Load Charts, Capacity Calculations, and Load Moment Indicators

Introduction

This chapter covers one of the most critical topics on the Red Seal exam for tower crane operators: reading load charts, performing capacity calculations, and using load moment indicators (LMIs). Mastering these skills isn't just an exam requirement — it's a matter of life-and-death safety on the jobsite. Overloading is a leading cause of many tower crane collapses. You need to understand not only how to read a chart, but also why each factor affects capacity.

2.1 Fundamentals of Lifting Capacity

2.1.1 Key Definitions

The rated capacity (or nominal load) is the maximum load a crane can lift under specific conditions, as defined by the manufacturer. This value is determined through testing and engineering calculations, then published in the operator's manual and on the load chart.

The net load is the weight of the load itself, excluding rigging. The gross load includes the net load plus all lifting accessories: hook, block, wire rope, sheaves, slings, spreader bars, buckets, and so on.

The safety margin is the difference between the rated capacity and the gross load. It must always be positive.

> Essential formula: Gross load = Net load + Weight of rigging. The gross load must never exceed the rated capacity shown on the load chart.

2.1.2 Load Moment

The load moment is the product of the load and the horizontal distance between the crane's centre of rotation and the load's centre of gravity (the radius). It is expressed in kilogram-metres (kg·m) or tonne-metres (t·m).

Load moment = Load × Radius

This concept is fundamental because a tower crane is a counterweighted machine. The overturning moment created by the load must be counterbalanced by the stabilizing moment of the counterweight and the crane's own mass. If the load moment exceeds the stabilizing moment, the crane tips over.

2.1.3 Stability Factors

The stability of a tower crane depends on several factors:

The counterweight mass and its distance from the centre of rotation

The jib mass and its distribution

The tower section mass and the base

The radius (horizontal distance of the load)

Wind conditions (wind pressure on the jib and the load)

The crane's inclination (base levelness)

The Canadian Electrical Code doesn't apply here; it's the CSA Z248 standard — Code for Tower Cranes — that governs the safe use of tower cranes in Canada. This standard requires that stability be verified for a specified wind load.

2.2 Reading the Load Chart

2.2.1 Structure of a Typical Load Chart

A tower crane's load chart is typically presented as a table or graph. It shows the maximum capacity at various radii. Here's a simplified example for a 100 t·m tower crane:

Radius (m)	Capacity (t)	Moment (t·m)
10	10.0	100
15	6.7	100
20	5.0	100
25	4.0	100
30	3.3	100
35	2.9	100
40	2.5	100
45	2.2	100
50	2.0	100

Notice that in this example, the load moment is constant (100 t·m). This is typical of lattice-jib tower cranes: capacity decreases proportionally as the radius increases, up to the crane's maximum capacity.

2.2.2 Chart Zones

The load chart typically has several zones:

33.Maximum capacity zone: between the minimum radius and the radius where capacity reaches its structural maximum. In this zone, capacity is limited by the structural strength of the jib, not by moment.

34.Constant moment zone: where capacity decreases along the moment curve (load × radius = constant).

35.Maximum radius zone: at the tip of the jib, where capacity is at its minimum.

2.2.3 Multiple Configurations

A tower crane can have several configurations: variable jib length, variable hook height, with or without a luffing jib (auxiliary jib). Each configuration has its own load chart. You must identify the crane's current configuration and use the corresponding chart.

Golden rule: Never interpolate between two different configurations. Always use the exact chart for the configuration in place.

2.3 Capacity Calculations

2.3.1 Calculating Gross Load

Gross load is calculated as follows:

Gross load = Weight of load + Weight of hook + Weight of block + Weight of wire rope (vertical portion) + Weight of slings and accessories

Example: You need to lift a steel beam weighing 4.5 t. The hook weighs 350 kg, the block weighs 180 kg, the slings weigh 120 kg, and the vertical wire rope weighs 40 kg.

Gross load = 4,500 + 350 + 180 + 120 + 40 = 5,190 kg = 5.19 t

If the radius is 25 m, the rated capacity according to the table above is 4.0 t. The gross load of 5.19 t exceeds the capacity — this lift is prohibited.

2.3.2 Calculating Effective Radius

The radius isn't simply the horizontal distance measured on the ground. You must account for:

The position of the trolley on the jib (for saddle-jib cranes)

The jib angle (for luffing-jib cranes)

The horizontal displacement of the hook due to wind

The distance between the centre of rotation and the load's centre of gravity

For a luffing jib: Radius = Jib length × cos(jib angle)

Example: 40 m jib at a 60° angle.

Radius = 40 × cos(60°) = 40 × 0.5 = 20 m

2.3.3 Capacity Reduction Factors

Several factors can reduce rated capacity:

Factor	Effect on Capacity
Wind above 20 km/h	Reduction per manufacturer's table
Extreme temperature (beyond -20 °C or +40 °C)	Reduction in material strength
Wire rope wear	Reduction in breaking strength
Base misalignment	Reduction in stability
Proximity to power lines	Minimum distance required, no capacity reduction but restricted zone

2.3.4 Calculating Percentage of Utilization

The percentage of utilization is the ratio of gross load to rated capacity, expressed as a percentage:

% utilization = (Gross load ÷ Rated capacity) × 100

Example: Gross load of 3.2 t at a radius where capacity is 4.0 t.

% utilization = (3.2 ÷ 4.0) × 100 = 80%

A utilization percentage above 100% indicates an overload. CSA Z248 requires that the operator never exceed 100% of rated capacity.

2.4 Load Moment Indicators (LMIs)

2.4.1 Definition and Function

The load moment indicator (LMI) is an electronic or mechanical device that continuously monitors the crane's load moment and alerts the operator when capacity is approaching or exceeding limits. It is mandatory on tower cranes in Canada under CSA Z248-17, Clause 8.4.2.

The LMI measures:

The load on the hook (via load cells or tension sensors)

The radius (via encoders on the trolley or the jib angle)

The calculated load moment (product of the two)

The jib angle (for luffing jibs)

The deployed jib length

2.4.2 LMI Components

A typical LMI includes:

77.Load sensors: installed on the block or hook, they measure tension in the wire rope.

78.Radius sensors: rotary encoders or ultrasonic sensors that determine the trolley position.

79.Angle sensor: inclinometer for luffing jibs.

80.Processing unit: calculates the moment and compares it to programmed limits.

81.Display: shows load, radius, moment, and percentage of utilization.

82.Alarms: audible and visual (yellow and red indicators).

83.Cut-out device: in some configurations, the LMI can stop dangerous movements.

2.4.3 How the LMI Works

The LMI continuously compares the actual load moment to the maximum allowable load moment for the current configuration. It performs this calculation at every instant, because the radius changes during lifting.

Progressive alarms:

Level	Percentage of Maximum Moment	Action Required
Pre-alarm	90%	Yellow indicator, intermittent alarm
Alarm	100%	Red indicator, continuous alarm
Cut-out	105–110%	Stop of hoisting and slewing movements

2.4.4 LMI Limitations

The LMI is an assistive tool, not a substitute for the operator's judgment. It can give inaccurate readings if:

The sensors are poorly calibrated

The wire rope is worn or damaged

The load is swinging (dynamic movement)

Temperature affects the sensors

The power supply is unstable

Regulatory requirement: The LMI must be checked and calibrated at the start of each shift, per CSA Z248-17, Clause 8.4.3. A test lift with a known load must be performed to confirm display accuracy.

2.4.5 LMI Verification Procedure

97.Visually check the condition of sensors and cables.

98.Perform a test lift with a known load (e.g., a calibrated counterweight).

99.Compare the LMI reading to the actual weight of the load.

100.If the discrepancy exceeds ±5%, stop the crane and have the LMI calibrated.

101.Document the verification in the crane's logbook.

2.5 Effects of Wind on Capacity

2.5.1 Wind Pressure

Wind exerts a horizontal force on the jib, the load, and the wire rope. This force increases the overturning moment and reduces effective capacity. Wind pressure is proportional to the square of wind speed:

Wind pressure (Pa) = 0.613 × V² (where V is in m/s)

Example: Wind of 72 km/h = 20 m/s.

Pressure = 0.613 × 20² = 0.613 × 400 = 245 Pa

2.5.2 Wind Limits for Tower Cranes

CSA Z248-17 specifies wind limits for different operations:

Condition	Maximum Wind Speed
Normal lifting	72 km/h (20 m/s)
Lifting large surfaces (panels, formwork)	45 km/h (12.5 m/s)
Crane in service with load	Per manufacturer, often 72 km/h
Crane out of service (weathervane)	100–130 km/h per manufacturer

Important: Wind affects the load itself. A flat, wide load (like a formwork panel) experiences much greater wind force than a compact load of the same weight. You must reduce the rated load accordingly.

2.5.3 Calculating Wind Reduction

The manufacturer typically provides a capacity reduction table based on wind speed and the exposed surface area of the load. In the absence of a table, a practical rule is to reduce capacity by 10% for each 10 km/h increase above 30 km/h.

2.6 Complete Calculation Examples

2.6.1 Example 1: Simple Lift

Situation: 100 t·m tower crane, 50 m jib. You need to lift a concrete bucket weighing 2.8 t (empty bucket) containing 3.5 m³ of concrete (density 2,400 kg/m³). The radius is 30 m. The hook weighs 300 kg, the block weighs 150 kg, and the slings weigh 80 kg.

Step 1: Calculate the weight of the concrete.

Weight of concrete = 3.5 m³ × 2,400 kg/m³ = 8,400 kg = 8.4 t

Step 2: Calculate the net load.

Net load = Weight of bucket + Weight of concrete = 2.8 + 8.4 = 11.2 t

Step 3: Calculate the gross load.

Gross load = 11.2 + 0.3 + 0.15 + 0.08 = 11.73 t

Step 4: Check capacity at 30 m.

Per the chart, capacity at 30 m = 3.3 t.

Conclusion: The gross load of 11.73 t far exceeds the capacity of 3.3 t. Lift not possible. You must reduce the amount of concrete or bring the load closer.

2.6.2 Example 2: Moment Verification

Situation: Same crane. Gross load of 3.0 t at a radius of 32 m.

Moment calculation:

Moment = 3.0 t × 32 m = 96 t·m

Verification: The crane has a maximum moment of 100 t·m. The moment of 96 t·m represents 96% of capacity. The LMI will trigger the pre-alarm at 90% (90 t·m) and the alarm at 100% (100 t·m). At 96%, you must be cautious, but the lift is permitted.

2.6.3 Example 3: Wind Effect

Situation: Same crane. Gross load of 2.8 t at a 40 m radius. Wind of 50 km/h. The load is a formwork panel with 12 m² of exposed surface area.

Capacity at 40 m: 2.5 t (per the chart).

Wind reduction: The manufacturer specifies a 15% reduction for a 50 km/h wind with a 12 m² load surface area.

Adjusted capacity = 2.5 t × (1 - 0.15) = 2.5 × 0.85 = 2.125 t

Conclusion: The load of 2.8 t exceeds the adjusted capacity of 2.125 t. Lift prohibited.

2.7 CSA Z248-17 Rules

CSA Z248-17 — Code for Tower Cranes is the national reference in Canada. Key clauses for this chapter:

Clause 8.4.1: Every tower crane must be equipped with a functioning load moment indicator.

Clause 8.4.2: The LMI must provide a visual and audible alarm when the load reaches 90% of rated capacity.

Clause 8.4.3: The LMI must be checked at the start of each shift and calibrated at least once a month.

Clause 8.4.4: The operator must never disable, bypass, or ignore the LMI.

Clause 9.2.1: The operator must know and understand the crane's load chart before starting work.

Clause 9.2.3: The operator must not lift a load whose weight is unknown.

Clause 10.1.1: Wind speed must be monitored continuously during operation.

2.8 Pitfalls to Avoid

151.Forgetting the weight of rigging: The hook, block, and slings often weigh several hundred kilograms. Always include them in the gross load.

152.Confusing net load and gross load: Net load is the weight of the load alone. Gross load includes everything suspended from the hook.

153.Interpolating between two radii: The load chart gives discrete values. If the radius falls between two values, use the lower capacity (the one for the greater radius).

154.Ignoring wind: Wind reduces capacity, especially for large-surface loads. Always check weather conditions before starting.

155.Relying solely on the LMI: The LMI can be miscalibrated. Always do your own verification calculations.

156.Using the wrong chart: Verify that the chart matches the crane's exact configuration (jib length, height, luffing jib).

157.Forgetting the weight of the wire rope: The vertical wire rope weighs several kilograms per metre. At great heights, this can represent a significant load.

158.Not accounting for dynamic effects: A load that swings or is lifted quickly creates dynamic forces greater than the static weight. Lift slowly and avoid sudden movements.

2.9 Exam Tips

Memorize the moment formula: Moment = Load × Radius. It's the basis of all calculations.

Practice conversions: kg to t (divide by 1,000), m³ to kg (multiply by density).

Know the LMI thresholds: 90% pre-alarm, 100% alarm, 105–110% cut-out.

Remember the wind limits: 72 km/h for normal lifting, 45 km/h for large surfaces.

Read questions carefully: The Red Seal exam often uses traps in the wording. For example, "net load" vs. "gross load," "radius" vs. "height."

Check your units: A unit error (kg vs. t) is a common mistake. Always convert before calculating.

Summary

Rated capacity is the maximum load allowed by the manufacturer for a given configuration and radius.

Gross load = net load + weight of all accessories suspended from the hook.

Load moment = load × radius. It must never exceed the crane's maximum moment.

The load chart is specific to each crane configuration. Always use the correct chart.

The LMI monitors the moment continuously and alerts you at 90% and 100% of capacity.

Wind reduces capacity, especially for large-surface loads. Wind pressure is proportional to the square of wind speed.

CSA Z248-17 is the regulatory reference in Canada for tower cranes.

You must perform your own verification calculations, even with a functioning LMI.

Pitfalls to Avoid (Exam Recap)

Pitfall	Consequence	Prevention
Forgetting rigging in gross load	Underestimating the load	Always add hook + block + slings
Using capacity at the lower radius	Potential overload	Use capacity at the greater radius (conservative)
Ignoring wind reduction	Possible tip-over	Check the manufacturer's reduction table
Confusing moment and load	Erroneous calculations	Remember: moment = load × radius
Neglecting LMI verification	False reading	Check at the start of each shift
Interpolating between configurations	Incorrect capacity	Use the exact chart for the configuration

This chapter prepares you for Red Seal exam questions on load charts, capacity calculations, and LMIs. Review the examples, memorize the formulas, and practice the calculations until they become second nature. Safety on the jobsite depends on your mastery of these concepts.

Load Charts, Capacity Calculations, and Load Moment Indicators