UNARCO, as well as other rack companies, offer both 3” wide and 4” wide columns. Often, a 3” column is quoted out of habit without considering the potential benefits of using a 4” wide column. The racking industry is so used to a 3” x 3” design sometimes we fail to look at stronger, more economical options.
4”x 3” closed tube columns that are 13 ga. steel thickness (t = 0.083”) have a moment of inertia of about 2.52 in^4 where a 3”x3” closed tube column of the same thickness has a moment of inertia of about 1.27”. This means that for the extra 1” of column width you are essentially doubling the stiffness of the column. The table below shows a comparison for a
90” column span for these two UNARCO column sections.
|Column||wt./ft.||Ix||Frame Cap. L = 90”|
|4×3 – 13 ga.||3.75 #/ft.||2.519||26,019 #/Frame|
|3×3 – 13 ga.||3.19 #/ft.||1.27||13,809 #/Frame|
|(Unarco II punching)|
For the case shown above, the capacity of the frame nearly doubles for a column weight difference of only 0.56#/ft. If the 3 x 3 frame selected needed to be 12 ga. its weight per foot would exceed the weight of the 4 x 3 13 ga. and its strength would still be less.
Here are some guidelines as to when to look at a 4” wide column:
1.) On pallet racks, pallet flow racks, Push Back racks or any beam-frame style rack when the column spans exceed about 72”, or anytime the 3 x 3 column gauge selection is a heavier gauge. When the 3 x 3 column is 10 ga. the 4” option is almost always more economical.
2.) On Drive-In racks greater than 22 feet to the top tie.
3.) On Pick Modules with more than one elevated floor. These tend to have longer column design lengths.
Many Pick Module racking designers and engineers have made the mistake of installing a module using 3” columns and have come back at a future time and made the request to re-configure or remove lower shelves only to find out that when they do so the column strength is not sufficient.
Another problem with building Pick Modules out of 3” columns is that “sway” beams often have to be installed to keep the column design lengths lower. These “sway” beams cannot be removed. Very often, the cost of the sway beams would have covered the upcharge for the 4” column. Many dealers prepare customer layouts using 3” columns and then later find that a 4” column would have been the right choice providing the customer with a stronger pallet rack design at a lower price.
In general, any application where the column span design length is longer than normal, a 4” column should be considered for both added story stiffness and economy.