To maximize resource utilization while satisfying customer requirements
To maximize customer service subject to a resource constraint
Storage Operations
The objective of storage and warehousing is:
To maximize resource utilization while satisfying customer requirements
To maximize customer service subject to a resource constraint
Storage and warehousing resources are:
Space
Equipment
Personnel
It is desirable to include the following factors in the design of storage and warehousing systems:
Maximize the space utilization
Maximize the equipment utilization
Maximize the labor utilization
Maximize the accessibility of all materials
Maximize the protection of all materials
Storage Space Planning:
The following chart can be used to determine the storage space requirements. The changes that are made in the unit loads after receipt or before shipping must be included in Part 1 of the storage analysis chart. The maximum and average quantities of unit loads stored (Part 2) are directly related to the method of controlling inventory and inventory control objectives. These quantities must be provided as inputs to the facilities planner by the inventory control function. The planned number of unit loads for each material to be stored may be determined by considering the receiving schedule and the method of assigning materials to storage locations. If all materials to be stored in a particular manner are to be received together, the planned quantity of unit loads stored must be equated to the maximum quantity of unit loads. If materials to be stored in a particular manner are to arrive over time, then the method of assigning materials to storage locations will determine the planned quantity of unit loads stored.
Part 1
|
UNIT LOADS |
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TYPE |
CAPACITY |
SIZE |
WEIGHT |
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Part 2
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QUANTITY OF UNIT LOADS STORED |
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MAXIMUM |
AVERAGE |
PLANNED |
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Part 3
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STORAGE SPACE |
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METHOD |
SPACE STANDARD |
AREA (SQ.FT) |
CEILING HEIGHT REQUIRED |
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Table 1. Storage Analysis Chart
Storage Location Methods:
There are two storage location methods:
Randomized storage
Dedicated storage
Randomized Storage:
Randomized storage is used when an individual stock-keeping unit (SKU) can be stored in any available storage location. When an inbound load arrives for storage, the closest available slot is designated as the storage location. Retrievals are performed on a first-in, first-out basis. Randomized storage results in less storage space than that required for dedicated storage.
Figure 1. Randomized Storage.
Dedicated Storage:
Dedicated storage is used when an SKU is assigned to a specific storage location or a set of locations. The term fixed slot is used to describe the dedicated storage. Two methods of dedicated storage are commonly used:
Store items in parts number sequence
Dedicate a location for an SKU based on its activity and inventory level.
The latter method is preferred when there are significant differences in either the activity level or the inventory level for SKUs.
Dedicated storage requires more space than randomized storage. If an out-of-stock condition exists for a given SKU the empty slot continues to remain "active" with dedicated storage; whereas , it would not with randomized storage. If multiple slots are assigned for a given SKU, as the inventory level decreases, the number of empty slots will increase.
Storage Layout Planning
:Storage layout is prepared by including the following factors:
Popularity
Similarity
Size
Characteristics
Space Utilization
Popularity:
The most popular materials should be stored at locations which will require the minimum travel distance. Popularity criteria is used to maximize the throughput. Travel distances may be minimized by storing items in deep storage areas and by positioning materials to minimize the total distance. As illustrated in Figure 2 by storing popular materials in deep storage areas the travel distance to other materials will be less than if materials were stored in shallow areas.
Figure 2. Effect of storage depth on travel distance.
Using the information from Figure 2, the travel distances will be as follows:
|
Distance from Reference Point |
Storage Depth 1 unit |
Storage Depth 2units |
Storage Depth 3 units |
|
Distance to A1 |
6 |
4 |
4 |
|
Distance to A6 |
1 |
1 |
1 |
|
Distance to B1 |
12 |
7 |
6 |
|
Distance to B6 |
7 |
4 |
3 |
|
Average Travel Distance |
6.5 |
4 |
3.5 |
Table 2. Travel distances for different storage depths.
If materials enter and leave a storage area from different points and are received and shipped in the same quantity, the most popular items should be positioned along the most direct route between the entrance and departure points.
Figure 3. Material storage by popularity
If materials enter and leave a storage area from different points and are received and shipped in different quantities, the most popular items having the smallest receiving/shipping ratio should be positioned close to the shipping point along the most direct route between the entrance and departure points.
The items having the largest receiving/shipping ratio should be positioned close to the receiving point along the most direct route between the entrance and departure points.
Receiving / Shipping ratio is the ratio of the trips to receive and the trips to ship a material.
Example:
The products given in Table 3 are the most popular in the warehouse shown in Figure 4. How should these items be aligned along the main aisle?
|
Product |
Trips to Receive |
Trips to Ship |
Receiving/Shipping |
|
A |
40 |
40 |
40/40=1.0 |
|
B |
100 |
250 |
100/250=0.4 |
|
C |
200 |
400 |
200/400=0.5 |
|
D |
30 |
43 |
30/43=0.7 |
|
E |
10 |
100 |
10/100=0.1 |
|
F |
250 |
125 |
250/125=2.0 |
Table 3
Figure 4
The receiving/shipping ratios (R/S ratio) are calculated in the last column of Table 3. An R/S ratio of 1.00 indicates that the same number of trips are required to receiving and shipping. The products that have the R/S ratios less than 1.00 must be assigned to locations close to the shipping area, and the products that have R/S ratios greater than 1.00 must be stored close to receiving area. Figure 5 illustrates the distribution of the items based on their R/S ratios.
Figure 5
Similarity:
The second principle for laying out a storage area relates to the similarity of the item stored. Items that are received and shipped together should be stored together. Even if items are not received together, it is almost always practical to store them together if they are shipped together.
Size:
Having small parts stored in spaces designed for large parts wastes storage space. It is also possible that large parts may not fit in a storage slot which is selected by random storage principle. If uncertainty exist about part sizes, adjustable racks and/or shelves should be utilized to accommodate the changing needs.
Characteristics:
Some material characteristics require a different type of storage method or policy because of the following reasons:
Perishable Materials (These are the type of materials that require a controlled environment, such as refrigeration)
Oddly shaped (For oddly shaped items usually open spaces should be available for storage. These items may not fit in any of the racks that are designed for other unit loads)
Crushable items (Materials that are crushable when exposed to high humidity should be stored separate from other unit loads. These items require a special unit load size which prevents crushing under a large weight)
Hazardous materials (Materials that present hazards, such as explosion, flammability require separate storage. Safety codes should be checked and strictly followed for all flammable and explosive materials. Hazard information can be obtained from the Material safety Data Sheets (MSDS) which are supplied by the manufacturers. Materials that present health hazards must be stored according to safety codes)
Compatibility (Some chemicals are not dangerous when stored alone, but become volatile if allowed to come into contact with other chemicals. These items should be stored according to the safety information which can be found on the Material Safety Data Sheets (MSDS)).
Space Utilization:
While considering popularity, similarity, size and material characteristics, a layout must be developed to maximize the space utilization as well as level of service provided. The factors that should be included in the determination of the layout are:
Conservation of space ( Minimize honeycombing)
Limitation of space ( Space utilization may be limited by the truss, sprinkler, ceiling heights, floor loads, post and columns, and the safe stacking heights of the materials.)
Accessibility ( Selection of sufficient aisle space that can handle various types of storage and retrieval tasks, storing parts in racks and shelves that are easily accessible )
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Last Update: July 29, 1999 |
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Prepared by : Serdar Z. Elgun |
