How to use the crane outrigger reaction calculator
Many crane tipping incidents start when the ground under one outrigger gives way. This calculator treats the four outriggers as a rectangular support pattern and applies moment equilibrium to find the peak reaction on a single leg for a given boom position, along with the resulting pressure under the pad.
The relation used is R = (Wc+WL)/4 + WL·R·sinθ/(2B) + WL·R·cosθ/(2L). A swing angle of 0 means over the front and 90 means over the side. When the side spread B is narrower than the front-to-rear spread L, side lifts drive the reaction up sharply. Include rigging and hook block weight in the lifted load, not just the load itself.
Contact pressure is the peak reaction divided by the pad area. Backfilled trenches, ground over utilities or vaults, and soil softened by rain all reduce allowable bearing pressure, so enlarge the cribbing or move the setup rather than accepting the number. If the contact pressure exceeds the allowable pressure, the lift must not proceed.
This is a simplified static estimate that places the machine weight at the center of the outrigger rectangle. The manufacturer outrigger reaction chart and load chart always govern, and a qualified person must review the lift plan before work starts.
Frequently asked questions
The worst case is when the boom points over a corner or over the narrow side of the outrigger rectangle, because the load concentrates on one leg. Side lifts are often more critical than over-the-front lifts because the side spread is usually the smaller dimension.
Size the cribbing so the computed contact pressure stays within the allowable bearing capacity of the supporting ground. Backfill, ground over buried utilities or vaults, and soil near storm lines can be far weaker, so confirm capacity with site data or a geotechnical engineer.
No. This is a simplified static estimate that assumes the machine weight acts at the center of the outrigger rectangle. The manufacturer outrigger load chart, which accounts for upperworks eccentricity and boom weight distribution, always governs, and a qualified person must approve the lift plan.