Outrigger pads, also called floats, of most cranes are small relative tothe loads they support. If the soil under a float yields, the float pressesinto the earth. The resulting abrupt ground settlement might cause thecrane to tip. For that reason, most floats have turned-up edges. Thesuddenly expanded bearing area engendered by the flared edges mayinhibit further settlement. Notwithstanding this protective feature,outriggers should not be supported directly on soil. Cribbing ought tobe placed under the outriggers to reduce the ground bearing pressure.Large cranes are widely understood to have substantial outriggerloads that need cribbing. Not unusually, plates or timbers for thispurpose are sent out with the crane on a truck carrying counter-weights or other accessories. But a small rough-terrain crane is notsent out with an accessory truck that can carry cribbing to the site.Though relatively light in weight, a rough-terrain crane usually hassmall floats that will develop very high unit bearing pressure. Theneed for sound cribbing is no less than for the large machine. Withfrequent repositioning, sometimes a few times a day, there is a temp-tation to skimp. Oftentimes the field crew is left to improvise by using scraps picked up around the site.
Though ground bearing capacity varies from place to place,standard cribbing can be developed for typical conditions. A reliablesolution, practicable for smaller cranes moving about a site, is tofabricate steel spreaders such as shown in Figure 5.21. These tem-porarily attach to the outrigger floats so that they can be carried by the crane from one working position to another.