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Support Cribbing – Part 2: Material Properties

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Once a load is picked up, you need to have a place to put it back down. Cribbing in one form or another often is that place.

Properly assembled cribbing constructed of a suitable material can provide a stable base for any object. If the cribbing is improperly assembled and/or made from inferior materials, it can provide the potential for dropped loads, damaged equipment, and injury to workers. The second part of this article gives an overview of the properties of material used for building proper support cribbing.

Part 2

Composite materials

Material that is convenient for decking an area of your home because it doesn’t require regular painting and maintenance is not suited to cribbing. If you look at the material properties that are published by the manufacturers, it typically has poor compression properties. It is simply not suitable as an engineering material.

Problems arise as it is easy for this material to get into a pile of cribbing on the skid that your mechanics hang onto because it’s lightweight. Another household reference I could use that I’ve seen in the field is wood used for flowerbeds. It is not appropriate for cribbing. The principle is the same. Leave it to the purpose for which it was manufactured.

Plastic

Typically, plastic cribbing is made out of high molecular weight polyethylene, filled nylon, or fiber reinforced plastic, which is commonly referred to as fiberglass even though it’s not.

It’s an engineered material like steel with predictable properties, and it can be cast and extruded into various shapes and worked with common woodworking tools. Those are desirable properties.

Two notable, large manufacturers of this make cribbing that fits together like blocks. You can produce a diamond surface on the top of it that gives it a very high coefficient of friction so it won’t slide apart. It also comes with notches cut into it that fits together. This can guard against cribbing becoming slippery, especially when it’s cold.

Steel

Steel is more predictable than anything else that we are liable to use. It’s heavy and dense. It can be custom-built for various purposes making it somewhat labor intensive in that you need it designed, welded, inspected, and painted. The trouble with that is it is often built to suit a single purpose. A steel rack built to hold a particular piece of kit should only be used for this purpose. Using it for something else is asking for trouble.

Wood

Wood is commonly available, inexpensive, environmentally friendly, and easy to work with. It is also lightweight compared to other materials. Both FEMA in the United States and Emergency Measures, Canada, tout the warning given by wood as it is about to fail in the form of creaking. Many rigging professionals are used to ductile materials and hooks and shackles that will bend a long way before they break. It may seem comforting but in reality it is not quantifiable. For example, if a white oak timber creaks seven times when you put eight tons on it, what does that mean? Really, it depends on the condition of the material, how many times it has been loaded, and how it has been loaded in this particular instance. You may not hear that. It is not always going to give you that warning.

One source I consulted indicated quite clearly that hardwood does not creak. The definitions of hard and softwood mostly have to do with international tariffs and not engineering properties. Different species can have vastly different properties far removed from their definition. Consider, for example, basswood and cherry are sold as hardwoods but they are very easy to work with and carvers particularly like basswood because it’s soft.

Additionally, a source suggested taking a value of 500 psi as an allowable load for wooden timbers. That represents 60% of capacity for Douglas fir and 50% of capacity for White Oak. Another source quoted 1,250 psi.

Wood can also suffer permanent damage without any visible clues. When you over-stress a piece of steel, it takes a permanent deformation; it sets. If you have a hook that is bent or twisted, it’s immediately obvious and you know you can’t use it.

If you overload a piece of wood in compression, you can severely damage its internal structure, and the strength of the wood relies on the chemistry that connects the wood fibers to one another through a material that replaces the plastic resin used in manmade composite materials. You can’t see that, and it doesn’t always show up externally.

Never use old wood that has been discarded. Leave it in a pile that’s marked firewood!

Simple conclusions to be drawn include : know when cribbing is required; select the right materials; and don’t forget to include cribbing in all of your lift plans.

 

Author: Fred Wolsey

Fred Wolsey is a mechanical and professional engineer. He has worked in the electrical power generation industry since 1985, and has been responsible for cranes and hoists at a power plant for the past 16 years. Wolsey has written maintenance and operating procedures for cranes and is the lead author of the Bruce Power Crane and Rigging Handbook.

Source: Crane & Rigging Hotline Magazine, July 2013 Issue

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