One question that is always asked is, do I need to put mats under this crane?

The default answer is always yes unless it can be proven otherwise.  Matting under cranes spreads the load from the crawlers or outriggers into the ground over a larger area.

In recent history, the existing soil conditions and the ground bearing pressures imposed on them by cranes have become increasingly scrutinized in the construction industry.  Crane manufacturers and rental houses usually have software to calculate the loads imposed by a specific crane in a particular configuration that is lifting a specific load at a specific radius.  This will only give one scenario out of the many dynamic loads that they crane will induce throughout the duration of a construction project. The ground bearing pressures should be looked at various radii, swing angles, and loads to find the worst-case scenario.  The heaviest load picked may not cause the worst ground bearing pressure and therefore is an ill-advised assumption to take.

The levelness of the crane pad is specified by the crane manufacturer for the crane.  This levelness is usually somewhere around one half of an inch vertically for every ten feet horizontally.  Depending on the existing site conditions, it may be easy to create a level pad, or it may come at a large cost.  The larger the crane pad, say for a crawler crane with a wheeled counterweight, it may be difficult to maintain this large crane pad throughout the duration of the job, and is something that should be looked at while planning the job.

The allowable ground bearing pressure, if given, or the mat limitations could steer the direction of the crane selection.  In most cases, as the capacity of the crane increases, so does the ground bearing pressure induced. There are some options such as wider tracks or possibly selecting a crane with a different track arrangement.  Another option that can help to reduce the loading is to add a sled or wheeled counterweight to the back of the crane. These backend counterweights add significant capacity to the crane, but they also help in keep the crane from leaning forward and therefore decreasing the ground bearing pressure of the crane.  This will have to be analyzed against the loading induced by the wheel counterweight or sled which may put down a greater ground bearing pressure than some of the lower capacity crane models.

Bearing pressures induced by crawler cranes can be lessened to an extent by distributing them with crane mats.  Crane mats are usually hardwood timber mats, but can be made out of steel, composites, and other materials. The mats can be stacked in layers to increase distribution of the loads.  For stack of more than two, each layer should be laid perpendicular to the layer above or below it to help distribute load. Crane mats are usually placed on a layer of compacted fill, usually crushed rock, to create a level pad to place the crane mats on and to also serve as a way to continue to distribute the crane loading to the existing grade.  The loading imposed by the crane when making a lift is usually a triangular distributed loading due to the crane leaning towards the direction of the boom.

There are many suggestions as to how to distribute the load through the layers of crane mats and compacted fill.  

They range from using finite element analysis to analyze it as a beam on elastic foundation to using assumptions to make the triangular distributed loading as a rectangular distributive loading.  To complicate things even further, putting another layer of crane matting on top of the first layer adds another set of distributions to manage.

The issue with the finite element analysis is that most construction companies do not have the capabilities of running such an analysis, nor do they usually have enough information or the capability to determine spring constants of the existing soils below the crane to complete the analysis.  The issue with assuming that the triangular loading is a distributive load is simple in that it is a major assumption. It is also usually assumed then that the loading from the crane is uniformly reacted against the loading from the crane pad, which again is a major assumption. David Duerr released a book titled “Mobile Crane Support Handbook” in 2015 which gives a few approaches as to how to handle the distribution of crane loads and compares the existing methods currently being used.  This is a must-read book for anyone do deals with mobile cranes and their setup.

Another issue that arises each time is what the allowable ground bearing pressures is for the site or that particular area where the crane will be located.  It should be understood that the load from the crane is a transient load and not a permanent load. Therefore, using calculations that are reserved for permanent building foundations is very conservative.  It is ok for the crane matting to settle a little bit over time because the matting can be reset if need be. It is a bigger concern if there is to be sudden or large settlements over a short period of time.  Another question will be how long will the crane be in that area.

We want to make sure you have a successful lift and that starts with proper planning and selecting the correct crane and setup for the job.  We can assist in this process by helping to create a lift plan, matting plan, and help size the correct crane for the job. Here at The Dothea Group, we will do our best to meet your schedule needs and we will provide efficient, engineered solutions to better your project, improve safety, and increase your bottom line.

If you are interested in learning more about our engineering services, contact us today.  We can partner with you to ensure a successful bid or project.