By Adel Al Ekkawi  |  Structural & FEA E.I.T. at ENA2

An industrial skid must support its equipment while transferring gravity and lateral loads through the frame and its connections. Drawing on a four-storey skid assessment, ENA2 Innovative Consulting Inc., a Calgary-based engineering consulting firm, outlines the load calculations, member checks, and serviceability considerations involved in structural skid design.

Structural skids are often required to support thousands of pounds of critical and high-value equipment, including pressure vessels, tanks, piping systems, pipe racks, access platforms, and other process components. In addition to these substantial equipment loads, skid structures must also be evaluated for applicable wind, snow, rain-related, and seismic loads. The magnitude and distribution of these loads must therefore be carefully determined in accordance with the applicable building codes and accurately represented in the structural analysis of the skid structure.

At ENA2, advanced structural analysis software is used to develop detailed models of skid structures and evaluate their response to governing combinations of gravity, equipment, operational, environmental, and seismic loads. The structural system is checked against applicable steel design standards for strength and serviceability. Lateral sway and drift are also reviewed against code limits and project-specific criteria to assess performance under the governing load cases.

At ENA2, a wide range of engineering standards and building codes are applied, with particular emphasis on the National Building Code of Canada (NBCC) and the corresponding provincial requirements. Whether a skid structure is located in Alberta, British Columbia, Saskatchewan, Nova Scotia, Quebec, Ontario, or elsewhere in Canada, the structural design is completed in accordance with the governing building code and regulatory requirements applicable to that province. Project calculations, reports, and drawings are then stamped by a professional engineer licensed in the corresponding jurisdiction, as required.

As part of ENA2’s quality-control process, structural analyses, when required, can undergo independent review by a panel of experienced senior engineers. These additional reviews provide another level of technical oversight and can be sealed by an appropriately licensed professional engineer, where required. This multi-level review process at ENA2 helps ensure that the structural analysis and design are aligned with the applicable codes, standards, and professional engineering requirements.

Protecting expensive equipment from failure

Heavy equipment can impose high concentrated loads on skid members, potentially causing localized failures such as web crippling, flange bending, and buckling. These failures can reduce structural integrity and increase deflections, potentially damaging the supported equipment.

Assessing the member and connection behaviour

Structural analysis of skid structures enables engineers to determine the forces acting on connections. Connections subjected to seismic forces, wind uplift, and concentrated equipment loads are therefore assessed through detailed engineering calculations and FEA to verify their strength and overall conformance with engineering standards.

Determining seismic and wind demands on the structure

Seismic and wind loads acting on both the skid structure and supported equipment, determined in accordance with applicable engineering standards, are critical to evaluating multi-storey skid performance. These loads govern key design checks such as lateral drift, member strength and stability, and the adequacy of the seismic force-resisting system.

Engineering decisions for safer industrial skids

Structural analysis results can be used to refine the design of deficient members, develop appropriate retrofit strategies, and verify compliance with applicable codes and standards. The final assessment is then documented in a stamped engineering report summarizing the analysis results, design modifications, and key engineering decisions.

Structural Resilience Against Lateral Loads: A Fundamental Design Requirement

Seismic and wind loads are significant lateral actions that must be considered in the structural analysis and design of skid structures. Heavy equipment supported on skids can generate substantial seismic forces due to its mass, which must be properly transferred into the skid members. In addition, equipment with a large, exposed profile, such as pressure vessels, engines, and tanks, can generate considerable wind loads. These equipment-induced wind forces are therefore calculated and transferred to the supporting skid structure to ensure adequate strength of the supporting skid members. The structural analysis should consider:

The calculated base shear, represented in the formulation below and adopted from NBC 2020 seismic design provisions Section 4.1.8.11, depends on the structure’s ductility, weight, structural period, and site’s spectral acceleration. In addition, the building importance factor also reflects the consequences of structural failure, including the potential risk to human life.

This formulation also:

• accounts for the ductility of the structure based on the selected seismic force-resisting system
• accounts for the overstrength of the resisting system, representing its reserve strength available beyond its nominal design resistance.

The calculated wind load, determined using the formulation presented below and adopted from NBC 2020 Section 4.1.7.3, is strongly influenced by structure geometry, size and distribution of openings, exposure conditions, surrounding topography, and the site-specific velocity pressure. The formulation also accounts for internal pressure effects, which may act as either positive pressure or suction depending on the skid building enclosure.

Where a structural skid forms part of a partially or fully enclosed building, internal pressure effects can be relevant to the overall wind loading. Proper consideration of both external and internal pressures is essential to accurately determine the net wind forces transferred to the skid structure.

The specified lateral seismic force on equipment and structural components is calculated in accordance with NBC 2020 Section 4.1.8.18 using the formulation below:

This formulation accounts for:

• Component response modification factor, : reflects the component’s expected inelastic behaviour and the performance of its connections under seismic loading, as defined by the applicable code provisions.
• Component weight, : represents the seismic weight of the equipment or component.
• Height factor, : accounts for the increase in seismic demand as the equipment is located at higher elevations within the supporting structure.
• Component factor, : accounts for the importance of the component and the potential consequences associated with its failure, including the hazard level of the equipment contents.
• Force amplification factor, : accounts for the dynamic amplification of the equipment response due to the flexibility of the equipment itself and/or the flexibility of its attachment to the supporting structure.

The wind force acting on equipment of different shapes is calculated using the formulation below in accordance with NBC Commentary I.

The wind pressure acting on equipment depends in part on its shape profile, reflected in the  factor.

This four-storey skid, 31 feet tall, was modelled in Dlubal RFEM 6, an advanced structural analysis and design platform suited to complex structural systems and load paths. The steel framing system was assessed for its structural response and applicable design checks. RFEM also supports the analysis and design verification of hot-rolled steel, cold-formed steel, timber, and concrete members and surfaces in accordance with relevant design standards.

As shown below, the complete steel framing system of the skid was modelled, together with major supported equipment such as pressure vessels, tanks, pumps, and pipe supports. These components were incorporated into the structural model so that the corresponding seismic and wind forces acting on the equipment could be accurately applied and transferred into the supporting skid members.

Some examples of load application are shown below.

ENA2 performs detailed structural analysis and design verification of all steel members to evaluate the governing failure modes, including flexure, shear, axial loading, flexural buckling, lateral-torsional buckling, and biaxial bending. In addition, the effective length of each member can be defined independently in each principal direction based on its actual support conditions, connection restraints, and field constructability, allowing the analytical model to more accurately represent the expected structural behaviour.

At ENA2, structural performance is evaluated not only in terms of strength but also through comprehensive serviceability checks. This includes assessing seismic and other lateral deflections and comparing the calculated response against the applicable NBC requirements for Canadian projects or relevant ASCE standards for projects in the United States.

The four-storey skid example illustrates why equipment loads and environmental and seismic actions should be considered together in structural analysis. By reviewing member strength, connection forces, and lateral deflections against the governing criteria, engineers can identify potential weaknesses and document the basis for design decisions. ENA2’s civil structural analysis and FEA services support these assessments for industrial equipment-support structures, from load evaluation through design verification.

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