A one-inch, 3-leg bridle wire rope sling, rigged at a 45° angle, is rated for 18 tons. The addition of a fourth leg will increase your capacity by:

When rigging a wire rope sling in a bridle configuration, the load capacity is affected by the angle at which the sling is rigged and the number of legs supporting the load. In this case, when a 3-leg bridle sling is rated for 18 tons at a 45° angle, adding a fourth leg does not increase the overall load capacity of the configuration when the angle remains constant.

For a bridle sling rigged at 45°, every leg supports an equal share of the load. The load capacity for each individual leg is dictated by the weakest link or the overall rating of the configuration. Since the initial rating of 18 tons already accounts for the load distribution across three legs, introducing a fourth leg at the same angle does not fundamentally alter the load capabilities of the system due to the existing limits imposed by rigging standards. Thus, the overall capacity remains the same; no additional capacity is gained by simply adding a leg without changing other parameters like the angle or using slings with a higher rating.

Therefore, the addition of a fourth leg in this scenario results in no increase in total capacity, leading to the correct conclusion that the increase in capacity when rigging this way is zero.