Benefits of triangular rigging tools
04 September 2019
In rigging applications, it’s often the triangle shape that is the most useful. Chad Fox reports exclusively for ACT.
Historically, the circle is the shape that seems to get all the attention. After all, it gave rise to technologies in transportation, agriculture and industry – and it certainly exists in construction tools of the trade.
In rigging applications, however, it’s the triangle that has a viable claim as the most useful shape. In the purest description, the triangle provides a medium for load equalization for multiple slings, offering a myriad of different uses. Design, construction and load testing of these triangular tools is a relatively inexpensive, simple endeavor, whether the user is ordering from a supplier or creating one from scratch.
Some of the triangle’s most useful applications include:
Load Handoffs: This application involves transferring loads between two crane hooks, whether mobile to mobile, or mobile to monorail/overhead. This may be especially useful when installing or removing equipment from a structure with a permanent or temporary monorail system that extends outside of the boundaries of the structure, or applications where setting down of the lifted object is not feasible prior to transfer.
Rigging system with shifting center of gravity: In this case a triangle allows for use of systems such as cantilever beams with an adjustable line where the center of gravity is shifted throughout the lifting process.
Rotation of a lifted object: This application allows a lifted object to be rotated by attaching two lines to the triangle plate, one of which lengthens or shortens to rotate the object. This may be of particular value during uprighting and/or tilting procedures from a single hook.
Short cantilever reaches: In this case the triangle allows for the use of a counterweight to access areas where a hook cannot reach directly overhead due to access limitations.
Third leg alignment adjustment: This application allows attachment of an adjustable third line to allow for rotational variation of the lifted object. The triangle can be helpful to achieve high accuracy alignment for certain installation or erection procedures.
Cascading system of slings: A triangle can be used to cascade rigging systems with multiple slings and avoid over-crowding in a hook. This can help to prevent sling overlap or bunching. Through minor rotation of the triangles this cascading system also allows for engagement of numerous slings to a lifted object, thus allowing for better load distribution throughout the system.
Load equalization between two hooks: In instances where multiple hooks must be utilized due to capacity limitations of the lifting equipment selected, the triangle provides an efficient way to distribute the lifted loads relatively equally to the two hooks, thus helping to avoid unintended overload caused by differences in relative hook elevations. This application may also allow for greater horizontal clearances between crane hooks and/or booms by spacing out hook attachment points.
When considering design and construction of these tools, The American Society of Mechanical Engineers (ASME) and the American Institute of Steel Construction (AISC) provide three standards that serve as invaluable resources.
- ASME B30.20: Below-the-Hook Lifting Devices:
This reference includes provisions that apply to the marking, construction, installation, inspection, testing, maintenance, and operation of Below-the-Hook lifting devices. The standard notes that load testing requirements should be at 125 percent of the rated load, and that the user should attempt to load test in actual intended use positions.
Load rotation allows a lifted object to be rotated by attaching two lines to the triangle plate, one of which lengthens or shortens to rotate the object.
- ASME BTH-1: Design of Below the Hook Lifting Devices:
This document sets forth design criteria for ASME B30.20 and serves as a guide to designers, manufacturers, purchasers and users of Below-the-Hook (BTH) devices. When designing lifting triangles according to BTH-1 the user must determine the appropriate Design Category as this will provide minimum required safety factors. Readers should pay special attention to the Pin Plate design in section 3-3.3, as most design checks are included in this portion of the document. Readers should focus on all the possible rotation positions to determine limiting design variables that are described in this section.
- AISC 360-16 Specification, Chapter J:
This standard describes the design of connections and serves as a solid secondary reference with design information regarding welds, bolts, pins and local checks on plate material.
Lifting triangles can provide efficient solutions to many rigging challenges, including scenarios requiring load handoffs, load rotation, reach of inaccessible loads or effective force distribution between slings. Their relative simplicity in design based on the provisions of ASME BTH-1, and inexpensive construction, marking and testing per requirements of ASME B30.20, make them an economical and valuable tool for any size project or budget.
Chad Fox, PE, SE is a project manager with Ruby+Associates Structural Engineers in Bingham Farms, MI.
He leads the firm’s Grand Rapids, Michigan office and its Heavy Lift service sector. Fox has over 15 years of experience in specialty steel design and fabrication, load testing, lifting and rigging equipment utilization, heavy lift engineering and field oversight.
In addition to his almost 10 years with Ruby, he also spent 7 years as the chief engineer with a crane, transport and specialty rigging contractor.
Fox can be reached at [email protected] or 616-206-4288