Construction Management - Coursework Example

rvey or once the quantities are determined, you divide those by the production rate to determine the activity duration. 2. Critical path method 3. Precedence diagram method 4. Program evaluation and review technique Network models There are two types of the networking models: Activity on Arrow- the arrows are arranged to show the plan or logical sequence in which the activities of the project are to be accomplished. There are several activity relationships between two activities. They include one-on-one relationship, many-to-many and dummies. Activity on Node-the activities are on node not on the arrow and there are links. There are no dummy activities. The resulting logical flowchart is a network of arrows, usually referred to as either the arrow diagram or the network. A CPM network is a graphical representation of a project. The tasks or activities that comprise the project are represented by arrows. The arrow tail and head represent the start and finish of an activity, respectively. AON is advantageous over AOA because it is easy to construct and modify. It does not need dummy activities and its precedence relationships with lag times are more effective in modeling activities. ACTIVITY RELATIONSHIPS The most simple and basic relationship that can exist between two activities is: one-on-one relationship. EVENT TIMES A computational process known as the forward pass is used to determine the early event time (EET) for each and every event. In the forward pass, calculations begin at the first node and travel toward the end until the early event times have been computed for every event on the CPM network. Usually, in normal calculations the early event time for the first node is zero. The early event time (EET) for the following event is found by simply adding the duration of the succeeding activity to the early event time of the preceding event. A computational process known as the backward pass is used to determine the late event times (LET) for each and every event. In the backward pass, calculations begin at the last node and travel toward the start until the late event times have been computed for each and every event on the CPM network. The EET at the last event is also taken as the LET of the last event. ACTIVITY TIMES In AOA, activity times are computed from event times and are usually presented either on the network or in a tabular form. The following six-step procedure can be used to compute and tabulate activity times: Step 1: List all activities in column 1 in ascending order by “i” node first, and then for each “i” node arrange the activities in ascending order of “j” node. Step 2: Record the description of each activity in column 2 and its duration in column 3. Step 3: Record the earliest start time for each activity in column 4. These are the early event times at the “i” nodes of activities. Step 4: Compute the earliest finish for each activity by adding its duration to its earliest start and place in column 5. Step 5: Record the latest finish time for each activity in column 7. These are the late event times at the “j” nodes of the activities. Step 6: Compute the latest start of each activity by subtracting its duration from its latest finish and place in column 6. CRITICAL PATH METHOD (CPM or AOA) CPM is used: • To track and coordinate projects • To determine the critical path that gives the shortest duration for the project. It is the path that has a float of zero and consists of the activities that cannot be delayed because if so they will delay the project completion. Critical Activity-It is the activity that has equal Early Start and Late Start. Float-It is the idle time of an activity that does not delay the project. Total Float (TF)- It is the maximum time that an activity can be delayed without delaying the project completion time (TF = LF – EF) Differences between CPM & PDM 1. Activities are on node not on arrow, 2. Links no arrows (logical relationship), 3. No dummy activities are used, and 4. Identification of activities is by single unique number, (no i&j) Advantages of PDM over CPM 1. Easy to construct and modify the network, 2. No need for dummy activities, 3. Less activities in presentation, and 4. Precedence relationships with lag times are more effective in modeling project activities PRECEDENCE DIAGRAMS The precedence diagram consists of a series of nodes with lines (links) connecting them to illustrate the activities to be executed and the sequence in which they must be done. The normal convention in the use of the precedence technique is that an activity can be started after all links drawn to its left starting) side has been traversed. When the project activities overlap, they must be split, to establish their logical sequence in AOA. The advantage of precedence in this case is demonstrated by the relative simplicity of its diagram. Precedence relationships Lead & Lag: relationships, Relationships between activities are shown by connecting arrows. The relatively concise representation of a precedence network is possible because the arrows show not only the sequence of activities but also the lead and lag times for the start and finish for each activity. The Lead and Lag times, often referred to a lead and lag factors, are indicated by pairs of subscripts, the first representing the preceding activity and the second the following activity. FFij = Lag time for a finish-to-finish relationship. (The succeeding activity finishes this amount of time after the completion of the preceding activity) SS = Lead time for a start to start relationship SSij=start-to-relationship. (The preceding activity starts this much earlier than the start of the succeeding activity) Optimistic completion time, which is the time the activity, the activity will take off if everything goes well. Pessimistic completion time, which represents the time the activity will take if something goes wrong. Most likely completion time, which is the time required under normal circumstances. Factors that can affect the activity duration are weather, learning curve and overtime and space congestion. The sources of construction delays or bad weather, change orders, equipment breakdowns and labor problems. Project evaluation and review technique This concept was developed based on Breaking the project down into individual components. Probabilistically estimating the times required to complete the work component. Defining the precedence relations between them. Performing a simple network analysis. Estimating the project completion time with an associated probability distribution. Excavation methods Earthmoving is the process of moving soil or rock from one location to another or processing it. Such tasks include excavation, loading, hauling, spreading and grading The sequence of Earthmoving includes: Clearing and grubbing Removing the top soil Excavation for utilities and building foundation. Removal of excess excavated material that is not needed for backfill. Grading of the site to meet contract requirements. Spreading top soil and landscaping. To obtain the desired properties for construction, the soil components and properties are studied. Soil properties have a direct effect on equipment selection and productivity. Soils are classified as: Gravel, Sand, Silt and Clay, The soil components are solids, water and air. The soil properties of importance are soil type, the soil density and the moisture content. The soil density is expressed in pounds per cubic foot, pounds per cubic yard, or kilograms per cubic meter. Soil density affects equipment productivity, because the equipment carrying capacities are limited by both volume and weight constraints. The moisture content is a percentage representing the weight of water in a soil mass divided by the weight of the solids. There are three principal conditions or states in which earthmoving material may exist: 1. Bank: Material in its natural state before disturbance. A unit volume is identified as a bank cubic yard (BCY) or a bank cubic meter (BCM). 2. Loose: Material that has been excavated or loaded. A unit volume is identified as a loose cubic yard (LCY) or loose cubic meter (LCM). 3. Compacted: Material after compaction. A unit volume is identified as a compacted cubic yard Trafficability is the ability of soil to support the weight of vehicles under repeated traffic. Loadability is the measure of the difficulty in excavating and loading soil. The increase or reduction in volume of the soil density is known as a swell. Swell (%) = {(weight / bank volume) / (weight/ loose volume) -1}* 100 Shrinkage is the reduction in soil volume due to compaction. Shrinkage (%) = {-(weight/ bank volume) / (weight/ compacted volume)} * 100 Load factor is used to convert from loose volume to swell. Load factor= {1/ (1+ swell)} Shrinkage factor is used to convert from bank volume to compacted volume. Shrinkage factor= 1- shrinkage Angle of repose is the angle that the sides of a spoil bank or pile naturally form with the horizontal when the excavated soil is dumped onto the pile. The angle of repose value varies with the soil’s physical characteristics and its moisture content. Trench Excavation: Volume = Cross-Sectional Area x Length (A). Procedures (Depth & Width are Constant) 1. Calculate the cross-section area (width x depth). 2. Calculate the volume by multiplying the cross section area by the trench length. (B).Procedures (Depth & Width are Variable) 1. Calculate the cross-section area (width x depth) at frequent linear intervals. 2. Calculate the volume between the locations computed in (1) using the above equation. ESTIMATING EARTHWORK VOLUME The procedures to be followed can be divided into three principal categories: 1. Pit excavations, 2. Trench excavation, and 3. Excavating or grading Pit Excavations: Volume = Horizontal Area x Average Depth Procedures 1. Divide the horizontal area into sets of rectangles, triangles or circular segments – Excavation Methods & Calculations segments, 2. Calculate the area for each segment, 3. Sum up all the segment’s areas in (2) to get the total area, 4. Calculate the average depth Trench Excavations: Volume = Cross-Sectional Area x Length Procedures (Depth & Width are Constant) 1. Calculate the cross-section area (width x depth). 2. Calculate the volume by multiplying cross section area by the trench length. Procedures (Depth & Width are Variable) 1. Calculate the cross-section area (width x depth) at frequent linear intervals. 2. Calculate the volume between the locations computed using the above equation. Excavation of Large or Complex Areas of Depth) x Weight Volume = Horizontal area x Average depth ESTIMATING EARTHWORK VOLUME Procedures: 1. Divide the area into a grid indicating the depth of excavation or fill at each grid. 2. Assign a weight for each corner or segment intersection according to its location 3. Calculate the average depth using the given equation. 4. Calculate the volume by multiplying the average depth by the horizontal area. Excavation of Large Areas in Case of Cohesive Soil: Volume = Length x Width x Depth Volume=H/3(A+B+{A*B}^1/2) Where, A = Top Area B = Bottom Area H = Vertical Height TYPES OF EQUIPMENT 1. Backhoe operation The hoe excavates by pulling the bucket back to the machine. It is mainly used for excavating below track level, trenching work and excavating basements. In trenching, a fall-in factor should be applied to excavator production to account for the work required to clean out materials that fall back in the trench. In trenching work, a fall in factor should be applied to excavator production to account for the work required to clean out materials that fall back into the trench from the trench walls. The normal excavator production should be mulitiplied by an appropriate value from the table below to get the effective trench production. The basic backhoe recommendation practices are: Minimize swing angle between digging and dumping. Consider the required maximum depth, working radius and dumping heights in selecting the proper backhoe. Select the bucket width that matches the trench width. Do not exceed the allowable load. Do not use a backhoe to fracture the rocks. Advantages and disadvantages of the backhoe The backhoe pulls material towards them, thus they need the lowest radius for their effectiveness; hence minimum force and power is needed unlike the front shovel which lifts with maximum radius. The backhoe has a better capacity during loading of trucks ecause it dumps materials in an organized manner; hence reduces the truck wear. 2. Hydraulic shovel operation The front shovel excavates into the ground using either the crowding force or the breaking force. This is used for excavating above the track level, excavating against a vertical face or excavating all classes of earth except solid rock. The front shovel recommendations are: Minimize swing angle between digging and dumping. Keep the shovel cut smooth to provide even footing for the shovel. Move the front shovel to keep optimum distance from the working face. Keep the teeth sharp. Advantages and disadvantages of the front shovel Loaders are not efficient in digging deep places because they can only dig up to the level of their wheels. They are wheeled thus having better speed and mobility; hence do not damage paved roads unlike the dozer. Large loaders use hydraulically actuated point that is set between the rear and the front axle unlike tractors and backhoes that use automotive steering. The mechanism enables the front load axle to be solid; hence carries a heavier weight than other equipment (Engineering, 2014). 3. Dragline operations This is an attachment used on t Read More