The recent failure of a cable system in a huge coast live oak prompts me to revisit this topic. I touched on it last year, (Feb. 20, 2021: Tree Cables Need Professional Installation and Maintenance) based on a case history where a sub-standard cable system gave way. It resulted in the collapse of large oak and caused severe property damage. With the proper cable hardware and periodic maintenance and revision, I believe that tree would be standing yet today, and with a low level of risk.
My current example involves a 60-inch trunk diameter coast live oak. Half of the tree fell when one component failed in the cable system. Fortunately, there was no “occupancy” under or near the tree — no target other than natural woodland and another similar oak. The second oak was struck but not severely damaged. Still, it was a sad loss of half of this beautiful old tree.
The system was well installed with standard hardware. The cables formed a triangle including three main limbs – a “closed box” cable system, designed to prevent excessive strain on the attachment of codominant secondary trunks by sharing the load among three limbs of similar size. But the hardware was not strong enough. The load exceeded the strength of one dead-end grip when the trunk, weakened by internal decay, began to split at the limb attachment. I was not able to obtain the details about the date of the failure, but I suspect rain and wind contributed to it. I found the broken grip, torn open.
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Tree cable systems installed according to ANSI Standards and ISA Best Management Practices may consist of various kinds of hardware. Here is an example of a common combination
• Cable: EHS (Extra High Strength) 7-strand zinc-coated steel of various diameters, depending on the estimated load. Quarter-inch is a common choice. It has a working load limit of 1,330 lbs.
• Dead end grip, also known as a pre-formed tree grip or “twisty." It resembles a giant hairpin with spiraling legs that wrap around and grip the cable. The dead-end of the cable terminates in the grip. I see no separate working load limit for these stated in the BMP. They are apparently lumped in with the cable for strength. This is the part that failed.
• Thimble, a steel, almost-closed-u-shaped saddle that sits inside the hook or eye. The u-shaped end of the dead-end grip is placed over the thimble. It prevents excessive stress and wear on the grip.
• Eye lags, lag hooks, or eye bolts installed into or through the limb. Options of type and diameter depending on limb size and condition.
So, the assembly is like this: Lag goes into the limb. Thimble goes into the lag eye or hook. U-shaped end of the dead-end grip rides in the thimble. Grip legs wrap along the cable.
Cabling systems are partly determined by industry standards but also by the arborist’s experience and choice of style. I have seen cable systems installed locally by two well-respected and reliable tree services but with very different styles. And every tree is different. As one mentor said: ”With cabling, you play the hand you are dealt."
I think the most fraught aspect in a case like this big oak failure is estimating the load.
Assuming the installing arborist determines a good design for cable placements and selects the appropriate type of hardware, they still need to select the appropriate strength cable, and lags or through bolts with respect to the load. It is tempting to just “eyeball” it, but we need to turn to the BMP for guidance. It provides a table of minimum hardware size requirements.
For example, ¼-inch EHS cable (and supposedly the dead-end grip) is shown as capable of bearing the load of a limb as large as 18-inch diameter. That’s where the arborist needs to stop, measure or estimate limb diameters and turn to the BMP for help in selecting the appropriate size cable and hardware. Even so, wood weight varies by species, limb length, dynamic wind loads, and it increases over time.
After the system is in place, the tree continues to grow larger and heavier, and cable system components are subject to wear and weathering. So, everything needs to be reassessed on a continuing basis. Three years is a good inspection interval.
There is science and art in tree cabling and there is never a guarantee of safety.
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