×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Outdoor Patio Tie Rod Connections

Outdoor Patio Tie Rod Connections

Outdoor Patio Tie Rod Connections

(OP)
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!

RE: Outdoor Patio Tie Rod Connections

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

RE: Outdoor Patio Tie Rod Connections

(OP)
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.

RE: Outdoor Patio Tie Rod Connections

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.

RE: Outdoor Patio Tie Rod Connections

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.

RE: Outdoor Patio Tie Rod Connections

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.

RE: Outdoor Patio Tie Rod Connections

(OP)
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.

RE: Outdoor Patio Tie Rod Connections

Quote (OP)

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.

RE: Outdoor Patio Tie Rod Connections

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.

RE: Outdoor Patio Tie Rod Connections

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.

RE: Outdoor Patio Tie Rod Connections

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

RE: Outdoor Patio Tie Rod Connections

(OP)
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.

RE: Outdoor Patio Tie Rod Connections

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

RE: Outdoor Patio Tie Rod Connections

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.

RE: Outdoor Patio Tie Rod Connections

(OP)
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.

RE: Outdoor Patio Tie Rod Connections

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).

RE: Outdoor Patio Tie Rod Connections

Quote (OP)

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.

Quote (OP)

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.

RE: Outdoor Patio Tie Rod Connections

Quote (KootK)

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.

RE: Outdoor Patio Tie Rod Connections

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.

RE: Outdoor Patio Tie Rod Connections

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

RE: Outdoor Patio Tie Rod Connections

Well I certainly didn't mean to imply that the the cable provided a pinned, lateral support to the top of the post. Hopefully that's not what the OP intended either.

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.

RE: Outdoor Patio Tie Rod Connections

(OP)
Haha. I apologize that I was not clear with my last post, I am obviously aware that the cables do not provide any support at the top of the post.

The location of this "patio" is a fairly high end winery, so the aesthetics are very important. I do agree that the wood column and cable design would be the way to go, I am just struggling to find something that looks good and I am comfortable with. After a handful of iterations, I have found that a 6x6 Douglas Fir - Larch, Select Structural grade post just passes my checks, with not much room for error.

RE: Outdoor Patio Tie Rod Connections

I was sorely tempted to just recommend "6x6 and let 'er be". I just didn't want to get cavalier-shamed. KootK approved.

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.

RE: Outdoor Patio Tie Rod Connections

If this is high end winery I change my answer to steel post 100%

RE: Outdoor Patio Tie Rod Connections

yea, steel post for sure or maybe an 8x8 wood post. Anything else will ultimately have visible creep.

RE: Outdoor Patio Tie Rod Connections

Civ-StructEIT:
The Arch. draws cute and sexy sketches, he just doesn’t have the vaguest idea what he is doing or what he is asking for. Sure, it can be done, but probably not with a 6x6 wooden post with some undefined connection and foundation at the base. He might even be able to point to some similar patio structures which are still standing. They just aren’t very old, haven’t likely seen full design loads, winds, ice, etc., or been used as chinning bars by some drunk, as mentioned above. They haven’t seen much weathering, aging, creep, long term deflection, etc. I’ve also seen some of these which, when you leaned on one post, you could see the top of the whole structure move laterally. I don’t like the large lateral loads induced at the upper tips of those posts, those will be an ongoing problem and an aging problem. I’d be inclined to use a light stl. 4x4 HSS, and then clad it in 1x6 wood for a finish, with a water shedding cap, bottom drainage and some venting features. Painted all sides before final assembly and final outer painting or staining. I’ll bet that at some point, they will want a fabric tent/sun screen over the top of this structure. The 8x8 HSS base detail is very complicated for what is does; placement, conc. forming, etc. What happens when this fills with water and frezzes?

I would take the approx. shape/arrangement that the Arch. shows on his elevation for the horiz. spanning members (20.5' and 23.5' lengths), if he is really set on that general shape, but rework it as follows. I would want these to act like light trusses, with a real top and bottom chord, with some diagonals (?), maybe just wrought iron infill, like a wrought iron fence or gate, maybe not as well defined or symmetrical as a std. OWSJ, but a truss nonetheless. The ends might have some lower wrought iron brackets below the bot. chord and finally end plates, 3"-4" wide, 1.5'-2' high bolted to the posts. These trusses carry some gravity load, and hold the posts together in tension or compression. The cable or tie rods aren’t required any longer so the large tip lateral load goes away. Some extra tube or some such, his scalloped shape, might be fixed below the bot. chord to act as conduit and base for the light fixtures. I would probably install a couple diag. cables and turnbuckles from corner to corner, for some stability.

RE: Outdoor Patio Tie Rod Connections

You could go the other way and try to out-architect your architect. There would be stuff to consider, of course, including your fee. And architects never seem to find my cool ideas as cool as their own.

Maybe some cable bracing in the middle two panels. One would have to let the numbers be their guide.



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.

RE: Outdoor Patio Tie Rod Connections

KootK, I think your patron is going to get crushed due to an LTB failure.

RE: Outdoor Patio Tie Rod Connections

Oh contraire, I think that she'll savour her gluten free salad and pumpkin spiced chi latte in peace. Vertical rods in combination with weighted/taught lower cable should stabilize the whole thing against LTB rotation if properly designed. It's the load that's gotta roll closer to the ground, not the top chord. At least that's it with a truss assumed weightless-ish.

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.

RE: Outdoor Patio Tie Rod Connections

Aaah, did not catch the cont. vertical rods - clever.

RE: Outdoor Patio Tie Rod Connections

It seems that I neglected to mention that the verticals would be continuous. It must be inferred by zooming in very closely and observing subtle graphical cues.

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources