Structural Design Considerations for Off-Site Manufactured (OSM) Utility Support Racks in Industrial Facilities

By Chad A. Reinemann, P.E., LEED AP

Introduction

The construction methods at new industrial facilities continue to advance as cutting-edge manufacturing processes demand larger and more complex equipment and associated infrastructure.  

Perhaps nowhere is this growing infrastructure complexity more visible at an industrial facility than the distribution network of utilities and their associated utility support racks.  These racks provide structural support for utility conveyance systems (such as process and waste piping, electrical tray and conduits, and HVAC ducts and piping) and transfer loads to the building structure.  The construction and installation of these racks demand precision, reliability, and significant multi-trade coordination for both space utilization and installation sequencing. In addition, the build out and installation of the various utilities within the support racks often occurs along the critical path of the construction schedule. 

Off-site manufacturing (OSM) of select utility support racks offers a promising solution to meet these complex requirements, providing an opportunity for enhanced safety, quality control, and space utilization efficiency, while reducing labor cost and construction timelines. However, the promise of OSM can only be fully realized if opportunities are identified and coordinated early in the design of the building.  This article explores the structural design considerations associated with OSM utility support racks and identifies important coordination items that should be aligned early during the design of the building structure prior to utility fit out design.

OSM Approach to Utility Support Design:

In an OSM approach, select utility support racks are built off site in modules that can be assembled in a controlled environment while the building shell is under construction.  The modules are fit-out with utilities to the greatest extent practical prior to being transported to the site.  The modules are then spliced together, positioned, and anchored to the building structure.  

For effective use of OSM, early identification and development of OSM opportunities by the design team is paramount.  This may be done during preliminary space planning and utility distribution mapping, with a focus on areas of high utility density and common routing paths.  Once the locations for OSM opportunities are identified, coordination with the structural design of the building may begin.  This coordination typically involves a number of important design considerations:

1. Structural Design Loads:

Because the OSM approach tends to bunch utilities together, early development of design loads on each OSM utility support rack is critical to ensure these loads are appropriately captured in the design of the building structure.  Design load assumptions for OSM utility support racks should be documented and mapped on the building structural drawings and the floor framing should be designed to support the areas of higher density utility loads.  The design loads for OSM utility support racks can include:

• Dead Loads: Includes self-weight of the support rack structure and operating weights of utilities and ancillary equipment.  Utility loads that will be installed on the rack during future fit outs are typically considered in the development of the design dead load. 

Since the final utility layout is not typically known during the early design stages, engineering judgment and experience is critical to develop design dead load assumptions.  More sophisticated building owners may provide design guidance on dead loads based on studies of similar facility spaces that have been fully fit out.   Process Industry Practices Structural Design Criteria (PIP STC01015) is another good reference for preliminary design loads, and suggests a uniformly distributed load of 20 psf to 60psf for each level on the utility support rack.  It is noted that 40 psf is approximately equivalent to 8-inch diameter, standard schedule carbon steel pipes, filled with water, at 15-inch on center spacing. 

It is this writer’s judgment that a minimum 40 psf uniform area load for the initial level of utilities and a minimum 20 psf for each additional level (30 psf if level consists of electrical tray) should used for early design dead load assumptions of OSM utility support racks.    

• Live Loads: Includes loads imposed by maintenance personnel on catwalks or temporary access planking and fall restraint tie-off loads.  It is important to identify early in design if the rack is intended to support permanent catwalks or will be used for fall restraint tie-off anchorages as these loads can drive the design of the connection details.
• Pipe Operational and Occasional Loads: Includes loads imposed by piping due to operating pressures, dynamic flow considerations, and temporary thermal movements.  These loads are typically developed later in the design process during pipe flexibility analysis and are not typically considered during early design.  However, the pipe anchor/guide/slide layout should be considered for large diameter piping when substantial concentrated pipe anchor loads are expected.
• Seismic Loads: Includes dynamic forces generated by seismic events.  These loads are dependent on facility location, soil characteristics, relative stiffness of the building and rack, importance of the utility systems to facility operation and safety, and the height of attachment of the rack within the structure.  OSM racks typically are considered non-building structures similar to buildings and Chapter 15 of ASCE 7: Minimum Design Loads and Associated Criteria for Buildings and Other Structures should be referenced when developing these loads.   In high seismic regions, these loads should be considered in preliminary design and a lateral load path for the rack should be identified.
• Environmental Loads: Includes exposure to ambient temperature variations and other weather considerations such ice, snow, ponding of water, and wind.  These load considerations are important for outdoor racks and trestles.
• Transportation and Handling Loads:  Includes loads associated with lifting, transporting, and installing the OSM rack modules.  These loads are not typically considered during preliminary design beyond confirmation that the OSM installation strategy is viable.

2. Structural Materials and Module Layout:

Both the materials used, and the module layout selected for OSM utility support racks is dependent on the design loads and specific support requirements of the various utilities.  It is important to develop an early strategy for the construction of the OSM rack to understand hanger or post layouts and the maximum point loads that may be present on the building structure.  Some general considerations to the selection of the rack materials and module layout are as follows:

• Transportability – The OSM rack modules need to be moved to the facility from the off-site assembly location, and once at the facility, need to be moved and positioned within the building.  Standard box trailers provide for module widths up to 8’-0” +/- and standard flatbed trailers provide for module widths up to 8’-6” +/-.  Modules exceeding these sizes may trigger oversized load requirements for transportation.  In addition, the OSM modules should not exceed the facilities Move-in, Move-out (MIMO) path dimensional limitations, which are typically set by the largest equipment being installed in the facility.  The buildings freight elevator size may also be another consideration.
• Maintenance accessibility:   The OSM rack modules need to provide for easy access for inspection, maintenance, and potential reconfiguration of utilities.  Vertical access spaces between modules or within modules should be considered.
• Metal channel strut components:  OSM rack modules constructed with metal channel strut are typically hung from the underside of the floor.  These racks are typically appropriate to support loads up to 80 psf on a 6’-0” x 4’-0” hanger grid layout and are appropriate for individual pipes up to 12-inch diameter.  Metal channel strut components are able to be connected to standard continuous channel strut embeds.
• Proprietary metal modular support rack components:  OSM rack modules constructed with proprietary support rack components are typically hung from the underside of the floor.  The racks may be appropriate to support loads in excess of 80 psf on a 6’-0” x 4’-0” hanger grid layout or individual pipe up to 18-inch or larger in diameter.   Proprietary support rack components are typically connected to discrete channel embeds that must be compatible with the proprietary system.
• Structural steel hanger components:  OSM rack modules constructed with structural steel hanger sections are typically hung from the underside of the floor.   These racks are typically appropriate for loads up to 120 psf on 6’-0” x 8’-0” hanger grid layout or individual pipe up to 24-inch diameter.  Structural steel hangers are typically connected to discrete steel plate embeds or pairs of discrete channel embeds.
• Structural steel post components:  OSM rack modules constructed with structural steel post sections are typically posted down to the floor below or set on seated plates embedded along the face of a column or wall.  Loads over 120 psf can be achieved with an almost unlimited post grid layout and individual pipe over 24-inch diameter may be safely supported. 

3. Module Connections to Building Structure:

Early design development of the OSM rack module connections provides an opportunity for efficient, standardized details that are fully integrated into the design of the building structure.  The module connection details to the building structure will be driven by the design loads and structural material assumptions for the utility support rack module.  Some general design considerations include:

• Installation flexibility – Module connections need to have built-in vertical and horizontal flexibility to accommodate various fabrication and construction tolerances in the concrete and steel building frame and within the modules themselves. 
• Structural Integrity: Module connections to be designed for adequate connection strength and stiffness to withstand design loads and minimize deflection.  Confirmation of the performance of connection details may involve early load testing of clamping devices or embedded channel components. 
• Coordination of embedded components:  Module connections that require embedded components into concrete should be coordinated with the structural building drawings.  These embed requirements should also be clearly specified for precast concrete components and other structural systems that are delegated to a specialty engineer.
• Coordination with concrete reinforcement:  Module connections that require post-installed anchors should be coordinated with the design and detailing of the building structure to avoid areas of heavy steel reinforcement and high shear stresses in the floor. These areas are difficult to coordinate and install the anchorages without conflicts with critical reinforcement.    
• Coordination with fire protection and high and tight utilities:  Module connections need to be coordinated with other utilities above the rack.  These utilities are often sequenced to be installed prior to the OSM utility support rack modules.  Bridge struts or other components at the top of the module that would potentially cross these utilities need to be installed before and separate of the module itself. 
• Cleanroom Compatibility: In cleanroom spaces, module connections need to be implemented in a way that is compatible with cleanroom protocols, ensuring minimal particle generation and contamination.  Connection details that minimize field welding and drilling are typically preferred in these situations.

4. Quality Control and Construction Tolerances in Building Structure:

The requirements for levelness, plumbness, and flatness of the building structure and accuracy in locating embedded components is typically prescribed in referenced standards based on accepted construction practice.  Construction tolerances applicable to OSM utility support racks are noted in the following referenced standards:

• ACI 117-10 Specification for Tolerances for Concrete Construction and Materials
• ACI 117.1R-14 Guide for Tolerance Compatibility in Concrete Construction
• PCI MNL 135-00 Tolerance Manual for Precast and Prestressed Concrete Construction
• AISC 303-16 Code of Standard Practice for Steel Buildings and Bridges
• MFMA-4 Metal Framing Standards Publication

Many of these tolerances focus on the top surface of the floor and may not be appropriate for efficient installation of OSM utility support racks connected to the underside/formed surface.  As a general rule of thumb, tolerance limits of ¾-inch +/- in any orthogonal direction along the length of the OSM module are reasonable for module installation.   It is important that any specific tolerances that are more stringent than the referenced standards and associated quality control procedures be identified early in design and communicated in the building structural drawings and specifications.

Conclusion:

Off-site manufacturing of utility support racks offers industrial facilities strategic advantages: improved quality control, efficient space utilization, accelerated construction timelines, and enhanced safety and flexibility to accommodate future modifications. Success of an OSM approach to utility support racks is dependent on addressing important design considerations such as the structural design loads, module layout and materials, module connection details, and building construction tolerances.  Working through these considerations early enough in the design to fully integrate the OSM strategy into the building structural design is imperative to maximizing the benefits of OSM and maintaining operation excellence on ever more complex industrial facilities.