Construction Cost Estimating Methods
Construction cost estimating is the process of forecasting the costs of labor, materials, equipment, and overhead required to construct a project. The accuracy of the estimate depends on the level of project definition at the time the estimate is prepared and the quality of the cost data used. Order-of-magnitude estimates are prepared during the conceptual phase using historical cost data per square foot or per unit of capacity, with accuracies of plus or minus 25 to 40 percent. Preliminary estimates are prepared during schematic design using unit cost methods based on the major building systems, with accuracies of plus or minus 15 to 25 percent. Detailed estimates are prepared after the design is substantially complete using quantity takeoff and unit pricing for every work item, with accuracies of plus or minus 5 to 10 percent. The estimating contingency decreases as the design progresses and the estimate accuracy improves.
Quantity takeoff is the measurement of the quantities of each material and work item from the contract documents. The takeoff is organized by the CSI MasterFormat divisions that classify construction work into standard categories from Division 1 general requirements through Division 48 electrical power generation. The quantities are measured in the appropriate units for each item such as cubic yards of concrete, tons of steel, square feet of roofing, and linear feet of piping. The takeoff must include allowances for waste, overlaps, and other factors that increase the quantity required beyond the net measured quantity. The waste factor for concrete is typically 5 to 10 percent depending on the complexity of the formwork. The waste factor for reinforcing steel is 10 to 15 percent for bars and 5 to 10 percent for welded wire fabric. The accuracy of the quantity takeoff depends on careful measurement and thorough review of all contract documents.
Unit pricing assigns a cost per unit of work to each item in the quantity takeoff. The labor cost is calculated from the crew composition, the crew hourly wage rates including burdens and benefits, and the productivity rate expressed in units per labor hour. The material cost is the delivered price of the materials including sales tax and the cost of storage and handling. The equipment cost is the hourly ownership and operating cost for each piece of equipment. The subcontractor costs are obtained from bids received from specialty contractors. The general conditions costs include the project management staff, temporary facilities, permits, bonds, and insurance. The overhead and profit markup is applied to the total direct cost to determine the final bid price. The markup varies from 10 to 25 percent depending on the project complexity, the level of risk, and the market conditions.
Construction Scheduling Methods
The Critical Path Method is the most widely used scheduling technique in the construction industry. The CPM schedule identifies the sequence of activities that determines the minimum project duration. The critical path is the longest path through the schedule network and has zero float, meaning any delay in a critical path activity directly extends the project completion date. Non-critical activities have float that represents the amount of time the activity can be delayed without affecting the project completion date. The float is calculated from the difference between the early start and late start dates or between the early finish and late finish dates. construction cost estimating contingency percentages. critical path method float calculation. lump sum versus guaranteed maximum price contracts. Total float represents the total delay available for an activity, while free float represents the delay available without affecting the early start of any successor activity.
Activity duration estimating assigns a time duration to each activity in the schedule based on the quantity of work, the expected crew productivity, and the available crew size. The activity duration equals the quantity of work divided by the daily production rate of the crew. The daily production rate depends on the crew size and composition, the productivity factors for the specific work conditions, and the anticipated learning curve effects. The estimate of activity durations must consider the weather conditions that will affect outdoor work, the availability of materials and equipment, and the coordination with other trades working in the same area. The duration estimate should be based on realistic productivity assumptions rather than theoretical maximums to create a achievable schedule.
Schedule updates compare actual progress against the baseline schedule at regular intervals. The update records the actual start and finish dates for completed activities, the percent complete for activities in progress, and any changes to the remaining duration or logic for activities not yet started. The update calculates the revised project completion date and identifies any activities that have lost float or become critical. The schedule update report includes the critical path, the activities with negative float that require management attention, and the project completion date forecast. Corrective actions such as adding resources, working overtime, or revising the construction methods are developed for activities that are behind schedule to recover the lost time. The schedule is an essential management tool that must be maintained throughout the project to support effective project control.
Construction Contracts and Legal Considerations
Construction contracts establish the legal relationship between the owner and the contractor, defining the scope of work, the contract price, the schedule, and the rights and responsibilities of each party. The most common contract types are lump sum contracts where the contractor agrees to perform the specified work for a fixed price, cost-plus contracts where the contractor is reimbursed for actual costs plus a fee, and guaranteed maximum price contracts where the contractor bears the risk of cost overruns up to the GMP while sharing savings below the GMP. The choice of contract type depends on the level of design completion, the owner’s risk tolerance, and the contractor’s willingness to accept risk. Lump sum contracts are suitable for well-defined projects with complete designs. Cost-plus contracts are used when the design is not complete or when the scope is uncertain.
The contract documents include the owner-contractor agreement, the general conditions, the supplementary conditions, the drawings, the specifications, and the addenda. The general conditions define the standard terms and conditions that apply to the project including the roles of the architect, engineer, and construction manager, the procedures for changes and claims, the payment application process, and the dispute resolution methods. The supplementary conditions modify the general conditions for the specific project requirements. The drawings and specifications define the technical requirements for the work. The specifications are organized by the CSI MasterFormat divisions and include the performance requirements, material standards, and installation methods for each work item. The technical specifications reference industry standards such as ASTM, ANSI, and ACI that define acceptable materials and testing methods.
Change order management is critical for controlling project costs and maintaining positive owner-contractor relationships. Changes to the contract scope must be documented through written change orders that describe the change, the impact on the contract price and schedule, and the signatures of authorized representatives from both parties. The change order pricing should include the direct costs of the changed work plus appropriate markups for overhead and profit. Claims arise when the contractor believes that the owner or the owner’s representatives have caused additional costs or delays that are not covered by the contract change order provisions. The contract should specify the procedures for submitting claims and the time limits for notice and documentation. Alternative dispute resolution methods such as mediation and arbitration are preferred over litigation because they are faster, less expensive, and preserve business relationships.
Project Closeout and Commissioning
Project closeout is the final phase of construction that includes the completion of all work items, the correction of deficiencies, the submission of documentation, and the final payment application. The punch list identifies the incomplete or defective work items that must be corrected before final acceptance. The punch list is developed through a joint inspection by the contractor, architect, and owner representatives. The contractor is responsible for completing all punch list items within the time specified in the contract. The certificate of substantial completion is issued when the work is sufficiently complete that the owner can occupy the facility for its intended use. Substantial completion triggers the start of the warranty period and the release of retainage.
Building commissioning is a systematic process of verifying that building systems are designed, installed, tested, and capable of being operated and maintained according to the owner’s requirements. The commissioning process begins during the design phase with the development of the commissioning plan and continues through construction with the verification of system installation and performance testing. The functional performance tests verify that each system operates correctly under all modes of operation including normal, emergency, and failure conditions. The commissioning report documents the test results and identifies any deficiencies that must be corrected. The training of the owner’s operation and maintenance staff is an essential part of the commissioning process.