ASTM E1946-2007 Standard Practice for Measuring Cost Risk of Buildings and Building Systems《估算建筑物和建筑系统成本风险的标准实施规范》.pdf

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1、Designation: E 1946 07Standard Practice forMeasuring Cost Risk of Buildings and Building Systems1This standard is issued under the fixed designation E 1946; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision.

2、 A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers a procedure for measuring cost riskfor buildings and building systems, using the Monte Carlosimulation techniq

3、ue as described in Guide E 1369.1.2 A computer program is required for the Monte Carlosimulation. This can be one of the commercially availablesoftware programs for cost risk analysis, or one constructed bythe user.2. Referenced Documents2.1 ASTM Standards:2E 631 Terminology of Building Construction

4、sE 833 Terminology of Building EconomicsE 1369 Guide for Selecting Techniques for Treating Uncer-tainty and Risk in the Economic Evaluation of Buildingsand Building SystemsE 1557 Classification for Building Elements and RelatedSiteworkUNIFORMAT IIE 2168 Classification for Allowance, Contingency and

5、Re-serve Sums in Building Construction Estimating3. Terminology3.1 DefinitionsFor definition of terms used in this guide,refer to Terminologies E 631 and E 833.4. Summary of Practice4.1 The procedure for calculating building cost risk consistsof the following steps:4.1.1 Identify critical cost eleme

6、nts.4.1.2 Eliminate interdependencies between critical ele-ments.4.1.3 Select Probability Density Function.4.1.4 Quantify risk in critical elements.4.1.5 Create a cost model.4.1.6 Conduct a Monte Carlo simulation.4.1.7 Interpret the results.4.1.8 Conduct a sensitivity analysis.5. Significance and Us

7、e5.1 Building cost risk analysis (BCRA) provides a tool forbuilding owners, architects, engineers, and contractors tomeasure and evaluate the cost risk exposures of their buildingconstruction projects.3Specifically, BCRA helps answer thefollowing questions:5.1.1 What are the probabilities for the co

8、nstruction contractto be bid above or below the estimated value?5.1.2 How low or high can the total project cost be?5.1.3 What is the appropriate amount of contingency to use?5.1.4 What cost elements have the greatest impact on thebuildings cost risk exposure?5.2 BCRA can be applied to a building pr

9、ojects contractcost, construction cost (contract cost plus construction changeorders), and project cost (construction cost plus owners cost),depending on the users perspectives and needs. This practiceshall refer to these different terms generally as “building cost.”6. Procedure6.1 Identify Critical

10、 Cost Elements:6.1.1 A building cost estimate consists of many variables.Even though each variable contributes to the total building costrisk, not every variable makes a significant enough contribu-tion to warrant inclusion in the cost model. Identify the criticalelements in order to simplify the co

11、st risk model.6.1.2 A critical element is one which varies up or downenough to cause the total building cost to vary by an amountgreater than the total building costs critical variation, and onewhich is not composed of any other element which qualifies asa critical element. This criterion is express

12、ed as:IF VY. VCRIT(1)AND Y contains no other element X where VX. VCRITTHEN Y is a critical elementwhere:1This practice is under the jurisdiction of ASTM Committee E06 on Perfor-mance of Buildings and is the direct responsibility of Subcommittee E06.81 onBuilding Economics.Current edition approved Ap

13、ril 1, 2007. Published April 2007. Originallyapproved in 1998. Last previous edition approved in 2002 as E 1946 02.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to t

14、he standards Document Summary page onthe ASTM website.3This practice is based, in part, on the article, “Measuring Cost Risk of BuildingProjects,” by Douglas N. Mitten and Benson Kwong, Project Management Services,Inc., Rockville, MD, 1996.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box

15、 C700, West Conshohocken, PA 19428-2959, United States.VY5 (2)Max. percentage variation of the element Y! * Ys anticipated cost!Total Building costVCRIT= Critical Variation of the Building Cost.6.1.3 A typical value for the total building costs criticalvariation is 0.5%4. By experience this limits t

16、he number ofcritical elements to about 20. A larger VCRITwill lead to fewercritical elements and a smaller VCRITwill yield more. A riskanalysis with too few elements is over-simplistic. Too manyelements makes the analysis more detailed and difficult tointerpret. A BCRA with about 20 critical element

17、s provides anappropriate level of detail. Review the critical variation usedand the number of critical elements for a BCRA against theunique requirements for each project and the design stage. Ahigher critical variance resulting in fewer critical elements, ismore appropriate at the earlier stages of

18、 design.6.1.4 Arrange the cost estimate in a hierarchical structuresuch as UNIFORMAT II (Classification E 1557). Table 1shows a sample project cost model based on a UNIFORMATII Levels 2 and 3 cost breakdown. The UNIFORMAT IIstructure of the cost estimate facilitates the search of criticalelements fo

19、r the risk analysis. One does not need to examineevery element in the cost estimate in order to identify thosewhich are critical.6.1.5 Starting at the top of the cost estimate hierarchy (thatis, the Group Element level), identify critical elements in adownward search through the branches of the hier

20、archy.Conduct this search by repeatedly asking the question: Is itpossible that this element could vary enough to cause the totalbuilding cost to vary, up or down, by more than its criticalvariation? Terminate the search at the branch when a negativeanswer is encountered. Examine the next branch unt

21、il allbranches are exhausted and the list of critical elements estab-lished. Table 1 and Fig. 1 show the identification of criticalelements in the sample project using the hierarchical searchtechnique.6.1.6 In the sample project, Group Element Superstructurehas an estimated cost of $915,000 with an

22、estimated maximumvariation of $275,000, which is more than $50,000, or 0.5 % ofthe estimated total building cost. It is therefore a candidate fora critical element. However, when we examine the IndividualElements that make up Superstructure, we discover that FloorConstruction has a estimated maximum

23、 variation of $244,500,qualifying as a critical element; whereas Roof Constructioncould only vary as much as $40,000, and does not qualify.Since Floor Construction is now a critical element, we wouldeliminate Superstructure, its parent, as a critical element.6.1.7 Include overhead cost elements in t

24、he cost model,such as general conditions, profits, and escalation, and checkfor criticality as with the other cost elements. Consider timerisk factors, such as long lead time or dock strikes for importedmaterial, when evaluating escalation cost.6.1.8 Allowance and contingency, as commonly used in th

25、ebuilding cost estimates, include both the change element andthe risk element. The change element in allowance covers theadditional cost due to incomplete design (design allowance).The change element in contingency covers the additional costdue to construction change orders (construction contingency

26、).The risk element in contingency covers the additional costrequired to reduce the risk that the actual cost would be higherthan the estimated cost. However, the risk element in allow-ance and contingency is rarely identified separately and usuallyincluded in either design allowance or construction

27、contin-gency. When conducting BCRA, do not include the riskelement in allowance or contingency cost since that will be anoutput of the risk analysis. Include design allowance only tothe extent that the design documents are incomplete. Includeconstruction contingency, which represents the anticipated

28、increase in the project cost for change orders beyond the signedcontract value, if total construction cost, instead of contractcost, is used. See Classification E 2168 for information onwhich costs are properly included under allowance and con-tingency.6.1.9 The sample project represents a BCRA cond

29、uctedfrom the owners perspective to estimate the constructioncontract value at final design. General conditions, profits, andescalation are identified as critical elements. Since the designdocuments are 100 % complete, there is no design allowance.The contingency in the cost element represents the r

30、isk elementand is therefore eliminated from the cost model. There is noconstruction contingency in the model since this model esti-mates construction contract cost only. If total project cost isdesired, add other project cost items to the cost model, such asconstruction contingency, design fees, and

31、 project managementfees.6.2 Eliminate Interdependencies Between Critical Ele-ments:6.2.1 The BCRA tool works best when there are no stronginterdependencies between the critical elements identified.Highly interdependent variables used separately will exagger-ate the risk in the total construction cos

32、t. Combine the highlydependent elements or extract the common component as aseparate variable. For example, the cost for ductwork and thecost of duct insulation are interdependent since both depend onthe quantity of ducts, which is a highly uncertain variable inmost estimates. Combine these two elem

33、ents as one criticalelement even though they both might qualify as individualcritical elements. As another example, if a major source of riskis labor rate variance, then identify labor rate as a separatecritical element and remove the cost variation associated withlabor rates from all other cost ele

34、ments.6.2.2 In the sample project, a percentage escalation istreated as a separate cost element, instead of having theescalation embedded in each cost element. The escalations forall cost elements are highly correlated because they all dependon the general escalation rate in material and labor. Ther

35、eforethe model is more accurate when taking escalation as a separatecost element. Treat escalation as a critical element if it causesthe total cost to vary by more than 0.5 %.6.3 Select Probability Density Function (PDF):4Curran, Michael W., “Range EstimatingMeasuring Uncertainty and Reason-ing With

36、 Risk,” Cost Engineering, Vol 31, No. 3, March 1989.E1946072TABLE 1 Sample Uniformat II Cost ModelGROUP INDIVIDUAL EST MAX/ITEM GROUP ELEMENT INDIVIDUAL ELEMENT ELEMENT ELEMENT VARIATIONCOST COSTA10 FOUNDATIONS $150,000 $45,000A1010 Standard Foundations $100,000A1030 Slab on Grade $50,000A20 BASEMEN

37、T CONSTRUCTION $70,000 $30,000A2010 Basement Excavation $20,000A2020 Basement Walls $50,000B10 SUPERSTRUCTURE $915,000 $275,000B1010 Floor Construction $815,000 $244,500 *B1020 Roof Construction $100,000 40,000B20 EXTERIOR ENCLOSURE $800,000 $250,000B2010 Exterior Walls $576,000 $172,800 *B2020 Exte

38、rior Windows $204,000 $102,000 *B2030 Exterior Doors $20,000 $8,000B30 ROOFING $54,000 $20,000B3010 Roof Coverings $54,000C10 INTERIOR CONSTRUCTION $240,000 $72,000 *C1010 Partitions $132,000 $45,000C1020 Interior Doors $108,000 $30,000C20 STAIRS $95,000 $40,000C2010 Stair Construction $75,000C2020

39、Stair Finishes $20,000C30 INTERIOR FINISHES $916,000 $300,000C3010 Wall Finshes $148,000 $45,000C3020 Floor Finishes $445,000 $178,000 *C3030 Ceiling Finishes $323,000 $129,200 *D10 CONVEYING $380,000D1010 Elevators and the low estimateabout 50 % of the most likely estimate. This serves as a checkon

40、 the range estimates.6.5 Create a Cost Model:6.5.1 The cost model is essentially the hierarchical costestimate. Treat all non-critical elements as constants. Simplifythe cost model by combining constants.6.5.2 In the sample project, the cost model becomes:( COSTCE1 $1,249,000! * 1 1 Profit! * 1 1 Es

41、calation! (5)where:COSTCE= variable cost for the critical ele-ments 1 through 18,$1,249,000 = total cost for all the non-criticalelements;Profit and Escalation = variable percentages.6.5.3 For triangular PDFs, the random cost of each criticalelement is calculated by the formula:COSTCE5 LE 1 RV * MLE

42、LE! * HELE!#0.5(6)if COSTCE#MLECOSTCE5 HE12RV! * HE MLE! * HE LE!#0.5(7)if COSTCE. MLEwhere:RV = a random variable between 0 and 1.Use the same random variable for each formula. Aftercalculating both formulas, use the one which satisfies thecorresponding condition on the right.6.5.4 For example, for

43、 the critical element Floor Construc-tion, if RV = 0.3, the two equations become:COST Floor Const.!5$652,000 1 0.3 * $815,000 (8)$652,000! * $1,059,500 $652,000!#0.55 $793,162, which satisfies the condition COST#$815,000COST Floor Const.!5$1,059,500 0.7 * $1,059,500 (9)$815,000! * $1,059,500 $652,00

44、0!#0.55 $795,410, which does not satisfy the condition COST . $815,000The result from the first equation will be used since itsatisfies the corresponding condition.6.6 Conduct a Monte Carlo Simulation:6.6.1 Run a Monte Carlo simulation once the risk in thecritical elements are quantified and the mod

45、el set up. TheMonte Carlo method builds up a PDF for the bottom linebuilding cost by repeatedly running the model with randomlygenerated numbers for the critical elements according to theindividual PDFs. Each Critical element will use a separaterandom number for the calculation. Each time the model

46、is run,one point is generated for the total building cost risk PDF. Theprocess is repeated until the total building cost risk PDF“converges” or settles into a final shape, which often requires1,000 or more iterations. See Guide E 1369, section 7.7, for amore detailed description of the simulation te

47、chnique.6.6.2 To implement a BCRA, use commercial softwareprograms or write your own simulation software code.6.7 Interpret the Results:6.7.1 By inspecting the converged PDF for the bottom lineconstruction cost and its corresponding Cumulative Distribu-tion Function (CDF), obtain the following infor

48、mation:6.7.1.1 Expected (mean) total cost, which is the average ofall the data points generated by the simulation.6Beeston, Derek T., “One Statisticians View of Estimating,” Property ServicesAgency, Department of Environment, London, UK, July 1974.FIG. 3 Skewed Triangular Probability Distribution Fu

49、nctionE19460766.7.1.2 Standard deviation on the total cost, which is thestandard deviation of all the data points generated by thesimulation.6.7.1.3 The confidence level, which is the cumulative per-centage corresponding to those data points generated by thesimulation which are less than or equal to the estimated amounton the CDF. Fig. 2 illustrates the concept of a confidence level.Denote the low estimate as point “a” and the high estimate aspoint “b.” Because point a corresponds to the 1stpercentile ofthe normal distribution, only 1 %