Warehouse Rack FAQ
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“RMI” are the initials of The Rack Manufacturers Institute. The Rack Manufacturers Institute is an independent, incorporated trade association formed in 1958 and affiliated with the Material Handling Industry. The membership of the RMI is made up of companies which produce the vast majority of industrial storage racks installed in USA. The RMI promotes the safe design and use of storage racks and related structural systems such as Welded Wire Rack Decking through research, testing, preparation of specifications, educational programs, and meetings. The RMI is the American National Standards Institute (ANSI) accredited developer of storage rack standards and administers the R-Mark Certification Program.
In 1997 the RMI issued a new specification for storage rack (later updated to become the 2002 edition and more recently adopted by American National Standard ANSI MH 16.1-2004). Shortly thereafter, RMI created the R-Mark Certification Program as a way for storage rack users and customers to clearly identify that rack frame and beam capacities shown in a load table were calculated in accordance with the new standard. A way of identifying special projects that were designed in accordance with the new specification was also established.
Copies of the most recent edition of the ANSI/RMI Specification for the Design, Testing and Utilization of Industrial Steel Storage Racks, Commentary, ANSI/RMI Specification for Welded Wire Rack Decking, and other useful information are available directly from the Rack Manufacturers Institute (704-676-1190), from any of the member companies, or from the RMI website at: www.mhi.org/rmi.
Beginning in the late 1930’s, Federal economic statistics for USA were collected and organized using the Standard Industrial Classification (SIC) System. In 1997, the SIC system was replaced by the North American Industry Classification System (NAICS) to normalize data-capture between USA, Canada and Mexico.
SIC and NAICS codes for racks and related items:Products/NAICS Code SIC Code
- Rack Accessories — 33715EYYW 2542300
- Drive In/Drive Through — 33715E111 2542341
- Cantilever — 33715E121 2542343
- Portable Racks/Frames — 33715E131 2542345
- Stacker Racks — 33715E141 2542347
- All Other (including Conventional) — 33715E151 2542349
The RMI/ANSI MH 16.1 is the best technical storage rack design resource available today. It may be desirable to have the whole rack system evaluated to this standard.
Repairs to any structural element of an existing rack structure should comply with the requirements of the RMI/ANSI specifications for new construction. Existing structural elements of a rack structure that do not require repair and are not adversely affected by the repair of other structural elements may not be required to comply with the ANSI/RMI requirements for new structures. It might be prudent to contact the local building department to determine if a new review is necessary.
IBC-2012 code reference: Chapter 34, Section 3403.1.
Concentrated Load – any static load which is not uniformly distributed over the entire surface of the decking section (Ref MH26.2 – 2004).
Point Load – any static load that is concentrated to particular points on the deck. (ie. A container with four small feet (point load) versus a container with two runner bars running the entire length of the container (concentrated load).
The zone designations are no longer used. This occurred with the introduction of the International Building Code in 2000 which affected the ANSI/RMI MH 16.1 Specification.
The old UBC seismic zones (0 to 4) were based upon the seismic ground motion, corresponding to a certain probability of occurrence within a zone. Therefore, all structures within a zone were designed for the same requirements.
The 2012 RMI Specification was modified in the following areas:
- In Sections 2.1 and 2.2 the load combinations were modified to include the redundancy factor and seismic product load coefficient.
- Section 126.96.36.199 was added explaining the redundancy factor.
- The seismic maps in Section 188.8.131.52 were updated.
- Information on the Seismic Design Categories utilized by the building code was added in Section 184.108.40.206.
- Section 2.6.4 regarding Connection Rotational Capacity was updated.
- Section 2.6.6 was added on the seismic separation between rack and the building columns in Seismic Design Category D and higher.
- The section on Beam-to-column connections was moved from Section 7 and added to Section 5, which deals with beam design.
- Section 5.5 was added on Pallet Supports.
- The maximum considered earthquake base rotation in Section 7.1.3 (formerly Section 7.2.3) was better defined.
- Section 7.2 on Slab and Subgrade Evaluation was added.
- Section 7.3 on Anchor Bolts was added.
- Section 9.6.11 on Evaluation of Test Results was added.
It is the responsibility of the owner to make sure that the new or existing floor slab in the building will support the loads that are imposed on it by storage racks, fork trucks and any other equipment that may be present.
The owner should consult with a qualified engineer who is able to evaluate the existing floor or design a new floor once the intended use of the building has been established and the expected loading on the floor has been determined.
The data required for designing a floor system or for evaluating an existing floor system should include information about the soil sub-grade. At a minimum, the designer typically needs the bearing capacity of the soil sub-grade expressed in pounds per square foot and the stiffness of the sub-grade (also known as the sub-grade modulus) expressed in pounds per cubic inch. Additional data such as the soil type may also be needed to evaluate slabs so that the soil site classification can be determined. This soil site classification can have a significant effect on the design of storage racks for earthquake resistance. This information should be given to the rack engineer and the building engineer, who is analyzing the floor slab.
The necessary slab data may include the strength of the concrete (compressive yield strength in pounds per square inch), the slab thickness, the strength and spacing of the steel reinforcement in the slab, the levelness and flatness of the floor, the joint locations, any other irregularities that may be present in the floor slab, and more. This data should also be given to the rack engineer and the building engineer who is analyzing the floor slab. It will be beneficial to the owner to provide all of the information on the slab and the sub-grade because doing so could reduce the chance of having problems with the slab or rack structure and could result in a more economical rack and floor slab design for new construction. In many cases the building engineer may communicate directly with the rack engineer at the request of the owner. The rack engineer may give the building engineer the loads imposed by the rack, and there can be agreement on items such as the base plate size and the anchor bolt locations. Often the location of the rack anchor bolts can be coordinated with rebar placement in the floor to reduce or eliminate interference.
Rack structural systems, not unlike buildings, are often subject to the building code review and permitting process. Most communities face the potential of earthquakes to varying degrees, magnitudes, and probabilities. Particular seismic requirements are site-specific, and the user should bring to the attention of the rack manufacturer the specific local requirements, including applicable building codes, the specific installation location, any knowledge of the supporting concrete slab, and any information about the below-slab soils and their properties.
Rack systems should be designed, manufactured, installed, and used in accordance with the site-specific requirements of the site; these requirements may include seismic effects and may also include the characteristics of the building in which the rack system is housed. (See also, ANSI/RMI, Specification section 2.7, and Commentary section 2.7). To find the requirements for your job site contact the local building authority.
The RMI defines the height to depth ratio for a single row of pallet rack to be the ratio of the distance from the floor to the top beam level divided by the depth of the frame. Normal anchoring as is used for double rows is usually adequate for racks whose ratio is 6 to 1 or less. If the height to depth ratio exceeds 6 to 1, the anchors and the base plates should be designed to resist overturning. The ANSI/RMI Specification in section 8.1 provides for the anchorage to resist an overturning force of 350# applied at the topmost shelf level (to an empty rack). If the LRFD method of design is used, this force should be treated as a live load and multiplied by 1.6.
If the height to depth ratio exceeds 8 to 1, the racks should be stabilized using overhead ties. If anchoring is used for this extreme case, the design of the anchors must be certified by an engineer.
Yes, ANSI standard MH26.2 – 2004 and can be purchased through www.MHI.org.
It is generally not a good idea to tie racks to the wall because forces from the building can be transferred to the racks and because forces from the racks can be transferred to the building, although wall ties are sometimes used in low seismic areas. If wall ties are used, there must be proper coordination between the building engineer and the rack engineer to ensure that the ties and any transmitted forces will not damage the rack or the building structures.
The connection to the wall must be capable of transferring the required forces, and the connectors must be compatible with the wall material. The seismic analysis of the rack and the building being tied together is extremely complex, and the connection is best avoided. If the height to depth ratio is such that a single row needs extra stability, heavy- duty anchor patterns with larger base plates or cross aisle tie configurations could be used rather than wall ties.
Wire decking is a decking system used on pallet rack shelves. Its purpose is to provide additional support for stored materials, as well as, becoming a safety net for unstable loads. Wire decking is fabricated from welded-wire mesh, and generally has reinforcements in the form of channels or support wires. Wire decks are supported by the rack beams at the front and rear and the strength and stiffness of the wire deck system provides support for the load between the beams. Decking designs vary greatly depending on the application. Wire thickness, grid pattern and number of channels all have an effect on performance.
Wire decking is unique to other types of shelving not only in appearance but also in performance. Because wire decks are made of steel, their integrity, capacity and performance remain constant. The advantages of wire mesh decks include safety, greater capacities, their ability to allow light, air, debris and water (very important in some states due to fire codes) to pass through the decks.
Racks that do not conform to the ANSI/RMI Specifications may not be as safe as racks that conform to the specification. The Rack Manufacturer’s Specification is the only recognized U.S. specification for the design, testing and utilization of industrial steel storage racks. If there should ever be an accident or other incident involving the storage racks, a responsible rack user may want to show that its racks have been designed to meet this recognized standard.
The RMI recommends purchasing racks that clearly meet the requirements of the ANSI/RMI Specification.
Column protectors are often used to protect rack columns from possible collision damage in traffic aisles of rack storage systems. The nature of column protection may depend on the particular rack system and the vehicles which are used to service it. With inattentive operation, columns may be struck by man-operated forklift trucks directly or by over-hanging loads being carried by those vehicles.
It is not always feasible to build, install, and operate rack systems that are immune to such dynamic operational abuse. Column-protectors, fenders, bumpers, or deflectors are often installed in front of each exposed rack column to attempt to keep such misuse from damaging the rack columns; aisle guides may also be used to attempt to keep a man-operated forklift from going astray; or reinforcement may be added to the exposed aisle-side columns with additional column sections, other reinforcing steel or other materials to improve their impact resistance. Automated or wire-guided vehicle systems are normally constrained on their intended path and are thus less likely to damage traffic-aisle rack columns. Users should consult their rack supplier about the various available protections, considerations, and options. (See ANSI/RMI, Specification section 1.4.9 and Commentary section 1.4.9).
- The most common type of wire deck is a waterfall style. The waterfall is the overlapping of the top deck wires running over and down the face of the support beams, resembling a waterfall. They usually have three to four support members or channels designed to fit within the step of the beam and support the load resting upon the deck. A waterfall deck for a box or structural beam is the same as above with the exception that the support members or channels are flattened or flared at the ends where they rest on the top of the rack beam.
- Another popular type of wire deck, similar to the above, is a flush or instep deck fitting step beams only. This deck sets on the step ledge between the beams, flush with the top of the beams. It can be flat or have formed instep waterfalls. The purpose of the design is to avoid any potential snag points and to leave the rack beam face unobstructed.*
- Also available is a non-waterfall deck that may span across the top of the front and rear load beams but does not waterfall down. This style of deck is not recommended for non-step beams due to the configuration being unstable.*
Installation and Safety
Yes. The rack frame bracing consists of horizontal and/or diagonal members that join the front column to the rear column. These members are very carefully designed by the rack manufacturer to stabilize the rack frame in the cross-aisle direction and to support each of the individual columns. Any damage to these components could jeopardize the stability of the frames and could degrade the strength of the column and result in possible rack failure.
If a frame brace is damaged, the first priority should be to immediately isolate the affected area, have a storage rack design professional evaluate the damage, and unload, replace or repair as directed by the professional.
A tunnel bay is a storage rack bay that has the lower beam levels removed to allow people or lift equipment to pass through the rack in the direction that is perpendicular to the storage rack aisles for the purpose of egress, and also to provide more efficient travel distance to stored loads. The person preparing the rack layout should work closely with the rack user to determine the most optimum locations for tunnel bays. Longer systems may require more than one tunnel bay location.
Special consideration should be taken for tunnel bays that are placed at the ends of the rows, because special design of the frame may be required to allow for the longer unbraced column length. Tunnel bay frames are often reinforced or equipped with protective devices, because they are more vulnerable to damage from cross-traffic or turning vehicles. The clear height of tunnel bays should be adequate to allow for the vehicle to pass through safely. Tunnel bays are often wider than standard bays. The first shelf level of tunnel bays is often protected by wire decks or other guarding.
A pick-module is a rack structure comprised primarily of vertical frames and horizontal beams, typically having one or more platform levels of selective, case-flow, or pallet-flow bays feeding into a central pick aisle(s) supported by the rack structure. Pick modules are used by authorized or trained personnel only, and are not open to the general public.
The materials required for a building permit normally include the details of the proposed rack system and its use, the various loads for which it has been designed, the “calculations” from an engineering analysis accomplished and “sealed” by a registered design professional, demonstrating the structural integrity of the proposed system and its conformance with all applicable building code provisions, details of the fabrication and installation processes, information about the building in which the rack system will be housed and used. The building information may include relevant information about the characteristics of the floor slab, the below-slab soils, and about the building structure if connections to the building are proposed.
Typically the owner works with the rack supplier to assemble and process this information through the permitting process. There may be costs associated with the development and processing of this information through the local permitting process and for a building permit itself. The magnitude of these costs and how they are shared are matters of negotiation between the owner and the rack supplier and may relate to the size, complexity, and site-specific requirements of particular projects.
The ANSI/RMI Specification shows the maximum out-of-plumb ratio for a loaded rack column as 1/2″ per 10 feet of height. Columns whose out-of-plumb ratio exceeds this limit must be unloaded and re-plumbed. Any damaged parts must be repaired or replaced. This ratio could be used for straightness also. In other words, the out-of-straightness limit between any two points on a column should not exceed 0.05″ per foot of length (1/2″ per 10 feet).
An out-of-plumb or out-of-straight condition will reduce the capacity of a rack column. The reduction can be significant. A rack that is out-of-plumb from top to bottom or a rack column that is not straight is likely to become further out-of-plumb or out-of-straight when it is loaded.
The out-of-straight limit is given to prevent excessive “bows” or “dogleg” conditions that may exist in a rack column. A column could be plumb from top to bottom but have an unacceptable bow at mid-height (see figure (a)), or a 20 ft. high column could be out 1″ from top to bottom, which could be acceptable using a simple top-to-bottom out-of-plumb measurement, but the entire out-of-plumb could be between the floor and the 5 ft. level (see figure (b)). This dogleg condition would be very harmful. This condition could be caused by fork truck impact. The column could have a sine wave shape and be out of straight as shown in figure(c). A column could also become bent and exceed this limit (see figure (d)). As re-written the specification now prevents these situations from being acceptable if they exceed the 0.05″ per foot out of straight limit.
If the reason for extending the height of the pallet rack upright frames involves a change in the existing beam elevations or the addition of one or more bean levels, the design configuration of the rack is being changed. Prior to making any such changes to the configuration or loads, the original and proposed rack design should be reviewed by the original manufacturer or by a qualified design professional.
All rack components and connections must be checked with the new loads and the revised configuration to ensure that all the requirements of the ANSI/RMI Specification are satisfied for the new configuration and loads. The splice connection used must adequately transfer all loads from the frame extension to the existing frame. The frame extension must have proper bracing and be compatible with the beams or other components that will connect to it for the new configuration. In some cases individual column extensions may be acceptable. If the rack configuration or load change is made and the extensions are added, it may be necessary to revise or replace the information on the load plaques and the rack application drawings.
If the reason for extending the frames is for non-structural purposes, the design review may not be required. If the racks are being extended to add cross-aisle ties for any reason, the design should be reviewed because the cross-aisle design model of the racks will be altered. If the racks are being extended for the purpose of tying the racks to the building, the design should be reviewed and the building design engineer must approve the connections. Any rack frames that are damaged must be properly repaired or replaced before the extensions are added.
It is important to install the frames oriented as the manufacturer recommends. However, there may be cases that the orientations are not identified as important design considerations.
When the orientation of the frames is not design critical the diagonal brace orientation in the bottom upright panels run from lower front to upper rear so that the diagonal braces go into tension should the base portion of the aisle column be damaged. This orientation also means that the aisle column usually has both a horizontal and a diagonal brace coming into the base portion of the aisle column for extra stiffness.
The other thought is to have the diagonal braces in the bottom upright panels run from upper front to lower rear so that the diagonal braces won’t be damaged or their welds broken if the base portion of the aisle post is damaged. The choice is basically a matter of personal preference. There are no studies which prove that one is better than the other and both cases have excellent track records.
To minimize damage to the aisle posts, your rack supplier will often recommend heavy-duty bottom braces, deflector angles, backer posts, post protectors, or some combination thereof.
Upright bracing members can be omitted to create openings. However, this should be included in the initial design and fabrication by the rack manufacturers.
It is also possible to retrofit existing uprights with openings. However, this is a substantial structural change to the uprights and must be reviewed by a qualified design professional. Removal of bracing may also require modifications to the surrounding bracing, columns, or both.
No, wire decking is not designed to be walked or stood upon. Walking and/or standing on a wire deck creates both dynamic (moving and varying) and concentrated loads. Wire decking is designed and assigned a load carrying capacity based on carrying uniformly distributed, static loads. While there is a safety factor designed and built into wire decking, dynamic and concentrated loading as a result of standing or walking on a wire deck is a use which falls outside its intended purpose.
In addition, the surface of a wire mesh deck is flexible and irregular and the open areas within the mesh may cause a person to trip. Furthermore, when subjected to lateral motion decks may slide upon the supporting rack beams or tip upward and become dislodged when loaded in a concentrated fashion on the outer extremities (beyond the outermost support members).
All storage rack systems are designed for the specified load in any location, and it is commonly assumed by the designer that the rack system will be loaded and unloaded in a random fashion during its lifetime. With that said, an appropriate approach to fully load a pallet rack is to start at the bottom middle of the rack row and to work outwards and upwards.
Research has shown that a generally appropriate protocol for loading a rack system is to store the heaviest product on the floor or lower levels toward the middle of the rack and then to work outward to the ends of the rows and then upward. Due to inventory systems and control, this may not always be possible, but it is often the most appropriate loading method for a given structure.
Wire decks are intended as an accessory to pallet rack. The dimensions of the wire deck must correspond with the rack upon which the decks are to be installed. There are a relatively large number of different rack manufacturers and a wide variety of beam styles and designs. If the dimensions are wrong, the wire deck may not fit on the rack or may fit but be unsafe. Generally wire deck manufacturers require a buyer to submit dimensional specification of the rack prior to production. This protects both the manufacturer and the buyer and assures that there is agreement upon precisely how the wire decks are to be utilized.
It is also a best practice to supply the wire deck manufacturer with the load capacity rating of the rack system so that the wire deck can be designed and built to meet or exceed the capacity of the rack system. Short of that the system is only strong as its weakest link. Generally speaking the deck capacity is specified to mirror that of the load beams of the rack system, for example a beam pair rated at 5,000 lbs. will require two wire decks rated at 2,500 lbs. each.
The storage rack system owner should establish and implement a program of regularly scheduled storage rack system inspections. The inspections should be performed by a qualified person familiar with the storage rack design and installation requirements retained or employed by the storage rack system owner.
Storage rack should be inspected periodically to check for any damage or abuse and immediately after any event that occurs that may result in damage to the rack. The frequency of inspections should be up to the discretion of the owner, depending on the conditions of use. As a minimum, inspections should be performed annually. The inspection schedule and results of the inspection should be documented and retained.