Rigging and lifting review

Rigging, hoisting and positioning is major work activity B, and at 30 percent of the exam it carries about 36 questions, more than any area except repair work. Dale takes the rigging night himself, with Wade Kozak, the lift director from the Northgate coker drum lift in BLR-120, as a guest. "Rigging questions are mostly arithmetic and judgement," Wade says. "The arithmetic is three or four formulas. The judgement is knowing when the numbers say stop." This lesson reviews load weight and centre of gravity, slings and hitches, sling angles and leg tensions, hardware, crane capacity and the critical lift, ground bearing, tandem lifts and signals.

Load weight and centre of gravity

Every lift starts with the weight. For steel, the weight is the volume times the density, 7,850 kg per cubic metre (490 pounds per cubic foot), and a plate's weight per unit area is a number worth remembering: 7.85 kg per square metre for every millimetre of thickness (about 40.8 pounds per square foot for every inch of thickness). Vessels and boiler components are built up from their parts: shell, heads, nozzles, internals, trays, insulation, refractory and anything left inside, such as water from a hydrotest or deposits. The manufacturer's data report or drawing gives an empty weight, but a vessel coming out of service can weigh much more than its nameplate. BLR-120 lesson 2 works through a full weight build-up.

The centre of gravity is the point where the weight acts. A load lifted from a single hook point settles with its centre of gravity directly below the hook, and a load whose pick points are not balanced about its centre of gravity tilts until it is. For a load built up from parts, the centre of gravity is found by taking moments: each part's weight times its distance from a reference line, added up and divided by the total weight. Exam questions often give a load with two parts and ask where its centre of gravity is, or give the centre of gravity and ask which leg of a bridle carries more load. The leg nearer the centre of gravity always carries more.

Slings, hitches and angles

A sling's rated capacity, or working load limit, is marked on its tag for each type of hitch. In a vertical hitch the sling takes the load straight. In a choker hitch the sling is wrapped round the load and passed through its own eye, and the bend at the choke reduces its capacity, typically to about three-quarters of the vertical rating for a choke angle of 120 degrees or more and less for a sharper choke. In a basket hitch the sling passes under the load with both eyes on the hook, and with the legs vertical it carries twice the vertical rating. A sling with no tag, or a tag that cannot be read, is removed from service, because its capacity cannot be known.

When two or more legs are used at an angle, each leg carries more than its share of the weight, because it pulls sideways as well as up. The angle is measured from the horizontal, and the tension in each leg of a symmetric bridle is the share of the load divided by the sine of that angle. At 60 degrees the factor is 1.155, at 45 degrees 1.414, and at 30 degrees 2.0, which means each leg carries the whole weight of the load. Below 30 degrees the factor climbs very fast, and most rigging practice does not allow sling angles below 30 degrees. This is the single most tested calculation in rigging, and the most common mistake is measuring the angle from the vertical.

Here T is the tension in each leg, W the load, n the number of legs carrying it, and θ the sling angle from the horizontal. When the legs are unequal, as with an off-centre load, the vertical share of each leg is found by moments about the centre of gravity, and each leg's tension is its vertical share times its length divided by the hook height above the pick points. The code below does both, then checks the gross load against the crane chart.

import math
# Rigging review: sling tension on a two-leg bridle and the crane's percent of capacity
LOAD = 18000          # load weight
U = "kg"
for angle in (90, 60, 45, 30):          # sling angle measured from the horizontal
    t = LOAD / 2 / math.sin(math.radians(angle))
    print(f"sling angle {angle:2d} deg  factor {1 / math.sin(math.radians(angle)):.3f}  tension per leg {t:7.0f} {U}")

# Unequal legs: the leg nearer the CG takes more of the load
a, b = 2.0, 3.0         # horizontal distances from the CG to each pick point
h = 4.0                 # height of the hook above the pick points
L1, L2 = math.hypot(a, h), math.hypot(b, h)
V1, V2 = LOAD * b / (a + b), LOAD * a / (a + b)
print(f"near leg: vertical {V1:.0f} {U}, tension {V1 * L1 / h:.0f} {U}")
print(f"far leg:  vertical {V2:.0f} {U}, tension {V2 * L2 / h:.0f} {U}")

RIG = 950             # block, slings, shackles and spreader
CAP = 24500             # chart capacity at the planned radius
gross = LOAD + RIG
print(f"gross load {gross:.0f} {U} = {100 * gross / CAP:.1f} % of chart capacity",
      "-> critical lift" if gross / CAP >= 75 / 100 else "")

sling angle 90 deg  factor 1.000  tension per leg    9000 kg
sling angle 60 deg  factor 1.155  tension per leg   10392 kg
sling angle 45 deg  factor 1.414  tension per leg   12728 kg
sling angle 30 deg  factor 2.000  tension per leg   18000 kg
near leg: vertical 10800 kg, tension 12075 kg
far leg:  vertical 7200 kg, tension 9000 kg
gross load 18950 kg = 77.3 % of chart capacity -> critical lift

At 45 degrees each leg of the bridle on the 18,000 kg drum carries 12,728 kg, so slings rated at less than about 12.7 tonnes in that hitch cannot be used even though the load per leg looks like 9 tonnes. The off-centre case shows the leg nearer the centre of gravity carrying 10,800 kg vertically and 12,075 kg along its length. And the gross load, which includes the block, slings, shackles and spreader, is 77 percent of the chart capacity, which makes it a critical lift under Ironbridge's rules.

Hardware and below-the-hook devices

Shackles, hooks, eyebolts, plate clamps and lifting lugs each have their own rules, covered in BLR-120 lesson 3. Shackles are rated for an in-line pull; a side load reduces the capacity, by half at 90 degrees under typical manufacturers' tables, and the pin must be fully screwed in or, for long-term use, a bolt-type shackle used. Eyebolts are rated for a straight pull; shoulder eyebolts can take an angled pull with a reduced rating, and plain-shank eyebolts should not be loaded at an angle at all. Hooks need a working safety latch, and the load must sit in the bowl of the hook, not on the tip. Below-the-hook devices such as spreader beams and lifting beams are marked with their rated load and their own weight, which is part of the rigging weight on the crane. Lifting lugs on vessels are designed by an engineer for a specific load and direction, and using a lug at a different angle from the one it was designed for is a common cause of failures.

Crane capacity, the lift plan and the critical lift

A crane's load chart gives the gross capacity at each radius and boom length, for the configuration stated: counterweight, outriggers fully extended or tracks, and the area over which the load is swung. The gross load includes the load, the hook block, the slings and hardware, the spreader, and any jib or attachment deductions stated on the chart. Radius is measured horizontally from the centre of rotation to the centre of the hook, and it grows when the boom deflects under load, so a lift planned at the edge of the chart can end up beyond it. BLR-120 lesson 4 covers charts in full.

Under the Alberta OHS Code an employer must have a written lift calculation for any lift at 75 percent or more of the crane's rated capacity, and many companies, Ironbridge among them, treat such lifts as critical lifts needing a full critical lift plan. Other triggers for a critical lift include tandem lifts, lifts over operating plant or occupied areas, lifts of personnel, and lifts of unusual or high-value loads. The plan states the load and rigging weights, the crane configuration, radius and capacity at each stage of the lift, the ground preparation, the rigging arrangement, the people and their roles, the signals, and the conditions under which the lift stops, such as wind speed. It is reviewed and signed before the lift, and the crew is briefed on it. BLR-120 lesson 11 covers critical lift planning.

Ground, tandem lifts and signals

The crane is only as stable as the ground under it. Outrigger loads on a large crane can be many times the bearing capacity of the soil, so mats spread the load over a larger area: the ground pressure is the outrigger load divided by the mat's effective area. Underground services, recent excavations and soft fill are the usual hazards. BLR-120 lesson 6 works through ground bearing.

A tandem lift uses two cranes, either to share a load or, when upending a vessel, with a tailing crane holding the bottom while the main crane raises the top. The load on each crane changes through the lift: as a vessel is upended the tailing crane's share falls and the main crane's rises until the main crane carries it all. Each crane must have capacity for its maximum share at the radius it will be at, plus a margin. BLR-120 lesson 7 covers tandem lifts.

Every lift has one designated signaller, who is the only person giving signals to the operator, except that anyone may give the emergency stop. Hand signals follow the standard chart; radios are used where the operator cannot see the signaller, with a dedicated channel and continuous talk during the lift. BLR-120 lesson 10 covers signals.

Traps in rigging questions

Wade's list of traps: measuring the sling angle from the vertical; forgetting the rigging weight in the gross load; using a sling's vertical rating for a choker hitch; dividing the load by four for a four-leg bridle when only two legs can be relied on to carry it, which is common practice because a rigid load rarely sits evenly on four legs; reading the chart at the planned radius instead of the radius after boom deflection; and assuming that the leg farther from the centre of gravity carries more. Each of these appears as a distractor in practice questions, and each has caused real accidents.