Not properly determining the design arm load for a cantilever rack application can easily result in insufficient arm strength for cantilever arms. Lack of sufficient arm strength is a severe safety concern. A common mistake that is made is to take the handled load and simply divide the weight by the number of arms that support the weight. The danger with this calculation is that it fails to account for the bundle being loaded unsymmetrically. Forklift drivers do not always place the load perfectly on the cantilever arms and this can lead to a unintended overloading.
For example: A 10 foot long bundle that weighs 5000# can be set on a cantilever rack where the columns (and arms) are on 5 feet centers. When this load is placed perfectly symmetrically the load per arm would be about 2500 lbs. per arm. If this load is placed on the two arms in an non-symmetric manner the result can be a significant overload on one arm. The more non-symmetric the placement, the worse the overload on one arm becomes. If the load is offset enough, it is possible to fail the overloaded arm if that arm was originally designed for only 2500 lbs.
The latest RMI Cantilever Specification allows three methods to determine the design load for the cantilever arm to account for non-symmetric load placement. Refer to the RMI Cantilever Specification for these methods. When this Specification language was written the RMI Specification committee did realize that there are many customers who have been buying and installing cantilever racks for a long time and may already know a design arm load that is working well for them and has been for years. The Specification allows them to specify this known arm load for their design criteria with the understanding that it is their responsibility that the load per arm is never exceeded. This Method #1 is the first of 3 listed in the new Cantilever Standard ANSI MH16.3_2025.
In instances where the arm design load needs to be calculated, the Specification offers two methods for this based on the customer’s stated load or bundle sizes. These methods are offered to reasonably account for non-symmetrically placed loads. Method #2 and Method #3 are summarized below.
Method #2 is more complex and allows the design per load to be calculated by the design engineer based on:
- The load sizes and weights given by the user
- The load weight distribution to the supporting arms
- Down-aisle and cross-aisle load placement tolerances
- Relative stiffness of the product and the arms
The column for method #2 is required to be designed for the sum of the arm design loads.
Method #3 provides some default equations to allow for uneven distribution of the load to the supporting arms in the down-aisle direction. This method is less complex than method #2 but still provides adequate conservatism for arm design in most cases. Method #3 can only be used where the down-aisle spacing of the arms is uniform and the load length is not greater than the number of arms times the arm spacing. For Method #3 the load per arm is given as 1.3W/n where W is the weight of the load and n is the number of arms supporting the load.
While user experience may lead some to choose Method #1, a second check with method #2 or method #3 may ensure that the effect of non-symmetric loading has been considered resulting in a safer arm design.