Pergola Engineering – Lateral Stability w/o Knee Braces
BobStrauss
| Posted in General Discussion on
Looking to reproduce something similar to the image (except no roof/glass), planning an attached 14×14 pergola with 6×6 posts, 2×12 (2 ply) beams, and 2×10 rafters – spaced roughly 34” oc.
Aside from deeply buried front posts, what’s the best way to beef up lateral stability without use of angle/knee braces? I’m concerned something significant will be needed, as no roof structure (or even purlins) will be present to provide support and prevent wobble.
Perhaps custom-made, thick steel flat L-braces on either side of each post and beam?
Replies
This is a good place to start your research.
https://www.strongtie.com/products/go/connectors/outdooraccents
If no braces are being used very strong tight connections are required. It can be achieved with classic mortise and tenon joinery. This is typically too advanced and time consuming for most people. Many people opt for metal hardware for rigid connections. Don’t skimp on the hardware if you want a solid structure.
Outdoor Accents by Simpson also include quite a few pergola plans free for download. I’ve used them for basic pergolas and they look good. It’s nice b/c they highlight hardware etc, very user friendly. They have plans w/out bracing. It will require the right hardware.
if you haven't found these yet, here is a link to steel hardware from Simpson that seems aimed at projects like yours.
https://www.strongtie.com/products/go/connectors/outdooraccents
It does use the knee braces which you want to avoid, however. If you want something more structural you might be looking at custom fabrication.
The advantage of Simpson is that all the engineering has already been done by the company. If you are making up custom hardware it needs to be engineered.
The engineering for the parts Simpson offers is done, but none of these connectors will provide the lateral strength necessary. They are basically just made for securing joints where timbers cross. A decent fabricator should have an engineer on call.
Set steel posts in concrete perimeter beam and wrap with your choice of wood.
What I never can understand is the number of people who are concerned enough about the strength of a structure that they post the situation in a builder forum to get advice for which the advice givers will bear zero responsibility, either financial or professional. If there is concern about whether a structure might wiggle; or worse, come crashing down on occupants, then pay a visit to a local civil/structural engineer who by law puts her/his reputation on the line and is subject to legal action if things go south.
This is a forum. In my opinion that is fairly self explanatory. Do all the engineers take on liability over on engineering forums? People are simply talking to one another for ideas. I won’t put my stamp on something discussed on this forum, would you? This isn’t a business transaction where we are getting paid for service and we do not accept taking on liability. If I can be held legally accountable for this I’m off! It’s just a few people talking things out. I for one like to talk things out with others before I hire help b/c it gives me a better idea of questions to ask of the professional being hired and minimize wasted time by going in to something totally blind. Like any forum, it’s the users discretion what they choose to do with the information. No one is legally obligated. We all only have the best of intentions being on here, using our experience to help. There will be some advice in forums that is not great, again it’s a forum. Example, advice from a contractor in FL is probably not the best for someone living in Canada (although it could be great advice for someone in FL).
On a side note the suggestion given by most in this chain to seek advice from Simpson is a good one. Simpson products are considered the industry standard and have a team of engineers designing and testing their products.