LISTSERV - BULLAMANKA-PINHEADS Archives

As a yout studying Archinecture at Cornell, I experienced one of the three great teachers one might be privileged to have in life. Neal Andrews, a structural engineer, had somehow just been hired into the Design Department, much to the consternation of the design staff elite.

Neal wanted to teach us basic structures so we could feel them in our guts. He knew that by passion we were design people, not engineers, but his professional experience as a structural engineer dealing with architects had made him aware of the need for rational design to be based on possible realities of structural physics before it got to his desk. He wanted us to at least think in engineering terms even though we would always hire professionals to do the calculations and accept the responsibility and liability for their work in getting our designs to stand up. He did not teach formulas, but taught by forensic observation of building failures and by creative observation of forms in nature. He taught the principles of cantilevers by having us observe cross sections of tree limbs, discovering how the cell structures differed in the areas under tension or compression, and by getting us to think about and do rough calculations of the loads on a giant tree limb extending 50 feet from the trunk when full of liquid sap and weight of summer leaves so we would be amazed at how effective the natural form really was. He had us study the bone and muscle structures of our arms to better comprehend cantilever action and tension and compression and flexure.

Neal's lectures were unending slide shows of building failures of all types, providing gut wrenchingly dynamic examples for us to analyse and identify with. One I especially recall was of a multi-story concrete parkade that had experienced a major fire in which all the cars burned up. The giant structure did not fall, it simply sagged when the steel reinforcing temporarily lost its tensile quality, only to "re-harden" in scalloped shapes when the fire subsided. The concrete never lost its compressive quality so the columns did not fail, only the floor decks, and their concrete just flowed in particle form until the steel went back to work, leaving the whole thing looking like a stack of curvy sea shells propped on sticks.

So, as to your tree, health and environment is everything, as is your intuition. I would go with the latter. In town here, there was a handsome row of 30" diameter 120' tall spruce trees along a driveway beside a recently restored heritage home. The trees, though aged, all looked wonderfully healthy - - - until an unusual wind storm came through one night and caused one to shear off horizontally 10' from the ground. Then we could see that only about 1-1/2" of solid wood was left all around the trunk and the entire inner core was hollow, obviously completely rotted and dissolved away long ago. Happily, the tree fell along the driveway and not on the house, but everyone in the neighbourhood was now seriously asking the same question you are. That whole row of trees all looked good, but......

A related picture comes to mind when you mention cutting off the big limb. At one of those large family Thanksgiving dinners, being the oldest, Grandmother was seated first while the aunts and uncles all wrestled over the hierarchy of who else was to sit where. A busy and nervous little lady, Granny fiddled with her hands under the table while everyone else delayed in their honoured sitting ritual. The table was piled high with food on platters. At one point, granny managed to fiddle out the prop for her side of the extended table leaf and that entire side of the table collapsed, piling food on her lap and on the still empty chairs beside her. Then, in the moment that followed, as everyone gasped and looked in awe at the mess of turkey and squash with marshmallow topping and peas and empty wine glasses on one side of things, the now unbalanced table followed the laws of gravity and balanced itself by tipping over to the other side, spilling everything left on to the opposite chairs. Cantilever action at work. I should have taken a slide for Neal to add to his show!

(still out on a limb)

----- Original Message -----

From: [log in to unmask] href="mailto:[log in to unmask]">[log in to unmask]

To: [log in to unmask] href="mailto:[log in to unmask]">[log in to unmask]

Sent: Tuesday, August 31, 2010 7:11 PM

Subject: [BP] A forensic tree engineer?

That's what I want, someone who understands the engineering tree.

We have a big, spreading oak, considered lovely by my wife's family. I have never been fond of it because I think it is likely to crush half of the house, over which one particularly giant limb looms.

All the "tree guys" have said "oh, don't take it off, it would hurt the tree" although they don't really have anything specific to say, it's just sort of seat of the pants. All the architects say "you're crazy, having that thing hanging over the building." Ditto.

I figure there must be a sort of technical approach to this. How much concealed rot is in each major branch? Does that relate to major failure? If the tree were to fail what would be the most likely event chain? If we took the limb off, what is the downside? Would that, in itself, destabilize the tree? How much of this is plant biology, and how much is engineering? Is this tree even, perhaps, at the end of its useful life?

Do you have a suggestion for how I could research finding someone with technical knowledge - a tree engineering failure expert?

Google doesn't work, has too many results for court experts, etc., especially ones who do fault-tree analysis.

Christopher