Outdoor Patio Tie Rod Connections

[Civ-StructEIT]

I just got tasked with looking over and reviewing a patio design from a Landscape Architect. It is a pretty simple setup, where there are 4 6x6 timber posts (10.5' high) spaced 20.5' x 23.5'. There is no roofing for this patio, but the architect is proposing 1/2" steel tie rods connecting to each of the timber posts at 9.5'. The tie rods would then be supporting various light fixtures. This seems sketchy to me, but I wanted to check to see if this is something anyone else has seen and worked well before, or if there was a good alternative to look at. Thanks!

 

[jayrod12]

What's the plan for the base connection? Most post bases specifically note they are not for cantilevered posts.

 

[Civ-StructEIT]

I have been going back and forth with them on the base connections. They want to sleeve the post with an 8" square steel tube and embed it 3 feet into a 24" diameter concrete foundation.

 

[jdgengineer]

If I understand the spans of the tie rods correctly they will behave essentially as cables. You will need large pretension forces in order to avoid excessive deflection. I think you will have trouble with this tension force with wood posts. You may have a chance with tube steel posts.

 

[jayrod12]

This all screams "This is a terrible idea".

In theory, the posts with a steel sleeve could work, provided they are willing to allow the sleeve to extend far enough up the post to ensure fixity. The second issue would be the pile/pier design and ensuring it's stiff enough to provide the fixity. The third issue is precisely as jdgengineer indicated, 20 or 23 ft for 1/2" cable is going to require significant pretension to stop it from sagging. This would likely cause the tops of the posts to pull inward without a member running between posts at the top designed to resist the compression.

 

[jdgengineer]

I agree with Jayrod it doesn't sound like a very good idea. Maybe a sketch would help as well. Getting a compression ring around the top as Jayrod mentioned would help dramatically. Steel posts I think would also be a better idea than wood.

 

[Civ-StructEIT]

Thank you for all your input. When I was first approached with the idea, I suggested steel posts and the client wasn't a fan of that at all, which is why they came back with this timber post and steel cable layout. I was just sent an updated sketch of the planned base connection. They are now showing the steel tube, along with a steel base plate and anchor bolts. I attached the new sketch, along with the section view I was given.

 

[KootK]

or if there was a good alternative to look at.

The first thing to get a handle on would probably be the expectation for rod straightness. If you can accommodate a healthy drape, then you've got a few viable options to play with as suggested above.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MotorCity]

Since the posts are cantilevers, I assume that you are not relying on the rods for any stability or bracing of the structure. If that's the case, I would not pretension the rods as others have stated because it could impose significant load on your posts. Calculate the sag in the rod that will occur due to the weight of the rod and the lights. Then calculate the end force on the rod (it will almost act as a cable and have a parabolic sag in the middle). As long as nobody objects to the sag in the rod and the posts are ok, then go with it.

 

[jdgengineer]

Without pretensioning I'd be a little surprised if the deflection wasn't in excess of 6" or so including weights of lights. You can install the columns out of plumb as well to allow them to deflect to plumb under tension load. I still think steel columns would be highly recommended and I don't love the approach.

 

[XR250]

Make sure you take into account the infinite force that will develop when some drunk decides to use it as a pull-up bar.

 

[Civ-StructEIT]

Assuming the client will be okay with 6" of deflection (I have a call with them later today), I used T=wL^2/8d with d being the allowable sag to calculate the tension acting on the post due to the cables to be roughly 600 lbs (using dead loads and an ice load). This turned out to be dramatically less than I was anticipating.

With Tension forces acting in 2 directions, and Wind Load being applied to the 6" x 6" wood post, it still passes an Axial & Bending Stress Check, with a ratio of 0.87:1.

Thank you all for your help and input.

 

[XR250]

It may pass, but long term creep will occur for sure. Residential stuff rarely fails but deflection is what clients usually dislike.

 

[jayrod12]

And 6" of deflection is a lot. It will be noticeably smiling the entire time. And once the wood posts creep inward, this sag will become larger as the tension force will be relieved slightly.

 

[Civ-StructEIT]

Update: The client has agreed to go away from the tie rods, and now are using light weight cables to string lightweight lights (similar to Christmas lights). With this new configuration, if I call out required sags and tensions of the new cable, would the top of the Wood Posts be considered a pinned connection? I am getting thoroughly frustrated with the Landscape Architect, who is refusing steel posts.

 

[jayrod12]

Still no. The posts would still be cantilevered without something capable of providing some compression resistance between adjacent poles. The lightweight cables are only partially an improvement, and may in fact be a downgrade as they will have zero bending resistance (compared to really minor bending resistance of rod).

 

[KootK]

With this new configuration, if I call out required sags and tensions of the new cable, would the top of the Wood Posts be considered a pinned connection?

I'd consider the connection between the top of the posts and the cables to be pinned with the cable exerting lateral and vertical load on the post.

The bottom of the post must still be fixed, of course, so that it can cantilever up from the base.

I am getting thoroughly frustrated with the Landscape Architect, who is refusing steel posts.

Can you blame them? Consider:

1) steel may introduce a new trade to the project and additional cost.

2) steel quite changes the aesthetic unless it's wood wrapped etc.

3) unless galvanized is okay, or there's budget for high end finish treatments, you're gonna have a rusty post out there in a few years.

I think that wood posts are probably the right design choice here. Ditto for the lightweight cable and lighting system.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

I'd consider the connection between the top of the posts and the cables to be pinned with the cable exerting lateral and vertical load on the post.

Maybe we have different definitions of the word pinned, but I would not consider these posts pinned at the top at all. Loaded yes, pinned no.

To me, pinned means there's some form of restraint at the top from buckling. I don't see that in this case.

 

[jdgengineer]

I believe KootK was referring to the cable modeling but I'll let him clarify. Seem like a wood post could work in this configuration if everyone is OK with a healthy amount of sag. This seems like it would be acceptable for light Italian string lights.

Sure would be nice to have some wood beams connecting the columns for compression though. Reduced demand at bottom of post fixity as well which may be tricky to get to work with standard wood hardware.

 

[jayrod12]

If that's the case, then I would agree. That's just not how I read the OP's question.