88 RIGGING, HOISTING, and SIGNALING PRACTICES
Bridle Hitch Considerations
SYNTHETIC WEB SLINGS
BRIDLE HITCH MADE FROM TWO BASKET HITCHES LOW SLING ANGLE
calculations must be made by a qualified person to determine whether these forces can be sustained by the load. Alternatively, a lifting beam can usually be used in order to avoid compressive forces on the load. Based on the sling angle and the portion of the load weight supported, the actual tension on the sling can be calculated. There are a number of methods for doing this.
Sling Loads
SLINGS TEND TO SLIDE TOGETHER
SLING LOAD (EQUAL TO LOAD PORTION)
LOAD BEARS ON SLING EDGES
½W
½W
Figure 6-17. When using a pair of basket hitches as a bridle hitch, they may easily slide together under load, possibly causing load instability. Also, if synthetic web slings are used, the load may bear disproportionately on one edge, causing sling damage. ½W
½W
SLING LOADS When selecting slings for a particular rigging application, the primary consideration is their rated loads. Each sling must be rated to withstand, with the appropriate safety factor, the maximum force that it will be subjected to in the proposed rigging configuration. Determining this sling loading involves two major factors: the portion of load weight it must support and the sling angle. A vertical sling experiences only the force of the portion of load weight it is supporting. See Figure 6-18. However, a sling loaded at an angle also experiences horizontal forces. The horizontal force is proportional to the sling angle; a smaller angle induces greater force. The vertical and horizontal forces combine to result in a total load on the sling that can be much greater than its portion of the load weight. The sling angle is used to adjust sling rated loads for nonvertical hitch configurations. The sling angle is the acute angle between horizontal and the sling leg. This angle is also referred to as the angle of loading or horizontal angle. The minimum recommended sling angle is 30°. Even if a sling had the rated load to withstand the extreme horizontal forces at angles less than 30°, the load or the lift points may still be damaged. If a sling angle of less than 30° is unavoidable, additional
W
SLING ANGLE
SLING LOAD (GREATER THAN LOAD PORTION)
>½W
HORIZONTAL FORCES
>½W
½W
½W
½W
½W
W
Figure 6-18. Vertical hitches experience tension equal to the weight they support. Slings used at an angle, however, experience tension greater than the load weight, due to the additional horizontal forces.