Automation of existing tower cranes: economic and technological feasibility

Automation in Construction 7 Ž1998. 285–298

Automation of existing tower cranes: economic and technological feasibility Yehiel Rosenfeld ) , Aviad Shapira Department of CiÕil Engineering and National Building Research Institute, Technion-Israel Institute of Technology, Haifa 32000, Israel

Abstract Tower cranes enjoy a long useful working life. Therefore, a vast population of cranes are still in use today that do not feature the advanced automation and sensor technologies such as those with which some of the new models are equipped. This paper examines the technological and economic feasibility of retrofitting existing tower cranes with semi-automatic devices for motion control. The proposed improvements are intended to enhance the cranes’ efficiency and their capacity to meet the challenges of today’s tightly scheduled construction projects. Based on work studies and analyses of craning cycles, the concept offered by the proposed improvements distinguishes between the long-distance navigation of the crane’s hook and the fine maneuvering in the loading and unloading zones. The expected economic benefits resulting from the enhancement of the crane’s performance, with regard to both types of motion, far exceed the cost of installing the various devices. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Automation; Construction; Economic analysis; Productivity; Tower cranes

1. Introduction Construction cranes, and in particular tower cranes, enjoy a long useful working life that far exceeds the common values of their economic life. Whether reconditioned or not, used machines 15–25 years old can be observed on construction sites, and they also fill the ‘For Sale’ advertisements in trade magazines w1,2x. Therefore, upgrading used cranes by adding advanced features that would enhance their efficiency, may have significant economic implications for their users and owners. Indeed, crane manufacturers equip some of their new models with such features w3x, but the automation and sensor technologies involved were not yet developed and )

Corresponding author.

widespread when many of the cranes used today were built. The few academic works published in recent years on crane automation also focus, for the most part, on new equipment rather than on older machines, e.g., Refs. w4–6x. This paper examines the technological and economic feasibility of furnishing existing tower cranes with electromechanical devices for motion control. The proposed improvements are intended to enhance the cranes’ capacity of meeting the challenges of today’s construction sites, given the growing importance cranes have in determining the efficiency of many on-site operations. While these improvements could in principle be applied to all types of cranes, they appear to be particularly suitable for tower cranes. This is because of these cranes’ electricallydriven and controlled mode of operation w7x, in addi-

0926-5805r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 5 8 0 5 Ž 9 8 . 0 0 0 4 9 - 1

286

Y. Rosenfeld, A. Shapirar Automation in Construction 7 (1998) 285–298

tion to their well-defined geometry and degrees of freedom. The paper first establishes the needs for the proposed improvements and describes the concept of the system involved. Next, the technological feasibility study is provided, with details of the proposed mode of operation and special attention to safety considerations. Then work studies and analyses of craning time Ži.e., crane involvement time., conducted to determine several implications of shorter cycle times, are presented. Finally economic viability is evaluated by examining the values of various economic parameters with regard to the expected costs and benefits.

2. Proposed improvements: needs and concept Fig. 1 presents a top-slewing crane, one of the common configurations of tower cranes w8x. It is interesting to note that in terms of its operation mode Žthough not in its electromechanical systems., this crane—in common with other tower cranes—has made only marginal progress since its introduction in Europe immediately after World War II w9–11x. In the crane’s conventional mode of operation, the operator first assesses the spatial location of the target, then actuates different joints, by joysticks andror buttons, until the hook arrives, with satisfactory accuracy, close to the target. This is done through a trial-and-error process, based on feedback provided by the operator’s own vision and assessment, hand signals of a designated crane or ground director w12x at the work zone, or radio communication from the scene. In many activities Že.g., casting of a fifth-floor

Fig. 1. Common tower crane Žarrows denote degrees of freedom. with a new MMI ŽB. replacing the old one ŽA..

concrete slab., a considerable percentage of the cycle time is apparently spent on maneuvering although the operator has, again and again, repeated almost the same path. If the operator only could ‘teach’ the crane a safe and efficient route between the fixed locations of the source and the target Že.g., concrete mixer and casting area., the crane could play back that route much faster and more accurately than repeated manual cycles. A solution to this obvious need is offered by the proposed semi-automatic navigation system. The geometric configuration of tower cranes features welldefined joints, which nowadays can be equipped with reliable, inexpensive, computer-based devices for motion control. A semi-automatic navigation system installed in a used crane can benefit from a synergy among three parties: Ža. the operator’s human intelligence, judgment, and improvisation skills; Žb. the computer’s programmability, vast memory, and rapid calculation capabilities; and Žc. the sensory devices’ accurate, real-time measurements and feedback. To complement the semi-automatic navigated path, the proposed system offers an additional enhancement that addresses the subsequent finemaneuvering phase of the craning cycle with a view to eliminating the need for a crane director. This enhancement is based on the observation that indirect operator-through-crane-director maneuvering is neither efficient nor adequately safe, which is especially true when the work zone is far away from the operator or obscured from hisrher view, or when the final craning phase demands high precision Že.g., concrete column casting, flying form assembly.. The idea, then, is to obtain assistance from foremen of major crews serviced by the crane, who are obviously situated at the optimal points to direct the load to its final destination Žthe elimination of crane directors, as a means to increasing efficiency, has also been the focus of other crane-automation-related R & D efforts, e.g., Ref. w12x.. Both features—the semi-automatic navigation and the direct fine maneuvering by a foreman—are technically detailed in Section 3. Numerous on-site observations conducted by both authors in other construction-site-related studies w13– 15x revealed that typical tower crane lifting cycles are characterized by workers waiting idly for the

Y. Rosenfeld, A. Shapirar Automation in Construction 7 (1998) 285–298

crane at both the loading and the unloading locations. On rare occasions only tower cranes were observed waiting in mid-cycle because workers were still busy completing parts of the previous work cycle unrelated to the crane Že.g., they were still spreading concrete from a previous bucket when the next bucket was already at the target location.. These observations were strongly confirmed by a systematic data collection, as will be discussed later on. It could therefore be concluded that cutting craning cycle time, as is expected from both features of the proposed system, will result in a shorter duration of entire activities. The translation of this effect into a whole array of direct and indirect benefits is detailed in Section 5.

3. Technological feasibility Electronic control devices have now been in use for some decades in a variety of automatic systems, and they abound in both manufacturing and assembly. Although more and more examples are extant, this technology is slow in being transferred to construction equipment, most probably due to noticeable difficulties related to the equipment’s physical size, operational environment, and safety considerations. The general reluctance of the construction industry

Fig. 2. Small-scale semi-automatic tower crane model.

287

Fig. 3. Full-scale semi-automatic electric overhead traveling crane.

to adopt changes w16,13,17x may also play a role here. However, automation technology is mature and reliable enough today to be adopted for construction cranes. In light of this notion, a preliminary feasibility study on tower cranes was conducted w18,19x, followed by the development of a prototype semi-automatic navigation system. This system was tested on two levels: Ž1. on a small-scale model of a tower crane ŽFig. 2.; Ž2. on a full-scale heavy-duty 5-tonpayload electric overhead traveling ŽEOT. crane, tested indoors in an experimental setting ŽFig. 3. w20,21x. The experiments provided fairly accurate estimations with regard to the fundamental technological feasibility of the system’s various components and paved the way for conducting the present study with regard to tower cranes. Referring again to Fig. 1, the position and path of the hook Žexcept its sway. can be determined accurately by measuring the movement and controlling it at each joint. With the semi-automatic navigation system, the actual position of each joint is fed into the computer in real time and compared with the target position. The computer makes the necessary calculations and instructs every motor to move accordingly, until the target is reached. An interface device Žcommonly termed MMI—Man-Machine-Interface. is also necessary for operator–computer communication. Tower cranes appear to be favored candidates for such a computer-integrated navigation enhancement: not only do they posses a well-defined geometry, they also are electrically operated, and hence they usually have sockets Že.g., for remote control. to which a computer can be readily connected. Accord-

288

Y. Rosenfeld, A. Shapirar Automation in Construction 7 (1998) 285–298

ing to the proposed system, the present manual control box Ži.e., the crane’s MMI., ‘A’ in Fig. 1, is replaced by an enhanced dual-purpose control box, ‘B’, which allows both manual and semi-automatic modes of operation: the former by means of the joysticks Žor buttons., the latter through a 10-digit keypad complemented by function keys Že.g., TEACH, PLAYBACK, GOTO, ENABLE COMPUTER, ENABLE FOREMAN. and a small screen. When deciding to use the semi-automatic option, the operator would first push the TEACH button and lead the crane as usual through hisrher own selected path. This safe path would be stored in the crane’s computer memory, trimming away most of the unnecessary maneuvering motions. For safety reasons, the operator would have to run at least one closely observed test cycle and make any necessary modifications prior to routinely using the playback mode. Thereafter, any number of successive cycles could be performed considerably faster Žusing high speed and better acceleration and deceleration rates.. For the fine-maneuvering enhancement feature, the improved MMI will support several pocket-size two-way radio devices, distributed among foremen of major crews. Each of these devices must be coded differently, so that the crane operator is able to choose one foreman at a time. The device will consist of both voice communication and remote control of the crane motion Žat low speed only.. While the load is semi-automatically craned toward a certain crew, the operator will maintain verbal communication with the foreman of the crew being served. Upon arrival of the load in the proximity of its target, the foreman will be given permission by the operator Žvia the ENABLE FOREMAN button. to direct the load to the exact place. This device can integrate existing technologies w22,23x in a miniaturized yet sturdy package. It can, for example, combine radio transceivers for voice communication with the more secure infra-red remote control for the short distances between the foremen and the hook. A different approach to bridging the gap between the operator and the scene utilizes video technology w24x. A camera mounted on a mobile crane and connected to a monitor installed in the cabin allows the operator a better view of the hook andror the signaling person. If adapted to tower cranes, this

technique can be used as an additional mode of operation, or together with the remote control finemaneuvering device described above. Once the basic system of computer and userfriendly MMI is installed on board the crane, it can be utilized for a whole array of additional advanced features Že.g., preprogrammed designation of restricted zones, laser-based hook homing, speed regulation for swing-and-sway damping, task assignment and monitoring of overlapping cranes.. Such features are being gradually introduced by crane manufacturers into their new models w25,26x. The term ‘semi-automatic’ Žrather than ‘automatic’. is used throughout this paper in order to emphasize that the operator will never relinquish responsibility or authority over the crane. The operator will merely delegate authority to the computer or to a foreman cooperating with himrher. Thus, for example, sending the hook automatically necessitates the prior use of the ENABLE COMPUTER button, while allowing a foreman to perform direct fine maneuvering is conditional on pressing of the ENABLE FOREMAN button. Granting such limited control over the crane to the computer or the foremen on the scene will not only enhance efficiency but will also improve safety. It will reduce chances for human errors and incorrect spatial assessments when the computerized playback mode is activated, and it will cut out misinterpretations of verbal directions or hand-signals when the foreman takes over.

4. Work studies and craning time analysis Since the major economic benefits associated with the proposed system are related, directly or indirectly, to an expected shortening of craning time, a separate study was needed for determining and substantiating the values of certain parameters to be used in the economic analysis. The present chapter describes this study. 4.1. Effect of cycle time shortening Work studies have found that, in general, tower cranes on construction sites are utilized for 50–80% of their nominal working hours w27,5,28x. This may

Y. Rosenfeld, A. Shapirar Automation in Construction 7 (1998) 285–298

lead to the Žfalse. impression that shortening cycle times will merely reduce even further the utilization rate but would not reduce the total length of time that the crane must remain on site Žlet alone shorten the duration of the project.. The fact is, however, that tower cranes Žunlike mobile cranes w15x. typically experience alternating periods of busier and less-busy times, on various scales: hourly, daily, weekly, or monthly. For example, on a monthly scale, the crane may be very busy, day in and day out for 3 successive weeks with one crucial activity, such as the erection of a steel skeleton. During these weeks other crews who need the crane can get only limited service, if any, while during the next few weeks they may enjoy very good service, since the crane is less in demand by others. Yet even during the less-busy weeks, on some days or during some hours of a certain day, the crane may again be forced to deny service to non-critical activities. In conclusion: whenever the crane is heavily involved in one specific activity—whether for several hours or for several weeks—it soon becomes a bottleneck that not only dictates the pace of that specific activity but, at the same time, also delays other activities. Thus, no matter how busy or relatively idle the crane is on average, i.e., whether it is occupied, for example, 90% of the time or only 50%—if both the length of the activity in which it is engaged and the waiting time of other activities can be shortened, its operation will become more efficient. With this in mind, it becomes clear that shortening individual cycles will ultimately add up to a shortening of both busy periods and delays. Eventually it will result in a shorter stay of the crane on site, and even shorter project duration, although not by the same percentage as the shortening of individual cycles. 4.2. Work studies Exhaustive work studies conducted at 11 construction sites w19x have led to quantitative estimates of time savings that may be obtained by the use of a semi-automatic navigation system. The data were collected in a direct Žor stopwatch. time study w29x, which was applied to nearly 800 lifting cycles. The profile of the projects surveyed was as follows: Ø Type of constructed facility: eight residential projects, three commercial projects.

289

Ø Project size: varying from two adjacent buildings, 300 m2 area per floor each to large complexes. Ø Building height: 10–30 m. Ø Construction method: six conventional cast-inplace concrete, five industrialized Ži.e., intensive use of large-panel forms andror precast concrete elements.. Ø Phase of construction at time of study: erection of concrete skeleton in all projects. Ø Cranes on site: top-slewing configuration, medium to large size, lifting capacity greater than 30 tm load moment. The results are summarized in Table 1. The percentage of maneuvering times out of the total cycle times for the observed activities were found to be in the range of 11–30%. Potential savings in crane cycle time would be derived mainly from these percentages. 4.3. Cycle-time analysis To estimate the time savings on a crane’s ‘typical work day’ Ži.e., one representing the crane’s operation over the entire duration of the project., the crane’s operation must be broken down into its various activities. If activity j is one of m activities in which the crane is engaged during a typical day, and Tj is the percentage of the duration of j out of the total craning time ŽÝTj s 100%., then the combined daily savings Žin percent. in crane time, DT, is: m

DT s

Ý Tj Ž a j a j q bj b j y c jg j .

Ž 1.

js1

where a j s mean percentage Žout of total cycle time. of maneuvering time in loading and unloading zones for activity j: a j s fraction of a j that can be saved: bj s mean percentage Žout of total cycle time. of long-distance navigation Žor travel. time for activity j; b j s fraction of bj that can be saved; c j s mean percentage Žout of total cycle time. of loading and unloading time for activity f ; g j s fraction of c f that can be saved. Note that DT in Eq. Ž1. takes into account only the productive time periods during the crane’s typical work day Ž50–80% on average, as mentioned

290

Description of activity

Casting of concrete slabs Assembly of precast walls Assembly of hollow-core slabs Installation of reinforcement Conveying of brick pallets a

Number of cycles observed

Total cycle time a Žmin. Mean Standard deviation

Maneuvering time in loading zone Žseconds.

Maneuvering time in unloading zone Žseconds.

Mean

Standard deviation

Mean

482

12

"2

10

"1

2.1

"0.3

17.5

122

19

"3

51

"5

5.1

"0.6

22.8

98

14

"3

55

"12

3.8

"0.4

30.0

27

11

"2

16

"3

2.4

"0.3

18.6

50

10

"2

17

"3

4.0

"0.5

11.1

Including long-distance navigation, maneuvering, loading and unloading.

Standard deviation

Percentage of maneuvering time in total cycle time Ž%.

Y. Rosenfeld, A. Shapirar Automation in Construction 7 (1998) 285–298

Table 1 Summary of results of a crane cycle time work study

Y. Rosenfeld, A. Shapirar Automation in Construction 7 (1998) 285–298

above., i.e., Eq. Ž1. is true only for busy periods, in which the crane constitutes the project’s bottleneck. The effect of the unproductive times, necessarily resulting in a smaller DT, is taken into account later in the analysis. As mentioned above, a is the major contributor to DT. In certain activities, the entire maneuvering time is expected to be eliminated Ži.e., a s 1. with the use of the proposed system Že.g., in repeated semi-automatic ‘soft’ landings of an empty concrete bucket at the chute-end of a truck mixer.. In other activities, where a foreman is likely to take over from the crane operator, as described earlier Že.g., in hollow-core slab placing., a would still be close to its upper limit Že.g., a s 0.8., even if not quite unity. However, since in yet other activities only a smaller fraction of the maneuvering time may be saved, a was conservatively assigned the value of 0.5. In the present analysis, the two other potential time-savers in Eq. Ž1., b and g , were taken to be zero. It should be stressed, though, that in full-scale testing of the proposed system when mounted on an EOT, savings of 15–40% were observed in times of long-distance navigation w20x Ži.e., b s 0.15–0.40.. However, long-distance time performance of the proposed system when mounted on a tower crane was not studied, hence no sufficient data were at hand to estimate b for tower cranes. Since, in any event, the system’s potential is intended to be fully demonstrated mainly in craning cycles in which long-distance navigation time is relatively short Že.g., low-rise and mid-rise buildings., any b-related potential savings were left unutilized in the present analysis. As for loading and unloading Ž c ., this component of the cycle is outside the scope of the enhancements offered by the proposed system. 4.4. Crane-operation analysis The other factor that may have an effect on the time savings, DT, is the breakdown of the crane’s operation and the values assigned to Tj . That is because, as can be also seen in Table 1, various lifting assignments Že.g., concrete, reinforcement, precast elements. typically have different maneuvering times. However, since the range of maneuvering times in Table 1 Ž11–30%. is limited, it appears that different activity breakdowns may not have a significant effect on the weighted time savings.

291

To investigate this point, several typical breakdowns were generated and examined. The data, intended to exemplify a wide spectrum of activity combinations, were adopted from detailed case studies and from work studies conducted within the present research project w27,5,19x. The combinations, shown in Table 2, represent three concrete construction methods: conventional cast-in-place, industrialized gang or flying forms, and industrialized precast. Two sets of data are given in Table 2 for each combination: Ž1. breakdown of the crane’s operation into various lifting assignments ŽTj , in percent., and Ž2. the weighted proportion of maneuvering times ascribed to these assignments ŽTj a j in percent.. The total ÝTj a j for each combination Žbottom line in Table 2. is the percentage of the crane’s productive time consumed by maneuvering, for that combination. The values of a j , the mean percentage of maneuvering time for each assignment, were adopted from Table 1. The value taken for precast elements Ž a 4 . was the mean of wall and slab components; for forms the value Ž a2 . was that of precast Ža conservative assumption, as fine-maneuvering operations with gang and flying forms at loading and unloading locations are commonly more time-consuming than with precast elements w30x.. Note that since the activity breakdowns presented in Table 2 cover only major lifting assignments, the total of Tj for each combination is less than 100%. It could have been reasonably assumed that other lifting assignments, not included in the breakdowns Že.g., site equipment, rubble skips, miscellaneous., also involve maneuvering times to a certain extent, and thus hold promise for time savings similarly to the assignments included. However, these other assignments were not taken into account, either because of lack of sufficient data or because they are specialized and difficult to analyze. 4.5. Results As can be seen from the results obtained for ÝTj a j ŽTable 2., the weighted cumulative time consumed by maneuvering is in the range of 12–20% of the crane’s effective time. Since the results for industrialized construction Ž19–20%. —the most common method nowadays—are clearly higher than those for

292

Lifting assignment Žactivity j .

Concrete Ž a1 s17.5%. Forms Ž a2 s 26.4%. Rebar Ž a3 s18.6%. Precasts Ž a 4 s 26.4%. Blocks Ž a5 s11.1%.Total ŽÝTj a j .

Conventional cast-in-place concrete

Industrialized Žgangrflying forms. cast-in-place concrete

Comb. a1a Tj Tj a j Ž%. Ž%.

Comb. a2 b Tj Tj a j Ž%. Ž%.

Comb. a3 b Tj Tj a j Ž%. Ž%.

Comb. a4 c Tj Tj a j Ž%. Ž%.

24

4.2

20

3.5

30

5.2

28

22

5.8

15

4.0

10

2.6

14

2.6

10

1.9

15

2.8

10

2.6

y

y

5

0.6 12.6

10

1.1 11.7

y

y

y

y 12.6

Precast concrete

Comb. a5c Tj Tj a j Ž%. Ž%.

Comb. a6 c Tj Tj a j Ž%. Ž%.

Comb. a7 c Tj Tj a j Ž%. Ž%.

Comb. a8 c Tj Tj a j Ž%. Ž%.

Comb. a9 a Tj Tj a j Ž%. Ž%.

4.9

51

8.9

57

10.0

43

7.5

35

6.1

18

3.1

7

1.2

43

11.3

17

4.5

24

6.3

36

9.5

33

8.7

7

1.8

2

0.5

13

2.4

15

2.8

15

2.8

18

3.3

21

3.9

3

0.6

3

0.6

9

2.4

y

y

y

y

y

27

7.1

45

11.9

y

y 18.6

y

y 19.1

y

y 20.3

0.7 19.4

y

y 12.6

y

y 14.2

y 9

y 1.0 19.6

Tj s Percentage of cumulative durations of lifting assignment j out of effective crane operation time. Tj a j Percentage of cumulative durations of maneuvering time of lifting assignment j out of effective crane operation time. a Source: w5x. b Source: w19x. c Source: w27x.

y 6

Comb. a10 a Tj Tj a j Ž%. Ž%.

Y. Rosenfeld, A. Shapirar Automation in Construction 7 (1998) 285–298

Table 2 Cumulative maneuvering times Žin percent of effective crane operation time. for various combinations of activities

Y. Rosenfeld, A. Shapirar Automation in Construction 7 (1998) 285–298

conventional and precast construction Ž12–14%., the value adopted for the remainder of the analysis is 18%, slightly higher than the mid-range for all combinations. When applying a s 0.5 to that value, according to Eq. Ž1. and the analysis in Section 4.3, the combined savings in effective crane time, DT, is obtained as 9%. Since, as already mentioned, cranes are found to be productive 50–80% on average of their on-site service time, the value of DT was moderated by a correction factor of 0.65 Žthe midrange of 50–80%.. Thus, a saving of 6% of the entire on-site crane time is used as the basis for the economic analysis that follows.

5. Economic feasibility To overcome some difficulties inherent in the conduction of an economic feasibility study of a system that has not yet been fully implemented, the following measures were taken: Ž1. a conservative approach was adopted Ži.e., one that yields high cost values on the one hand and low benefit values on the other.: Ž2. sensitivity analyses with even more conservative assumptions were conducted; Ž3. data from previous studies relating to other crane types w20,21x Žwith the required modifications. were used only as long as they were applicable to tower cranes; and Ž4. appropriately modified data from laboratory experiments were used only as long as they applied to a construction site environment. Unless otherwise mentioned, the economic analysis that follows pertains to the tightly scheduled construction sites common today, in which craning constitutes the major bottleneck in the work’s progress. It particularly pertains to building types and lifting assignments in which hook maneuvering time around loading and unloading locations is substantial relative to the hook’s long-distance travel time. The cost–benefit analysis is conducted from the viewpoint of the construction company, which would purchase one or more units of the proposed system and install them on tower cranes it owns and operates. ŽNote that, unlike mobile crane services, which are procured mainly through a developed subcontracting or rental market, tower cranes—especially

293

in the mature tower crane culture common in Europe but to a great extent also in the US—are usually owned by the construction companies that operate them w15x.. 5.1. Costs The expected cost per unit can be assessed with reasonable accuracy on the basis of the actual development, assembly, and testing of the full-scale EOT-mounted prototype presented in Fig. 3 w20x. A tower-crane-mounted prototype is expected to cost US$25,000, as follows: US$7000 total cost Žcomponents, assembly, and installation. of the basic semiautomatic control system, US$5000 for the radio remote control option for fine maneuvering by foremen, US$6000 for optional speed regulators Žfor smoother motion., and an estimated US$7000 for installation on a tower crane Žincluding multiple weather-proof cable connectors and weather-proof wiring.. Quantity production would certainly lower the unit cost, albeit inconsiderably, since virtually all the device’s components are already existing, and standard, economies-of-scale cost reduction that may materialize is assumed to be offset by possible marketing costs. ŽSelling the system by a company offering several other, similar products as well as the nature of its advertisement in the construction equipment market, would render marketing costs insignificant. Therefore this assumption is on the conservative side.. On-site training and start-up costs are estimated at US$2000, bringing the total cost to US$27,000. While this cost is based on reliable data and reasonably conservative estimates, the proposed system’s market price would be subject to a high level of uncertainty and is likely to vary substantially according to market conditions, mainly on the level of barriers to entry. Hence, the prediction of the retail price is inherently speculative. With no barriers to entry, i.e., assuming an industry operating in a near-perfectly competitive market Žas is the common reality in the construction equipment market., the system’s market price will be driven toward its marginal cost, US$27,000. In order to serve as the basis on which the initial cost–benefit analysis would credibly be conducted, this figure

Y. Rosenfeld, A. Shapirar Automation in Construction 7 (1998) 285–298

294

was raised to US$30,000; any likely price increases for the end user Žand these could be substantial. are addressed in Section 5.4. 5.2. Benefits The major benefits to the user may be attributed to shortening the cycle time of the crane due to the semi-automatic navigation system. Additional savings are incurred by other features, as described above. 5.2.1. Better utilization of crane To assess the economic value of better crane utilization stemming from shorter cycle times, the equivalent time-dependent annual cost of the crane, R, has first to be calculated, as follows w31x: R s P = Ž ArP ,i , N . y S = Ž ArF ,i , N . q As Ž P y S . = Ž ArP ,i , N . q S = i q A

Ž 2. where P s purchase price of the crane; S s expected salvage value Žprice of the crane after N years of use.; i s annual interest rate; N s expected economic life of the crane Žin years.; Ž ArP, i, N . s Capital Recovery Factor that converts the initial investment, P, into an annual equivalent for given i and N; Ž ArF, i, N . s Sinking Fund Factor that converts the salvage value, S, into an annual equivalent for given i and N; and A s other time-dependent annual costs. The values of Ž ArP, i, N . and Ž ArF, i, N . can be readily found in tables in books on engineering economics, or calculated by the following formulae w31x:

Ž ArP ,i , N . s Ž ArP ,i , N . s

iŽ1 qi.

N

N Ž1qi. y1

i N Ž1qi. y1

,

Ž 3.

.

Ž 4.

R was first obtained for initial values of the variables P, S, N, i, and A, and then sensitivity tests were conducted. The initial values of the variables were determined as follows. v P s US$350,000—Since the price range of tower cranes is quite broad Žcommonly US$100,000 to US$600,000 w30x., a mid-range price was used for the initial calculations ŽUS$350,000 is also the pur-

chase price provided by Means w32x for the ‘most widely used’ tower crane.. It should be stressed that the values used for P could be those of either a new or a refurbished crane. This, however, may have only little effect on the outcome of the analysis, since the life expectancy N of a given crane is correlated to its purchase price Ži.e., N is longer for a new machine than for an old one of the same type.. v S s 0—There are no common estimates for the market value of used cranes w33x. However, since the price of a crane at the end of its economic life is very low relative to its purchase price, the effect of its salvage value on R is marginal. Hence, S was initially taken as nil. v N s 10 years—The life expectancy of intensively utilized tower cranes, according to common economic models w34,30x. This value is also applicable to reconditioned cranes with their extended life span w2x. v i s 8%—This value of the effective interest rate reflects a nominal interest of 11% adjusted for 3% inflation. v A s US$92,000—The components of the time-dependent annual cost considered are as follows w34,32,30x: maintenance Žservice and repairs. —9% of the purchase price ŽUS$31,500.; insurance, registration, and other fees—3% ŽUS$10,500.; cost of operator Žto the crane owner. —US$50,000. Computing R by Eq. Ž2. with these values of P, S, N, i, and A, showed that the equivalent time-dependent annual cost of a crane is US$184,150. By installing the proposed semi-automatic features, the crane would, on average, perform its assignments 6% faster, as concluded above in the craning time analysis. Consequently, a few days or a few weeks would be saved on each project. These time savings can be used to provide lifting services, with the same crane, on additional projects without incurring additional annual costs. Thus, the economic value, B1 , of this benefit can be quantified as: B1 s 6% = R s 0.06 = 184,150 s 11,049 ( US$11,050 per year.

Ž 5.

5.2.2. Reducing labor cost Another direct benefit of faster crane cycles are the resulting savings in labor cost. A faster crane

Y. Rosenfeld, A. Shapirar Automation in Construction 7 (1998) 285–298

means higher labor productivity, because the crews served by the crane will produce the same output in a shorter time Žconsidering the usual scene of workers waiting for the crane rather than vice-versa., so that the entire job will require fewer workerhours altogether. To estimate the economic value of this aspect, the following conservative assumptions were made: Ž1. The crew working with the crane and served by it Žat both loading and unloading locations. consists of four workers; Ž2. the crane is on on-site duty 1800 h per year Žthe total of both effective and ineffective time.. Combined with the 6% shortening of the entire on-site crane time, as explained earlier, and with a construction worker’s hourly cost of US$25.35 w32x, these assumptions yield a yearly labor cost savings, B2 as follows: B2 s 4 = 1800 = 0.06 = 25.35 s 10,951 ( US$10,950 per year. Ž 6. 5.2.3. Eliminating the need for a full-time crane director With the proposed system, the crane operator would require considerably less guidance and signaling. Data appearing in Table 2 ŽTj columns. indicate that, on projects for which the proposed system is intended, at least half the effective crane time would be consumed in lifting assignments performed by fairly large crews. As described in Section 3, the foremen of these crews would be equipped with a remote control device, rendering the task of a crane director redundant. Considering 1800 h of on-site crane service time per year, moderated by the 0.65 correction factor Žconverting overall to effective crane time. discussed earlier, a total of at least 585 crane director hours Ž0.5 = 1800 = 0.65. would be saved annually. Adopting a more conservative approach, half of these hours are translated to tangible benefits in the present analysis. Thus, the savings, B3 , of this aspect, considering US$27.35 hourly cost of a crane director Žtaken as a foreman w32x. will result in: B3 s 300 = 27.35 s 8205 ( US$8200 per year. Ž 7 . 5.2.4. Reducing wear of crane Installation of the proposed system would result not only in faster operation, but also, as explained earlier, in much smoother crane motions, which reduce the wear and tear of the crane’s cables, motors,

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and other mechanical parts, as well as its structural fatigue Ždeformations, cracks, etc... As a result, the crane is expected to require less repair and maintenance, experience fewer andror shorter downtime periods, and enjoy an extended useful life. Assuming that annual maintenance and repair costs Žincluding the equivalent cost of downtime. will only drop from 9% of the purchase price to 8% and that the crane’s overall economic life will be prolonged by only 1 year Žfrom ten to eleven., the annual savings B4 due to this aspect can be calculated as follows: Prolonged economic life: B4,1 s US$350,000 = Ž ArP ,8%,10 years . y US$350,000 = Ž ArP ,8%,11 years . s US$3150. Ž 8. Reduction of maintenance and repair: B4,2 s US$350,000 = Ž 9–8% . s US$3500. Ž 9. Total: B4 s B4,1 q B4,2 s 3150 q 3500 s US$6650 per year. Ž 10 . 5.2.5. Summary of direct benefits The sum total of the direct benefits, B, as expressed by Eqs. Ž5. – Ž7. and Ž10., reaches: Bs B1 q B2 q B3 q B4s11,050 q 10,950 q 8200 q6650s 36,850 ( US$37,000 per year. Ž 11 . 5.2.6. Indirect benefits Several other benefits which are, however, harder to quantify, can be envisaged. v A shortening of the total duration of the project, which saves overhead costs and management fees. It may also permit earlier occupancy or other use of the constructed facility, with possibly substantial economic implications. v More free crane time, which has the advantage that the crane can be utilized instead of other types of equipment Že.g., a concrete pump. which would otherwise have had to be brought in as an alternative for a busy crane. v A reduction of the need for scheduled overtime, a fairly common necessity, either to overcome process-induced peak workloads or to catch up with the schedule as originally planned. v Safer crane operation, resulting mainly from lower chances for human errors, as explained later on.

Y. Rosenfeld, A. Shapirar Automation in Construction 7 (1998) 285–298

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Table 3 Summary of sensitivity analyses of economic criteria Combination

Economic criterion

Number

Initial cost ŽUS$.

Annual benefit ŽUS$.

Payback period months

Benefit–cost ratio

Net present worth ŽUS$.

Annual rate of return. Ž%.

1 2 3 4

30,000 60,000 30,000 60,000

37,000 37,000 17,700 17,700

10 20 21 41

8.3 4.1 4.0 2.0

218,300 188,300 88,800 58,800

123 62 59 29

Although these indirect benefits are substantial and may, in some cases, even exceed the direct benefits, they are left here in qualitative form for the readers’ own appreciation, over and above the US$37,000 per year of clearly quantifiable benefits. 5.3. Cost–benefit analysis The cost–benefit analysis was conducted according to several common economic criteria w31x and yielded the following results: v Payback period—A US$37,000 annual benefit is about US$3100 per month. Hence the entire US$30,000 cost of the system will be recovered within 10 months. v Benefit–cost ratio—The present value of the US$37,000 annual benefit wfor N s 10, i s 8%, and Ž PrA, 8%, 10 years. s 6.71.x is US$248,300. Thus, the benefit–cost ratio Žfor a US$30,000 cost. is 8.3. v Net present worth—The dollar value of the whole venture, obtained by subtracting the present cost of the investment, US$30,000, from the present value of the benefit, US$248,300, is US$218,300. v Rate of return—A US$30,000 investment that yields an annual US$37,000 benefit over 10 years, reflects an annual rate of return of 123%. By any measure, the values obtained for all the above indices are exceptionally good. 5.4. SensitiÕity analyses Table 3 summarizes the results of sensitivity tests conducted under assumptions considerably lessfavorable than the above cost–benefit analysis. Rather than address each of the numerous variables separately, a few aggregated combinations of costs and benefits were generated. The following combinations are presented in Table 3.

Ž1. The original assumptions remain unchanged: cost of US$30,000 and annual benefits of US$37,000 Žpresent value: US$248,300.. Ž2. The benefits, US$37,000, remain unchanged Žpresent value: US$248,300., but the cost of the system is doubled, to US$60,000, as a provision for possible cost increases due to changing market conditions, as well as for various other costs initially based on estimation. Ž3. The cost, US$30,000, remains unchanged, but the benefits are reduced to B1 q B4 only US$17,700 Žpresent value: US$118,800.. This change refers to cases where the crane owner is only interested in savings of crane time, operator time, and reduced wear. Ž4. Combination of items 2 and 3 above, namely inflated cost and reduced benefits. The results obtained for the various economic criteria show that, even under extremely conservative assumptions, as reflected in the last combination, the economic performance of the system is still attractive: the cost of the system is recovered in about 3.5 years, the direct tangible benefit is twice as high as the cost; the net present worth of the system is approximately US$60,000, and the annual rate of return on the initial investment is 29%. Note that, in most cases, not only are the true benefits likely to be higher than these minimal figures, but at least part of the abovementioned unquantified indirect benefits will most probably be obtained.

6. Conclusion and future research This paper has shown that equipping existing tower cranes with semi-automatic navigation features is not only technologically feasible, but it also has a

Y. Rosenfeld, A. Shapirar Automation in Construction 7 (1998) 285–298

potential for productivity enhancement. Furthermore, as the cost of high-technology components continues to decline and that of construction labor continues to rise, the proposed system is bound to become increasingly economic. The promise lies mainly in the ability to upgrade the large population of used cranes. When these cranes were manufactured 20 and even 10 years ago, the related advanced technologies were not readily available, whereas such ‘old’ cranes can still enjoy 5 to 15 years of useful service. Eventually, the initial limited purpose of a semi-automatic navigation system will gradually transform tower cranes —and other types of construction equipment, for that matter—into more efficient, user-friendly, and safer machines. Based on the results of this feasibility study, a full-scale tower-crane-mounted system should be tested in a real construction site environment with a view to developing a practical prototype. The effort would require the active cooperation of a construction company. The results of the current study, and not least the findings of the work studies and craning time analyses conducted, would be instrumental in obtaining such cooperation. The proposed workplan for the development of this prototype is as follows. v Analyze the overall system on the basis of the tower crane selected Že.g., its structural and electromechanical configuration, wiring system, speed of hoisting, trolleying and slewing, central control panel, command system in the operator’s cabin.. v Characterize the components to be installed Že.g., central processing unit, input–output interface, power supply, speed regulators, encoders, sensors. with specific regard to tower cranes and with special attention to weather proofing. v Expand the capacity of the multi-task navigation software to handle the typically dynamic assignments of tower cranes. v Adapt the man–machine interface to fit a variety of tower cranes and improve its ergonomics and user-friendliness. v Install the hardware Žincluding wiring and connection to the central control panel. and integrate it with the software. v Prepare a testing plan, in consultation with the site manager, the crane operator, and foremen of major crews. Run tests to check the system’s techno-

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logical performance Žmainly reliability and accuracy., safety, ergonomics, and efficiency. The concept inherent in the proposed system makes a distinction between two parts of the crane’s hook motion: long-distance navigation Žbetween the loading and unloading zones. and fine maneuvering Žin the loading and unloading zones.. Whereas longdistance navigation is handled in a semi-automatic operation that essentially keeps the operator in-sole charge, fine maneuvering is accomplished through the delegation of part of the controls to others Ži.e., foremen of major crews.. This latter mode of operation may require Žat least in some countries. the modification of existing crane work regulations. It will also, of course, necessitate the procurement of a special permit to conduct the proposed full-scale testing in an active construction site setting.

Acknowledgements This research was supported by the Israel Ministry of Construction and Housing under grants number 017-438 and 017-454 and by the Fund for the Promotion of Research at Technion-Israel Institute of Technology. Assistance in data collection was provided by the authors’ former graduate students, S. Berkovitz and S. Chazon. The authors are indebted to S. Selinger, ZIV Systems, Haifa, Israel, and to N. Sicherman, Graduate School of Business, Columbia University, NY, for their constructive comments on this paper.

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