Unit Load Storage

Policies

Marc Goetschalckx marc.goetschalckx@isye.gatech.edu

PASI 2013, Santiago, Chile

1

Born in Belgium

Georgia Tech

Total research career in logistics, material

handling, facilities design, supply chains

Material Handling Research Center & CICMHE

Supply Chain Engineering (Springer 2011)

Educational interactive software (TSP, VRPB,

block layout)

Soccer, tennis, photography, beer & wine

Instructor Information

2

Storage Policies Overview:

Introduction

Introduction to Storage Policies

5

7-Aug-13

Warehouse Operations

Flow Path Schematic (FFN)

Sharp et al. 1991 Receiving

Cross Docking

Storage

Case PickingFull Pallet

PickingItem Picking

Sortation

Packaging

Shipping

Definitions

Storage Policy

Set of rules that determine where to store arriving

SKUs in a warehousing system

Unit Load

A collection of materials that can be transported,

stored, and controlled (managed) as a single unit

• Examples

• Vast majority of discrete goods

7

Block Stacking Operation With

Counterbalanced Truck

8

Sainsbury’s Grocery

Distribution Center

10

Example: Walmart DC Pallet

Rack

7-Aug-13

Forward-Reserve:

Restocking with Reach Truck

7-Aug-13

Forward

Reserve with

Unit Load

Replenishment

Restock:

Person

remains on

floor,

forklift &

reach truck

Forward Reserve:

Order Picking on Higher Levels

7-Aug-13

Order Picking:

Person in cab

travels up,

Order picking

truck

Empty Single-Deep Pallet Rack

with Four Levels

15

ASRS Pallet Unit Load High-Rise

Storage

16

Volkswagen Automated Finished

Goods (Cars) Warehouse

17

Handling Units Example:

Collapsible Pallet

18

Wine Barrels in a Rack

19

Container Port Overview

(Hamburg Port Altenwerder)

20

Twin Yard Cranes (Hamburg

Port Altenwerder Quay)

21

Amazon Forward Reserve

Warehouse

7-Aug-13

SKUs (Products) Classification

Bulk Storage

Liquids , gases, dry bulk

Discrete Units Storage

Unit Loads

• Free standing

• Rack supported

Loads of different sizes

25

Warehousing Storage Objectives:

Back to Basics

Minimize the expected travel time & cost

for given input-output operations

Minimize MH equipment and personnel

Variable (marginal) costs

Minimize the required storage space for

given stored inventory

Minimize capital investment

Fixed costs

28

Storage Policies Overview:

Unit Load Storage Policies

Introduction to Storage Policies

Unit Load Storage Policies

29

Unit Load Main Principle to

Maximize Storage Capacity

Use the “Cube” by

utilizing the height

of the warehouse

and keeping it

filled

33

Unit Load Main Principle to

Minimize Travel Time

Place unit loads that

generate the

highest frequency

of access

in locations

with the lowest

expected access

time

34

Warehousing Storage

Objectives

Minimize the Expected Travel Time

Minimize MH Equipment and Personnel

Minimize the Required Storage

Space

Minimize Capital Investment

Min f tj jj

Min N

36

Occupancy Gantt Chart:

Rack Based Direct Access

Time Periods

J1

J2

J3

J4

J5 I

6

I1

I9

I3

I7

I4

I8

I2

I10

I5

37

Unit Load Storage Formulation

Single command, Direct Access

min

. .

{ , }

c x

s t x i

b x j

x

ij ijj

N

i

M

ijj

N

i iji

M

ij

11

1

1

1

1

0 1

38

Occupancy and Constraint

Matrices

B bi

L

NMMM

O

QPPP

[ ]

1 1 1 0 0

0 1 1 1 0

0 0 1 1 1

A

I I I I

B

B

B

L

N

MMMMMM

O

Q

PPPPPP

..

..

..

.. .. .. .. ..

..

0 0 0

0 0 0

0 0 0

39

Vector Assignment Formulation

Neither AP nor 3DAP nor BPP

Consecutive ones in occupancy matrix B

Block diagonal structure of constraint

matrix

Integrality property not satisfied

41

VAP Conclusions

Very large integer optimization problem

Very tight LP relaxation

Efficient sub problem and problem size

indicate decomposition

Very small gap for Lagrangean relaxation

upper bound

Highly primal and dual degenerate

Acceptable penalty for primal heuristic

44

Dedicated vs. Shared Storage

Policies: Product Dedicated

Product Dedicated Storage Policies

Static

Simple

Space inefficient (maximum)

Unconstrained replenishment

maxDED p p pt MAX

tp p

N q si I N

45

Dedicated vs. Shared Storage

Policies: Product Shared

Product Shared Storage Policies

Dynamic

Requires inventory map

Simple (COL) or complex (DOS)

Space efficient

N ISHAt

ptp

RS|T|UV|W|

max

46

Unbalanced Warehouse Size for

Various Policies

0

2

4

6

8

10

12

14

16

18

1 6 11 16 21 26 31 36 41 46 51 56

A

B

C

D

All

Ded

Sha

47

Storage Policy Size Ratio

Storage Policy Size Ratio

a

N

NDED

051. ,

49

Storage Policy Size Ratio

18, 16

160.89

18

DED SHAN N

a

51

Perfectly Balanced Warehouse

Size for Various Policies

0

2

4

6

8

10

12

14

16

1 6 11 16 21 26 31 36 41 46 51 56

A

B

C

D

All

Ded

52

Perfect Balanced Storage Space

Ratio for Shared Storage Policies

16, 10

100.625

16

DED SHAN N

a

54

Storage Policies Overview:

Unit Load Storage Policies

Introduction to Storage Policies

Unit Load Storage Policies

Product Based Storage Policies

56

Unit Load Warehouse

1 2

P1

3 4 5 6

7

13

19 20 21 22 23 24

8

14

9

15

10 11 12

16 17 18

P2

P3

57

Safety Inventory for Product

Turnover Based Storage

Safety inventory si aded to

replenishment order size q to

compute locations required

Safety and cycle replenishment

order size q determined by supply

chain factors

p p

p

p p p

F Ff

N q si

58

Product Information (Factoring)

Product A B C

Characteristic Symbol p

Storage Requirement q 12 2 10

Pallets Received Monthly Through

Door P3 e3 400 60 200 0.500

Pallets Shipped Monthly Through

Door P1 e1 300 45 150 0.375

Door P2 e2 100 15 50 0.125

Total 800 120 400

59

Factoring Condition

All products have identical

probability mass functions for the

selection of input/output points

(docks)

F e pe

Fp pk

kpk

pk

p

p p ppk k

60

Travel Independence

If factoring condition is satisfied:

expected travel time is independent

of the product stored in location

g1 = 0.375*80+0.125*60+0.500*25 = 50

g13 = 0.375*60+0.125*80+0.500*15 = 40

g p dj k kjk

61

Warehouse Factoring Distances

1

50

2

50

P1

3

50

4

50

5

50

6

57.5

7

42.5

13

40

19

42.5

20

42.5

21

42.5

22

42.5

23

42.5

24

50.0

8

42.5

14

40

9

42.5

15

40

10

42.5

11

42.5

12

50.0

16

40

17

40

18

47.5

P2

P3

62

Locations Sorted by Increasing

Distance

1

50

2

50

P1

3

50

4

50

5

50

6

57.5

7

42.5

13

40

19

42.5

20

42.5

21

42.5

22

42.5

23

42.5

24

50.0

8

42.5

14

40

9

42.5

15

40

10

42.5

11

42.5

12

50.0

16

40

17

40

18

47.5

P2

P3

63

Three Dedicated Storage

Policies

Fast and furious

“Fast movers closest to the door”

Small is beautiful

“Small inventory closest to the door”

But higher turns beats them all

Frequency of access ratio of demand

rate divided by maximum inventory

“Fastest turning closest to the door”

“Short stays closest to the door”

64

Demand Based Layout

Commonly Called “Place Fast

Movers Closest to the Door”

Products Ranked by Decreasing

Demand or Operations

FA = 800, FC = 400, FB = 120

Assigned to Locations by

Increasing Expected Travel Time

65

Demand Based Layout

1

50

C2

50

C

P1

3

50

C4

50

C5

50

C6

57.5

B

7

42.5

A

13

40

A

19

42.5

A20

42.5

A21

42.5

A22

42.5

A23

42.5

A24

50.0

C

8

42.5

A

14

40

A

9

42.5

C

15

40

A

10

42.5

C11

42.5

C12

50.0

B

16

40

A17

40

A18

47.5

C

P2

P3

66

Total Travel Time for Demand

Based Layout

Total Travel = Sum of Product

Travels

TA = 2·800·41.46 = 66,333

TB = 2·120·53.75 = 12,900

TC = 2·400·47.50 = 38,000

T = 66,333+12,900+38,000 = 117,233

%T = 0.8 % over 116,333

67

Inventory Based Layout

Commonly Called “Place Low

Inventory Products Closest to the

Door”

Rank Products by Increasing

Number of Required Locations

qB = 2 , qC = 10, qA = 12

Assign to Storage Locations by

Increasing Expected Travel Time

68

Inventory Based Layout

1

50

A2

50

A

P1

3

50

A4

50

A5

50

A6

57.5

A

7

42.5

A

13

40

B

19

42.5

C20

42.5

C21

42.5

C22

42.5

C23

42.5

C24

50.0

A

8

42.5

A

14

40

B

9

42.5

A

15

40

C

10

42.5

C11

42.5

C12

50.0

A

16

40

C17

40

C18

47.5

A

P2

P3

69

Total Travel Time for Inventory

Based Layout

Total Travel = Sum of Product

Travels

TA = 2·800·48.54 = 77,667

TB = 2·120·40 = 9,600

TC = 2·400·41.75 = 33,400

T = 77,667+9,600+33,400 = 120,667

% T = 3.7 % over 116,333

70

Frequency of Access

Sort and Locate Products by

Decreasing Frequency of Access to

Locations with Expected Distance

fA = 800/12 = 66.66

fB = 120/2 = 60

fC = 400/10 = 40

p p

p

p p p

F Ff

N q si

71

Product Turnover Dedicated

Storage Layout (Factoring)

1

50

C2

50

C

P1

3

50

C4

50

C5

50

C6

57.5

C

7

42.5

A

13

40

A

19

42.5

A20

42.5

A21

42.5

A22

42.5

A23

42.5

A24

50.0

C

8

42.5

A

14

40

A

9

42.5

B

15

40

A

10

42.5

B11

42.5

C12

50.0

C

16

40

A17

40

A18

47.5

C

P2

P3

72

Total Travel (Factoring)

Total travel = sum of product

travels

TA = 2*800*41.46 = 66,333

TB = 2*120*42.5 = 10,200

TC = 2*400*49.75 = 39,800

T = 66,333+10,200+39,800 = 116,333

2

2 2 2p

p

p

p p p p j p p

p p j Z p

T T

f q g f g F g

73

Product Information

(Non-Factoring)

Product A B C

Characteristic Symbol

Storage Requirement q 12 2 10

Pallets Handled Monthly Through

Door P1 e1 300 6 100

Door P2 e2 100 24 240

Door P3 e3 400 90 60

Total 800 120 400

75

Product-Door Probabilities

(Non-Factoring)

Product A B C

Characteristic Symbol

Storage Requirement q 12 2 10

Pallets Handled Monthly Through

Door P1 e1 0.375 0.050 0.250

Door P2 e2 0.125 0.200 0.600

Door P3 e3 0.500 0.750 0.150

76

Product A Expected Distances

1

50

2

50

P1

3

50

4

50

5

50

6

57.5

7

42.5

13

40

19

42.5

20

42.5

21

42.5

22

42.5

23

42.5

24

50.0

8

42.5

14

40

9

42.5

15

40

10

42.5

11

42.5

12

50.0

16

40

17

40

18

47.5

P2

P3

77

Product B Expected Distances

1

34.75

2

39.75

P1

3

44.75

4

49.75

5

54.75

6

60.75

7

28.75

13

30.25

19

39.25

20

44.25

21

49.25

22

54.25

23

59.25

24

65.25

8

33.75

14

35.25

9

38.75

15

40.25

10

43.75

11

48.75

12

54.75

16

45.25

17

50.25

18

56.25

P2

P3

78

Product Storage Assignment

Formulation (Non-Factoring)

min 2

. .

1

0

i ij ij

i j

ij i i

j

ij

i

ij

f g x

s t x q si i

x j

x

capacity

service

requirement

naturally

integer

80

Product Turnover Dedicated

Storage Layout (Non-Factoring)

1

34.75

B2

52.75

C

P1

3

45.75

C4

38.75

C5

31.75

C6

29.75

C

7

28.75

B

13

40

A

19

42.5

A20

42.5

A21

42.5

A22

42.5

A23

42.5

A24

40.25

C

8

42.5

A

14

40

A

9

42.5

A

15

40

A

10

40.75

C11

33.75

C12

31.75

C

16

40

A17

40

A18

35.25

C

P2

P3

81

Total Travel (Non-Factoring)

Total Travel = Sum of Product Travels

TA = 2*800*41.46 = 66,333 TB = 2*120*31.75 = 7,620 TC = 2*400*38.05 = 30,440 T = 66,333+7,620+30,440 = 104,393

2 2

2 2p

p p p p p

p p

p pj p p

p j Z p

T T f q si g

f g F g

82

Product Turnover Class Based

Storage

Pure product dedicated is very

space inefficient

3 to 5 classes based on frequency

of access

Dedicated space for each class

Inside class use random (RAN) or

closest open location (COL)

Shared storage policy inside a class

83

Turnover Based Class Storage

Policy

Class space determined by

simulation

Class space estimated based on

service level statistics

84

Classes based on Frequency of

Access

Sort and aggregate products by

decreasing Frequency-of-Access

fA = 800/12 = 66.66

fB = 120/2 = 60 sequence[A,B,C]

fC = 400/10 = 40

2 classes {{A,B},{C}}, but {{A},{B,C}} may

be better alternative

p p

p

p p p

F Ff

N q si

85

Inventory Measures and

Relationships

Safety

Inventory

Inv

en

tory

Time

Average

Cycle

Inventory

Average

Cycle

Inventory

iq=ci+si

ci=q/2

miq=2ci+si=q+si

86

Inventory Distribution

Middle of Period Withdrawal

Probability of i units of inventory is

distributed [0,q]+si, nearly uniformly

except two extreme periods

2

2

2

2

12

2

12

ii

i

i

qE i si

qVar i

q

Inventory

Time

q

1

87

Zone K Size Determination

Based on Service Level a

2

2

2

2

12

p

K p p

p K p K

p

K p

p K p K

qI I si

q

Z I z

Px x

z

K K K

LNM

OQP

a

91

Average Zone Size Calculations

1

2

[12,2,10] 24

2

6,1,5 , 12

, 6 1 7

5

MAX DED

K p p p

p K p K

p

A B

C

q N N

I I q si

I I

I I A B I I

I I C I

92

Standard Deviation Zone Size

Calculations

2 2

2 2

2 22 2

2

2 2

1

2

2

12,2,10

2 12 2, 12.167,

12 12

2 2 10 20.5, 8.500

12 12

,

12.167 0.500 12.667 3.559

8.500 2.915

ii A

B C

K p

p K

A B

C

q

q

93

Zone Size Calculations

1 1

1

2

, 0.95 1.65

7 1.65 3.559 7 5.87 12.87 13

5 1.65 2.915 5 4.81 9.81 10

13 10 23

23 24 96%

p p

k

k

z N N

N I z

N

N

N N

space ratio

a

94

2-Class Layout

1

50

22

50

2

P1

3

50

24

50

25

50

26

57.5

7

42.5

1

13

40

1

19

42.5

120

42.5

121

42.5

122

42.5

223

42.5

224

50.0

2

8

42.5

1

14

40

1

9

42.5

1

15

40

1

10

42.5

111

42.5

112

50.0

2

16

40

117

40

118

47.5

2

P2

P3

95

Average Travel Time for Each

Zone

1

2

12 2

5 40 8 42.5 54041.54

13 13

2 42.5 47.5 7 50 482.5

10 10

k k k

k

k

k p z p j

p Z p Z j Zk

T T

T F g F gN

g

g

96

Travel Time (Random inside 2

Zones with Spillover)

1

1

2

2 1

2 800 120 0.95 41.54 0.05 48.25

2 920 41.87 77,048

12 400 0.95 48.25 0.05 57.5

10

2 400 48.71 38,970

77,048 38,970 116,018

% 0.3 from 116,333 (optimal full dedicated)

k

k p z z

p Z

T F g g

T

T

T

T

a a

97

Modeling Extensions

(Research)

Number of classes and product

partitions

Skewness of the frequency of access

(turnover) distribution

Dual command

Quadratic programming

Warehouse layout/configuration

Problem size (10K SKUs, 20K unit

locations) and dynamic (8 periods) 98

Storage Policies Overview:

Unit Load Storage Policies

Introduction to Storage Policies

Unit Load Storage Policies

Product Based Storage Policies

Unit Load Based Storage

Policies

99

Shared Storage Example:

Frequency of Access Distribution

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Fre

qu

en

cy o

f A

ccess

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Location

DOS

COL

DED

111

Duration of Stay Shared

Storage Observations

Exploits that first and last unit load

in batch are different

Cross docking (DOS = 0)

Minimizes both storage space and

travel time for a perfectly balanced

warehouse

Very constrained perfectly

balanced replenishment pattern

np(t) 114

Perfectly Balanced Warehouse

Balanced = Minimum Space

Perfectly Balanced = Minimum

Space and Minimum Time

n t n t p t p

z n i

p p

p pi

p

( ) ( ) ,

( )

1

n t n t tin out( ) ( )

115

Duration of Stay Example:

Product Schedule Information

product daily reorder replenishment

demand quantity day

A 1 4 3

B 0.25 2 2

C 1 4 2

D 1 4 1

E 0.25 3 3

F 0.25 3 7

G 1 4 4

H 0.25 2 6

I 0.25 3 11

Total 5.25 29117

Duration of Stay Example:

Statistics

Product dedicated required storage

space = 29

Total daily demand rate = 5.25

Warehouse size = 24 locations

118

Duration of Stay Example:

Layout with Expected Distances

1

50

2

50

P1

3

50

4

50

5

50

6

57.5

7

42.5

13

40

19

42.5

20

42.5

21

42.5

22

42.5

23

42.5

24

50.0

8

42.5

14

40

9

42.5

15

40

10

42.5

11

42.5

12

50.0

16

40

17

40

18

47.5

P2

P3

119

Balanced Inventory Pattern

Product

Period A B C D E F G H I Total

1 2 1 1 4 1 2 3 2 3 19

2 1 2 4 3 1 2 2 1 3 19

3 4 2 3 2 3 1 1 1 2 19

4 3 2 2 1 3 1 4 1 2 19

5 2 2 1 4 3 1 3 1 2 19

6 1 1 4 3 3 1 2 2 2 19

7 4 1 3 2 2 3 1 2 1 19

8 3 1 2 1 2 3 4 2 1 19

9 2 1 1 4 2 3 3 2 1 19

10 1 2 4 3 2 3 2 1 1 19

11 4 2 3 2 1 2 1 1 3 19

12 3 2 2 1 1 2 4 1 3 19

120

Duration of Stay Example:

Unit Load Duration of Stay

Arrival Period

1 2 3 4 5 6 7 8 9 10 11 12 zone

1 D C A G 1

2 D C A G 2

3 D C A G 3

4 D C, B A, E G H F I 6

5 0

6 0

7 0

8 B E H F I 4

9 0

10 0

11 0

12 E F I 3

Total 19

Dura

tion o

f S

tay

121

Unit Load Duration of Stay

Shared Storage Layout

1

50

122

50

P1

3

50

4

50

5

50

6

57.5

7

42.5

4

13

40

1

19

42.5

420

42.5

421

42.5

422

42.5

423

42.5

324

50.0

12

8

42.5

4

14

40

2

9

42.5

8

15

40

2

10

42.5

811

42.5

812

50.0

12

16

40

317

40

318

47.5

8

P2

P3

122

Duration of Stay Example:

Total Travel (Duration-Of-Stay)

Total Travel = sum of Duration-Of-

Stay zone travels

Frequency of Access (FOA)

2 * for storage and withdrawal access

4 * for withdrawal access (1/p) only

T T f z gp

gpp

p p pp

jj Zp p

FHGGIKJJ

2 41

123

Duration of Stay Example:

Total Travel (Duration-Of-Stay)

DOS z FOA g Travel

1 1 1.0000 40.00 160.00

2 2 0.5000 40.00 160.00

3 3 0.3333 40.83 163.33

4 6 0.2500 42.50 255.00

5 0 0.2000 0 0.00

6 0 0.1667 0 0.00

7 0 0.1429 0 0.00

8 4 0.1250 43.75 87.50

9 0 0.1111 0 0.00

10 0 0.1000 0 0.00

11 0 0.0909 0 0.00

12 3 0.0833 50.00 50.00

Total 875.83 124

Duration of Stay Example:

Duration-Of-Stay Performance

T1 = 4*40/1 = 160

T2 = 4*80/2 = 160

T3 = 4*122.5/3 = 163.33

T4 = 4*255/4 = 255

T8 = 4*175/8 = 87.5

T12 = 4*150/12 = 50

T = 160+160+163.33+255+87.5+50 =

875.83

Space ratio = 19 / 29 = 66 % 125

Closest-Open-Location (COL)

Storage

Only 19 best locations

1 1 1

1

1

1 1 1 1 1

1

1

1

1

1 1

1 1 1

1

50

2

50

P1

3

50

4

50

5

50

6

57.5

7

42.5

13

40

19

42.5

20

42.5

21

42.5

22

42.5

23

42.5

24

50.0

8

42.5

14

40

9

42.5

15

40

10

42.5

11

42.5

12

50.0

16

40

17

40

18

47.5

P2

P3

126

Closest-Open-Location Travel

Time

Random access to any used location

Overall expected travel distance

1

4 4 4N

i i i i i i j

i j

T T T gf q r g r g N

(5 40 10 42.5 47.5 3 50) /19

822.5 /19 43.29

4 43.29 (4 1 5 0.25) 909.08

3.8% 875.83

g

T

T vs DOS

127

Closest-Open-Location

Performance Characteristics

Space ratio = 19 / 29 = 66 %

Zone z FOA g Travel

1 19 0.2763 43.29 909.08

2 5 0.0000 51.50 0.00

Total 909.08

128

Duration of Stay Example:

Two Class Product Turnover

Product turnovers

Product classes{{A,C,D,G},{B,H,E,F,I}}

1 1 1 1 1 1 1 1 1, , , , , , , ,

4 8 4 4 12 12 4 8 12pf

1

2 2

2

1

1

4 4 2 8

4 2 12 1.50

4 6 2.45

8 1.65 2.45 8 4.04 12.04 13

A

A

I

N

129

Two Class Product Turnover:

Size of Class 2

2

2 2

2 2

2

2

2

2 2 2 3 3 2 6.5

2 2 12 0.50

3 2 12 0.917

2 0.5 3 0.917 3.75 1.94

6.5 1.65 1.94 6.5 3.20 9.70 10

13 10 23

23 29 79%

B

E

CL

I

N

N

space ratio

130

Duration of Stay Example:

2CL Warehouse Layout

1

50

22

50

2

P1

3

50

24

50

25

50

26

57.5

7

42.5

1

13

40

1

19

42.5

120

42.5

121

42.5

122

42.5

223

42.5

224

50.0

2

8

42.5

1

14

40

1

9

42.5

1

15

40

1

10

42.5

111

42.5

112

50.0

2

16

40

117

40

118

47.5

2

P2

P3

131

Duration of Stay Example:

2CL Expected Travel

1

2

1

2

5 40 8 42.5 13 540 13 41.54

2 42.5 47.5 7 50 10 482.5 10 48.25

4 4 0.95 41.54 0.05 48.25

4 4 41.87 669.98

4 1.25 0.95 48.25 0.05 57.5

4 1.25 48.71 243.56

669.98 243.56 913.55

4.3% 875.83

g

g

T

T

T

T vs DOS

132

2 Class Performance

a 0.95

Zone z f g g(a) Travel

1 10 0.4000 41.54 41.87 669.98

2 13 0.0962 48.25 48.71 243.56

3 1 0.0000 57.50 0.00

Total 23 913.55

Space ratio = 23 / 29 = 79 %

133

Shared Storage Policies:

Comparison Summary

Comparison criteria

Space, travel time, information

requirements, implementation simplicity

Policy Space Space Travel Time Time

DOS 19 875.83

COL 19 0% 909.08 3.8%

2CL 23 21% 913.55 4.3%

134

Not Perfectly Balanced

Systems

Static Greedy Heuristic

Sort by Increasing Departure Time

Adaptive, Dynamic Heuristic

Combine DOS into classes

Remedial Action for Full Classes

z p E np p

135

Storage Policies Overview:

Unit Load Storage Policies

Introduction to Storage Policies

Unit Load Storage Policies

Product Based Storage Policies

Unit Load Based Storage Policies

Experimental Comparison

Conclusions

137

Unit Load Storage Policy

Conclusions

Unit load systems are very common

Single or dual command cycles

Two main objectives:

• storage space,

• total travel time

Three planning problems

• Strategic sizing

• Tactical storage policy

• Operational storage & retrieval sequence 138

Conclusions (2):

Three Types of Storage Policies

No information (RAN, COL)

• Space efficient, simple storage, retrieval with inventory

map, medium travel efficiency

Product Turnover Information (DED, 2CL, 3CL)

• DED: space inefficient, simple

• xCL: good tradeoff between space, travel, and

simplicity

Unit Load Time Information (DOS, 2TZ, 3TZ)

• DOS: theoretical best, but not practical

• xTZ: tradeoff not as good as xCL in practice

139

Easier

Do more than three classes make a difference?

Does dual command make a difference?

Do aisle configurations make a difference?

Harder

Does this apply to deep lane, block stacking (not

rack supported) ?

When to relocate?

Extensions to container ports

Research Questions

140

Unit Load Storage Policy

Conclusions

Savings magnitude depends on

replenishment pattern, # products,

product correlations

Data requirements indicate

automated warehouses or WMS

141

May I answer any questions?

142