COST ESTIMATING ON BUILDING PROJECTS BASIC TECHNIQUE
How To Do Cost Estimates On Building Projects?
During the late 1950s the technique of elemental cost planning on buildings was established. This technique enabled the client to obtain a more reliable pre-tender estimate and gave the design team a template in order to control the cost during the design development stages.
The technique was embraced by the Hertfordshire County Council and used successfully on the CLASP modular school building projects in the 1960s.
The technique is now well established in the building sector and has been further developed by the Building Cost Information Service of the RICS (BCIS) to include a national database of elemental cost analyses, which can be accessed using online computer techniques.
Such information can be used to aid the pre-contract estimating process in the building sector as well as helping to ensure VFM by aiding the designer to ensure the most appropriate distribution of costs within the project.
Cost management is the total process, which ensures that the contract sum is within the client’s approved budget or cost limit. It is the process of helping the design team design to a cost rather than the QS costing a design.
The basis of the design cost control using the cost-planning technique is the analysis of existing projects into functional elements in order to provide a means of comparison between projects planned with data from existing projects. A building element is defined as part of a building performing a function regardless of its specification.
Elemental analysis allows the comparison of the costs of the same element to be compared between two or more buildings.
As the cost element under consideration is performing the same function, an objective assessment can be made as to why there may be differences in costs between the same elements in different buildings. There are four main reasons why differences in costs occur:
1. Differences in time (inflation)
2. Quantitative differences
3. Qualitative differences
4. Differences in location.
On a major project it is necessary to consider individual buildings or parts of buildings. A major shopping centre may be split into common basement, finished malls, unfinished shells, hotel and car parking. The parts of the whole may be physically linked and difficult to separate, but separation will ease estimating and control.
The costs of the identifiable parts can then be compared against other schemes. For example, a composite rate per square metre is meaningless when you mix the cost of finished atrium malls with unfinished shells.
It is not only important to separate out parts of the building that serve different functions but it is equally important to separate for phasing. Many major projects have to be built around existing structures, which increase the cost because of temporary works as well as inflation.
The client’s and project’s status with regard to VAT will also need to be established. In the UK VAT is currently payable on building work other than constructing new dwellings and certain buildings used solely on both residential and non-business charitable purposes and also on all consultants and professional fees. The current VAT rate is 17.5%.
It is customary to exclude this amount from estimates and tenders, a practice that is well understood in the construction industry. However, this must be pointed out to any client who otherwise may think that the estimate is their total liability (Ferry and Brandon, 1999).
Thursday, April 5, 2012
TOWER CRANES BASIC AND CIVIL ENGINEERING TUTORIALS
TOWER CRANES BASIC INFORMATION
What Are Tower Cranes?
Much like a mobile crane, a tower-mounted crane moves loads by executing three motions: the hook is raised and lowered by means of a winch and fall, carried in a circular path by the swing gear, and carried in a radial motion by either luffing the jib or rolling a trolley carriage on its underside. The simplest of tower cranes have only these motions, but more complicated arrangements include mechanisms that allow the base to roll on a track, the crane to change elevation by climbing, or the jib to articulate on a hinge point.
In the twentieth century, tower crane and mobile-crane industries developed independently on opposite shores of the Atlantic Ocean in response to needs and cultural proclivities of their respective lands. Simply put, Europe was the realm of tower cranes, North America the realm of mobile cranes, and the rest of the world a patchwork of the two.
The globalized marketplace and rapid diffusion of knowledge in the present century have eroded differences in construction practices worldwide so that the selection of a crane type now is more likely to be determined by its suitability to the work than by the country where the work is taking place. At least that is the broad picture; contractors in some localities cling longer to traditional practices than others.
Tower cranes are the lifting machines of choice worldwide for most mid- and high-rise building construction. They are used also on expansive sites where the broad hook sweep and the relative ease of coordinating multiple tower cranes is an advantage.
There are niche markets for these as well; cable stay and suspension bridges, offshore oil platforms, and power plants are some examples. In most of the world outside U.S., small tower cranes are used for modest-size residential and commercial projects.
Many of these rigs are self-erecting machines that are pulled to the site by truck. In North America, similar work would utilize a small telescopic crane.
Gradually, however, self-erecting tower cranes are penetrating the U.S. market. There is some doubt whether these machines should be classified as tower cranes; though their operating motions fit thepattern, their deployment—setup time, operation, inspection, maintenance, and demobilization—is more like that of mobile cranes.
Freestanding hammerhead tower cranes range up to about 300 feet (91 m) in height; for luffing tower cranes the limitation is less. Though most tower cranes free-stand and remain at a fixed height, various self-climbing arrangements permit a tower crane to attach to a building under construction and rise with it.
With such supplemental means of support, a tower crane can ascend to any building height. Very high line speeds up to 1000 ft/min (5 m/s) available with some models yield good production rates even at extraordinary heights.
Some machines can operate in winds up to 45 mi/h (70 km/h), which is far above mobile-crane wind limits. Lift capability of tower cranes is gauged by a moment rating expressed as tonne-meters. The tonne-meter rating is obtained by multiplying rated capacity in metric tons by the working radius in meters.
This is done according to a method that averages a range of boom lengths and working radii. The smallest machines used for light construction have ratings of about 20 meter-tons and the very largest in production exceed this by a factor of about a hundred. Most used for heavy construction are in the range of 150 to 650 meter-tons.
The cost associated with installing and removing a tower crane is small for the self-erecting type but can be considerable for most others. At minimum, those costs would include trucking, hiring a rigging crew and an assist crane, construction of a foundation, electrical hookup, and the services of a trained technician. More complicated installations such as those which climb have considerable additional expenses.
The high costs of installation, as well as the considerable investment of time and planning, make all but the smallest tower cranes a tool for longer-term projects where these expenditures can be amortized.
Excluding self-erectors, tower crane designs follow the “erector set” concept; that is, they are composed of multiple components of common design. Components connected together by pins or bolts are often interchangeable among an array of models.
As a tower is a cantilever subjected to high-bending moments that shift and reverse, each tower leg must be designed to resist alternating compressive and tensile forces. The usual stress levels and the expected number of lifetime loading cycles are not exceptional; thus leg design does not pose a difficult fatigue problem for designers.
The connections between tower sections are another matter, being a perpetually nettlesome design and maintenance concern. Some designs use bolts and others use pins to join the legs of one section to the next. Bolts must be preloaded to carefully controlled levels.
Failure to establish and maintain the preload can result in bolt fatigue failure. The alternative pin-type connection requires tight fabrication tolerance; imperfect towers can be either too sloppy or difficult to fit together. They also must be carefully designed to avoid the pitfall of premature fatigue failure.
With the exception of a few diesel-powered machines with hydrostatic drives, tower cranes are powered by electricity and motors driving all the machinery are almost universally electric, too. The hoist motors on older machines have high- and low-speed ranges with stepped increments in each range.
More recent models have variable-frequency drive or other forms of continuously adjustable speed motors with friction or eddy-current brakes and creep speed. Automatic acceleration for all motions is typical on many cranes. Remote controls are sometimes offered.
A tower crane is said to be top-slewing—slewing being the preferred tower crane term for swing motion—if the swing circle is mounted near the tower top and it’s said to be bottom-slewing if the swing circle is near the. Bottom-slewing machines might have become largely obsolete had the concept not been revived by recent developments with self-erecting machines.
What Are Tower Cranes?
Much like a mobile crane, a tower-mounted crane moves loads by executing three motions: the hook is raised and lowered by means of a winch and fall, carried in a circular path by the swing gear, and carried in a radial motion by either luffing the jib or rolling a trolley carriage on its underside. The simplest of tower cranes have only these motions, but more complicated arrangements include mechanisms that allow the base to roll on a track, the crane to change elevation by climbing, or the jib to articulate on a hinge point.
In the twentieth century, tower crane and mobile-crane industries developed independently on opposite shores of the Atlantic Ocean in response to needs and cultural proclivities of their respective lands. Simply put, Europe was the realm of tower cranes, North America the realm of mobile cranes, and the rest of the world a patchwork of the two.
The globalized marketplace and rapid diffusion of knowledge in the present century have eroded differences in construction practices worldwide so that the selection of a crane type now is more likely to be determined by its suitability to the work than by the country where the work is taking place. At least that is the broad picture; contractors in some localities cling longer to traditional practices than others.
Tower cranes are the lifting machines of choice worldwide for most mid- and high-rise building construction. They are used also on expansive sites where the broad hook sweep and the relative ease of coordinating multiple tower cranes is an advantage.
There are niche markets for these as well; cable stay and suspension bridges, offshore oil platforms, and power plants are some examples. In most of the world outside U.S., small tower cranes are used for modest-size residential and commercial projects.
Many of these rigs are self-erecting machines that are pulled to the site by truck. In North America, similar work would utilize a small telescopic crane.
Gradually, however, self-erecting tower cranes are penetrating the U.S. market. There is some doubt whether these machines should be classified as tower cranes; though their operating motions fit thepattern, their deployment—setup time, operation, inspection, maintenance, and demobilization—is more like that of mobile cranes.
Freestanding hammerhead tower cranes range up to about 300 feet (91 m) in height; for luffing tower cranes the limitation is less. Though most tower cranes free-stand and remain at a fixed height, various self-climbing arrangements permit a tower crane to attach to a building under construction and rise with it.
With such supplemental means of support, a tower crane can ascend to any building height. Very high line speeds up to 1000 ft/min (5 m/s) available with some models yield good production rates even at extraordinary heights.
Some machines can operate in winds up to 45 mi/h (70 km/h), which is far above mobile-crane wind limits. Lift capability of tower cranes is gauged by a moment rating expressed as tonne-meters. The tonne-meter rating is obtained by multiplying rated capacity in metric tons by the working radius in meters.
This is done according to a method that averages a range of boom lengths and working radii. The smallest machines used for light construction have ratings of about 20 meter-tons and the very largest in production exceed this by a factor of about a hundred. Most used for heavy construction are in the range of 150 to 650 meter-tons.
The cost associated with installing and removing a tower crane is small for the self-erecting type but can be considerable for most others. At minimum, those costs would include trucking, hiring a rigging crew and an assist crane, construction of a foundation, electrical hookup, and the services of a trained technician. More complicated installations such as those which climb have considerable additional expenses.
The high costs of installation, as well as the considerable investment of time and planning, make all but the smallest tower cranes a tool for longer-term projects where these expenditures can be amortized.
Excluding self-erectors, tower crane designs follow the “erector set” concept; that is, they are composed of multiple components of common design. Components connected together by pins or bolts are often interchangeable among an array of models.
As a tower is a cantilever subjected to high-bending moments that shift and reverse, each tower leg must be designed to resist alternating compressive and tensile forces. The usual stress levels and the expected number of lifetime loading cycles are not exceptional; thus leg design does not pose a difficult fatigue problem for designers.
The connections between tower sections are another matter, being a perpetually nettlesome design and maintenance concern. Some designs use bolts and others use pins to join the legs of one section to the next. Bolts must be preloaded to carefully controlled levels.
Failure to establish and maintain the preload can result in bolt fatigue failure. The alternative pin-type connection requires tight fabrication tolerance; imperfect towers can be either too sloppy or difficult to fit together. They also must be carefully designed to avoid the pitfall of premature fatigue failure.
With the exception of a few diesel-powered machines with hydrostatic drives, tower cranes are powered by electricity and motors driving all the machinery are almost universally electric, too. The hoist motors on older machines have high- and low-speed ranges with stepped increments in each range.
More recent models have variable-frequency drive or other forms of continuously adjustable speed motors with friction or eddy-current brakes and creep speed. Automatic acceleration for all motions is typical on many cranes. Remote controls are sometimes offered.
A tower crane is said to be top-slewing—slewing being the preferred tower crane term for swing motion—if the swing circle is mounted near the tower top and it’s said to be bottom-slewing if the swing circle is near the. Bottom-slewing machines might have become largely obsolete had the concept not been revived by recent developments with self-erecting machines.
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