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DETAILS OF A CAST IRON BASE. The main parts of a cast iron base are as follows:
The Barrel, is the central part of the base, and has the form of a closed chamber, usually circular or rectangular. The rectangular form is shown in the base in Fig. 25.
The upper part of the barrel is covered by the Top Flange of the base, upon which rests the column. This top flange is provided with a hole in the centre, for grouting purposes. This grouting hole is cored, and is at least 3 inches in diameter. In addition there are four bolt holes near the corners of the top flange. These holes are very accurately drilled and are used in achoring the column to the base by means of 34 in. or I in. diam. bolts.
The lower part of the barrel rests on the Bottom Flange of the base. This flange spreads the load over the pier area. Like the top flange, the bottom flange is provided with a 3 in. grouting hole in the centre, and with a number of IJ/4 in. diam. grouting holes all around the barrel. All these holes are cored, not drilled.
The barrel and the two flanges are tied together into one Solid mass by means of a number of Ribs. These ribs are at least I in. in thickness, and the longer ribs at corners are usually of greater thickness than the interior ribs.
All around the edges of the bottom flange, the cast iron bases are usually provided with a vertical rim, about 3 inches in height and at least one inch in thickness. This rim is known as the Compensation Flange. It greatly increases the load carrying capacity of the base, and stiffens the joints between ribs and the bottom flange.
TESTING CAST IRON BASES. In the case of small loads the bases may rest on a steel plate on top of a pier, or even directly on the concrete pier as in Fig. 25. These bases are often dumped into the excavation before the derrick is set. They are tested by tapping with a hammer. Good bases will stand the hardest blows a man could deliver with a light sledge hammer. The same method of testing applies also to cast iron columns. Good castings give a clear ringing sound on tapping. The casting is gray, soft, with small crystals and is
T Top Flange. B Bottom Flange. K Barrel. C Compensation
Flange. R Ribs. G. Grouting Holes. WW Wooden Wedges. MM Bricks. Arrows indicate the flow of grout.
easily indented. Sand holes or blow holes are detected by a dullness in the sound. Cracked bases give also a characteristic sound which is easily distinguished. Warped bases are likely to have internal stresses due to unequal contraction or to other defects in manufacture and should be rejected ; the same applies to bases of incorrect dimensions.
Repeated Inspections. Cast iron work must be repeatedly irospected. Bases which have been approved upon delivery may be cracked in handling.
In one case bases were made to slide on two timber guides running from the sidewalk to the bottom of the excavation. One base left the guides and struck a boulder. Luckily the base was smashed. Should the base have been cracked Only, it is doubtful whether the particular iron foreman would have had the honesty to notify the inspector to this effect. He would have probably taken chances. As it was, a new base had to be obtained, but this caused several days delay in that part of the job.
In another case in a 12 story building, a steel beam fell down into the cellar and broke part of a base and more than half of the lower flange of a heavy I5 inch case iron column. The structure already eight stories high, had to be shored up and the column replaced.
SETTING CAST IRON BASES. Bases resting directly on grillage are bolted on top with four 34 in. bolts to the steel or cast iron columns as in Fig. 25. Four pairs of wooden wedges are placed under the base when it rests directly on the pier. The base is centred and raised to the proper height. The clearance between base and pier should not exceed 34 inch. Next a mixture of one part cement to one or two parts sand is prepared and this grout is poured through the top of the barrel, whence it penetrates under the base, comes out through the grout holes and overflows the compensation flange. Bricks are placed on edge all around the base to stop the grout from spreading.
Common Defects in Setting. It sometimes happens that the holes in the bottom of the column do not match with the holes in the top flange of the cast iron base. A drift pin can not be used to enlarge the holes and make them match, as this may crack the casting. In poor work, one of these methods is followed :
I. Omitting bolts altogether.
2. Using bolts of smaller diameter and with or without washers.
3. Using bent bolts.
In good work such holes are made to match by enlarging the opening by means of a hand or a compressed air reamer. After the holes have been lined up, the proper size bolts are put in.
Bolts without nuts and loose bolts are very common on jobs which are poorly supervised.
The columns mostly used in tall buildings are steel columns. During the past few years there have still been erected several loft buildings with cast iron columns, some of them being twelve stories in height.
Steel columns usually come in two story lengths. This gives added stiffness and saves some field work. Cast iron columns are generally one story long in order to avoid cold shuts in castings. A longer column will also be more liable to be rejected due to defects near either end or near the centre.
CAST IRON COLUMNS are tested by tapping with a hammer just as in the case of cast iron bases. The most common defects found in cast iron columns are as follows:
1. Eccentricity. In casting the column, the core has shifted. This makes the column heavier on one side and lighter on the other. Eccentricity is easily detected by drilling several 3% inch test holes and by measuring the thickness of the column at several points. All closed cast iron columns must have test holes as required by the Code. These test holes are drilled in the shop, generally about three in number, and about two feet from the bottom flange. The Building Department has the right to demand extra test holes to be drilled in columns or bases at doubtful points.
The eccentricity can be easily detected in a round column without test holes, by causing such column to be rolled on top of two smooth edges slightly sloping downward. These may be two steel beams laid nearly horizontally. With eccentric columns the rolling is irregular. If the column has a tendency to settle along a certain side, drill a test hole in the part exactly opposite and measure the thickness of the metal. This will give the thickness of the lighter part of the column.
The Code prescribes that in case of eccentric columns whenever the core has shifted more than one-fourth the thickness of the shell, the strength shall be computed assuming the thickness of the metal all around, equal to the thickness of the thinnest part, and the column shall be condemned if this computation shows the strength to be less than required by the Code.