Designing a block stacking storage layout

Block stacking of unit loads is the optimal storage philosophy in many facilities. This concept stacks unit loads on top of each other and places them on the floor in storage lanes (blocks), generally 2-10 loads deep.

12/01/1999


Block stacking of unit loads is the optimal storage philosophy in many facilities. This concept stacks unit loads on top of each other and places them on the floor in storage lanes (blocks), generally 2-10 loads deep. Depending on the weight and stability of the loads, the stacks range from two high to a figure determined by acceptable safe limits, load stackability, vehicle lift-height capacity, crushability of loads, and building clear height.

Block stack storage alternatives are well suited for areas with:

- Low ceiling (clear) heights

- Large quantities of stock-keeping units (SKUs) where the load is stackable and relatively stable

- Full unit load putaways and picks

- First in/first out (FIFO) is not an operational requirement.

Planning

When considering block stacking, designers must understand that there is more to planning alternatives than defining heights and lane depths.

The first step is defining why bulk storage is the method of preference. At a minimum, a cursory review of other storage systems -- such as selective, pallet-flow, double-deep, or drive-through rack -- should be done.

While these more sophisticated options may have better selectivity and FIFO controls, the investment is higher, and both flexibility and space are underutilized. Additionally, physical constraints of the plant and product storage container may make these other storage types nonfunctional.

Space requirements of the alternatives are directly related to the volume of material to be stored and use-of-space characteristics. The two most important use-of-space characteristics are aisle and honeycombing allowances.

Aisle allowance is the space percentage occupied by aisles within a storage area (aisle area/total area). Honeycombing allowance is the percent of storage space lost due to ineffective use of the storage capacity. Honeycombing occurs whenever a multiunit storage location is partially filled with material. The unoccupied area is honeycombed space, and can occur both horizontally and vertically (below).

Honeycombing is a result of the operational philosophy that once product is removed from storage, the location can only be used with the same product (and often lot number) or locked stock occurs. Locked stock results when access to a particular SKU is physically blocked by another.

Once the decision is made to block stack inventory, the next step is to define the optimal lane depths. Varied depths maximize space use and improve labor efficiency.

If a small variety of products is stored and throughput is high, space use is more important than product selectivity. The bulk storage area resembles layout 1 (below).

On the other hand, if a great variety of products is stored and throughput is relatively low, then selectivity is more important than space use. This bulk storage area resembles layout 2 (below).

When deciding the number of positions and lane depths to use in a bulk storage design, consider the "A," "B," and "C" impact. For warehouse layout planning purposes, the three categories are based on transaction frequencies rather than dollars invested.

"A" items are the 20% of the SKU population constituting the top 80% of activity (transactions). "B" items make up the next 15% of the activity, and are generated by 30% of the population. "C" items make up the final 5% of activity and encompass the remaining 50% of the population.

If activity is the only basis on which lane depth is based, the design is skewed to accommodate the "A" items, since they represent 80%. Doing so results in a design similar to layout 1. Consequently, the "C" items become locked stock. Gains achieved in space use are negated by the labor increase needed to gain access to the "C" items, which become buried within the lanes and reduce product accessibility.

Conversely, if population is used as the basis for which lane depth is designed, the layout looks similar to layout 2. Gains in product accessibility are offset by poor space use resulting from the large number of lanes.

In determining lane depths, it is important to look at each class independently. All else being equal, "A" items are generally stored in deeper lanes, while "C" items are placed in shallower lanes with increased accessibility. By analyzing each class separately, it is possible to determine the best lane depth resulting in increased space use and selectivity, as well as reduced honeycombing.

Analysis

It is important to define the storage parameters (stack height, load width, load depth, aisle allowances, etc.) of the product being stored. Once these parameters are determined, the analysis can be performed.

The first step is assigning each SKU to either an "A," "B," or "C" class. The inventory class level is evaluated to determine the optimum lane depth, which is the one with, or near, the lowest space standard. Analysis tools are available to assist in calculating the lowest space standard and optimum lane depth for each inventory class level. The following plot is a graphical representation of an analysis performed using hypothetical parameters.

For this example, the optimal lane depth is represented by the lowest point (2). Once an analysis is done for each inventory class level, the lanes should be grouped into three or four "ideal" depths. Watch for storage depths most frequently providing the best space utilization across various inventory levels. From this analysis, total space requirements are calculated by multiplying the number of loads within an inventory level by the predetermined space standard.

There are tools available to help get the most out of space and labor in a unit-load, block-stacking environment. To keep the blocked-storage area from becoming an inefficient black hole for merchandise, consider at a minimum honeycombing, activity levels, and varied lane depths as a part of the planning methodology.

-- Edited by Ron Holzhauer, Managing Editor, 630-320-7139, rholzhauer@cahners.com

Key concepts

Block stacking puts unit loads on top of each other and places them on the floor in storage lanes.

Block stacking is good for plants with low ceilings, many SKUs, full load puts and picks, and when FIFO is not an operational requirement.

Stack height, load width and depth, and aisle allowances should be defined.

More info

The author is willing to answer technical questions concerning this article. Mr. Barnes is available at 678-657-6103.

The complete text of all feature articles is available on our web site, PE Online: www.plantengineering.com.

Calculating honeycomb allowance

To determine the honeycomb allowance, look at the probability of having a given number of units in a storage location at any given time.

Consider a bulk storage area in which material will be stored three high and two deep. For this example, the probability of having a given number of units in storage at any given time is the same for any possibility. In other words, all onhand inventory possibilities, 0-6, have a 1-in-7 chance of occurring. If there happens to be two units in storage, then there are four honeycombed positions; five units in storage, one honeycombed position, etc.

The expected number of honeycombed positions is equal to the number of honeycombed positions for each storage multiplied by the probability of having the corresponding level of storage. In our case, there are 2.14 (1/7 X [1 + 2 + 3 + 4 + 5]) honeycombed positions. Honeycombing is 36% of installed capacity. On average, 2.14 out of 6 positions are lost as a result of honeycombing. A honeycombing factor is determined and used to incorporate location losses into the total space required.

In this example, a six-load capacity storage location stores, on average, 3.86 (6 - 2.14) pallets of inventory. Hence the honeycombing factor is 1.55 (6/3.86).

In summary, plan for ten (6 X 1.55) locations to store the planned level of six loads. Honeycombing allowances (and losses) are reduced for a SKU as a lot is stored over multiple locations.

Comparing space requirements

With storage space at a premium, it is important to maximize what and how much are available. The layout planner should determine the amount of space required to safely and effectively store inventory. The storage alternative with the lowest space standard maximizes the use of this physical inventory space. The space standard is defined as the ratio of square feet/unit of storage. The multiple of the space standard times planned inventory is an estimate of the required floor area.

The XYZ Co. needs to store 320 pallets. The three storage methods being considered are selective rack, double-deep rack, and block. The layout constraints for each alternative have already been determined and are shown.

The unit load, rack, and aisle characteristics are:

Unit load length: 48 in. Unit load width: 40 in.

Rack beam length: 98 in. Rack frame depth: 48 in. - 6 in. = 42 in.

(3-in. overhang/side of pallet)

Rack flue space: 12 in. Rack upright width: 3 in.

Storage aisle: 104 in. Intersecting aisle: 144 in.

Selective rack analysis

1. Estimated floor space (width): [6 in. + 12 in. + 6 in. + (42 in. X 4) +

(104 in. X 2)]/12 in. = 33.33 ft

Estimated floor space (length): 144 in. + [10 bays X (98 in. + 3 in.)] +

3 in./12 in. = 96.42 ft

Estimated total area: 33.33 ft X 96.42 ft = 3214 sq ft

2. Calculate space standard: 3214 sq ft/320 pallets = 10.04 sq ft/pallet

Aisle allowance area 1: 33.33 ft X (144/12) ft = 400 sq ft

Aisle allowance area 2: [(104 X 2)/12] ft X {[10 X (98 in. + 3 in.)] +

3 in./12 in.} ft = 1463 sq ft

Total aisle area: 400 + 1463 = 1863 sq ft

Aisle allowance: Total aisle area/total area = 1863/3214 = 58%

Allowance factor: 1/(1 - 0.58) = 2.38

Double-deep rack analysis

1. Estimated floor space (width):

[6 in. + (12 in. X 5) + 6 in. + (42 in. X 8) + (104 in. X 2)]/12 = 51.33 ft

Estimated floor space (length):

144 in. + [5 bays X (98 in. + 3 in.)] + 3 in./12 = 54.33 ft

Estimated total area: 51.33 ft X 54.33 ft = 2789 sq ft

2. Determine honeycombing factor: 1.09 (assume 2 positions/SKU)

3. Calculate space standard:

2789 sq ft/320 pallets X 1.09 = 9.50 sq ft/pallet

When honeycombing is taken into account, 3040 sq ft are required

to store 320 pallets.

Aisle allowance area 1:

[(104 X 2)/12] ft X [5 bays X (98 in. + 3 in.)] + 3 in./12 ft = 734 sq ft

Aisle allowance area 2: 51.33 ft X (144/12) ft = 616 sq ft

Total aisle area: 734 + 616 = 1350 sq ft

Aisle allowance: Total aisle area/total area = 1350/3040 = 44%

Allowance factor: 1/(1 - 0.44) = 1.79

Block stacking analysis

1. Estimated floor space (width): [(8 X 48 in.) + 120]/12 = 42 ft

(assume 10-ft storage aisle)

Estimated flor space (length): [(10 X 40) + 144 in.]/12 = 45.33 ft

Estimated total area: 42 ft X 45.33 ft = 1904 sq ft

2. Determine appropriate honeycombing factor:

1.283 (assume 2 slots/SKU)

3. Calculate space standard: 1904 sq ft/320 pallets X 1.283 = 7.6 sq ft/pallet

When honeycombing is taken into account, 2432 sq ft are required

to store 320 pallets.

Aisle allowance area 1: [(10 X 40)/12] ft X (120/12) ft = 333 sq ft

Aisle allowance area 2: 42 ft X (144/12) ft = 504 sq ft

Total aisle area: 333 + 504 = 837 sq ft

Aisle allowance: Total aisle area/total area = 837/2432 = 34%

Allowance factor: 1/(1 - 0.34) = 1.52

Storage alternative characteristics

Storage methodology Sq ft/pallet Total sq ft Relative selectivity Aisle allowance

Selective rack 10.04 3214 High 58%

Double-deep rack 9.5 3040 Medium 44%

Block stack 7.6 2432 Low 34%





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