Construction Planning & Scheduling
One of the most important responsibilities of construction project management is the planning and scheduling of construction projects. The key to successful proﬁt making in any construction company is to have successful projects. Therefore, for many years, efforts have been made to plan, direct, and control the numerous project activities to obtain optimum project performance. Because every construction project is a unique undertaking, project managers must plan and schedule their work utilizing their experience with similar projects and applying their judgment to the particular conditions of the current project. Until just a few years ago, there was no generally accepted formal procedure to aid in the management of construction projects. Each project manager had a different system, which usually included the use ofthe Gantt chart, or bar chart. The bar chart was, and still is, quite useful for illustrating the various items of work, their estimated time durations, and their positions in the work schedule as of the report date represented by the bar chart. However, the relationship that exists between the identiﬁed work items is by implication only. On projects of any complexity, it is difﬁcult, if not virtually impossible, to identify the interrelationships between the work items, and there is no indication of the criticality of the various activities in controlling the project duration. A sample bar chart for a construction project is shown in Fig
The development of the critical path method (CPM) in the late 1950s provided the basis for a more formal and systematic approach to project management. Critical path methods involve a graphical display (network diagram) of the activities on a project and their interrelationships and an arithmetic procedure that identiﬁes the relative importance of each activity in the overall project schedule. These methods have been applied with notable success to project management in the construction industry and several other industries, when applied earnestly as dynamic management tools. Also, they have provided a much- needed basis for performing some of the other vital tasks of the construction project manager, such as resource scheduling, ﬁnancial planning, and cost control. Today’s construction manager who ignores the use of critical path methods is ignoring a useful and practical management tool.
Planning and Scheduling
Planning for construction projects involves the logical analysis of a project, its requirements, and the plan (or plans) for its execution. This will also include consideration of the existing constraints and available resources that will affect the execution of the project. Considerable planning is required for the support functions for a project, material storage, worker facilities, ofﬁce space, temporary utilities, and so on. Planning, with respect to the critical path method, involves the identiﬁcation of the activities for a project, the ordering of these activities with respect to each other, and the development of a network logic diagram that graphically portrays the activity planning. Figure 2.2 is an I-J CPM logic diagram. The planning phase of the critical path method is by far the most difﬁcult but also the most important.
It is here that the construction planner must actually build the project on paper. This can only be done by becoming totally familiar with the project plans, speciﬁcations, resources, and constraints, looking at various plans for feasibly performing the project, and selecting the best one.
The most difﬁcult planning aspect to consider, especially for beginners, is the level of detail needed for the activities. The best answer is to develop the minimum level of detail required to enable the user to schedule the work efﬁciently. For instance, general contractors will normally consider two or three activities for mechanical work to be sufﬁcient for their schedule. However, to mechanical contractors, this would be totally inadequate because they will need a more detailed breakdown of their activities in order to schedule their work. Therefore, the level of activity detail required depends on the needs of the user of the plan, and only the user can determine his or her needs after gaining experience in the use of critical path methods.
Once the activities have been determined, they must be arranged into a working plan in the network
logic diagram. Starting with an initial activity in the project, one can apply known constraints and reason
that all remaining activities must fall into one of three categories:
1. They must precede the activity in question.
2. They must follow the activity in question.
3. They can be performed concurrently with the activity in question.
The remaining planning function is the estimation of the time durations for each activity shown on the logic diagram. The estimated activity time should reﬂect the proposed method for performing the activity, plus consider the levels at which required resources are supplied. The estimation of activity times is always a tough task for the beginner in construction because it requires a working knowledge of the production capabilities of the various crafts in the industry, which can only be acquired through many observations of actual construction work. Therefore, the beginner will have to rely on the advice of superiors for obtaining time estimates for work schedules.
Scheduling of construction projects involves the determination of the timing of each work item, activity, in a project within the overall time span of the project. Scheduling, with respect to the critical path methods, involves the calculation of the starting and ﬁnishing times for each activity and the project duration, the evaluation of the available ﬂoat for each activity, and the identiﬁcation of the critical path or paths. In a broader sense, it also includes the more complicated areas of construction project man agement such as ﬁnancial funds, ﬂow analyses, resource scheduling and leveling, and inclement weather scheduling. The planning and scheduling of construction projects using critical path methods have been discussed as two separate processes. Although the tasks performed are different, the planning and scheduling processes normally overlap. The ultimate objective of the project manager is to develop a working plan with a schedule that meets the completion date requirements for the project. This requires an interactive process of planning and replanning, and scheduling and rescheduling, until a satisfactory working plan is obtained
Tuesday, April 17, 2012
Construction Planning & Scheduling
Tuesday, January 17, 2012
This Staad book covers the following topics
Features Affecting the Pre-Processor (Modeling Mode)
ASME NF Steel Design Codes
Floor Response Spectrum
Russian Wind Loading
Additional Standard Profile Databases
Features Affecting the Analysis and Design Engine
Time History Animation
Enhanced Plate Stress Results
ProjectWise is an engineering project team collaboration system which is used to help
teams improve quality, reduce rework, and meet project deadlines. One of the major
pieces of functionality provided by ProjectWise is an Integration Server which allows
data to be managed and shared across a distributed enterprise.
STAAD.Pro has been enhanced so that the model STD data file can be managed on a
Four integration functionalities have been added. These are
• Open a STAAD model from a ProjectWise repository.
• Save a local STAAD model into a ProjectWise repository.
• Update an existing model from ProjectWise.
• Review model properties (meta-data) which has been opened from a
Note that access to all of these functionalities is available from ProjectWise sub-menu
under the general File menu described below.
Installation and management of a ProjectWise server is beyond the scope of this
document and should be obtained from the ProjectWise installation.
A local ProjectWise client should be installed which allows access to ProjectWise
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Sunday, January 15, 2012
Post-tensioning is a method of reinforcing (strengthening) concrete or other materials with high-strength steel strands or bars, typically referred to as tendons. Post-tensioning applications include office and apartment buildings, parking structures, slabs-on-ground, bridges, sports stadiums, rock and soil anchors, and water-tanks. In many cases, post-
tensioning allows construction that would otherwise be impossible due to either site constraints or architectural requirements.
Although post-tensioning systems require specialized knowledge and expertise to fabricate, assemble and install, the concept is easy to explain. Imagine a series of wooden blocks with holes drilled through them, into which a rubber band is threaded. If one holds the ends of the rubber band, the blocks will sag. Post-tensioning can be demonstrated by placing wing nuts on either end of the rubber band and winding the rubber band so that the blocks are pushed
tightly together. If one holds the wing nuts after winding, the blocks will remain straight. The tightened rubber band is comparable to a post-tensioning tendon that has been stretched by hydraulic jacks and is held in place by wedge-type anchoring devices.
To fully appreciate the benefits of post-tensioning, it is helpful to know a little bit about concrete. Concrete is very strong in compression but weak in tension, i.e. it will crack when forces act to pull it apart. In conventional concrete construction, if a load such as the cars in a
parking garage is applied to a slab or beam, the beam will tend to deflect or sag. This deflection will cause the bottom of the beam to elongate slightly. Even a slight elongation is usually enough to cause cracking. Steel reinforcing bars (“rebar”) are typically embedded in the
concrete as tensile reinforcement to limit the crack widths. Rebar is what is called “passive” reinforcement however; it does not carry any force until the concrete has already deflected enough to crack. Post-tensioning tendons, on the other hand, are considered “active” reinforcing. Because it is pre stressed, the steel is effective as reinforcement even
though the concrete may not be cracked. Post-tensioned structures can be designed to have minimal deflection and cracking, even under full load
There are post-tensioning applications in almost all facets of construction. In building construction, post-tensioning allows longer clear spans, thinner slabs, fewer beams and
more slender, dramatic elements. Thinner slabs mean less concrete is required. In addition, it means a lower overall building height for the same floor-to-floor height. Post-
tensioning can thus allow a significant reduction in building weight versus a conventional concrete building with the same number of floors. This reduces the foundation load and can
be a major advantage in seismic areas. A lower building height can also translate to considerable savings in mechanical systems and façade costs. Another advantage of
post-tensioning is that beams and slabs can be continuous, i.e. a single beam can run continuously from one end of the building to the other. Structurally, this is much more efficient
than having a beam that just goes from one column to the next. Post-tensioning is the system of choice for parking structures since it allows a high degree of flexibility in the column lay-
out, span lengths and ramp configurations. Post-tensioned parking garages can be either stand-alone structures or one or more floors in an office or residential building. In areas
where there are expansive clays or soils with low bearing capacity, post-tensioned slabs-on-ground and mat foundations reduce problems with cracking and differential settlement. Post-tensioning allows bridges to be built to very demanding geometry requirements, including complex curves, variable superelevation and significant grade changes. Post-tensioning also allows extremely long span bridges to be constructed without the use of temporary intermediate supports. This minimizes the impact on the environment and avoids disruption to water or road traffic below. In stadiums, post-tensioning allows long clear spans and very creative architecture. Post-tensioned rock and soil anchors are used in tunneling and slope stabilization and as tie-backs for excavations. Post-tensioning can also be used to produce virtually crack-free concrete for water-tanks.
A post-tensioning "tendon" is defined as a complete assembly consisting of the anchorages, the prestressing strand or bar, the sheathing or duct and any grout or corrosion-inhibiting coating (grease) surrounding the prestressing steel. There are two main types of post-
tensioning: unbonded and bonded (grouted). An unbonded tendon is one in which the prestressing steel is not actually bonded to the concrete that surrounds it except at the anchorages. The most common unbonded systems are monostrand (single strand) tendons, which are used in slabs and beams for buildings, parking structures and slabs-on-ground. A monostrand tendon consists of a seven-wire strand that is coated with a corrosion-inhibiting
grease and encased in an extruded plastic protective sheathing. The anchorage consists of an iron casting and a conical, two-piece wedge which grips the strand.In bonded systems, two or more strands are inserted into a metal or plastic duct that is embedded in the concrete. The
strands are stressed with a large, multi-strand jack and anchored in a common anchorage device. The duct is then filled with a cementitious grout that provides corrosion protection to the strand and bonds the tendon to the concrete surrounding the duct. Bonded systems are more commonly used in bridges, both in the superstructure (the roadway) and in cable-stayed bridges, the cable-stays. In buildings, they are typically only used in heavily loaded beams such as transfer girders and landscaped plaza decks where the large number of strands required makes them more economical. Rock and soil anchors are also bonded systems but the
construction sequence is somewhat different. Typically, a cased hole is drilled into the side of the excavation, the hillside or the tunnel wall. A tendon is inserted into the casing and then the casing is grouted. Once the grout has reached sufficient strength, the tendon is stressed. In slope and tunnel wall stabilization, the anchors hold loose soil and rock together; in excavations they hold the wood lagging and steel piles in place.
There are several critical elements in a post-tensioning system. In unbonded construction, the plastic sheathing acts as a bond breaker between the concrete and the prestressing
strands. It also provides protection against damage by mechanical handling and serves as a barrier that prevents moisture and chemicals from reaching the strand. The strand coating material reduces friction between the strand and the sheathing and provides additional corrosion protection. Anchorages are another critical element, particularly in unbonded systems. After the concrete has cured andobtained the necessary strength, the wedges are inserted nside the anchor casting and the strand is stressed. When the jack releases the strand, the strand retracts slightly and pulls the wedges into the anchor. This creates a tight lock on the strand. The wedges thus maintain the applied force in the tendon and transfer it to the surrounding concrete. In corrosive environments, the anchorages and exposed strand tails are usually covered with a housing and cap for added protection.
In building and slab-on-ground construction, unbonded tendons are typically prefabricated at a plant and delivered to the construction site, ready to install. The tendons are laid out in the forms in accordance with installation drawings that indicate how they are to be spaced, what their profile (height above the form) should be, and where they are to be stressed. After the concrete is placed and has reached its required strength, usually between 3000 and 3500 psi (“pounds per square inch”), the tendons are stressed and anchored. The tendons, like rubber bands, want to return to their original length but are prevented from doing so by the anchorages. The fact the tendons are kept in a permanently stressed (elongated) state causes a compressive force to act on the concrete. The compression that results from the post- tensioning counteracts the tensile forces created by subsequent applied loading (cars, people, the weight of the beam itself when the shoring is removed). This significantly increases the load-carrying capacity of the concrete. Since post-tensioned concrete is cast in place at the job site, there is almost no limit to the shapes that can be formed. Curved facades, arches and complicated slab edge layouts are often a trademark of post-tensioned concrete structures. Post-tensioning has been used to advantage in a number of very aesthetically designed bridges.
ENSURING QUALITY CONSTRUCTION
The amount of post-tensioning strand sold has almost doubled in the last ten years and the post-tensioning industryis continuing to grow rapidly. To ensure quality construction,
the Post-Tensioning Institute (PTI) has implemented both a Plant Certification Program and a Field Personnel Certification Training Course. By specifying that the plant and the installers be PTI certified, engineers can ensure the level of quality that the owner will expect. PTI also publishes technical documents and reference manuals covering various aspects of post-tensioned design and construction.
Monday, January 9, 2012
Analysis and Design Using Staad Pro
Step by Step example for analysing and designing a beam and structure using staad pro
Analysis and Design of Structure using Staad