Load spread angle
Also called Dispersion angle, Load dispersal angle, 45 degree rule
The angle at which a load is assumed to disperse outwards as it passes down through a mat, pad or layer of engineered fill, used to work out the area over which it finally bears on the ground. A 45° dispersion is the assumption most commonly seen in UK lift planning, but it is an assumption the temporary works designer makes and owns — it is not a property of the mat.
In practice
The assumption earns its keep in proportion to the depth it acts through, because what it adds is that depth to each side of the loaded patch. Through a metre of engineered platform it transforms the bearing area; through a mat a few tens of millimetres deep it adds very little, and the area that matters is the plan size of the mat itself. It also presumes the spreading element is stiff enough to carry load out to its edges — a panel that deflects under the foot delivers much of the load close to the foot whatever angle the calculation used.
Where this comes from: Common UK temporary works and lift planning convention. The angle that may be assumed on a particular lift is the temporary works designer’s decision; we do not publish an angle for our mats, because it is not something a mat has.
Where it matters on site
When two suppliers size different mats from the same outrigger reaction, the assumed angle is usually where they parted company, so the useful question is what each assumed rather than which answer is bigger. Whatever figure is used belongs in the temporary works design and the lift plan in writing, together with the thickness it was applied through, because the two only mean anything as a pair. Where load passes through more than one layer — a pad on a mat on a platform — each interface is a decision, not something to compound by habit.