Part 43 (1/2)
Cores similar to these were used in molding the ribbed floor for the Bush terminal factory building described in a succeeding section. These cores are capable of repeated use so that while they are somewhat expensive to frame they give a very low cost of form work when the beam and girder s.p.a.cing is arranged largely in duplicate from floor to floor.
It will ordinarily be cheaper to have these cores made to pattern by regular woodworking shops, and s.h.i.+pped to the building ready to erect.
~WALL FORMS.~--Wall work in modern commercial and manufacturing buildings, when we come to eliminate windows and wall columns and girders, is confined very largely to isolated curtain wall panels between windows and framework. In such buildings, therefore, wall forms consist merely of wooden panels, one for each face of the wall, constructed to fit the s.p.a.ces to be walled up. Where these s.p.a.ces are duplicated from bay to bay or story to story the same form panels will serve repeatedly. For residences and other buildings having greater proportionate area of blank wall the builder has a choice between continuous forms carried by staging and movable panel forms.
[Ill.u.s.tration: Fig. 201.--Continuous Form for Wall Construction.]
For one and two-story buildings, with the usual variation in architectural detail, panel work and window work, the continuous form has many advantages, and the superior economy of movable panels in retaining and other plain wall work is by no means always true here. One good type of continuous wall form construction is shown by Fig. 201. The gallows frames are s.p.a.ced about 6 ft. apart along the wall and connected by horizontal stringers nailed to the uprights or by diagonal bracing.
Each frame may be made up of 66-in. posts connected by 24-in.
cross-struts and diagonals with bolted connections so that the frame can be taken down and put together easily and so that the bracing can be removed as the wall is built upward. The other details of the form work are shown by the drawing. This construction leaves a clear s.p.a.ce for placing the concrete and the cross pieces give support to runways; it has been successfully used in a large amount of low building work.
[Ill.u.s.tration: Fig. 202.--Sectional Form for Wall Construction.]
Movable panel forms are of great variety in detail but are generally of either one or the other types shown by Figs. 202 to 204.
The form shown by Fig. 202 was used in constructing a church at Oak Park, Ill. For the back of the wall it consists of continuous lagging held by 24 studs. For the face 16-in. lagging 12 ft. long was nailed to 24-in. studs to form panels. It will be noted that the ends of the studs are scarfed so as to interlock in succeeding panels. This construction also shows a method of supporting the reinforcing bars inside the form.
The form shown by Fig. 203 was used in constructing a large factory building, and consisted of two side pieces or panels 3 ft. high and 16 ft. long, the distance between wall columns. For the first course these were seated on the carefully leveled and rammed ground and securely braced by inclined or horizontal struts inside and outside of the building. After the concrete had set for three days the molds were loosened and lifted until the lower edges were 2 ins. below the top of the concrete and there they were held by horizontal bolts through their lower edges and across the top of the concrete by ties nailed across their tops every 3 ft. and by bracing to the falseworks supporting the column and floor forms. The cross bolts pa.s.sed through pasteboard sleeves which were left permanently embedded in the wall. By starting the molds level and finis.h.i.+ng each course level with their tops no difficulty was had in keeping the forms plumb and to level as they were moved upward. This type of form has to be exteriorly braced to staging or adjacent column forms, etc.
[Ill.u.s.tration: Fig. 203.--Sectional Form for Wall Construction.]
The type of movable panel form shown by Fig. 204 depends for all support on the wall alone. The sketch shows the form filled ready to be s.h.i.+fted upward; this operation consists in removing the bottom bolts and loosening the top bolts enough to permit the studs to be slid upward the full length of the slots. The lagging boards left free are then removed and placed on top and the bolts are tightened, completing the form for another section of wall.
[Ill.u.s.tration: Fig. 204.--Movable Panel Form for Wall Construction.]
[Ill.u.s.tration: Fig. 205.--Sullivan's Plank Holders for Wall Forms.]
A type of wall form construction intended to do away with studding and bracing is ill.u.s.trated by Figs. 205 and 206. In both cases metal plank holders are used in place of studs, and practically the only difference between the two is in the shape and material of the holders. The mode of procedure is to work in horizontal courses one plank high around the wall, removing the bottom plank and placing it on top as each new course is begun after the first few courses have been laid. Using the arrangement shown by Fig. 205 in constructing a building 10054 ft. in plan and 36 ft. high with 12-in. walls, a height of two 122-in planks was all the form work that was ever necessary at any one time, so that the amount of form lumber required for the building was 2,464 ft. B. M.
plus 205 ft. B. M. of 24-in. flooring strip, or altogether 2,669 ft. B.
M., or 0.24 ft. B. M. per square foot of exterior wall surface, or 6 ft. B. M. per cubic yard of concrete. This same form lumber with 16 additional plank was then used to construct a building 100100 ft.16 ft. high, so that some 3,000 ft. of form lumber sufficed for 17,548 sq.
ft. (exterior surface) of wall or for 617 cu. yds. of concrete in 12-in.
wall, which gives 0.17 ft. B. M. per square foot or 4.8 ft. B. M. per cubic yard of concrete.
[Ill.u.s.tration: Fig. 206.--Farrell's Plank Holders for Wall Forms.]
~ERECTING FORMS.~--The organization of the erecting gang will depend very largely on the manner in which the forms have been constructed. If they have been constructed in sections which go together with wedges and clamps common laborers with a foreman carpenter in charge to direct and to line and level the work will do the erecting, but if they have to be largely built in place carpenters are necessary for all the work except carrying and handing. There should be at least one foreman for every 15 to 20 men and a head foreman in charge of all form work. The mode of procedure will differ for every job, but the following general directions apply to all work in greater or less measure.
Clamps, bolts and wedges and not nails should be used wherever possible in a.s.sembling parts of forms in erection; these devices are not only quickly and easily applied in erection but they are just as quickly and easily loosened in taking down forms and they can be loosened without jarring the concrete member.
Lining girder forms and lining and plumbing column and wall forms is high-cla.s.s carpenter work and should be directed by competent carpenters. A column or girder which is out of line or plumb not only looks bad but may be required to be removed and corrected by the engineer. The expense for one such correction will be many times that which would have been involved by proper care in the first place.
Supports or staging for the forms should be used freely and well braced in both directions. Uprights should be set on wedges and bear against a cap piece and on a sill piece to distribute the load.
Erect, line and plumb the column forms first; then erect, line and level the girder forms and set the girder staging, and finally erect and level the slab centers and their supports.
Leave the foot of each column form open on one side at the bottom so that the column reinforcement can be adjusted and connected up and so that a clear view can be had through the form to detect any object that may have fallen into the form and become wedged; this same opening makes it possible to clean the form.
Give the forms a final inspection before concreting to check line and level, to close open joints and to tighten up clamps and wedges. Finally clean each form and wet it down thoroughly before placing the concrete--do this just before placing the concrete.
~REMOVING FORMS.~--Good judgment and extreme care are essential in removing centers. It goes without saying that forms should never be removed until the concrete has set and hardened to such strength that it will sustain its own dead weight and such live load as may come upon it during construction. The determination of this condition is the matter that calls for knowledge and judgment. Some cements set and harden more rapidly than others, and concrete hardens more and more slowly as the temperature falls. These and other circ.u.mstances must all be taken into account in deciding upon the safe time for removal. Many large contractors mold a cube of concrete for each day's work and leave it standing on the finished floor exposed to the same conditions as the concrete in the forms; examination of this sample block gives a line on the condition of the concrete in the work and on the probable safety of removing the forms at any time. In all cases it should be the superintendent's duty to determine when to remove forms, and he should satisfy himself by personal inspection that the concrete is in condition to stand without support. It is also wise at least as a matter of precaution for the contractor to secure the engineer's or the architect's approval before removing any formwork.
Care in removing forms is essential for the reason that green concrete is particularly susceptible to injury from shock or sudden strain. It is well, therefore, to have a separate gang always doing the work. These men will in a few days become trained under an experienced foreman so that they will not only do the work with greater safety but also more rapidly. This gang should, furthermore, be required to follow a regular system in its work; a system which may not be departed from without direct orders from the superintendent. An example of such a system is outlined below.