Cabling and Bracing

A recently approved American National Standard for Tree Care Operations identifies the methods and materials arborists should use when installing tree support systems.

by Michael Oswald

American National Standards Institute (ANSI) A300 (Part 3)-2000 presents performance standards for the care and maintenance of trees, shrubs, and other woody plants, and is a supplement to ANSI A300-1995. ANSI A300 (Part 3)-2000 should be used in conjunction with the rest of the A300 standard when writing specifications for tree care operations.

The new standard is intended as a guide in the drafting of maintenance specifications for federal, state, municipal, and private authorities including property owners, property managers and utilities.

The A300 standard stipulates that specifications for tree work should be written and administered by a professional possessing the technical competence to provide for, or supervise, the management of woody landscape plants. Users of this standard must first interpret its meaning, then apply their knowledge of the growth habits of certain plant species in a given environment. In this way, the user ultimately develops their own specifications for plant maintenance.

Support Systems

The new standard states that support systems are used to provide supplemental support to leaders, individual limbs, and/or the whole plant, and that the objectives for support systems should be clearly defined prior to installation.

"When writing the new standard," states Craig Greco, candidate for Masters of Forest Resources at Clemson University, "people were asking if what we have been doing in past decades really works. Is this or that step really necessary? Is it the best way? Can we improve on these methods? New technologies such as computer simulations can help today's arborists look at trees from an engineering perspective, and the standard will improve as more research is conducted."

Ron Rubin, operating officer at The Care of Trees, says the new ANSI standard raises some additional questions, which can be answered with more research. "For example," he notes, "we left enough room to add the Cobra system--a harness-type system consisting of polyurethane rope, rather than cables and bolts. There hasn't been a tremendous amount of research on cabling hardware, and systems will become morecomprehensive as they evolve."

"The standard is very worthwhile, in that it gives the industry some guidelines from which to work," Rubin says. "It includes some basic practices we all strive for."

According to Dr. Tom Smiley, researcher at Bartlett Tree Research Laboratory, the International Society of Arboriculture will be coming out with a series of books entitled Best Arboricultural Practices. "One of the books in that series will cover this new tree support systems standard," he says.

Pruning

According to the new standard, all necessary pruning should be performed prior to installing a tree support system. "This step is important," Rubin explains, "because you want to get all the weight off before installing the cable. Pruning first, allows the worker to make the cable properly taut. You also want to make sure that, after you do the pruning, it's still appropriate to put the cable in."

Liability

Prior to installation, according to the new standard, the owner or owner's agent should be notified of the need for periodic inspection by an arborist. The standard goes on to say that inspections should be the responsibility of the tree owner and should include system, condition, position, cable tension, and the tree's structural integrity.

"Putting a cable in doesn't exonerate the property owner, or the arborist for that matter, of any liability," Rubin notes. "It merely indicates that a structural flaw has been identified. And it's the property owner's or manager's responsibility to have the support system inspected periodically. If potential liability is of the utmost importance, the best option may be to remove the limb or tree."

Greco furthers this point. "Many people want to get the hazard out of the way," he relates. "But arborists need to make it clear to the property owner that installing a tree support system isn't a concrete method for stabilizing the tree. It's merely preventative."

How frequently these periodic inspections should be conducted depends on several factors, according to Greco. "The tree species, its environment and the age of the tree all affect how often the support system should be inspected," he says. "For instance, a
Silver Maple might need more frequent inspection than a Willow Oak."

Hardware

Steel hardware should be corrosion resistant, and synthetic fiber cable systems should be ultra-violet (UV) light resistant, according to the standard. "Sunlight breaks down the plastics in synthetic fibers if the cable's not UV resistant," Greco says. "Likewise, plain steel cabling shouldn't be used. Zinc-plated, or galvanized, steel cable has a defense against corrosion. These cables cost more, but the extra money is well worth it."

Cavities, the standard indicates, should not be filled to provide support. "Filler material is only cosmetic," Greco says. "It's not a good material for strengthening."

Installation

According to the new standard, the diameter of the hole shall not be greater than one-sixth (1/6) the diameter of the limb, trunk, or branch at the point of installation, and the longitudinal alignment of anchors and/or braces should be avoided.

Dr. Tom Smiley says the one-sixth guideline is intended to prevent very large hardware from being installed in small trees. "Also, anchors and braces shouldn't be placed one underneath the other," he adds. "By offsetting them, you won't split the bark between them."

Heavy-duty or heat-treated heavy duty round steel washers, the standard states, should be installed between the nut(s) and the wood. "In tests," Smiley reports, "we've found that standard-duty washers can bend and pull right through the hole in the bolt. It's better to use heat-treated, heavy-duty washers, which tend to be thicker and larger in diameter. The washer is important because it spreads out the force of the pull on the bolt."

Holes for through-hardware should not be drilled greater than 1/8" (3 mm) of the diameter of the hardware being installed, states the standard. "Through hardware should slip easily into the hole you drill," Smiley explains. "From a tree health perspective, you'd like to drill the hole the same size as the hardware when you can. In some cases, however, you need to drill the hole a slightly larger diameter than the hardware. This is especially true of pines, which tend to close down after you drill the hole. All the standard is saying is that an arborist may need to drill the hole 1/8-inch larger in diameter, at most, than the diameter of the hardware."

Terminations formed by eye-splice configurations should incorporate thimbles, according to the standard. "The heavy-duty thimble is a piece of metal that prevents the cable from wearing," Smiley states. "Cables tend to wear where the I-bolt and the cable touch, and use of a thimble can keep the cable from wearing in that spot."

Installations, the standard indicates, shall follow manufacturers' recommendations. "The dead-end grip and Cobra systems are the only types we know of that come with instructions," Smiley states. "This portion of the standard simply points out that if a tree support system comes with instructions, those instructions should be followed by the installer."

Cabling

The standard says that cabling systems should be designed using one or more of the cabling system types in Figure 39.2. "The cabling system type used is totally dependent on the situation of the specific tree you're working on," says Rubin. "The box and triangle methods distribute the weight very well. Although hub and spoke is a good method, few people I've spoken to use that method, due to the cost in time and materials."

a. Direct. Direct cabling consists of a single cable between two tree parts, e.g. two limbs, two stems, or a trunk and a limb (three direct cables shown).

b. Triangular. Triangular cabling consists of connecting tree parts in combination of threes. This method should be preferred when maximum support is required (two triangular systems shown).

c. Box. Box cabling consists of connecting four or more tree parts in a closed series. This system should be used only when minimal direct support is needed.

d. Hub and Spoke. Hub and Spoke cabling consists of a center attachment (hub) with spans (spokes) of cable radiating to three or more leaders. Hub and Spoke cabling should only be used when other installation techniques cannot be used.

According to the standard, steel cables should be taut following installation. "The cabling system is supposed to be taut when you install it, but the time of year factors into that assessment," Rubin says. "If you install the system in the summer it may be very taut due to the weight of the leaves, but it may slacken in the winter due to the loss of leaves."

Anchor(s) should be installed at or near a point two-thirds (2/3) of the length of the limb or leader to be supported, the standard says, measured from the trunk or crotch. "What you're trying to do is get as close to a 45 degree angle as possible," Rubin explains, "because it gives you the best leverage and support."

The standard says that if existing cables are to be replaced, they should not be removed until after the new system is installed. "If you cut the existing cable first, it could split the tree," says Greco, "or it could become hazardous to the arborist. If the existing cable is taut, even with a little bit of tension, it should be left on. Also, if the old cable is removed, it should not be re-used on another tree."

Bracing

Bracing systems, according to the new standard, should be designed using one or more of the bracing system types in Figure 40.2. "The bracing type selected varies based on the size of the tree, species, how it compartmentalizes and what the objective is," Rubin reports. And Greco adds: "The bracing type used depends on the heredity factors of the tree."

"There are not very many arborists doing bracing--cabling is much more widespread--so there's not a high level of understanding out there," Rubin says. "With bracing, you need to have a very thorough understanding of trees, especially because you're creating multiple wounds."

The standard indicates that a cabling system should be used to provide supplemental support for the limbs forming the crotch being braced. "Cabling is always stronger than bracing," Smiley says, "because it is higher up in the tree and has better leverage. In other words, bracing should almost always be used as a supplement to cabling. The most common example of where bracing may need to be done alone, without cabling, is the Bradford Pear. With that tree, cabling often cannot be done."

The preferred location for a single rod for a non-split crotch, according to the standard, should be one to two times the branch diameter above the crotch. "That distance helps you get the optimal leverage for the brace rod," Greco notes.

Bracing should be installed in either a through brace or dead-end brace configuration, the standard indicates. Through-braces should be used when bracing through decayed area/wood or in trees that are poor compartmentalizers or have weak wood characteristics, according to the standard.

"Most arborists should be using through bracing, rather than dead-end bracing," Smiley says. "It's the better system because it has nuts on either end and you don't have to worry about decay as much."

The new standard says the brace should be installed completely through the smaller or equal portion and at least halfway into the other portion. "If you're bracing a crotch of different sizes," Smiley explains, "you should drill all the way through the smaller side and half way into the larger side. You want to end up with the maximum number of threads in the smaller side. If the crotch sides are equal, however, this point is not important."

Editor's Note: In this article, Arbor Age only mentions specific portions of ANSI A300 (Part 3)-2000. ANSI A300 (Part 3)-2000 is published by the National Arborist Association (NAA). No part of this publication may be reproduced in any form without prior written permission of the publisher. To purchase a copy ($15 for NAA and ISA members, $25 retail), contact the NAA at (800) 733-2622. Free shipping for online orders at www.natlarb.com

Suggestions for improvement of the standard should be mailed to: A300 Secretary, c/o National Arborist Association, 3 Perimeter Rd. - Unit 1, Manchester, NH 03103; or e-mailed to naa@natlarb.com