Most outrigger mat materials perform well under compression load but to perform as intended the tensile strength of an outrigger mat is also critical. The suitability of the outrigger mat used is determined by:
• The size of the mat being suitable to distribute the load to the ground at a stress level less than the ground bearing capacity
• The strength and integrity of the mat and its ability to evenly distribute the load exerted by the outrigger
We’ve recently carried out a simple point load test utilising Finite Element Analysis software on several mat modules capable of being manually handled, to compare the stiffness and load bearing capability of the various materials.
Here’s what we tested:
| Material | Beam Span | Point Load | Max Deflection | ||
|---|---|---|---|---|---|
| ALIMATS® (Aluminium) | 1.5m | 10T | 24mm | ||
| Single Sleepers (Timber) D40 Oak | 1.5m | 10T | 88mm | ||
| Ultra-High-Molecular-Weight Polyethylene (Plastic) UHMW-PE | 1.5m | 10T | 972mm |
The tests showed the amount of deflection and stress build up within each mat material, with startlingly different results when it came to deflection. Deflection or ability to adjust to the ground has both benefits and disadvantages so selecting the right type of material for the job is critical.
It’s clear from our research that some manufacturers and distributors of outrigger mats are promoting the compressive strength of the material, even though all outrigger mat materials have high compressive strength. A far better question to ask is: ‘are they capable of safely spreading the outrigger load horizontally before distribution to the ground? Which is related to the stiffness of the material.
We don’t understand how we’re in an industry where in many cases an outrigger mat area is calculated above all else, while their make-up is largely ignored – especially when you consider different materials perform very differently. This doesn’t happen within other design procedures, for example, a beam size is designed in tandem with the material.
We believe that going forward outrigger mat manufacturers and distributors, including ourselves, declare a Safe Work Load (SWL) for each configuration. We have already done this on a handful of our configurations and plan on completing this project next year across our range. In our opinion, the only way to do this is via Finite Element Analysis, as physical testing would not be viable on full outrigger configurations. But for this process to be credible, it has to be visible to designers, therefore transparency of design assumptions are essential. This point is extremely pertinent to the UHMW-PE system, as it promotes a higher SWL as the mat area increases, which defies Physics.
By adopting the SWL approach, when the outrigger load is calculated along with the outrigger mat area, selecting a suitable outrigger system would thereafter be seamless, with available Safe Working Loads from outrigger mat manufacturers.