l9dtrm_plan_sp04 - Planning and Deterministic Scheduling...

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Unformatted text preview: Planning and Deterministic Scheduling Nathaniel Osgood 3-8-2004 Topics Problem set background Planning CBS OBS WBS Scheduling Motivations and context Gantt charts CPM Float Critical path Float ownership A Word on the Problem Set Construction methods Assume tie-backs required to support structure during construction (need invert slab formwork) Slab on grade Not responsible for slab on grade Casting using invert forms Can cast beams and slabs at same time Wall casting (illustration only) Don’t worry about formwork reuse Several types of anchor bolts in structure Casting a concrete slab on grade Sequence: 1. 2. 3. 4. (You are not responsible for this formwork in the assignment) 5. 6. 7. Form and edges Reinforcement and embedment Striking off or straightedge Floating (if smoother surface is needed) Control joints Troweling (if very smooth surface is needed) Curing (under damp conditions) Casting a concrete wall Sequence: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. Coated form (one side only) Reinforcing Placement of Ties Placement of construction joints (if needed) Inspection Coated form (2nd side) Placing concrete Curing Stripping of formwork and snapping off ties Point and Patch Rub Planning Components What: Scope (Plans and specifications) How much $: Budget (via CBS – and estimate) Who: OBS How: WBS When: Schedule Topics Problem set background Planning CBS OBS WBS Scheduling Motivations and context Gantt charts CPM Float Critical path Float ownership Cost Breakdown Structure (CBS) Canonical way of accounting for costs in the project Assigns accounts for different types of expenditures Should permits tracking expenditure by activity (work item) Often includes WBS-based characterization (e.g. CSI Masterformat) CSI Masterformat (Building Const) Bidding Requirements, Contract Forms, and Conditions of the Contract 00010 Pre-bid Information 00100 Instructions to Bidders 00200 Information available to Bidders 00300 Bid Forms 00400 Supplements to Bid Forms 00500 Agreement Forms 00600 Bonds and Certificates 00700 General Conditions 00800 Supplementary Conditions 00900 Addenda 02150 02160 02170 02200 02300 02350 02450 02480 02500 02600 02660 02680 02700 02760 02770 02780 02800 02900 03100 03200 03250 03300 03370 03400 03500 03600 03700 03800 04100 04150 04200 04400 04500 04550 04600 04700 Shoring and Underpinning Excavation Support Systems Cofferdams Earthwork Tunneling Piles and Caissons Railroad Work Marine Work Paving and Surfacing Utility Piping Materials Water Distribution Fuel and Steam Distribution Sewerage and Drainage Restoration of Underground Pipe Ponds and Reservoirs Power and Communications Site Improvements Landscaping Concrete Framework Concrete Reinforcement Concrete Accessories Cast-In-Place Concrete Concrete Curing Precast Concrete Cementitious Decks and Toppings Grout Concrete Restoration and Cleaning Mass Concrete Mortar and Masonry Grout Masonry Accessories Unit Masonry Stone Masonry Restoration and Cleaning Refractories Corrosion Resistant Masonry Simulated Masonry 05050 05100 05200 05300 05400 05500 05580 05700 05800 05900 06050 06100 06130 06150 06170 06200 06300 06400 06500 06600 06650 07100 07150 07180 07190 07195 07200 07240 07250 07270 07300 07400 07480 07500 07570 07600 07700 07780 07790 Metal Fastening Structural Metal Framing Metal Joists Metal Decking Cold Formed Metal Framing Metal Fabrications Sheet Metal Fabrications Ornamental Metal Expansion Control Hydraulic Structures Fasteners and Adhesives Rough Carpentry Heavy Timber Construction Wood and Metal Systems Prefabricated Structural wood Finish Carpentry Wood Treatment Architectural Woodwork Structural Plastics Plastic Fabrications Solid Polymer Fabrications Note: The items listed above are not specification sections and are referred to as "Documents" rather than "Sections" in the Master List of Section Titles, Numbers, and Broadscope Section Explanations. Division 6 - Wood and Plastics Specifications Division 1 - General Requirements 01010 01020 01025 01030 01035 01040 01050 01060 01070 01090 01100 01200 01300 01400 01500 01600 01650 01700 01800 02010 02050 02100 02140 Summary of Work Allowances Measurement and Payment Alternates/Alternatives Modification Procedures Coordination Field engineering Regulatory Requirements Identification systems References Special Project Procedures Project Meetings Submittals Quality Control Construction Facilities and Temporary Controls Material and Equipment Facility Startup/Commissioning Contract Closeout Maintenance Subsurface Investigation Demolition Site Preparation Dewatering Division 3 - Concrete Division 7 - Thermal and Moisture Protection Waterproofing Damproofing Water Repellents Vapor Retarders Air Barriers Insulation Exterior Insulation and Finish Systems Fireproofing Firestopping Shingles and Roofing Tiles Manufactured Roofing and Siding Exterior Wall Assemblies Membrane Roofing Traffic Coatings Flashing and Sheet Metal Roof Special Ties and Accessories Skylights Joint Sealers Division 4 - Masonry Division 2 - Site Work Division 5 - Metals 05010 Metal Materials 05030 Metal Coatings Cost Code Mirrored by cost hierarchy Commonly include standardized and project components Project id (often has useful info to avoid lookup) Often omitted from internal project references Area-facility code (geographically distributed projects, or areas of a facility unique to project) Work-type code: WBS May be standard code (e.g. CSI Masterformat) if uniform across projects Distribution code: Cost type associated with work (e.g. Materials, Equipment, Labor, Subcontract, etc.) Cost Code Illustration Topics Problem set background Planning CBS OBS WBS Scheduling Motivations and context Gantt charts CPM Float Critical path Float ownership Organizational Breakdown Structure In Broader Context (Matrix Org) Topics Problem set background Planning CBS OBS WBS Scheduling Motivations and context Gantt charts CPM Float Critical path Float ownership Work Breakdown Structure Central role in project monitoring, control Create through collaboration of Estimation team Project control team Field operations maintenance group Often do not include procurement But do need to reflect in schedule! WBS Phase 1 WBS Refinement Refined WBS Combinations CBS + OBS: Budget monitoring of crews, etc. WBS + OBS: Task assignments Schedule + OBS: Crew assignments Schedule + CBS: Cost monitoring Topics Problem set background Planning CBS OBS WBS Scheduling Motivations and context Gantt charts CPM Float Critical path Float ownership Motivations for Scheduling Key: Both lowers chance of delay and assists in recovering from delay, resolving responsibility Assistance in reasoning about huge number of details (e.g. 1000s of activities) Delays often result simply from poor planning Resources are most Valuable components Hard to manage Can identify resource conflicts far ahead of time Formalization necessary but not sufficient for managing Ubiquitous Role of Schedule Importance of schedule Design (preliminary schedule) Establish finish, milestone times for choreographing activities Procurement time, subcontractor presence, tenant occupancy Importance for thinking through issues Identify critical path Communication tool between parties Framework for monitoring Role in control Assessing impacts of changes Allows demonstration of indirect costs Legal importance Link to resources Payments Resource usage Identify exposure to crowding, weather conditions Legal Ramifications In some cases, must produce schedule by law Precedence-encoding schedules pay key role in addressing Impact of change Responsibility for delay Schedule considered by court need not be used in field Schedule proposed by contractor can be taken as approach – even if only passively accepted by owner Mega Construction Co. Inc. v. United States 29 Fed. Cl. 396 (1993) “Plaintiff’s bar chart depicted its version of the numerous work items. However, it failed to prove that the claimed delays occurred along the critical path, because it does not indicate the interdependence of any one or more of the work items that were on the critical path while the project was ongoing, but offered no credible evidence of the interdependence of the project’s activities” Linkage to Estimation Scheduling allows understanding of cash flow over time Given time value of money, scheduling critical to understanding present value of estimate Quantity takeoff reasoning can be used to inform both Estimation Scheduling Scheduling Considerations Risk of Imbalanced use Use early on Discarding later Central office use only Danger scheduling information not propagated from CM/owner/Designer to contractors Need buy-in by superintendents Want shared schedule Small projects may not need–but collection does Contractor Scheduling Contractor scheduling very simple, short-term E.g. meet once a week to plan next two weeks Focus is on keeping crews busy If master schedule doesn’t accomplish this, may perform work out of synch Important Scheduling Factors Delay time for reviews, approvals Submittals Permitting Procurement Planning for changes Coordination of labor & equipment GC coordination of subcontractors Critical – and difficult due to interfaces Design scheduling difficult Highly iterative Hard to know when design, cost will converge Procurement Scheduling Especially key in urban areas Custom production items difficult Latencies uncertain Different parties Complex workflow (CM, structural engineer) Quality checks Different classifications Bulk materials : Fast delivery (1-5 days) Commodity fabrication (3-12 weeks) Customized fabrication (10-16 weeks) Long-Lead Items Conveyance Elevators Mechanical/plumbing Fire protection Pumps Boilers Cooling towers Control systems Air handling units Chillers/refrigeration unit Structural steel Reinforcing rods Precast panels/decks Special cladding Electrical Transformer Motors Switch boxes Special conduits Critical Role of Resources Key: Mutual dependence bt. schedule, resources Schedule depends on activity durations, which assume some resource availability Resource availability depends on scheduling Highly complex problem Key for effective work Informal means of handling: Iteration A later lecture will focus on this topic Topics Problem set background Planning CBS OBS WBS Scheduling Motivations and context Gantt charts CPM Float Critical path Float ownership Gantt/ “Bar Charts” WWI Origin (systematized earlier work) Very effective communication tool Very popular for representation of simpler schedules Can be cumbersome when have >50 activities No dependencies captured Most effective as reporting format rather than representation Simple Gantt Chart Gantt: Sequential vs. Phased Hierarchy of Gantt Charts Topics Problem set background Planning CBS OBS WBS Scheduling Motivations and context Gantt charts CPM Float Critical path Float ownership Critical Path Method (CPM) Origin at Dupont (1956) First application to construction in early 1960s Sometimes narrow term, sometime more general Directed acyclic graph Drawn (topologically sorted) left to right Specify activities and associated information (e.g. duration) and run scheduling algorithm to yield scheduling recommendations/constraints Gantt vs. CPM Network Methods: Basic Steps Define activities from WBS work packages Estimate $, time, resources for each activity Define precedence relationships between activities Iterate Perform CPM scheduling Estimate time, cost, resource usage over project If acceptable, terminate If not acceptable, impose dependencies or added/reduced resources Important Considerations Durations depend implicitly on many things Amount of work Productivity (environment, skill, learning, mgmt,… # of people assigned Equipment assigned Costs from cost estimate Human resources from OBS/WBS and takeoff Materials, Equipment from takeoff May want to estimate durations via several ways Recall WBS Listing of Tasks Precedence Considerations Unless impose constraints, assuming that activities can be performed in parallel Relationships between activities reflect constraints Regulatory/Contractual Physical Resource/Financial Safety Managerial Environmental Identification of Direct Precedences Representation Scheme:AON Also called “precedence diagram method” (PDM) and “bubble diagram method” Easier to visually recognize opportunities for concurrency Most popular for software Requires no dummy nodes Diagram should encode EST,LST,EFT,LFT Allows for representation of richer semantics S2S, F2F, S2F, F2S AON Example Recall Direct Precedences Corresponding AON Schedule Representation Scheme: AOA Historically most popular Very similar to Gantt format when perform “manhattan” layout Requires dummy nodes Arrows can only come from/go to single node Only one arrow between two given nodes Workarounds for generality built into AON E.g. Concurrency can be enforced via bracketing Disaggregation for alternative relationships AOA Simple Case (No Dummy Arrows) Violation of Uniqueness of Node Connections Dummy Arrows: Case 1 Options for Representing Dummy Arrows: Case 2 Common Successors with Distinct Sets of Predecessors Because Arrow can only have 1 unique source and destination, must introduce dummy arrow Dummy Nodes Recall Direct Precedences Corresponding AOA Schedule CPM Algorithm Derives early, late finish/start for nodes Can run on AOA or AON diagrams O(n) [Linear time] NB: LF, LS are latest could start/finish without delaying whole project These are not the latest could start when “keeping busy” with other activities AOA Scheduling Notation AON Scheduling Notation Because each node represents entire activity at one point, must specify •Early Start (ES) •Late Start (LS) •Early Finish (EF) •Late Finish (LF) Float=(LS-ES)=(LF-EF) Different notations typical The Subdivided Bubble Symbol for a Task Earliest Start Earliest Finish Date Date Start Date Task #3 2 hrs Finish Date #3 Task EST # EFT LST LFT Dur. Start End Task #3 Fly to Reno Day 3 Day 3+ 3 3.1 6.4 2 6.5 hrs or Earliest Latest Latest Start Latest Finish Date Date Duration Option "A" Option "B" Other Symbols for Tasks 035 Task Name Resource Start Date End Date Task No. EST Task's Name LST EFT DURATION LFT Task No. Title of Task EST LST Dur. TF FF } Responsibility EFT LFT Passes Forward pass Because all preceding activities must finish before a successor, early start of a given node is maximum of early finishes of preceding nodes Overall project duration (late finish) is defined as maximum of early finishes for nodes Backward pass Because preceding activity must finish before any following activity, Late finish of a given activity is minimum of late starts of following activity Passes: Pseudocode (AOA) Earliest Event Time Algorithm Step 1: Let E(0) = 0. Step 2: For j = 1,2,3,...,n (where n is the last event), let E(j) = maximum {E(i) + Dij} where the maximum is computed over all activities (i,j) that have j as the ending event. Latest Event Time Algorithm Step 1: Let L(n) equal the required completion time of the project. Note: L(n) must equal or exceed E(n). Step 2: For i = n-1, n-2, ..., 0, let L(i) = minimum {L(j) - Dij} where the minimum is computed over all activities (i,j) that have i as the starting event. Float/Slack Fundamentals Intuitively, measures leeway in scheduling Degree of freedom in timing for performing task Length of difference between when we “have to” finish activity and how long it takes to finish Types of float differ in how define “have to” NB: While it may be possible to schedule an activity at many different points, some points may be far preferable to others! Float Total float: max time can delay w/o delaying project Min((L(j)-Dij)-E(i)) i.e. (Latest time that could start and still finish project on time)-(Earliest possible time that could start) Free float: max time can delay w/o delaying successors Min((E(j)-Dij)-E(i)) :(latest time that could start but still finish before early start of next activity)-(Earliest possible time that could start) Independent float : Max(0,Min((E(j)-Dij)-L(i))) Like Free float but assuming worst-case start Topics Problem set background Planning CBS OBS WBS Scheduling Motivations and context Gantt charts CPM Float Critical path Float ownership Critical Path Definition: Longest 0-float path of activities For algorithm as described, at least one such path Must be completed on time or entire project delayed Essentially indicates minimum time required for project Want to consider near-critical activities as well! Typically evolves over time, as activity durations unfold No flexibility to shift for e.g. resource leveling Contingency buffer + Critical chain buffering Topics Problem set background Planning CBS OBS WBS Scheduling Motivations and context Gantt charts CPM Float Critical path Float ownership Using Float by Sequentializing Items Arbitrary Use of Network Logic for Resource Constraint Place Footing 1 Place Footing 2 CONCURRENT Place Footing 1 Place Footing 2 SEQUENTIAL Topics Problem set background Planning CBS OBS WBS Scheduling Motivations and context Gantt charts CPM Float Critical path Float ownership Float “Ownership” Tension between owner and contractor Significant legal implications Problem: Owners seek to push contractors on tight schedule Contractors seek flexibility, claims against owner Motivations for Contractor Float Ownership Feel owed higher compensation b/c ownercaused delays would be much worse w/o heroics Seek flexibility in scheduling (e.g. for resource leveling) Flexibility has value! Create multiple critical- or near-critical-paths Deliberately inflate durations ($ charge to speed up) Insert artificial precedence constraints “preferred way of doing things”) ($ charge to change) Resent owner interference in construction Motivations for Owner Float Ownership Seeks to lower risk by getting work done earlier Too many late starts risks overall project duration May seek to impose unrealistically short schedule on contractor May contractually limit flexibility of contractor Specify owner rights to use float Right of owner to select scheduling procedure Right to object to “unreasonable” durations Right to remove artificial constraints from diagram Force redrawing of critical path if contractor behind ...
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