An Approximate Cost Estimation Model based on ... - Springer Link

KSCE Journal of Civil Engineering (2013) 17(5):877-885 DOI 10.1007/s12205-013-0287-z

Construction Management

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An Approximate Cost Estimation Model based on Standard Quantities of Steel Box Girder Bridge Substructure Chi Don Oh*, Chansik Park**, and Kyong Ju Kim*** Received September 30, 2011/Revised July 3, 2012/Accepted September 18, 2012

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Abstract Cost estimation during early stages of construction should be swift and accurate. However, approximate estimation of the costs of bridge construction in Korea is flawed because it is calculated using average construction costs per length or area and do not reflect bridge characteristics. For quick and accurate cost estimates of the substructure of steel box girder bridges with minimum input data, this study proposes an approximate cost estimate model that is based on standard quantities of major work items. The data for 52 steel box girder bridges in Korea were collected. 42 bridges were analyzed for the model development and 10 for validation. Five major work items were identified that accounted for 80% or more of construction costs. The developed estimation model utilizes only six input data: overall height of the bridge abutment, number of piers, overall height of piers, types and numbers of bridge shoes, number of locations where steel pipes are used, and length of steel pipe. It shows an error range of -5.78% through +3.93%. Keywords: steel box girder bridge, approximate cost estimating, quantity of standard work, major work item, major cross section ··································································································································································································································

1. Introduction Cost estimates at the early stage of construction project are bases for feasibility study and evaluation of design alternatives. The accuracy of early cost estimates is extremely important to both the owner and engineer (Kim et al., 2009b; Kim et al., 2009c; Kim 2011, Kim and Kim 2010). It generally depends on the information available at the stage of a construction project. Early cost estimates are usually inaccurate due to lack of information (Bell and Kaminsky, 1987; Carr, 1989; Flyvbjerg et al., 2002; Trost and Oberlender, 2003; Kim et al., 2009c). Presently, approximate cost estimates for bridge construction in Korea utilizes the linear analysis method based on construction costs per length or area. These previous methods have limitation on the evaluation of design alternatives at the early stage of detailed design because they cannot reflect various characteristics of a bridge (Kim et al., 2009c). This study classifies the structure

of a bridge into its major elements such as superstructure, substructure, civil work, and other categories of work. For more accurate cost estimate at the early stage of detailed design, this research suggests separate models for its major elements. Steel box girder bridges and PSC beam bridges account for the highest percentage (24%) of bridges constructed along national roads in Korea as shown in Table 1. For this reason, this study targets an approximate cost estimate model for the steel box girder bridge. The whole model for a bridge is too large to describe in a paper. This paper limits the scope of the estimating model to the substructure of the steel box girder bridge only, not the whole bridge. This model utilizes standard quantities of major work items that influence overall construction cost. Then the model estimates total quantities of work items with minimum input data and assigns unit prices to them. This model is expected to support quick and accurate cost estimates using only the

Table 1. Types and Share Rates of Bridges (Kim, 2009a) Type No. Share Rate Type No. Share Rate

PSC Beam 71 24% RC Box 6 2%

Steel Box Girder 71 24% PSC Slab 3 1%

RC Rigid-Frame 67 22% PSC Box 3 1%

Preflex 32 11% Composite rigid-frame 2 1%

IPC Girder 24 8% Others 18 6%

*Ph.D. Candidate, School of Architecture & Building Science, Chung-Ang University, Seoul 156-756, Korea (E-mail: [email protected]) **Member, Professor, School of Architecture & Building Science, Chung-Ang University, Seoul 156-756, Korea (Corresponding Author, E-mail: [email protected]) ***Member, Professor, Dept. of Civil & Environmental Engineering, Chung-Ang University, Seoul 156-756, Korea (E-mail: [email protected]) − 877 −

Chi Don Oh, Chansik Park, and Kyong Ju Kim

Table 3. Criteria for Approximate Cost Estimates of Bridges

limited information available during preliminary design stages.

2. Literature Review As shown in Table 2, many researchers have suggested approximate cost models to accurately estimate construction costs during early project stages, such as parametric models based on statistics (Singh, 1990), regression model (Kim et al., 2004; Kouskoulas and Koehn, 1974; Trost and Oberlender, 2003), etc. In particular, Hegazy and Ayed (1998) developed a parametric cost model for road and bridge construction based on past highway construction costs and the neural network. Kim et al. (2000) proposed a cost model for road construction through time series analysis on the construction cost index and multi-regression analyses on historical unit price data. Park and Lee (2002a, 2002b) suggested a regression model based on historical data for road construction. In bridge construction area, Kim and Kim (2010) proposed a cost estimation model for preliminary design stage. The model is based on Case-Based Reasoning (CBR) and Genetic Algorithm (GA). Kim (2011), also, suggested the cost estimation model which utilizing CBR for the railroad bridge. Kim et al. (2009c) suggested an approximate cost estimate model for superstructure of the PSC beam bridges that is based on unit quantity of standard works. Pure multiplicative formulations (Wilmot and Cheng, 2003), extrapolations of past trends/time-series analyses (Hartgen and Talvitie, 1995) and cost indexes (Park and Lee, 2003) have also been used to forecast overall construction costs. For road construction, in Korea, cost estimates are typically based on the average prices of unit quantities such as cost per length (cost/ Table 2. Current Approximate Cost Estimate Models Author(s) (Year) Method Kouskoulas and Koehn Regression Analysis (1974) Parametric Models based on StaSingh (1990) tistics Building Trost and Oberlender Construction Regression Analysis (2003) Kim et al. (2004) Regression Analysis Neural Networks Genetic AlgoKim et al. (2005) rithms Stevens (1995) Unit Price Hartgen and Talvitie Time-Series (1995) Hegazy and Ayed Neural Network Genetic Algo(1998) rithms Road Kim et al. (2000) Multiple Regression Analysis Construction Park and Lee (2002) Unit price Park and Lee (2003) Cost Index Wilmot and Cheng Pure multiplicative Formulation (2003) Kim et al. (2009) Quantity of Standard Work Case Base Reasoning (CBR) and Bridge Kim and Kim (2010) Genetic Algorithms Construction Kim (2011) Case Base Reasoning(CBR)

Class Guideline for Investment Evaluation (MOCT, 2007a) Guide for Road Construction (MOCT, 2007b)

Average Unit Price USD (KRW1,000)

Criteria

182 (2,358)

per ft2 (m2)

190 (2,461)

per ft2 (m2)

2-lane road 133 (1,723) 4-lane road 130 (1,681) per ft2 (m2) 6-lane road 133 (1,729) Guideline for Preliminary 8-lane road 118 (1,533) Feasibility Study 2-lane road 5,157 (20,421) (KDI, 2004) 4-lane road 10,422 (41,271) per ft (m) 6-lane road 13,752 (54,456) 8-lane road 15,454 (61,196) Table 4. Criteria for the Calculation of Outlined Construction Costs for Bridge Construction Proper Length of Span (feet) RC Slab 16-44 RC T-Beam 40-60 RC Box 50-120 CIP/PS Slab 40-65 CIP/PS Box 100-150 PC/PS Slab 20-50 PC/PC 30-120 Bulbut T Girder 90-145 PC/PSI 50-120 PC/PS Box 120-200 SRTUCT Steel I-Girder 60-300 Type of Bridge

Cost Range $/ft2 (1000KRW/m2) 85-120 (1,096-1,548) 90-180 (1,162-2,322) 100-170 (1,290-2,194) 95-130 (1,266-1,678) 80-150 (1,032-1,936) 120-180 (1,548-2,322) 100-170 (1,290-2,516) 100-195 (1,290-2,516) 115-175 (1,484-2,258) 140-250 (1,806-3,226) 150-215 (1,936-2,774)

m) or area (cost/m2), as specified by the Guideline for Investment Evaluation (MOCT, 2007a), Guide for Road Construction (MOCT, 2007b), and Guideline for Preliminary Feasibility Study (KDI, 2004) as shown in Table 3. The California Department of Transportation (Caltrans) in the United States provides various bridge types, their optimum span lengths, and average historical unit prices as shown in Table 4. This data can be used for approximate cost estimates. These models consider only cost per length (cost/m) or area (cost/m2). None of these models reflect the characteristics of each bridge. As a result, these models cannot support the evaluation of design alternatives on a bridge during early stage of detailed design. This study proposes an approximate cost estimate model that reflects the characteristics of bridge elements. The substructure of steel box girder bridges is the target of the model.

3. Collection of Data and Selection of Major Work Items 3.1 Overview of Data Collection This study collected design documents for 52 steel box girder bridges from 21 road construction projects constructed in Korea since 2000 for analyses of cost data. Fig. 1 through 5 show

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KSCE Journal of Civil Engineering

An Approximate Cost Estimation Model based on Standard Quantities of Steel Box Girder Bridge Substructure

distributions of completion year, local area, maximum span, composition of span, and width. 42 bridges out of 52 were used for analysis, and 10 for validation.

Fig. 1. Completion Year

3.2 Classification of Construction Cost Contents Lee (2003) pointed out that in building construction area, 20% of total number of Bill of Quantity (BoQ) accounted for more than 80% of the entire construction costs. This study identifies major work items that accounts for 80% or more of the entire construction costs among work items included in substructure works. The work of classifying the whole quantity, as shown in Fig. 6, was performed into superstructure, substructure, civil work, secondary work, and other categories. Substructure works were divided into bridge abutment, pier and foundation.

Fig. 2. Local Area

Fig. 6. Classification of Total Quantity

Fig. 3. Maximum Span

3.3 Identification of Major Work Items Major work items are selected for over 80% of the total construction costs of the substructure. The ratio of work items should be calculated using an expression such as the following: Construction cost by each work item (Abutment or Pier or Foundation) Ratio of construction = ×100 costs by the work items Total construction cost (Abutment or Pier or Foundation)

(1)

Fig. 4. Composition of Span

As shown in Table 5, the bridge abutment and pier were divided into the five work items: major material cost, rebar assembling, formwork, bridge shoe and concrete placing. The foundation Table 6 was estimated to occupy 80% or more by Table 5. Ratios of Major Work Item Costs in Bridge Abutment and Pier Bridge Abutment (Average) A. Major material cost 41.54% B. Rebar assembling 19.09% Major work C. Formwork 11.14% items D. Bridge shoe 7.11% E. Concrete placing 5.93% Sub-total 84.81% Other work items 15.19% Total 100% Work Items

Fig. 5. Range of Bridge Width Vol. 17, No. 5 / July 2013

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Pier (Average) 40.50% 23.23% 7.89% 12.70% 4.48% 88.80% 11.20% 100%


Table 6. Ratios of Major Work Item Costs in Foundation Foundation (Average)

Work Items Major work items

A. Steel pipe pile material cost B. Steel pipe pile driving or mounting C. Head/end rebar Other work items Total

98.70% 1.30% 100%

material cost, of the driving or mounting, and head and end reinforcement work items. As a result, major work items for the substructure can be identified as shown in Table 7 and 8. Therefore, approximate cost estimates for substructures of steel box girder bridges can be estimated through calculations of the construction costs for these major work items by utilizing standard quantity measure.

Fig. 7. Correlation of Bridge Abutment Overall Height and Concrete Placing Quantity

4. Approximate Cost Estimating Model 4.1 Method of Standard Quantity Measure The superstructure, substructure and foundation types of steel box girder bridges are determined at the early stage of detailed design phase. The width and rough height of the bridge and abutment, and the height of pier are also set at this phase. Therefore, this available information can be utilized in calculating standard quantities described as follows. 4.1.1 Concrete Placement The quantity of the abutment has the characteristic of increasing or decreasing with overall height as shown in Fig. 7. Therefore, the standard quantity of concrete placement for the abutment is calculated as the concrete quantity (m3/m) per height of the abutment. Piers can be divided into three parts: coping, column and footing.

Fig. 8. Distribution of Coping Height

Pier quantities increase or decrease depending on column height. The approximate height of a pier can be estimated, but there is a limitation on calculating specific column height during the early design stage. For this reason, a standard section for a pier was analyzed from collected documents. As shown in Fig. 8, copings are constructed in the range of 2.0 m through 4.459 m with 3.0 m as the most frequent height. In

Table 7. Selection of Major Work Items in Bridge Abutment and Pier Class Major Work Material cost Rebar assembling Formwork

Concrete placing Bridge shoe

Detailed Contents Bridge Abutment Pier Ready-mix concrete Ready-mix concrete Rebar Rebar Complicated Very complicated Veneer Pump car Plain concrete

Veneer Steel form Pump car User specified

Specification

Unit

24 MPa D16 or above D16 or above 3 times 4 times 24 MPa 16 MPa

m3 ton ton m2 m3 EA

Table 8. Selection of Major Work Items in Foundation Class Major Work Items Steel pipe piling work

Detailed Contents Bridge Abutment Pier Steel pipe piling material cost Steel pipe pile driving or mounting Head and end reinforcement − 880 −

Specification

Unit

D = 508 mm, t = 12 mm D = 508 mm Bolt type (D = 508 mm)

m m column



having a coping with 3 m height, a circular shape of column and a square shape of footing with 2.5 m height (Fig. 11). As a result, the standard quantity of concrete placing work for the pier can be calculated as shown in Table 9. 4.1.2 Rebar Assembling The standard quantities of rebar assembling for the abutment and pier can be calculated using the ratio of rebar assembling quantity (ton) over the concrete placing amount (m3). Here, the quantity of rebar assembling is the net quantity.

Fig. 9. Distribution of Footing Height

Rebar assembling quantity (ton) Rebar Assembling = ×100 Standard quantity (ton/m3) Concrete placing quantity (m3)

(2) 4.1.3 Formwork The form works use steel form or veneer types (3 times and 4 times). The standard quantity (m2/m3) also can be calculated using the ratios of their quantity (m2) over the concrete placing quantity (m3). Fig. 10. Distribution of Footing Width

4.1.4 Material Cost (Concrete and Rebar) The work quantities of concrete placing and rebar assembling are net quantities. The materials of the two items, however, have loss in work. The calculation for the costs of the materials should apply surcharge factor considering their loss. As a result, for the calculation of material cost, surcharge ratios are applied to the quantities of concrete placing (1%) and rebar assembling (3%). 4.1.5 Bridge Shoe The steel box girder bridge has different type, size and performance of shoe. Therefore, the user directly input the information on the type and the quantity of the shoe.

Fig. 11. Standard Section of Pier

addition, footings are constructed to 2.5 m in height and 8.0 m, 9.0 m or 10.0 m in width, as shown in Fig. 9 and 10. Their shape is usually square. From this analysis, this study established a standard cross section of a pier for a steel box girder bridge

4.1.6 Steel Pipe Pile Work The major work items of steel pipe pile work include the material quantity of steel piles, pile driving or mounting and head/end reinforcement. As a result of analyzing the BoQ, the quantity of steel pile in the abutment was a minimum of 14 pieces, and a maximum of 48 pieces. The average of 28 pieces of pile was used as the standard quantity of steel pile in the abutment. For the pier, 32 piles were used for 8 m footing width, 42 piles for 9 m and 49 piles for 10 m. As described above, the methods used to estimate the standard quantities of major work items for each element (bridge abutment,

Table 9. Method of Calculating the Standard Quantity of Concrete Placement Part Bridge abutment Coping Pier Column Footing

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Calculating Method Calculate standard quantity of concrete placement (m3) with bridge abutment height (m) Concrete Placing (m3) / 3 m 3 Concrete Placing (m ) / (Pier overall height - coping height (3 m) - footing height (2.5 m)) Calculate footing height (2.5 m) and width (8 m or 9 m or 10 m) − 881 −


Table 10. Calculation Table for Standard Quantity of Bridge Abutment Major Work Item

Detailed Work Item

Calculation Expression

Unit

Concrete placing Rebar assembling

Pump car Complicated Veneer (3 times) Veneer (4 times)

Concrete placing quantity (m3)/Abutment overall height (m) Rebar assembling quantity (ton)/concrete placing quantity (m3)

m3/m ton/m3

Standard Quantity 26.086 0.108

Form quantity ( m2)/concrete placing quantity (m3)

m2/m3

1.372

EA m3 ton

-

Formwork Bridge shoe Major material cost

User input Concrete placing quantity ( m3) Rebar assembling quantity (ton)

Ready-mix concrete Rebar

Table 11. Calculation Table for Standard Quantity of Pier Major Work Item

Coping Concrete placing Rebar assembling Formwork


Unit

Concrete placing quantity (m3)/3.000 m Concrete placing quantity (m3)/(Pier overall height (m) - coping height (3.0 m) - footing height (2.5 m)) Footing width that user selects Rebar assembling quantity (ton)/concrete placing quantity (m3)

m3/m

Standard Quantity 19.869

m3/m

6.591

3

m /m ton/ m3

0.158

Form quantity (m2)/concrete placing quantity (m3)

m2/m3

0.874

EA m3 ton

-

Detailed Work Item

Pump car

Column

Footing Very complicated Steel Coping Circular column Column Veneer (4 times) Footing

Bridge shoe Major material cost

User input Concrete placing quantity (m3) Rebar assembling quantity (ton)

Ready-mix concrete Rebar

Table 12. Calculation Table for Standard Quantity of Foundation Major Work Item

Steel pipe pile work

Detailed Work Item


Unit

Steel pile material cost

-

Column

Steel pipe pile driving or mounting Head and end reinforcement

-

Column Column

pier and foundation) are shown in Tables 10-12. 4.2 Methods of Quantity Calculation and Model for Estimating Construction Cost The quantities of abutment, pier and foundation can be calculated using the standard quantity estimate tables provided in Section 4.1. However, the standard quantity is quantity for each one. In general, a bridge has four of abutments and a different number of piers according to the number of superstructure spans. Therefore, the user must input the height of each abutment and piers to estimate the entire quantity of the abutments and piers. For the quantity of steel pipe pile, the user also specify the numbers of abutments and piers in which steel pipe pile will be installed because steel pipe pile foundation can be installed in only part of the abutment and pier. The quantity of concrete placing of an abutment can be calculated by multiplying the standard quantity (m3/m) by an

Standard Quantity Bridge abutment: 28 columns Pier footing width 8 m 32 columns Pier footing width 9 m 42 columns Pier footing width 10 m 49 columns Number of columns used in steel pipe pile material cost Number of columns used in steel pipe pile material cost

overall abutment height (m) that is entered by the user. The quantities of rebar assembling and formwork are calculated by multiplying the standard quantity by the calculated total quantity of concrete placing (m3). The material cost is calculated by multiplying the surcharge rate by the calculated total quantity of concrete placing (m3). Here, the quantities of formwork for 3 time use and 4 time use are calculated by multiplying the composition ratio of each item because the quantity of formwork means the total quantity (Table 13). For the calculation of pier quantity, overall pier height (m) is entered and the width of the pier footing is selected. Coping is calculated by multiplying the standard quantity by 3.0 m, as determined in the standard section. Columns are calculated using values that subtract coping and footing heights from overall height. In particular, since almost all footings of bridge piers are square, the quantities are calculated using the input of footing width. The total quantity of concrete placing is

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Table 13. Expressions for Calculating the Quantity for a Bridge Abutment Major Work Item Concrete placing Rebar assembling Formwork Major material cost

Detailed Work Item Pump car use Complicated Veneer (3 times) Veneer (4 times) Ready-mix concrete Rebar

Bridge shoe

Standard Quantity Quantity Calculation Expression 26.086 (m3/m) Bridge abutment overall height (m) × Standard quantity 0.108 (ton/m3) Concrete placing quantity (m3) × Standard quantity 2 3 Concrete placing quantity (m3) × Standard quantity × composition ratio (84.61%) 1.372 (m /m ) 2 3 1.372 (m /m ) Concrete placing quantity (m3) × Standard quantity × composition ratio (13.07%) Concrete placing quantity (m3) × Extra charge rate (1%) Rebar assembling quantity (ton) × Extra charge rate (3%) User input

Table 14. Expressions for Calculating the Quantity for a Pier Major Work Item Concrete placing Rebar assembling Formwork

Major material cost

Detailed Work Item Coping Rebar assembling Molding Very complicated Steel Circular column 4 times of plywood Ready-mix concrete Rebar

Standard Quantity 19.869 (m3/m) 6.591 0.158 (ton/m3) 0.874 (m2/m3) 0.874 (m2/m3) -

Bridge shoe

Quantity Calculation Expression 3.000 m × unit quantity (Pier overall height(m) - 3.000 m - 2.5 m) × Standard quantity (User selected width)2 × 2.500 m Concrete placing total quantity(m3) × Standard quantity (ton/m3) Concrete placing quantity (m3) × Standard quantity × composition ratio (30.98%) Concrete placing quantity (m3) × Standard quantity × composition ratio (34.09%) Concrete total quantity (m3) × Standard quantity × composition ratio (33.48%) Concrete placing quantity (m3) × Extra charge rate (1%) Rebar assembling quantity (ton) × Extra charge rate (3%) User input

Table 15. Expressions for Calculating the Quantity for a Foundation Major Work Item Steel pile work

Detailed Work Item Pile material cost Driving/mounting Head/end reinforcement

Standard Quantity -

Quantity Calculation Expression No. of steel pipe piles × pile length × extra charge rate (5%) No. of steel pipe piles × (pile length - 0.2) No. of steel pipe piles

Fig 12. Approximate Cost Estimating Model for Substructure of a Steel Box Girder Bridge

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calculated by summing the concrete placing quantities by the parts calculated. In addition, the quantities of rebar assembling, form and major material are calculated using the calculated concrete placing quantity as with abutments, and each quantity of form is calculated by multiplying the composition ratios of steel form, steel circular column form and 4 times veneer (Table 14). Various types of bridge shoes are used for the abutment and pier as described in Section 4.1. Thus, the user should input the quantity directly. The quantity of steel pipe pile foundation is calculated using the same method as in Table 15 using the number of steel pipe piles and pile length that the user inputs. The approximate cost estimating model based on the standard quantity of major work items can be summarized as shown in Fig. 12. An approximate cost of each item can be estimated by multiplying the total quantity by unit price. The construction costs of a steel box girder bridge substructure can therefore be estimated using six input values: number of piers, overall height of each abutment and pier, types and number of shoes, number of steel pipe piles and length of steel pipe piles.

5. Model Validation To validate the suggested cost model, this study selects 10 steel box girder bridges (Table 16) which reflect various

characteristics of a bridge substructure and compares the estimated cost of the suggested model with the results from the detailed BoQ. The error range of the estimate was +3.93% ~ -5.78% (Fig. 13).

6. Conclusions For quick and accurate cost estimates of the substructure of steel box girder bridges with minimum input data, this study proposes an approximate cost estimate model that is based on standard quantities of major work items for the substructure of steel box girder bridges. The data for 52 steel box girder bridges in Korea were collected. 42 bridges were analyzed for the model development and 10 for validation. This study identified the available information during early stage of the design phase. Five major work items were identified that accounted for 80% or more of the total construction costs through an analysis of BoQ and a quantity production sheet. The developed estimation model utilizes only six input data: overall height of the bridge abutment, number of piers, overall height of piers, types and numbers of bridge shoes, number of locations where steel pipes are used, and length of steel pipe. The process for cost estimate was established using the standard quantity of major work items and unit price, and was validated by applying it to 10 real projects. The results of this validation study indicated that the model was acceptable with an estimation error less than 6%. However, this scope of this model was intended to the substructure, excluding superstructure and secondary work. Further studies are being performed for entire steel box girder bridges.

Acknowledgements This Research was supported by the Chung-Ang University Research Grants in 2010.

References Fig. 13. Error Rates for the Bridges Table 16.Basic Information of the Bridges for the Model Validation Test Bridge No. A B C D E F G H I J

Ordering Organization Busan Seoul Busan Daejeon Iksan Seoul Seoul Busan Seoul Wonju

Project Segment Jeongchon-Hotan Hanam-Hoido2 Jeongchon-Hotan Geumsan-Dogye Muju-Seolcheon Hanam-Hoido2 Hanam-Hoido2 Nongso-Eomo Hanam-Hoido2 Shinmuk-Yongsan

Maximum Span 65 m 60 m 55 m 55 m 55 m 50 m 50 m 50 m 45 m 45 m

Span Span1 Span3 Span5 Span4 Span6 Span7 Span3 Span6 Span2 Span6

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