Structural Design for Mobile Equipment

Understanding Structural Design for Mobile Equipment

Mobile equipment forms the backbone of numerous industries across Atlantic Canada, from forestry and mining operations in Nova Scotia's resource sector to agricultural machinery working the fertile lands of the Annapolis Valley. The structural design of this equipment presents unique engineering challenges that differ significantly from stationary structures, requiring specialized knowledge and careful consideration of dynamic loads, material selection, and operational environments.

At its core, structural design for mobile equipment involves creating frameworks and supporting structures that can withstand not only static loads but also the complex dynamic forces encountered during operation, transportation, and varying terrain conditions. Engineers must account for factors such as vibration, impact loading, fatigue cycling, and the harsh environmental conditions common throughout the Maritime provinces.

Key Design Considerations for Mobile Equipment Structures

Dynamic Load Analysis

Unlike stationary structures where loads are relatively predictable, mobile equipment experiences constantly changing forces. A piece of heavy machinery operating in a Cape Breton mining operation, for instance, may encounter impact loads up to 3-4 times the static weight when traversing rough terrain. Engineers must analyse these dynamic amplification factors using methods such as:

• Modal analysis to determine natural frequencies and avoid resonance conditions, typically targeting frequencies above 25 Hz for primary structural members

• Transient dynamic analysis for impact events such as sudden stops or terrain obstacles

• Fatigue analysis considering the cumulative damage from millions of load cycles over the equipment's service life

• Random vibration analysis for equipment exposed to continuous irregular excitation

The dynamic load factor (DLF) for mobile equipment typically ranges from 1.5 to 3.0, depending on the application and operating conditions. For equipment designed to operate in the challenging terrain conditions found throughout Nova Scotia's forestry regions, engineers often apply DLFs at the higher end of this spectrum.

Material Selection and Environmental Factors

The Maritime climate presents particular challenges for mobile equipment structures. The combination of salt air, freeze-thaw cycles, and high humidity levels requires careful material selection and corrosion protection strategies. Common material choices include:

• High-strength low-alloy (HSLA) steels such as CSA G40.21 350W or 480W, offering yield strengths of 350-480 MPa while maintaining good weldability

• Weathering steels (CSA G40.21 350A) for exposed components where the protective patina can develop

• Aluminum alloys (6061-T6 or 5083-H116) for weight-critical applications, offering strength-to-weight ratios approximately three times that of steel

• Duplex stainless steels for highly corrosive environments, particularly in coastal operations

Protective coatings meeting CSA standards are essential for equipment operating in Maritime conditions. Hot-dip galvanizing to CSA G164 specifications provides zinc coating thicknesses of 85-100 micrometres for structural steel, while modern epoxy and polyurethane coating systems can achieve service lives exceeding 15-20 years with proper surface preparation.

Finite Element Analysis in Mobile Equipment Design

Modern structural design for mobile equipment relies heavily on finite element analysis (FEA) to optimise designs and verify structural integrity before fabrication. This computational approach allows engineers to simulate complex loading scenarios and identify potential failure points with remarkable accuracy.

Mesh Development and Convergence

Proper mesh development is critical for accurate FEA results. For mobile equipment structures, engineers typically employ:

• Shell elements (4-node quadrilateral) for plate and sheet metal components, with element sizes of 10-25 mm for global models

• Solid elements (10-node tetrahedral or 20-node hexahedral) for complex 3D components such as cast nodes or machined fittings

• Beam elements for preliminary frame analysis and member sizing

• Refined mesh zones at stress concentrations, weld toes, and connection points, often requiring element sizes of 2-5 mm

Convergence studies should demonstrate that stress results change by less than 5% with further mesh refinement. For fatigue-critical locations, sub-modelling techniques allow detailed analysis of local stress distributions while maintaining computational efficiency.

Load Case Development

A comprehensive FEA study for mobile equipment typically includes 15-30 load cases representing various operating conditions. These commonly include:

• Maximum operational loads at rated capacity

• Dynamic impact scenarios (2.0-3.0× static loads)

• Transportation loads including tie-down reactions

• Lifting and handling configurations

• Thermal expansion effects (-40°C to +40°C for Canadian operations)

• Emergency braking and cornering loads

Connection Design and Welding Requirements

Connections represent the most critical elements in mobile equipment structures. Statistical analysis of equipment failures consistently shows that 70-80% of structural failures originate at connections, making their design paramount to overall structural integrity.

Welded Connections

Welding for mobile equipment structures in Canada must comply with CSA W47.1 for steel and CSA W47.2 for aluminum. Key considerations include:

• Weld category selection based on fatigue loading—Category C details (ground flush welds) can provide fatigue strengths 2-3 times higher than Category E (as-welded fillet welds)

• Preheat requirements particularly critical for equipment fabricated during Nova Scotia's cold winters, with minimum temperatures of 10-150°C depending on material thickness and carbon equivalent

• Post-weld heat treatment for components exceeding 50 mm thickness or where residual stress relief is required

• Inspection requirements including visual, magnetic particle, ultrasonic, and radiographic testing as specified by CSA W59

For fatigue-critical applications, specifying weld toe grinding or TIG dressing can improve fatigue performance by 30-50%. Full penetration welds should be specified for primary structural connections, with partial penetration welds limited to secondary members and non-critical attachments.

Bolted Connections

High-strength bolted connections using ASTM A325 or A490 bolts remain common in mobile equipment for field-assembled components and maintenance accessibility. Design considerations include:

• Slip-critical connections for applications subject to vibration and load reversal

• Tension control bolts for consistent preload achievement

• Thread-locking compounds or safety wiring for vibration-prone assemblies

• Corrosion allowances of 1-2 mm for exposed connections in coastal environments

Fatigue Design and Life Prediction

Mobile equipment structures must be designed for finite fatigue life, typically ranging from 10,000 to 1,000,000 load cycles depending on the application. The fatigue design approach significantly influences structural weight, cost, and reliability.

S-N Curve Application

The stress-life (S-N) approach remains the most common method for fatigue design of welded structures. CSA S16 and S6 provide design curves for various weld categories, with typical endurance limits at 10 million cycles ranging from:

• Category A (base metal): 165 MPa

• Category B (longitudinal welds): 110 MPa

• Category C (transverse butt welds): 90 MPa

• Category E (fillet welds): 40 MPa

For equipment operating in the demanding conditions found throughout Atlantic Canada's resource industries, applying appropriate safety factors (typically 2.0-4.0 on life or 1.25-1.5 on stress) ensures adequate reliability margins.

Cumulative Damage Assessment

Miner's rule provides a practical approach for assessing cumulative fatigue damage under variable amplitude loading. The accumulated damage index (D) should typically be limited to 0.5-1.0, with lower values specified for safety-critical components. Modern approaches using cycle counting methods such as rainflow analysis provide more accurate predictions for complex load histories.

Regulatory Compliance and Standards

Mobile equipment operating in Canada must comply with numerous federal and provincial regulations. Engineers must navigate a complex framework including:

• Canada Labour Code requirements for workplace equipment safety

• Nova Scotia Workplace Health and Safety Regulations for provincial compliance

• CSA standards including Z150 (mobile cranes), B167 (overhead cranes), and specific industry standards

• Transport Canada regulations for road-transportable equipment

• Professional Engineers Act requirements for engineering certification and documentation

Equipment intended for export may also require compliance with international standards such as ISO, EN, or AS/NZS specifications, each with distinct requirements for factors of safety, load combinations, and testing protocols.

Quality Assurance and Documentation

A robust quality assurance programme is essential for mobile equipment structural design. Documentation requirements typically include:

• Design calculations signed and sealed by a Professional Engineer licensed in the jurisdiction of operation

• Fabrication drawings with complete welding symbols, tolerances, and material specifications

• Welding procedure specifications (WPS) and procedure qualification records (PQR)

• Material test reports with full traceability to heat numbers

• Non-destructive examination records for critical welds

• Final inspection and load testing reports where required

For safety-critical equipment, third-party inspection and certification by organizations such as CSA, Bureau Veritas, or Lloyd's Register may be required. Maintaining comprehensive design files protects both manufacturers and end users while facilitating future modifications and life extensions.

Partner with Sangster Engineering Ltd. for Your Mobile Equipment Projects

Structural design for mobile equipment demands specialized expertise combining mechanical engineering principles, materials science, and practical fabrication knowledge. From initial concept development through finite element analysis, detailed design, and fabrication support, professional engineering guidance ensures your equipment meets performance requirements while maintaining safety and regulatory compliance.

Sangster Engineering Ltd., based in Amherst, Nova Scotia, brings decades of experience serving clients throughout Atlantic Canada and beyond. Our team understands the unique challenges of designing equipment for Maritime conditions, including corrosion resistance, temperature extremes, and the demanding operational environments found in the region's forestry, agriculture, mining, and manufacturing sectors.

Whether you require structural analysis of existing equipment, design of new mobile machinery, or engineering certification for regulatory compliance, our Professional Engineers are ready to assist. Contact Sangster Engineering Ltd. today to discuss your mobile equipment structural design requirements and discover how our expertise can contribute to your project's success.

Partner with Sangster Engineering

At Sangster Engineering Ltd. in Amherst, Nova Scotia, we bring decades of engineering experience to every project. Serving clients across Atlantic Canada and beyond.

Contact us today to discuss your engineering needs.