Walls in ma.s.s work, 1 to 3 days, or until the concrete will bear pressure of the thumb without indentation.
Thin walls, in summer, 2 days; in cold weather, 5 days.
Slabs up to 6-ft. span, in summer, 6 days; in cold weather, 2 weeks.
Beams and girders and long span slabs, in summer, 10 days or 2 weeks; in cold weather, 3 weeks to 1 month. If sh.o.r.es are left without disturbing them, the time of removal of the sheeting in summer may be reduced to 1 week.
Column forms, in summer, 2 days; in cold weather, 4 days, provided girders are sh.o.r.ed to prevent appreciable weight reaching columns.
Conduits, 2 or 3 days, provided there is not a heavy fill upon them.
Arches, of small size, 1 week; for large arches with heavy dead load, 1 month.
The method of removing forms will vary in detail with the character of the structure. With proper design and lubrication of forms they will ordinarily come away from the concrete with a moderate amount of sledge and bar work. If the work will warrant it, have a special gang under a competent foreman for removing forms. The organization of this gang and the procedure it should follow will vary with the nature of the form work, and they are considered in succeeding chapters for each kind of work.
~ESTIMATING AND COST OF FORM WORK.~--It is common practice to record the cost of forms in cents per cubic yard of concrete, giving separately the cost of lumber and labor. This should be done, but the process of a.n.a.lysis should be carried further. The records should be so kept as to show the first cost per 1,000 ft. B. M. of lumber, the number of times the lumber is used, the labor cost of framing, the labor cost of erecting and the labor cost of taking down, all expressed in M. ft. B.
M. In this way only is it possible to compare the cost of forms on different kinds of concrete work, and thus only can accurate predictions be made of the cost of forms for concrete work having dimensions differing from work previously done. It is well, also, to make a note of the number of square feet of exposed concrete surface to which the forms are applied.
Some of the items mentioned demand brief explanation. Framing and erecting costs are kept separate for the reason that the framing is done only once, whereas the erecting occurs two or more times. The lumber cost, where the material is used more than once, can be computed in two ways. An example will ill.u.s.trate the two modes of procedure. In one of the buildings described in Chapter XIX the lumber cost $30 per M. ft. B.
M. and was used three times. As 34,000 ft. B. M. were required to encase the 200 cu. yds. of concrete in one floor, including columns, it would have required 34,000 200 = 170 ft. B. M. of lumber at $30 per M. per cubic yard of concrete if it had been used only once. But since it was used three times we may call it 170 ft. B. M. at $10 per M. per cubic yard of concrete, or we may call it 170 3 = 57 ft. B. M. at $30 per M.
per cubic yard of concrete. The authors prefer the first method, due to the fact that it is 170 ft. B. M. that is handled and taken down each time and it is more consistent to have the lumber cost on the same basis thus:
170 ft. B. M. of lumber at $10 per M $1.70 170 ft. B. M. handled at $2 per M 0.34 170 ft. B. M. erected at $7 per M 1.19 ----- Total 170 ft. B. M. per cu. yd $3.23
Returning to our main thought, there are three ways of recording the cost of form work: (1) In cents per cubic yard of concrete; (2) in cents per square foot of concrete face to which forms are applied, and (3) in dollars per 1,000 ft. B. M. of lumber used. In all cases the cost of materials and of labor should be kept separate. It is well if it can be done to attach a sketch of the forms to the record. So much for the general method of recording costs in form work.
In estimating the probable cost of forms for any job the following method will be found reliable: Having the total cubic yards of concrete in the work and the time limit within which the work must be completed determine the number of cubic yards that must be placed per day, making liberal allowances for delays. Next estimate the number of thousands of feet board measure of forms required to encase the concrete to be placed in a day. This will give the minimum amount of lumber required, for it is seldom permissible to remove the forms until the concrete has hardened over night. Now we come to the very important and puzzling question of the time element, particularly in work where it is possible to use the same forms or the same form lumber two or more times.
It has already been pointed out that wet concrete sets more slowly than dry concrete; that all concrete sets more slowly in cold than in warm weather, and that the support of forms is necessary a longer time for pieces subject to bending stress like arches and girders. General suggestions as to specific times for removing forms have also been given. Where the specifications state the time of removal the contractor has a definite guide, but where they do not, as is most often the case, he must depend very largely on judgment and previous experience. Another matter which deserves consideration is the use of the forms as staging for runways or tracks. Such use may result in forms having to stand on work for sake of their service as trestles much longer than there is any necessity so far as supporting the concrete is concerned. A derrick or cableway may often prove cheaper than tieing up form lumber by trying to make it serve the double purpose of a trestle.
The possibilities of repeated use of forms and of unit construction of forms have already been noted. This is the next point to be considered in estimating form lumber. At the expense of a little planning movable forms can be used to materially reduce the amount of lumber required.
The reader is referred particularly to the chapters on retaining wall, conduit and building work for specific data on movable form work.
Having estimated the amount of lumber required and the number of times it can be used the labor cost of framing, erecting and taking down can be figured. In ordinary retaining wall work forms will cost for framing and erection from $6 to $7 per M. ft. B. M. To tear down such forms carefully and to carry the lumber a short distance will cost some $1.50 to $2 per M. ft. B. M. We have then a cost of $7.50 to $9 per M. ft. B.
M. for each time the forms are erected and torn down. Where movable panels are used and the forms not ripped apart and put together again each time there is of course only the cost of moving, which may run as low as 50 cts. per M. ft. B. M. Framing and erecting centers for piers will run about the same as for retaining wall. At this point it may be noted that in estimating the cost of forms for plain rectangular piers the following method will give very accurate results. Ascertain the surface area of the four sides of the pier. Multiply this area by 2, and the product will be the number of feet board measure of 2-in. plank required. Add 40 per cent. to this, and the total will be the number of feet board measure of 2-in. plank and of upright studs (46), s.p.a.ced 2 ft. centers. Sometimes 36-in. studs are used, and s.p.a.ced 2 ft. centers, which requires practically the same percentage (40 per cent.) of timber for the studs as where 46-in. studs are used and s.p.a.ced 2 ft. centers.
No allowance is made for timber to brace the studs, since, in pier work, it is customary to hold the forms together either with bolts or with ordinary No. 9 telegraph wire, which weighs 0.06 lb. per foot. The foregoing data can be condensed into a rule that is easily remembered:
_Multiply the number of square feet surface area of the sides and ends of a concrete pier by 2.8, and the product will be the number of feet board measure required for sheet plank and studs for the forms._
If the form lumber can be used more than once, divide the number of feet board measure by the number of times that it can be used, to ascertain the amount to be charged to each pier. Forms can be erected and taken down for $8 per M. carpenters being paid $2.50 and laborers $1.50 a day.
Since there are 2.8 ft. B. M. of forms per square foot of surface area of concrete to be sheeted, it costs 2 cts. for the labor of carpenters per square foot of surface area to be sheeted. If lumber is worth $24 per M., and is used three times, then the lumber itself also costs 2 cts. per sq. ft. of surface area of concrete. By dividing the total number of cubic yards of concrete into the total number of square feet of area of surface to be sheeted with forms, the area per cubic yard is obtained. Multiply this area by 4 cts., and the product is the cost per cubic yard for material in the forms (a.s.sumed to be used three times) and the labor of erecting it and taking it down.
The cost of framing and erection of forms for building work and of centers for large arches is a special problem in each case and is considered in the chapters devoted to those cla.s.ses of work.
CHAPTER X.
METHODS AND COST OF CONCRETE PILE AND PIER CONSTRUCTION FOR FOUNDATIONS.
Two general methods of concrete pile construction are available for engineering work. By one method a hole is formed in the ground by driving a steel sh.e.l.l or by other special means and this hole is filled with concrete. By the other method the pile is molded in suitable forms and after becoming hard is driven as a wood or steel pile is driven.
Piles constructed by the first method may be either plain or reinforced, but piles constructed by the second method are always reinforced to strengthen them for handling and driving. Concrete piers for foundation work are simply piles of enlarged diameter.
~MOLDING PILES IN PLACE.~--Molding piles in place requires the use of special apparatus, and this apparatus is to a very large degree controlled by patents. Pile work of this kind is thus generally done by concerns which control the use of the apparatus employed and the general contractor can undertake it only by permission of the proprietary companies. The methods of work followed and the cost of work are thus of direct interest only as general information.
~Method and Cost of Constructing Raymond Piles.~--The machinery and processes employed in the construction of Raymond concrete piles are patented and all piling work by this method is controlled by the Raymond Concrete Pile Co. As detail costs of construction are not given out by the company the following figures collected by the authors are subject to revision. They are believed to be fairly approximate, having in one case been obtained by personal watch on the work and in the other case from authentic records of the engineers on the work.
The pile is made as follows: A collapsible steel core 30 ft. long, 20 ins. diameter at the top and 6 ins. diameter at the bottom, encased in a thin sheet steel sh.e.l.l, is driven into the ground by an ordinary pile driver. When it has reached the proper depth, a wedge is loosened, permitting the two sections of the core to come closer together so that the core can be pulled out of the hole, leaving the steel sh.e.l.l behind as a casing to prevent the sides from caving in. The sh.e.l.l is made of No. 20 gage steel, usually in four or more sections, which telescope one over the other. A nest of sections is slipped over the lower end of the core as it hangs in the leads, a rope is. .h.i.tched around the outer section and the engine hoists away until the sections are "un-telescoped" and drawn snug onto the core. The rope is then unfastened and the driving begins. Figure 49 shows the usual pile driving rig used. The following are examples of pile construction in actual work:
_Example I._--In this work, for a building foundation in New York City, the pile driver was mounted on a turntable, the framework of the turntable in turn resting on rollers traveling on timbers laid on the ground. The driver was moved along and rotated when necessary by ropes pa.s.sing around the winch head of the engine. The driver had 50-ft. leads and a 3,100-lb. hammer operated by an ordinary friction clutch hoisting engine. The hammer blow was received by an oak block fitting into a recess at the top of the steel core. This block was so battered by the blows that it had to be renewed about every five or six piles driven. A -in. wire rope pa.s.sing over a 10-in. sheave lasted for the driving of 130 piles and then broke. When the work was first begun the crew averaged 10 piles per 10-hour day, but the average for the job was 13 piles per day, and the best day"s work was 17 piles. The cost of labor and fuel per pile was as follows:
5 men on driver at $1.75 $ 8.75 2 men handling sh.e.l.ls at $1.75 3.50 1 engineman 3.00 6 men mixing and placing concrete 10.50 1 foreman 5.00 Coal and oil 2.50 ------- Total, 13 piles, at $2.55 $33.25
[Ill.u.s.tration: Fig. 49.--Pile Driver Rigged for Constructing Raymond Concrete Piles.]
Deducting the cost of placing the concrete we get a cost of $1.75 for driving the cores. The pile, 25 ft. long, 6 ins. at the point and 18 ins. at the head, contains 21 cu. ft., or 0.8 cu. yd., of concrete, and has a surface area of 77 ft. As No. 20 steel weighs 1.3 lbs. per sq.
ft., each sh.e.l.l weighed approximately 100 lbs. The cost per pile may then be summarized as follows:
1.2 bbls. cement in 0.8 cu. yd., at $1.75 $2.10 0.8 cu. yd. stone at $1.25 1.00 1/3 cu. yd. sand at $1.05 0.35 100 lbs. steel in sh.e.l.l at 3 cts. 3.50 Labor and fuel as above 2.55 ----- Total per pile (38 cts. per lin. ft.) $9.50
_This cost, it should be carefully noted, does not include cost of moving plant to and from work or general expenses._
_Example II._--In constructing a building at Salem, Ma.s.s., 172 foundation piles, 14 to 37 ft. long, 6 ins. diameter at the point and 20 ins. diameter at the top, were constructed by the Raymond process. The general contractor made the necessary excavations and provided clear and level s.p.a.ce for the pile driver, braced all trenches and pier holes, set stakes for the piles and gave all lines and levels. The piles were driven by a No. 2 Vulcan steam hammer with a 3,000-lb. plunger having a drop of 3 ft., delivering 60 blows per minute. Figure 49 shows the driver at work. Sixteen working days were occupied in driving the piles after the driver was in position. The greatest number driven in one day was 20, and the average was 11 piles per day. When in position for driving, the average time required to complete driving was 12 minutes.
The total number of blows varied from about 310 to 360, the average being about 350. The piles were driven until the penetration produced by 8 to 10 blows equaled 1 in. When in full operation, a crew of 5 men operated the pile driver. Seven men were engaged in making the concrete and 5 men working upon the metal sh.e.l.ls.
a.s.suming the ordinary organization and the wages given below, we have the following labor cost per day:
1 foreman at $5 $ 5.00 1 engineman at $3 3.00 4 laborers on driver at $1.75 7.00 6 laborers making concrete at $1.75 10.50 5 laborers handling sh.e.l.ls at $1.75 8.75 ------ Total $34.25
As 172 piles averaging 20 ft. in length were driven in 16 days, the total labor cost of driving, given by the figures above, is 16 $34.25 = $548, or practically 16 cts. per lineal foot of pile driven.
The concrete used in the piles was a 1-3-5 Portland cement, sand and 1-in. broken stone mixture. A 20-ft. pile of the section described above contains about 20 cu. ft. of concrete, or say 0.75 cu. yd. We can then figure the cost of concrete materials per pile as follows:
0.85 bbl. cement at $1.60 $1.36 0.36 cu. yd. sand at $1 0.36 0.60 cu. yd. stone at $1.25 0.75 ----- Total per pile $2.47
The steel sh.e.l.l has an area of about 72 sq. ft., and as No. 20 gage steel weighs 1.3 lbs. per sq. ft., its weight for each pile was about 94 lbs. a.s.suming the cost of coal, oil, etc., at $2.50 per day, we have the following summary of costs:
Per lin. ft.
of pile.
Labor driving and concreting $0.16 Concrete materials 0.123 94 lbs. steel sh.e.l.l at 3 cts. 0.145 Coal, oil, etc. 0.011 ------ Total $0.439
_The cost does not include interest on plant, cost of moving plant to and from work and general expenses._
[Ill.u.s.tration: Fig. 50.--Sketch Showing Method of Constructing Simplex Concrete Piles.]
~Method of Constructing Simplex Piles.~--The apparatus employed in driving Simplex piles resembles closely the ordinary wooden pile driven, but it is much heavier and is equipped to pull as well as to drive. A 3,300-lb.
hammer is used and it strikes on a hickory block set in a steel drive head which rests on the driving form or sh.e.l.l. This form consists of a -in. steel sh.e.l.l 16 ins. in diameter made in a single 40-ft. length.