It takes approximately 90 years to grow a 12-inch by 12-inch oak timber twenty feet long. High quality hardwood timbers are becoming harder to find. This is widely known but rarely discussed. The shortage of wood timbers is a national problem.
Crane mats have typically been constructed using oak or other select hardwood timbers. In recent years, as supply dwindles, timbers do not measure a full 12-inch cross section. As they are milled, we often see one or more sides are not square but rounded due to the smaller diameter of the raw cut logs. In some cases, we are seeing timber mats made from mixed wood species, and some with combined hard and soft woods.
Ground bearing pressure and load distribution calculations for timber mats are based on a full cross section of matched hardwood with no cracks or wood rot. When mats are constructed from timbers with less than a full cross section we have lower strength levels than desired. This affects the bearing area and bending properties, ultimately reducing load distribution. In this case, the calculations can be off by as 25 percent or more. In addition to the above, timber mats degrade, or rot, over time and the structural strength decreases as they decompose. The engineers have no way of knowing if the mats are good quality or not.
Photos Courtesy of Quality Mat Company
The demand for crane mats is extremely high and we don’t have time to wait for the hardwood trees to grow to suitable size.
One reputable crane mat manufacturer, Quality Mat Company, is dealing with the problem of undersized and inferior wood products. They have a strict quality control process in place and return inferior timbers to the saw mill. Quality Mat will only use wood products that meet their design specifications. The question is, how long can they continue the current mat construction design until the large diameter timbers run completely out.
To help manage the problem, Quality Mat Company designed an engineered mat structure. Their patented design combines a structural steel frame, synthetics, certified lifting rings, and treated wood products.
Photos Courtesy of Quality Mat Company
Although Becht Engineering has not analyzed or tested the Quality Mat Company Hybrid Design, it appears to be well thought out and will certainly solve the problem of rounded sides and allow for use of smaller cross sections of high quality, select, wood products.
We are all aware of another world-wide problem - Used Plastic Parts, Plastic Containers and Plastic Bottles
As an alternative to wood timbers, AXION Structural Innovations has developed a full line of crane mats that are manufactured from recycled materials. They use almost 100% recycled materials with 2 main ingredients, post-consumer High Density Polyethylene (detergent bottles, milk jugs, shampoo bottles) and post-industrial glass filled PP (automotive waste).
Their mat design includes steel plate inserted between or sandwiched between the composite boards to provide the necessary stiffness needed for proper distribution. Axion uses special MatLok® hardware that was value engineered for use with their material. It is not countersunk, keeps all stresses on the outside of the boards.
The product has undergone comprehensive engineering analysis and been tested by Texas A&M University. Becht Engineering analyzed the product and the results are favorable. We found that the although the Axion products perform slightly different than timber mats, they provide acceptable distribution properties.
Photos Courtesy of Axion Structural Innovations
The crawler crane mats are currently available with 7-inch, 9-inch and other sizes of boards. Axion has also designed shorter mats for use under crane outrigger pads.
As with wood timbers, depending on the applied load and allowable ground bearing pressures, the composit mats can be assembled in multipul layers and or combined with steel mats or steel plate to achieve higher distrabution properties and maintain structural integrety over time.
Axion is reusing millions of pounds of discarded plastic waste and their products are 100 percent recycable. Although they are extremely durable, some contractors will find a way to damage them. In that case, the mats can be recycled and used over and over again.
Steel mats are being used under very large cranes with extremely high ground bearing pressures. If properly engineered and fabricated, bending is not measurable. Therefore, they provide the highest distribution of the three products discussed here. They are increasingly popular for outrigger equipped cranes as they virtually last forever and can be easily stacked for transport and placement.
Huge steel mats are being used under crawler and outrigger equipped cranes with higher capacities from 800 to 5000-tons. The up side is they are practically indestructible. The tradeoff is; the large steel mats are expensive to manufacturer and heavy to transport and handle. In many cases, smaller steel mats are combined with timber or composite mat products to limit bending and increase load distribution. This is often a more economical approach.
Any type of matting, whether wood, steel or composite, requires and engineering analysis to determine the load distribution values. The analysis must take into consideration the type and capacity of the soil under the mats, the pressure applied to the top of the mats, the resistance to crushing and the bending properties.
There are currently no regulations or standards to guide industry for the use of existing products and new products that will be created in the future. There are often disagreements between geotechnical engineers and civil/structural engineers over crane foundation design and applied safety factors. The author is reaching out to crane industry leaders to develop a standard based on the knowledge and tools available today and into the foreseeable future.
The mat manufacturers feel strongly that North America will exhaust our large timber resource within 10-years. The mat industry must start now to develop new methods of distributing the ground pressure before the timber resource is depleted.
I believe in 20-years industry will be exclusively using composite materials combined with other products to achieve the required stiffness and load distribution. The timber shortage will drive this.
In 50-years, cranes could be hovering in air or levitating loads electromagnetically and may not require any matting at all.
Joseph (Joe) Collins is the Heavy Lift Division Manager for Becht Engineering Co., Inc. He is providing Consulting and Design Services for Lifting and Transport of process equipment, machinery, chemical, refining and nuclear vessels and components.His duties include consulting to clients using the world’s largest cranes and super heavy lift projects.
Mr. Collins has over 45-years’ experience in the heavy lift industry. He served as Commissioner and later as Vice President for the National Commission for the Certification of Crane Operators (NCCCO). Joe also served as a member of the Cranes and Derrick Advisory Committee (C-DAC) to OSHA, which wrote and delivered the initial draft of the new OSHA Crane Safety Standard. In January 2011, Engineering News Record magazine honored Joe as one of the “Top 25 Newsmakers” for his work with C-DAC.
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