Wind Load Landscaping Trees

[bookowski]

I searched around and found some old threads on here without resolution - just checking if there is any new or add'l. info.

My condition is numerous large trees (30ft. - 40ft.) in planters on building terraces and roofs. My stance is that the design of the tree supports (planter box, tie-downs, guying, netting, etc.) should all be provided by the arborist's engineer with loads imposed on the base building provided to me. Does this seem reasonable? As expected they think this is crazy and and don't know where to begin.

I can't find any US code provisions for wind on trees, just some articles but nothing definitive. There seems to be an argument that trees shed wind and therefore have much smaller forces than what would be calculated using a projected area. However I can also imagine an argument that the A > Aproj due to the overlapping/density of exposed branches/leaves.

We have two projects with this condition - seems like an emerging trend.

Anyone dealt with/resolved this on past projects?

 

[Archie264]

I've certainly never dealt with that and I'll be watching this thread to learn about it.

I would think that A = Aproj would be a reasonable value to use, absent other information. And I expect that the forces involved will be large since a tree in the ground has a fixed base whereas a tree in a planter has a pinned base.

Say, with that in mind that could be your upper bound: the force required to break the cantilever tree trunk. Then again, most trees that I've seen blown over by wind were uprooted, not broken.

Anyway, it sounds interesting.

 

[Ron]

bookowski...your issue is similar to a satellite dish or other appurtenance on a building. I agree with your approach. It is a delegated engineering function that should be returned to you as the SEOR.

The planter-mounted trees should meet overturning moment requirements as would any other appurtenance.

 

[KootK]

Some thoughts that come to mind:

1) The axial load of the trees is easy enough to estimate and probably not all that much relative to the weight of the planter boxes filled with saturated soil. Easy.

2) If the trees are guyed, then the moments are resolved over a lever arm equal to the cable spread. Probably not a big deal for your typical plaza / terrace structure.

3) If the trees are free standing and depending on their root system for stability, the moment is delivered into the planter boxes in a relatively spread our manner via the root system. This must be true as running APP.D shear pryout calcs on soil wouldn't yield much capacity. Again, this means that resolving the moment into your plaza / terrace structure probably isn't a big deal.

4) In a RACI chart sense, yes, determining the reactions is the responsibility of the landscape folks. It's worth keeping in mind, however, that their structural engineer probably isn't a structural engineer in the same sense that you are. He or she is probably a civil engineer who knows enough statics to give it a go. In terms of having numbers that are safe but not wildly conservative, you might be better off making your own estimates. In similar situations, I've volunteered to be the Landscaper's structural engineer for an appropriate, additional fee. This brings in some extra work, gives me reasonable loads, and usually makes the design team happy. Since we're all strangers here, I'll admit that my next step is to hand the work off to some patsy colleague in my office as I detest dealing with landscaping structural issues.

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.

 

[dhengr]

Just think about it.... now the SEOR is also responsible for the wind load analysis and the wind stresses on all the trees in the neighborhood, once he gets involved in a project. Here comes another couple hundred pages of code b.s. to wade through, what with all the different kinds of trees possible, with their various canopies, exposed areas to the wind, various heights, etc. etc. etc.. Just imagine, when you start considering all the possible branching points for various sized limbs, with various amounts of foliage on them. The torsional problem alone will boggle the mind. And, here comes a whole new bunch of software to solve this problem for us. Just think of it, what is the difference in the von Mises stresses in a fine piece of ash and those in a piss elm? A whole new area of research and PhD study is right around the corner. The father tree said to the little boy tree, your mother was a fine piece of ash.

 

[sdz]

Interesting. I've never thought about this before.

You certainly have to design for the imposed gravity loads. Planters and such are generally not fixed to the building and under high winds the trees would topple over. Trees in the ground do this too. If that's acceptable to the client then let it happen but if not then the anchors have to be designed and I think it is reasonable to ask the plant supplier for the design loads.

One issue is what danger would these plants cause if they do fall over. Would they fall off the roof or balcony and land on someone or something below?

 

[bookowski]

Kootk - I agree and internally I have already run my own guesstimates of what the forces could look like. You're right that whoever they get to do it isn't likely to be an expert or come up with anything much better - but if someone is going to sell a client 40ft. trees on a roof they should come with someone equipped to engineer a safety system for it. I generally don't nickel and dime over who is responsible and prefer to just get things done, but this seems like a very high risk item and without any code provisions to fall back on it really leaves you flapping in the wind if anything goes wrong (managed to work 'leaves' and 'flapping in the wind' into my tree rant!).

With no code guidance or recommendations I'm ok making some assumptions to apply to the base structure (although I wouldn't say that and still asked for loads imposed), I feel confident I could ensure that never budges. What I am more interested in is who designs a system to ensure it doesn't uproot out of the planter, or snap at the base, or branches fly off. These trees are on roofs in an urban area, they vary from 5 to 20ft from the roof edges (various roofs - 2nd, 8th, 10th, 15th etc) so if they went over they are really going over.

The forces can get quite large. Assume a 40ft. tree has a 15ft diameter perfectly round canopy, use 40psf as a rough starting point. Area of canopy = 175sf, x 40psf = 40kips, apply that at centroid of 32.5ft and you have 230 k-ft. I would assume that this would be guyed down to give a nice moment arm but at our first meeting they hadn't thought of anything like this.

Sounds like there isn't any code guidance/provision that I'm missing. I'll post an update if I find out anything interesting - seems like this will be a growing issue with green roofs etc.

 

[manstrom]

I doubt the Arborist has his own engineer; this is going to get kicked to you. Since it is not part of the building, create a new contract for it with a high price tag to cover your exposure.





When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[SAIL3]

Today I looked at some trees that are still without leafs even though spring is on the doorstep.....I would guess that the solid to gross ratio would average, say 50%....then add in the affect of leafs say another 20% for a total of 70%....what drag coeff to use?...most of the branches are of circular x-section, so I would try a drag coeff of 0.8....if the tree is on the roof of a bldg, then I would add in the increase req'd by code....I would consider this a lower bound value.....to get a handle on an upper bound value, I would try a few approaches...the one mentioned by Archie would provide a clean approximation
by checking the trunk out as a cantilevered col subject to an axial dead load and bending moment due to wind.....another approach would be to examine a single branch of the tree and caculate the bending moment on it neglecting any axial load...by comparing it's capacity versus the load one could find out if the wind loading assumed is within a reasonable range of possibility....this all would be based on visual observation of existing trees around your neighborhood...another method of getting a feel for a conservative upper bound woud be to assume a drag coeff of 2.0 on the solid area....the results of all of these would give a range of values and using engineering judgement settle on some intermediate value....if you happen to have a tree in the backyard, one could do an experiment and load up one of the branches to failure and calculate the moment involved and from that figure out an upper bound wind load and from that figure out the corresponding drag coeff.....I am sure there are many more approaches one could take and look forward to other ideas ....