Rack Codes & StandardsRack Design & Installation

Updated Methodology for Seismic and Stability Calculations in Rack Designs Adopted by International Building Code, Part 1

Seismic Calculations

ANSI MH16.1-2023Dedicated to continually improving the safety and performance of industrial steel storage racks, the members of the Rack Manufacturers Institute (RMI) routinely publish rack design standards. Among them is ANSI MH16.1-2023: Design, Testing, and Utilization of Industrial Steel Storage Racks, referenced by the International Code Council’s International Building Code (IBC) as governing the safe design and installation of steel storage racks. That means RMI’s most recently updated methodology to perform seismic and stability calculations in storage rack designs is part of the newest IBC, published in 2024.

New Calculation Methodology Provides More Comprehensive Analysis

Most notably, ANSI MH16.1-2023 includes a more comprehensive and sophisticated method for analyzing the stability of storage racks in seismic environments of varying intensity. This is crucial for ensuring the safety of both the stored items and personnel. These checks typically involve analyzing the ability of the storage rack’s design and configuration to withstand seismic forces. Those forces can include both lateral and vertical loads.

“Throughout the U.S., the most widely used building code in the rack industry until recent months has been the IBC 2021. That publication references the ANSI MH16.1 standard from 2012,” explained Christopher Aprile, Senior Structural Engineer with Steel King Industries, a member of RMI.

“MH16.1-2012 used a different methodology for seismic and stability calculations when designing pallet storage rack systems,” he said. “But in the years since the approval of that standard, RMI revised its approach for seismic and stability calculations. The latest calculation methodology incorporates newer finite element analysis modeling methods.”

The newest IBC 2024, continued Aprile, adopted the 2021 edition of ANSI MH16.1 standard.

“Ideally, rack engineers need to start to comply with the new methodology for seismic and stability calculations soon — if they have not already,” he explained. “This new method may, in certain project areas with a higher likelihood of seismic events, result in a more robust storage rack design. That, in turn, could increase the cost of the rack system as a whole.”

Seismic and Stability Calculations Evolve from Effective Length to Direct Analysis

ANSI MH16.1-2021 introduced a new means to perform seismic and stability calculations in storage rack design called the direct analysis method (DAM). Before 2021, ANSI MH16.1 prescribed using the effective length method (ELM).

“ELM is a simplified first-order analysis design approach commonly used in traditional structural analysis and design. It models storage rack structures as linear elastic systems with idealized conditions. For example, by using only one down-aisle effective length factor of K=1.7 in the buckling analysis of an upright column post, regardless of its unbraced length, or boundary conditions at the beam shelf (story) levels,” Aprile explained.

“It also makes simplified load distribution and load path assumptions,” he continued. “Ultimately it could yield nonconservative rack designs that may not accurately represent the true response of the structure under seismic loading.”

Conversely, ANSI MH16.1-2021 (and likewise the newest version of the standard published in 2023) recommends rack engineers use the direct analysis method (DAM) or second-order analysis to determine a better seismic response of storage rack structures. Further, with DAM the down-aisle effective length factor of K=1.0 can be used in the buckling analysis of an upright column post.

“Both calculation methods demonstrate what a seismic event does to a loaded rack,” Aprile said. “But DAM enables engineers to account for nonlinear behavior second-order effects and load redistribution mechanisms. The intent is to help engineers obtain more accurate seismic analysis results.”

Seismic and Stability Calculations

Applying Second-Order Effects to Seismic and Stability Calculations

In addition to prescribing DAM when performing seismic and stability calculations, ANSI MH16.1-2021 also specifies the application of second-order effects to the analysis.

“When performing DAM using second-order effects, the standard directs a rack design engineer to use the average pallet load weight in the calculations. They also must apply notional loads — which is a horizontal or lateral load applied at each shelf level — derived from a small percentage of the existing product gravity load,” said Aprile.

“Notional loads simulate the effects of geometric imperfections that occur during fabrication and installation. In addition, seismic calculations are performed with both the flexural and axial stiffnesses of all the steel members of the rack structure reduced. This accounts for the reduction in member stiffness due to material yielding or geometric nonlinearities,” he continued.

“By considering second-order effects, DAM provides a more accurate representation of how structures respond to seismic forces,” added Aprile. “This includes accounting for the redistribution of internal forces and the development of inelastic behavior, both of which are crucial for assessing the seismic performance of structures.”

Other Aspects of Seismic Calculations Consider Location-Specific Factors

In addition to the total weight of the loaded structure, the seismic calculations consider the degree of seismicity at the installation location. They also account for the thickness of the supporting concrete slab and the soils under the slab. These calculations inform the overall strength and rigidity of the system and the number of levels of pallet loads it can support.

“Applying second-order effects to the seismic calculations for very tall, heavily loaded storage rack systems could result in the rack structure’s fabrication being a more rigid rack system,” continued Aprile. “This could include heavier beams and upright frames, larger base plates, more anchors, and an increase in the number of rivets and/or bolts used in the beam-to-column post connectors.”

That makes having the most accurate average pallet load weights — not maximum pallet load weights — supplied to the rack engineer by the purchasing dealer or warehouse owner/end user in the preliminary design quotation phase of a project significantly more important.”

Learn More About Safe Rack Installations in Earthquake Zones

RMI has assembled a list of frequently asked questions and answers specific to seismic considerations for rack design. In addition to ANSI MHI16.1-2023: Design, Testing, and Utilization of Industrial Storage Racks, they address rack design reviews, seismic design categories, and soil classifications. Also covered are seismic factors and site coefficients, seismic separation, redundancy, and the newest seismic maps. RMI offers a video on the topic, too: Seismic Considerations for Rack Designs.