Mobile cranes depend on stability through their outriggers. These are the four “legs” that are deployed onto soil or other working surfaces adjacent to the crane. When a mobile crane is set up at a site, the outriggers are deployed by a hydraulic mechanism that extends the four outriggers beyond the crane body and then jack the crane free of its wheels so that it is supported by the outriggers only.
The ability of the crane to be supported to its rated loads is sometimes governed by the bearing capacity of the working surface to resist the loads imposed by the crane outriggers. The outriggers are equipped with pads to distribute those loads, but the ability of the working surface under the pads to withstand those loads is often unknown.
Detailed soil testing and evaluation is sometimes done to determine the ability of the working surface to withstand crane loads; however that is often impracticable at sites where the crane is used for only one lift or is moved frequently around a site. Instead, it is common to use a visual examination as well as crane mats to further distribute the outrigger loads. The mats provide a strong surface with a larger bearing area than provided by the outrigger pads on a crane. These mats have traditionally consisted of wood cribbing or wood pads made by the crane owner. More recently, companies have begun selling crane mats constructed of steel or other materials that are more portable and longer lasting than traditional mats.
Any investigation of a mobile crane turnover involves an evaluation of the working surface supporting the crane outriggers. This is not always an easy evaluation because the outriggers often dig in to the surface as the crane tips. It must be determined whether any damage to the working surface under the outriggers was the cause or result of the crane tipping.
John Phillips, senior consulting engineer at Warren, has more than 30 years of crane and heavy equipment experience and more than 17 years of experience in forensic engineering. A licensed professional engineer in South Carolina, North Carolina, Georgia, Louisiana and Ohio, he’s NCEES registered both as a model engineer and with The United States Council for International Engineering Practice, USCIEP. John has designed crane systems, supervised installation, tested and certified lifting equipment even serving as a project engineer for maintenance and certification of nuclear weapon lifting and handling systems. John is a certified fire and explosion investigator and fire and explosion investigator instructor by the National Association of Fire Investigators. John is a member of the American Society of Materials and American Society of Testing and Materials, as well as a voting member of ASTM Ships & Marine Forensic Sciences, Forensic Engineering, and Performance of Buildings committees.