Concrete Construction: Methods and Costs by Gillette and Hill

METHOD AND COST OF BUILDING CONSTRUCTION OF SEPARATELY MOLDED MEMBERS. This chapter deals exclusively with the methods and cost of molding and erecting separately molded wall blocks, girders, columns and slabs. The structural advantages and disadvantages of this type of construction as compared with monolithic construction will not be considered. The data given in succeeding paragraphs show how separate piece work has been done and what it has actually cost to do it in a number of instances. ~COLUMN, GIRDER AND SLAB CONSTRUCTION.~--European engineers have developed several styles of open web or hollow girder and column shapes, but in America solid columns and girders have been used except in the comparatively few cases where one of the European constructions has been introduced by its American agents. ~Warehouses, Brooklyn, N. Y.~--In constructing a series of warehouses in Brooklyn, N. Y., the columns and girders were molded in forms on the ground. For molding the columns, forms consisting of two side pieces and one bottom piece, were used, saving 25 per cent. in the amount of lumber required for a column form, and doing away with yokes and bolts, since only simple braces were required to hold the side forms in place. It was found that the side forms could readily be removed in 24 to 48 hours, thus considerably reducing the time that a considerable portion of the form lumber was tied up. It was figured by Mr. E. P. Goodrich, the engineer in charge of this work, that this possible re-use of form lumber reduced the amount required another 50 per cent. as compared with molding in place. Girders were molded like columns in three-sided forms; the saving in form work was somewhat less than in the case of columns, but it was material. In general, Mr. Goodrich states, the cost of hoisting and placing molded concrete members is higher per yard than when the concrete is placed wet. That is in mass before it is hardened. [Illustration: Fig. 239.--Sketch Showing Forms and Reinforcement for Visintini Girder.] ~Factory, Reading, Pa.~--In constructing a factory at Reading, Pa., an open or lattice web type of girder invented by Mr. Franz Visintini and extensively used in Austria was adopted; columns were molded in place in the usual manner with bracket tops to form girder seats. The girders were reinforced with three trusses made up of top and bottom chord rods connected by diagonal web rods; one truss was located at the center of the beam and one at each side. The method of molding was as follows: The trusses were made by cutting the chord rods to length and threading the web diagonals and verticals onto them. To permit threading the web pieces were bent, when rods were used, with an eye at each end; when straps were used the ends were punched with holes. The work was very simple and was done mostly by boys in the machine shop of the company for which the building was being erected. The girders were molded two at a time in forms constructed as shown by the sketch. Fig. 239. A form consisted of a center board, two side boards, two end pieces and the proper number of cast iron cores, all clamped together by three yokes. Triangular cast iron plates, A, were screwed to the bottom boards for spacers. The side, center and end boards were then set up and the end clamps were placed. The cast iron hollow cores, B, were then set over the spacers, and the form was ready for pouring. A layer of concrete was placed in the bottom of the mold and the first side truss was placed; the concrete was then brought half way up and the middle truss was placed; concreting was then continued up to the plane of the second side truss which was placed and covered. Cores and forms were all cleaned and greased each time they were used. The cores were removed first by means of a lever device and generally within three or four hours after the concrete was placed. The remainder of the form was taken down in two to four days and the beam removed. ~Kilnhouse, New Village, N. J.~--In constructing a kiln house for a cement works one story columns with bracket tops and 50-ft. span roof girders were molded on the ground and erected as single pieces. The columns by rough calculation averaged about 2 cu. yds. of concrete and 675 lbs. of reinforcement each or about 337 lbs. of steel per cubic yard. The girders averaged by similar calculation 5 cu. yds. of concrete and 2,260 lbs. of steel, or 452 lbs. per cubic yard of concrete. The average weight of columns was thus not far from 41.3 tons and of girders fully 11 tons. [Illustration: Fig. 240.--Arrangement for Molding Ten Single-Bracket Columns.] Several combinations of arrangements were used for molding the columns and girders. For wall columns having one bracket the arrangement shown by Fig. 240 was adopted. The concrete slab molding platform was covered with paper, and on this the two outside and the middle columns were cast in forms. When those columns had set the forms were removed, the intervening spaces were papered and the two remaining columns were cast. Ten columns, five sets of two columns in line, were cast on each base. The remaining columns were cast in combination with girders as shown by Fig. 241. The two outside lines of columns (1) were molded in forms, allowed to stand until set and then stripped. Using a column surmounted by a shallow side form for one side and a full depth side form for the other side molds were fashioned for the two outside girders, Nos. 2 and