Tower Cranes for civil engg projects.

Parts of a Tower Crane

All tower cranes consist of the same basic parts:

• The base is bolted to a large concrete pad that supports the crane.
• The base connects to the mast (or tower), which gives the tower crane its height.
• Attached to the top of the mast is the slewing unit -- the gear and motor -- that allows the crane to rotate:

On top of the slewing unit are three parts:

• The long horizontal jib (or working arm), which is the portion of the crane that carries the load. A trolley runs along the jib to move the load in and out from the crane's center:

• The shorter horizontal machinery arm, which contains the crane's motors and electronics as well as the large concrete counter weights:

• The operator's cab:

The machinery arm contains the motor that lifts the load, along with the control electronics that drive it and the cable drum, as shown here:

The motors that drive the slewing unit are located above the unit's large gear:

Construction Site Environment

While the general information about the construction site is usually available at the planning stage of a project, it is important for the design professionals and construction manager as well as the contractor to visit the site. Each group will be benefited by first-hand knowledge acquired in the field.

For design professionals, an examination of the topography may focus their attention to the layout of a facility on the site for maximum use of space in compliance with various regulatory restrictions. In the case of industrial plants, the production or processing design and operation often dictate the site layout. A poor layout can cause construction problems such as inadequate space for staging, limited access for materials and personnel, and restrictions on the use of certain construction methods. Thus, design and construction inputs are important in the layout of a facility.

The construction manager and the contractor must visit the site to gain some insight in preparing or evaluating the bid package for the project. They can verify access roads and water, electrical and other service utilities in the immediate vicinity, with the view of finding suitable locations for erecting temporary facilities and the field office. They can also observe any interferences of existing facilities with construction and develop a plan for site security during construction.

In examining site conditions, particular attention must be paid to environmental factors such as drainage, groundwater and the possibility of floods. Of particular concern is the possible presence of hazardous waste materials from previous uses. Cleaning up or controlling hazardous wastes can be extremely expensive.

Example1 : Groundwater Pollution from a Landfill

The presence of waste deposits on a potential construction site can have substantial impacts on the surrounding area. Under existing environmental regulations in the United States, the responsibility for cleaning up or otherwise controlling wastes generally resides with the owner of a facility in conjunction with any outstanding insurance coverage.

A typical example of a waste problem is illustrated in Figure1. In this figure, a small pushover burning dump was located in a depression on a slope. The landfill consisted of general refuse and was covered by a very sandy material. The inevitable infiltration of water from the surface or from the groundwater into the landfill will result in vertical or horizontal percolation of leachable ions and organic contamination. This leachate would be odorous and potentially hazardous in water. The pollutant would show up as seepage downhill, as pollution in surface streams, or as pollution entering the regional groundwater.

Figure 1: Cross-Section Illustration of a Landfill

Before new construction could proceed, this landfill site would have to be controlled or removed. Typical control methods might involve:

• Surface water control measures, such as contour grading or surface sealing.
• Passive groundwater control techniques such as underground barriers between the groundwater and the landfill.
• Plume management procedures such as pumping water from surrounding wells.
• Chemical immobilization techniques such as applying surface seals or chemical injections.
• Excavation and reburial of the landfill requiring the availability of an engineered and environmentally sound landfill.

The excavation and reburial of even a small landfill site can be very expensive. For example, the estimated reburial cost for a landfill like that shown in Figure1 was in excess of $ 4 million in 1978.

Importance of PLANNING for any project.

In the planning of facilities, it is important to recognize the close relationship between design and construction. These processes can best be viewed as an integrated system. Broadly speaking, design is a process of creating the description of a new facility, usually represented by detailed plans and specifications; construction planning is a process of identifying activities and resources required to make the design a physical reality. Hence, construction is the implementation of a design envisioned by architects and engineers. In both design and construction, numerous operational tasks must be performed with a variety of precedence and other relationships among the different tasks.

Several characteristics are unique to the planning of constructed facilities and should be kept in mind even at the very early stage of the project life cycle. These include the following:

• Nearly every facility is custom designed and constructed, and often requires a long time to complete.
• Both the design and construction of a facility must satisfy the conditions peculiar to a specific site.
• Because each project is site specific, its execution is influenced by natural, social and other locational conditions such as weather, labor supply, local building codes, etc.
• Since the service life of a facility is long, the anticipation of future requirements is inherently difficult.
• Because of technological complexity and market demands, changes of design plans during construction are not uncommon.

In an integrated system, the planning for both design and construction can proceed almost simultaneously, examining various alternatives which are desirable from both viewpoints and thus eliminating the necessity of extensive revisions under the guise of value engineering. Furthermore, the review of designs with regard to their constructibility can be carried out as the project progresses from planning to design. For example, if the sequence of assembly of a structure and the critical loadings on the partially assembled structure during construction are carefully considered as a part of the overall structural design, the impacts of the design on construction falsework and on assembly details can be anticipated. However, if the design professionals are expected to assume such responsibilities, they must be rewarded for sharing the risks as well as for undertaking these additional tasks. Similarly, when construction contractors are expected to take over the responsibilities of engineers, such as devising a very elaborate scheme to erect an unconventional structure, they too must be rewarded accordingly. As long as the owner does not assume the responsibility for resolving this risk-reward dilemma, the concept of a truly integrated system for design and construction cannot be realized.

It is interesting to note that European owners are generally more open to new technologies and to share risks with designers and contractors. In particular, they are more willing to accept responsibilities for the unforeseen subsurface conditions in geotechnical engineering. Consequently, the designers and contractors are also more willing to introduce new techniques in order to reduce the time and cost of construction. In European practice, owners typically present contractors with a conceptual design, and contractors prepare detailed designs, which are checked by the owner's engineers. Those detailed designs may be alternate designs, and specialty contractors may also prepare detailed alternate designs.

Example 1: Responsibility for Shop Drawings

The willingness to assume responsibilities does not come easily from any party in the current litigious climate of the construction industry in the United States. On the other hand, if owner, architect, engineer, contractor and other groups that represent parts of the industry do not jointly fix the responsibilities of various tasks to appropriate parties, the standards of practice will eventually be set by court decisions. In an attempt to provide a guide to the entire spectrum of participants in a construction project, the American Society of Civil Engineers issued a Manual of Professional Practice entitled Quality in the Constructed Project in 1990. This manual is intended to help bring a turn around of the fragmentation of activities in the design and construction process.

Shop drawings represent the assembly details for erecting a structure which should reflect the intent and rationale of the original structural design. They are prepared by the construction contractor and reviewed by the design professional. However, since the responsibility for preparing shop drawings was traditionally assigned to construction contractors, design professionals took the view that the review process was advisory and assumed no responsibility for their accuracy. This justification was ruled unacceptable by a court in connection with the walkway failure at the Hyatt Hotel in Kansas City in 1985. In preparing the ASCE Manual of Professional Practice for Quality in the Constructed Project, the responsibilities for preparation of shop drawings proved to be the most difficult to develop. [1] The reason for this situation is not difficult to fathom since the responsibilities for the task are diffused, and all parties must agree to the new responsibilities assigned to each in the recommended risk-reward relations shown in Table 1.

Traditionally, the owner is not involved in the preparation and review of shop drawings, and perhaps is even unaware of any potential problems. In the recommended practice, the owner is required to take responsibility for providing adequate time and funding, including approval of scheduling, in order to allow the design professionals and construction contractors to perform satisfactorily.

Table 1 Recommended Responsibility for Shop Drawings

Task	Responsible Party
	Owner	Design Professional	Construction Contractor
Provide adequate time and funding for shop drawing preparation and review	Prime
Arrange for structural design	Prime
Provide structural design		Prime
Establish overall responsibility for connection design		Prime
Accomplish connection design (by design professional)		Prime
Alternatively, provide loading requirement and other information necessary for shop drawing preparation		Prime
Alternatively, accomplish some or all of connection design (by constuctor with a licensed P.E.)			Prime
Specify shop drawing requirements and procedures	Review	Prime
Approve proper scheduling	Prime	Assisting	Assisting
Provide shop drawing and submit the drawing on schedule			Prime
Make timely reviews and approvals		Prime
Provide erection procedures, construction bracing, shoring, means, methods and techniques of construction, and construction safety			Prime

Example 2:Model Metro Project in Milan, Italy [2]

Under Italian law, unforeseen subsurface conditions are the owner's responsibility, not the contractor's. This is a striking difference from U.S. construction practice where changed conditions clauses and claims and the adequacy of prebid site investigations are points of contention. In effect, the Italian law means that the owner assumes those risks. But under the same law, a contractor may elect to assume the risks in order to lower the bid price and thereby beat the competition.

According to the Technical Director of Rodio, the Milan-based contractor which is heavily involved in the grouting job for tunneling in the Model Metro project in Milan, Italy, there are two typical contractual arrangements for specialized subcontractor firms such as theirs. One is to work on a unit price basis with no responsibility for the design. The other is what he calls the "nominated subcontractor" or turnkey method: prequalified subcontractors offer their own designs and guarantee the price, quality, quantities, and, if they wish, the risks of unforeseen conditions.

At the beginning of the Milan metro project, the Rodio contract ratio was 50/50 unit price and turnkey. The firm convinced the metro owners that they would save money with the turnkey approach, and the ratio became 80% turnkey. What's more, in the work packages where Rodio worked with other grouting specialists, those subcontractors paid Rodio a fee to assume all risks for unforeseen conditions.

Under these circumstances, it was critical that the firm should know the subsurface conditions as precisely as possible, which was a major reason why the firm developed a computerized electronic sensing program to predict stratigraphy and thus control grout mixes, pressures and, most important, quantities.

Design considerations.

While the conceptual design process may be formal or informal, it can be characterized by a series of actions: formulation, analysis, search, decision, specification, and modification. However, at the early stage in the development of a new project, these actions are highly interactive as illustrated in Figure 1 Many iterations of redesign are expected to refine the functional requirements, design concepts and financial constraints, even though the analytic tools applied to the solution of the problem at this stage may be very crude.

Figure 1: Conceptual Design Process
(Adapted with permission from R.W. Jensen and C.C. Tonies, Software Engineering,
Prentice Hall, Englewood Cliffs, NJ, 1979, p.22)

The series of actions taken in the conceptual design process may be described as follows:

• Formulation refers to the definition or description of a design problem in broad terms through the synthesis of ideas describing alternative facilities.
• Analysis refines the problem definition or description by separating important from peripheral information and by pulling together the essential detail. Interpretation and prediction are usually required as part of the analysis.
• Search involves gathering a set of potential solutions for performing the specified functions and satisfying the user requirements.
• Decision means that each of the potential solutions is evaluated and compared to the alternatives until the best solution is obtained.
• Specification is to describe the chosen solution in a form which contains enough detail for implementation.
• Modification refers to the change in the solution or re-design if the solution is found to be wanting or if new information is discovered in the process of design.

As the project moves from conceptual planning to detailed design, the design process becomes more formal. In general, the actions of formulation, analysis, search, decision, specification and modification still hold, but they represent specific steps with less random interactions in detailed design. The design methodology thus formalized can be applied to a variety of design problems. For example, the analogy of the schematic diagrams of the structural design process and of the computer program development process is shown in Figure 2

Figure 2: An Analogy Between Structural Design and Computer Program Development Process
(Reprinted with permission from E.H. Gaylord and C. N. Gaylord, eds., Structural Engineering Handbook,
2nd Ed., McGraw-Hill Book Company, New York, 1979.)

The basic approach to design relies on decomposition and integration. Since design problems are large and complex, they have to be decomposed to yield subproblems that are small enough to solve. There are numerous alternative ways to decompose design problems, such as decomposition by functions of the facility, by spatial locations of its parts, or by links of various functions or parts. Solutions to subproblems must be integrated into an overall solution. The integration often creates conceptual conflicts which must be identified and corrected. A hierarchical structure with an appropriate number of levels may be used for the decomposition of a design problem to subproblems. For example, in the structural design of a multistory building, the building may be decomposed into floors, and each floor may in turn be decomposed into separate areas. Thus, a hierarchy representing the levels of building, floor and area is formed.

Different design styles may be used. The adoption of a particular style often depends on factors such as time pressure or available design tools, as well as the nature of the design problem. Examples of different styles are:

• Top-down design. Begin with a behavior description of the facility and work towards descriptions of its components and their interconnections.
• Bottom-up design. Begin with a set of components, and see if they can be arranged to meet the behavior description of the facility.

The design of a new facility often begins with the search of the files for a design that comes as close as possible to the one needed. The design process is guided by accumulated experience and intuition in the form of heuristic rules to find acceptable solutions. As more experience is gained for this particular type of facility, it often becomes evident that parts of the design problem are amenable to rigorous definition and algorithmic solution. Even formal optimization methods may be applied to some parts of the problem.