Concrete Construction Article Pdf_ Estimating Formwork.pdf

Estimating formwork A realistic appraisal must be made of the kinds and amounts of materials and the needed size of crew

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he final profit or loss on a structure is often decided in the estimating or bidding stage before a job actually starts. Basically the cost of forming depends upon thickness, height and length of wall but there are also many other factors involved that are not always easy to evaluate and some of which are even likely to be unforeseen . The knowledge necessary for successful formwork estimating is gained by trial and error—though some is obtained by schooling. A highly useful supplement is the assistance offered by most manufacturers of concrete forming equipment who can give estimates, cost quotations and layouts for the use of their equipment. Such information can be included in estimating the overall cost of the job. Since concrete work may be as high as 75 percent of the total cost on some jobs and since forming may represent 60 to 75 percent of the concrete cost the help given by the manufacturer should be valuable in keeping these costs low. Occasionally complaints are heard that a manufacturer has misfigured the job or has not supplied enough equipment so that delays have occurred and the schedule has had to be adjusted. When this happens it is usually because the contractor and manufacturer have not understood one another, have not supplied each other enough information or have not planned properly. The contractor and man-

Typical of much of the forming today is this job with its many offsets, projections, ledges and box-outs. Cut-up forming such as this makes estimating more complex and overall forming costs higher.

ufacturer have to be both honest and realistic with each other.

Estimating material requirements To begin the estimate the contractor must consider what forming equipment is available, how much of the needed equipment he owns and how much he must rent or purchase. The next step is to analyze the job in terms of what types of equipment are needed—prefabricated, job-built or specials. The manufacturer should be asked for assistance and for this purpose he should be given all the information needed to make a realistic estimate. There are two types of formwork by which the job can be handled: job-built or prefabricated. The choice can considerably affect the p re p a ration of the bid and its amount. Sometimes the design dictates the type of equipment required. For example, if columns are

to be of unusual design, it may prove more economical to use special forms if they can be reused enough times to justify the cost. Once the estimate is started it proceeds according to the following four steps, usually in this sequence but not always: • analysis of the total job • examination of equipment or systems available • consideration of the effect of the choice of formwork on the total job • consideration of factors other than formwork that can affect the results Reviewing these in detail we find the following: 1. Analysis of the total job. To underestimate a job and receive the contract can prove to be a financial problem to the contractor and even lead to business failure.

The blueprints should be reviewed to analyze each area or segment of the job that involves conc re t e. If the job is large or cut up, detailed notes should be made regarding any questionable areas as a safeguard that nothing be overlooked. Many jobs will have some factors about them that will immediately indicate the job plan, the equipment to use and sequence of work. Others will not, and particular attention must be given to these jobs. Overlooking one small item or misfiguring a size or sizes can have a bearing on the success of the job. Usually the big errors are not caused by misfiguring but by omitting or overlooking some key item or phase of work. Many estimators prepare a rough estimate at this point to determine the amount of concrete to be formed, expressing it in terms of square feet of contract area. This will provide a rough indication of what percent of the total job is concerned with forming. It will also aid in selection of forming equipment. 2. Examination of available equipment or systems. If the contractor owns forming equipment he may prepare a detailed take-off of the job to see if his inventory is adequate or if additional equipment and materials are necessary. If new equipment is to be rented or purchased, the take-off will be of great assistance in planning and scheduling. The take-off will be especially helpful if he plans to build the forms himself. Best economy requires balancing the effects of original investment, reuse value of forms and efficiency of erection and stripping cycles. Here again, a little extra thought and work can make a big difference. The choice of job-built or prefabricated forms may be dependent upon these factors. The original cost of equipment must be weighed against expected performance, cost per reuse, life expectancy, and cost of finishing. Although there are many other influencing factors the

In preparing the estimate or take-off of areas to be formed, careful attention must be paid not only to the equipment and sizes needed but to the amount of labor required. Job-building of forms for conditions such as this requires careful planning to minimize costs

Gang forming of large sections of forms, when possible will reduce the square foot cost in both erection and stripping, Crane handling of forms should be planned to coordinate with crane handling of materials to minimize idleness of equipment.

Equipment should be located as close as possible to where it will be used to reduce moving time and costs.

others are more of a subject for economy than estimating. At this point assistance and estimates from prefabricated form manufacturers, most of whom offer this service, can be of great help. Manufacturers often work closely with the contractor in finalizing the cost quotation, bill of materials and layout drawings. In order for the manufacturer to p re p a re a formwork estimate, he must be furnished certain supplementary information along with the blueprints. He will want to know the size of carpenter and labor crews, number of concrete placements, the equipment available for material handling, tie specifications and whether the job is to be hand-set or gang-formed. From this he can prepare a preliminary estimate with complete layouts and bill of materials when the job is awarded. 3. Consideration of the effect of

formwork on the total job. With estimates on prefab forms or job-building, or both, the contractor can compare figures and reach a conclusion on the best and most economical way to proceed. Howe ve r, the cost of forming cannot be considered completely independently because it can affect the other operations or work from the very beginning of a job. During the estimating period consideration should be given to how forming operation will affect such other operations as material handling, steel work, plumbing and finishing. When forming equipment is first delivered to the jobsite it should be located close to where it will first be used. If it has to be moved any distance at all and if the quantity is large, the material handling cost can become significant. If it is possible that problems might arise that would reduce efficiency, cost allowances should be made or

High-rise construction calls for careful planning of equipment cycles and smooth scheduling sequence of trades

alternate plans prepared in case they are needed. No one can foresee e ve rything, but the key to cost reduction is reducing labor and increasing the reuse of equipment. Since the concrete work usually precedes almost all other work, a delay of any nature can affect the whole job. 4. Consideration of factors other than formwork that can affect results. In making estimates calculations are often used from a previous job where the work appeared to be the same, where the design was similar or where other factors suggest that the figures are applicable again. Yet it is rather hard to conceive that two jobs can be identical, so previous estimates should be used only with caution. One factor not directly related to materials and job operations is del i ve ry of equipment. When it runs into unexpected delays from strikes, bad weather, accidents, or shortness of supply it can cost the contractor money. The cost of moving onto the jobsite and the efficiency of workers should be evaluated in terms of productivity. This includes the cost of supervision at all levels. Labor relations affect the output of work considerably. The contractor must be able to anticipate a certain amount of work each day. If that work can become repetitious, it adds to the economy, but only if careful scheduling has been done so all crews are kept moving without undue pressure that detracts from m o ra l e. Good working conditions and safety aids have a bearing on the mental attitude and efficiency of the crews. Se ve ral of the many other factors that have an indirect bearing on forming include job conditions, weather, condition of equipment, and strikes among the trades. The estimator must be able to make an educated evaluation of all such factors that influence or might influence the job. He may obtain additional help from any of several estimating books available for figuring

labor, materials costs, wage rates, fringe benefits and all types of construction equipment. These books allow for adjustment of labor rates and the cost of equipment for various parts of the country. Generally speaking, the smaller the work force, the lower the labor unit cost. In order to determine the correct size of labor force an analysis should be made, the requirements set and the work force hired to meet these requirements.

Determining the size of crew It is hard to lay down exact rules to follow in estimating formwork but the following is an example of how the size of a labor force can be determined. Job conditions will have to be ascertained and provided for. Assume that a contractor wants to place concrete in 4,800 square feet of forming every third day and is able to strip the following morning. The ratio of carpenters to laborers is one to one. The carpenter rate is $4.50 per hour including fringe benefits and the laborer’s rate including fringe benefits is $3.50 per hour—a total of $8 for a two-man team. Let us further assume that 20 cents per square foot would be adequate for the labor of erecting and stripping. Working backward we see that erecting and stripping would cost $960; spread over the three-day period this would be $320 per day. The cost of a team of one carpenter and one laborer is $64 per eighthour day. If five teams of carpenters and laborers are used the daily cost will be $320, the same amount that was allowed on the assumption that the cost should be 20 cents per square foot. Over the three-day period the carpenters and laborers will have to average 1,600 square feet per day or 320 square feet per carpenter-laborer team. This amounts to 20 two- by eight-foot forms including waling and bracing during each eight-hour period. This is a very reasonable fig-

ure. Depending upon job conditions it could perhaps be increased considerably, thereby reducing the labor force and square-foot cost. This kind of analysis made by working in reverse can insure the most efficient production. If the requirements are set first and the work force is hired to meet these req u i re m e n t s, the costs can be controlled. The method facilitates early recognition of problems so that corrective measures may be taken before the work has gone too far.

• length of time stringers and shoring must be supported • cycle for stripping and handling • adaptability of equipment from floor to floor • whether equipment can be used at any location or must remain in the same position for each floor In going over this check list it will be necessary to acquire the following information: • how many floors are typical and how many atypical

Check list

• floor heights and slab thicknesses

The following is a general check list of items that must be established when figuring the forming: • number of concrete placements and average volume of each

• size and location of beams

• wall dimensions

• type of slab or its design • sequence of erecting and casting columns, shear walls, core walls, spandrels and other elements

• surface treatment

• number of concrete placements per floor

• construction joints

• location of bulkheads

• treatment of reinforcing bars

• treatment of other materials, such as whether plumbing is sleeved or cast in deck

• handsetting or gang forming • crane time, if any • ties: breakbacks, cones, washers • length of time forms are required to remain in place • any special conditions or equipment Analysis of high-rise work or deck forming follows much the same p ro c e d u re. Howe ve r, the biggest considerations in estimating the working cycle for such work are the criteria for shoring and how long the forms must remain in place. Forming costs can vary up to 50 percent because of differences in specifications and severity of engineering requirements. In order to estimate shoring equipment for slabs with a reasonable degree of accuracy, the following check list is needed: • casting schedule • length of time forms must be supported

• type of ceiling finish • allowable deflection Each region of the country has a different set of standards not only for labor rates but also for working conditions. These should be carefully explored when moving from one area to another. But no matter where the work is done the basic steps outlined here provide an easyto-use formula that can serve as a guideline for all jobs. Use of such a standard procedure will help the estimator develop material and labor cost sheets for record-keeping purposes and these will be of immeasurable value for each new job.

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