OHS Code Explanation Guide

Published Date: July 01, 2009
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Part 6 Cranes, Hoists and Lifting Devices

Section 92.2 Outriggers

Proper outrigger support is an important part of crane setup. Crane collapse can result from

(a) failure to fully extend the outriggers,
(b) not extending all outriggers,
(c) failure to get completely “off rubber”,
(d) not accounting for poor ground conditions, or
(e) failure to level the crane.

Outriggers improve crane stability. Accurate use of the “on-outriggers fully extended” load chart requires that outriggers be fully extended and raise the crane completely off the tires. If the tires are touching the ground, then the “on rubber” load chart is used. Manufacturers do not recommend extending only one or two outriggers. The load charts of some manufacturers now permit partially-extended outriggers, so use of the correct load chart is critical.

All crane outriggers are designed for good ground conditions. Poor ground conditions reduce the amount of load a crane can safely place on an outrigger. Because of this, many crane operations require additional supports or “floats”. These floats may be of wood, steel or synthetic material but must be of sufficient size, strength and rigidity to transfer the outrigger load to the full area of the float. A float that is smaller than the outrigger pad will increase the pressure on the ground and will cause an outrigger to “punch through”. If the outrigger is set on an unlevel float, the pad may slide causing the crane to tip. Many manufacturers stipulate that the crane must be within 1 percent of level before their load chart applies. In a 20-foot span this is only 2 inches off-level. Past that, the crane can lose 15 to 20 percent or more of its capacity.

When sizing outrigger floats, it is important to know the maximum pressure applied by the outrigger as well as the required float area. The area (square feet) of float can be estimated by dividing the maximum capacity of the crane by five (5). [Construction Safety Association of Ontario]

A reasonable approximation for maximum ground pressure (worst case) applied by the outrigger is:

Pressure (tons per ft2) applied by outrigger =
0.85 x total crane mass + maximum crane capacity
individual outrigger area

[Construction Safety Association of Ontario]

This can be then be used to calculate the required thickness of float.