola builder at age 75 and am fast realizing this may not be as simple as I thought. My dau has asked me to build her a 12x12 pergola in CA no risk of freezing.
Her design suggestions include having these posts support a hammock or swing. We are thinking of 4x4 posts with 2x10 doubled girders and 2x6 rafters. Will this pergola require heftier 6x6 posts and 2x12 girders? Any suggestions on a good how to book? Thanks for your help. We have already added a generator to our shopping list.

[Bill Berklich]

My dau has asked me to build her a 12x12 pergola in CA

First, check with the community building inspector. Some have seemingly strange rules and codes (Mine would require 8x8's). Also check with the Home Owners Assoc. Again weird rules. I'd use 6x6's 4 feet in the ground in gravel. You will beholding up a fair amount of weight with minimal cross bracing in an earthquake area.

Post pics when you are done.

[Lee Schierer]

I don't know if pergolas have to meet seismic requirements, but most likely you will need a permit, which will also dictate how the structure has to be supported.

[Sam Murdoch]

No - you should not bury posts in cement - PT or otherwise. That is a system doomed to fail. Use these or similar - http://www.fastenersplus.com/Simpson-ABU66Z-6x6-Adjustable-Post-Base-Zmax-Finish?gclid=Cj0KCQjwub7NBRDJARIsAP7wlT8nGkcVMhiwQ b7lF4oEFlY6COYeMWzK4CaRgReigfs84V0-nNS5m9UaAkTDEALw_wcB available in lumber yards and box stores throughout Maine. There are other Simpson post bases that you could use too. Your post remain above ground but are secure to the concrete footing/pad. 8" dia. Sonotube for a 6x6 post. Yes, please check with the town code or at least ask what is typical depth for soil conditions in your area to remain below frost line. Sandy soil could require a broader pad then simply a Sonutube. A pergola, as a stand alone/unattached structure, can be subject to wind and other forces that would require a more stable base than simply posts on concrete pads regardless of the depth.

[Jamie Buxton]

No - you should not bury posts in cement - PT or otherwise. That is a system doomed to fail. Use these or similar - http://www.fastenersplus.com/Simpson-ABU66Z-6x6-Adjustable-Post-Base-Zmax-Finish?gclid=Cj0KCQjwub7NBRDJARIsAP7wlT8nGkcVMhiwQ b7lF4oEFlY6COYeMWzK4CaRgReigfs84V0-nNS5m9UaAkTDEALw_wcB available in lumber yards and box stores throughout Maine. ....

I agree that wood in ground contact is not a long-term solution. That's why fences fall down, and why most building codes forbid houses to be built with wood touching dirt. However, those Simpson post bases are only part of the solution. The issue is that the post base provides very little resistance to sideways loading -- loading that would come from a hammock, or wind. You need additional bracing against those loads. For example, if you have two posts, you can put a beam running horizontally at the tops, and braces running at fortyfive degrees from each post up to the beam. The triangles formed by the braces are very strong against the horizontal load.

[Jim Barstow]

We just built a large trellis and the anchors and framing had to be specified by an engineer. The posts were set on concrete pads held in place
with a large steel rod. The real strength came from a perimeter of steel connecting all top of the posts. The concern was
racking.

Better safe than sorry, I'd have an engineer review the plans.

[Sam Murdoch]

I agree that wood in ground contact is not a long-term solution. That's why fences fall down, and why most building codes forbid houses to be built with wood touching dirt. However, those Simpson post bases are only part of the solution. The issue is that the post base provides very little resistance to sideways loading -- loading that would come from a hammock, or wind. You need additional bracing against those loads. For example, if you have two posts, you can put a beam running horizontally at the tops, and braces running at fortyfive degrees from each post up to the beam. The triangles formed by the braces are very strong against the horizontal load.


We just built a large trellis and the anchors and framing had to be specified by an engineer. The posts were set on concrete pads held in place
with a large steel rod. The real strength came from a perimeter of steel connecting all top of the posts. The concern was
racking.

Better safe than sorry, I'd have an engineer review the plans.

...and now you have the rest of the story. :) Can't go wrong with this advice.

[Bill Berklich]

Unfortunately the Simpson linked is for compression loading (think deck posts) not for bending (think wind/Hammock/side loading). If you really want to use something by Simpson Strong Tie then use the MPZB LINK >>http://seblog.strongtie.com/2017/02/new-moment-resisting-post-base/ It's the standard for open air Pavilions. Notice however that it requires part of the MPZB and post base be buried in the concrete. So the whole "OMG Wood is touching blah blah blah) is pretty much out. Todays materials and treatments are 30-40 year life. We just installed an 80ft light vehicle bridge for a local municipality on 6 treated 8x8s sunk 6' in Concrete.

[Jamie Buxton]

That MPBZ post base is something I have not seen before. It sure does look like it solves the problem. It keeps the wood out of the concrete, and strongly resists side loads. However, the only price I can see online is $88 for one. That might explain why I haven't seen it in local stores. I think I'd stick to knee bracing -- those triangulating braces I mentioned above.

[Sam Murdoch]

These appear to be an excellent product for the purpose. Though reading the specs and studying the illustrations show that the wood post is not buried - just a section of the stand off is in the concrete. The wood posts sits above the pour. This below, from the installation specs. for this product. Note the line highlighted in red. Your municipality will be changing out those 8 x 8 bridge support posts way sooner than acceptable if they are just standard local lumber yard PT 8x8s. No blah blah about it.



Installation: • Use all specified fasteners; see General Notes.
• Install MPBZ before concrete is placed using embedment level indicators and form board attachment holes.
• Place post on tabs 1" above top of concrete.
• Install Simpson Strong-Tie Strong-Drive 1/4" x 21⁄2" SDS
Heavy-Duty Connector screws, which are supplied with the MPBZ. (Lag screws will not achieve the same load.)
• Concrete level inside the part must remain 1/4" or less above
embedment line for water drainage.
• Annual inspection of connectors used in outdoor application is
advised. If significant corrosion is apparent or suspected, then the wood, fasteners and connectors should be evaluated by a qualified engineer or inspector.

[Clark Harbaugh]

I did a pergola about 6 or 7 years ago, and had it engineered professionally. The challenge, as stated above, is two-fold: 1) wood in/on concrete is going to fail. It's not if, it's when. Ground water and moister in the concrete will be absorbed by the post and will eventually cause the post to rot. 2) using a bracket, similar to what has been linked above, is not that great for lateral forces from wind. For mine, we used 8x8 posts using the Simpson post bases (http://www.fastenersplus.com/Simpson...UaAkTDEALw_wcB (http://www.fastenersplus.com/Simpson-ABU66Z-6x6-Adjustable-Post-Base-Zmax-Finish?gclid=Cj0KCQjwub7NBRDJARIsAP7wlT8nGkcVMhiwQ b7lF4oEFlY6COYeMWzK4CaRgReigfs84V0-nNS5m9UaAkTDEALw_wcB) ) on the front of the pergola and then using a ledger board with stand-off brackets we anchored the back side to the roof joists. The roof tie-in removed any concern for lateral movement/forces (I have a 90deg roof, so my pergola is anchored to the roof on two sides). Ours has been up 7 or 8 years and has survived typical N. Texas spring weather - 4 major hail storms, 50-60 straight line winds, and one close call with a tornado.