Cantilever Racks New Design Requirements

New Design Requirements for Cantilever

Recently the first ever RMI Cantilever Specification was completed and adopted by the RMI (Rack Manufacturers Institute). Following this adoption the RMI submitted this document to ANSI for approval to become an ANSI (American National Standards Institute Specification) standard. Before ANSI approves a specification, the specification has to go out and be approved by a canvas committee that is made up of various interest groups that include manufacturers, consultants, users and those who may have a general interest in the document. At the conclusion of the review, all comments that are submitted are addressed prior to final ANSI approval.

During the final review of the Cantilever Specification, an engineer from the seismic engineering community realized that there were no special seismic detailing requirements for cold-formed steel cantilever racks. Due to the absence of these detailing requirements, he commented that greatly reduced R values needed to be required until these detailing requirements could be developed and met. Detailing requirements for structural steel can be found in the AISC 341 Specification. These requirements place strict limits on the b/t (width-to-thickness) ratios for stiffened and unstiffened web flanges to prevent local buckling and add ductility. These limits are well exceeded by most normally designed cold-formed cantilever racks. Below is a table from the new RMI Cantilever Specification showing the requirement for reduced R values:

RMI Seismic Design Parameters

Notice in the table above the cross-aisle R value changes from 3.0 to 1.0 for category D which essentially triples the amount of seismic force the rack is required to resist. The down-aisle R value changes from 3.0 to 1.5 which doubles the forces on the bracing. In column and base design for category D, the column and base design will be governed by the seismic load case. This means the cross-aisle moment will be tripled by this requirement. This will add significant cost to these structures and possibly make them impossible to design in some cases. The computed seismic uplift will also be extreme.

A reasonable question to ask would be, “Why didn’t the RMI just refuse to change the R requirement in their document?” The answer is two-fold. First, ANSI approval would not have been possible with the comment from the seismic community being ignored. Secondly, if the RMI was to be adopted by the building codes (ASCE7 and IBC) without the reduced R values, an exception probably would have been added to the ASCE7 stating, “Use the 2016 RMI Cantilever Specification except use the requirement for the lower R values for Category D and above.”

Another question would be, “What is the effect on Structural Cantilever racks?” Structural cantilever racks will have to meet the detailing requirements set forth in AISC 341. This will disqualify sections that exceed the b/t limits in AISC 341 TableD1.1. They also are required to have the column to base connection designed to include the over-strength multiplier (Ω) or the full Mn (Ultimate moment strength) of the column.
The first footnote in the table above does allow cantilever racks designed using cold-formed steel if they are designed in accordance with AISC 341. The difficulty here is that the b/t requirements require the plate elements to be so thick that the sections cannot be easily fabricated using tooling that exists in most fabrication shops.

Question: How do I know the seismic design category for my site?
To determine the seismic design category, the engineer must first determine the Sds and the Sd1 values for the site. To do this, the soil site class is needed (if available). If the soil site class is not available the ASCE7 requires that the default site class (Class D) be used. Using the soil site class and the Ss and S1 values obtained from the USGS website for the address of the site, the Sds and Sd1 values are determined in accordance with chapter 11 of the ASCE7 Specification. The storage rack engineer can quickly determine the seismic design category if the soil site class and the address are known. If the soil site class is not known the engineer will have to use D. It can be very advantageous if the soil site class is A, B or C. The table below (from ASCE7) then defines the seismic design category. For storage racks the Risk Category for the table would be the first column labeled I or II or III.

Question: What are my options if I find that my site is in seismic category D?
Since the seismic forces will be extreme, the design of the cantilever structure will likely need to change. More columns can be used to reduce the force per column. The structure may have to be shorter with fewer arm levels. Heavier loads could be required at low elevations in the rack and lighter loads at higher elevations.

Another option would be to look at the load lengths and use pallet rack for any load that can be short enough to possibly be stored on pallet type rack.
Compliance to the new R values given in the ANSI/RMI Cantilever Specification will significantly increase the cost of racks going into seismic design category D.

Note that another option is to choose stockier members (less slender elements) so that the width-to-thickness limits in Table D1.1 of the AISC 341 specification are not exceeded. This will allow more reasonable R values.