CHAPTER 17 ESTIMATING PROCESS.

Presentation on theme: "CHAPTER 17 ESTIMATING PROCESS."— Presentation transcript:

1 CHAPTER 17 ESTIMATING PROCESS

2 17.1 Estimating Construction Costs
Estimating is the process of looking into the future and trying to predict project costs and resource requirements. Project costs and Resource requirements  정확한 예측은 손해를 예방하고 이익을 촉진.

3 17.2 Types of Estimates 설계 완료 전  개산 견적 : 공사 수량  공사 단가  10% 정도의 오차  설계 완료 후 폐기 전체 설계의 40% 완료 : 예비(기본)설계 완료 후  예비 견적 산출 상세 설계 완료 후  설계도 및 시방서 작성 완료  상세 견적 산출  3% 정도의 오차 공사가 발주자의 예산 범위 내에 있는지, 시공자의 견적이 적절한지 체크하는 용도로 사용 견적 비용은 전체 공사비의 0.25% 정도 In building construction, the four types of estimates are commonly used: 1. Conceptual estimate : 설계 없이 2. Preliminary estimate : 기본 설계 후 3. Engineer’s estimate : 상세 설계 후 4. Bid estimate : 입찰 견적

4 17.2 Types of Estimates FIGURE 17.1 Costs based on a representative unit. From Building Construction Cost Data 2008.

5 17.2 Types of Estimates PREFERATION degree Tender: 입찰
(공식적 형식적 문서에 사용하는 용어)

6 17.3 Detailed Estimate Preparation
The preparation of a detailed bid-level estimate requires that the estimator break the project into cost centers or cost sub-elements. 전체 프로젝트를 비용 추정이 가능한 작은 비용 단위로 분해 Resource : man hours, materials, subcontracts, equipment hours, and dollars needed to accomplish the work or meet the requirements associated with the cost center. The cost center relates to some physical subcomponent of the project, such as foundation piles, excavation, steel erection, interior dry wall installation, and the like. Certain nonphysical components of the work generate costs, however, and these cost centers must also be considered. Many of the items listed as “indirects” are typical of costs that are not directly connected with physical components or end items in the facility to be constructed: insurance, bonding premiums, fees, expense, home office overheads

7 17.3 Detailed Estimate Preparation
The process of estimating: Break the project into cost centers. Estimate the quantities (quantity takeoff) required for cost centers that represent physical end items (e.g., cubic yards of earth, linear feet of pipe, etc.) For nonphysical items, determine an appropriate parameter for cost calculation (e.g., the level of builder’s risk insurance required by the contract or the amounts of the required bonds). Price out the quantities determined in Step 2 using historical data, vendor quotations, supplier catalogs, and other pricing information.  Price per unit (unit cost) basis or a lump-sum (one job) basis. Calculate the total price for each cost center by multiplying the required quantity by the unit price. This multiplication is commonly called an extension, and this process is called running the extensions.

8 17.3 Detailed Estimate Preparation
Labor Hour 17.3 Detailed Estimate Preparation Subcontactor + = 간접비를 항목별로 따로 계산

9 17.4 Definition of Cost Centers
The subdivisions (cost centers) into which the project is divided for detailed cost estimation purposes are variously referred to as 1. Estimating accounts 2. Line items 3. Cost accounts  공사가 시작된 이후 관리 목적으로 사용: 코스트 센터라 할 수 없음 4. Work packages  원가관리와 일정관리를 위해 사용

10 17.5 Quantity Takeoff The development of the quantities of work to be placed in appropriate units (e.g., square feet cubic yards, etc.) is referred to as the quantity takeoff or quantity surveying.  물량 단위(ft2, yard2)로 표현된 작업 물량 The first step in the quantity takeoff procedure is to identify the materials required by each estimating account or work package.  소요 자재량 파악

11 FIGURE 17.4 Small wall construction (CMU is concrete masonry unit)
17.5 Quantity Takeoff 설계도 Materials (자재) FIGURE 17.4 Small wall construction (CMU is concrete masonry unit)

12 17.5 Quantity Takeoff 자재 소요량

13 17.5 Quantity Takeoff Recap Sheet 여유율을 감안하여 소요량 산정 공사 중 파손되어 못 쓰는 양 포함
60개중 10% 낭비  54개 유효

14 17.6 Methods of Detailed Cost Determination
The two methods of cost determination most frequently used: (a) unit pricing  공종에 대한 단가 개념 (b) resource enumeration 소요되는 자원을 일일이 계산 Quantity take off  Unit price = Estimate  100 lineal feet  $65/lineal feet = $6,500 단가 제공 기관 1. R. S. Means Company’s Building Construction Cost Data 2. R. Walker’s Building Estimator’s Reference Book 3. Richardson Construction Estimating Standards

15 17.6 Methods of Detailed Cost Determination
자재비+인건비 + Overhead & Profit Material + Installation = Total + O&P를 포함한 인건비 단가 O&P를 포함하지 않은 순수한 인건비 단가 (24시간)  = (8시간)  = +2.641.1 190 190 Square Foot Contact Area O&P: 자재비  10% markup (profit) 추가 FIGURE 17.7 Line item cost development using R. S. Means data.

16 17.7 Problems with Unit-Cost Method
Cost / unit: 평균 단가라는 문제  생산성 반영  단가 조정 필요 Resource hours (RH) per hour / Units per hour = RH / unit Cost / unit = RH / Unit  Cost / RH : 단위당 비용보다는 단위당 자원 예측이 보다 안정적임  물가 상승의 영향을 덜 받음(변경된 단가를 적용함으로써)

17 17.7 Problems with Unit-Cost Method
단가에 생산성 Factor를 고려하여 조정단가 생성 생산성 Factor

18 17.8 Resource Enumeration Almost every project has unique or special features for which unit-pricing data may not be available. Cost center  개별 하위 구성요소(자원)로 분해 Resource Enumeration : 보다 세부적인 요소로 분해하여 세부요소 단위로 비용을 추정하는 방법 Efficient factor가 90%라면, 인건비 $30.83는 $ (= $30.83/0.9)로 반영되어야 함

19 17.8 Resource Enumeration 장점: 정확한 추정 가능 단점: 보다 많은 시간과 노력 필요 활용 사례
단점: 보다 많은 시간과 노력 필요 활용 사례 단가가 존재하지 않는 경우에만 사용 예산에서 차지하는 비중이 높아 정확한 원가 추정이 필요한 Item에만 사용 기존 사례를 참조할 수 없는 유일한 프로젝트에만 사용 $30.83/0.9

20 17.9 Work Package or Assembly-Based Estimating
Missing FIGURE Construction systems concept—concrete footing

21 17.9 Work Package or Assembly-Based Estimating
컴퓨터를 이용한 자동화 FIGURE Work package concept

22 17.9 Work Package or Assembly-Based Estimating
수작업 FIGURE Work package collection sheet—concrete slab

23 17.9 Work Package or Assembly-Based Estimating
수작업 FIGURE Work package collection sheet—excavation

24 FIGURE 17.15 Steps in the estimating process
17.10 SUMMARY Three estimation methods 단순한 프로젝트 Unit-pricing or catalog lookup method 복잡한 프로젝트 2. Resource enumeration 3. Work package/assembly method 최소의 비용으로 최대의 정확도를 경험 & 판단 필요 단순계산 경험 필요 FIGURE Steps in the estimating process