CHAPTER VI. STRENGTH OF MASONRY. By the term "strength of masonry we mean its resistance to a crushing-force, as that is the only force to which masonry should be subjected. The strength of the different stones and materials used in masonry, as determined by experiment, is given in the following table. (For Architectural Terra-Cotta, see page 186a.) Crushing Resistance of Brick, Stone, and Concretes. (Pressure at right angles to bed.) Flagging: North River, N. Y Portland cement 1, sand and stone 9, 6 months 10,000 17.783 12,156 13,425 Concrete: Rosendale ceinent 1, sand and stone 74, 46 months old. *This stone should not be set on edge. 1,544 2,000 The stones in this table are supposed to be on bed, and the height to be not more than four times the least side. Of the strength of rubble masonry, Professor Rankine states that "the resistance of good coursed rubble masonry to crushing is about four-tenths of that of single blocks of the stone it is built with. The resistance of common rubble to crushing is not much greater than that of the mortar which it contains." Stones generally commence to crack or split under about one-half of their crushing-load. Crushing-Height of Brick and Stone. — If we assume the weight of brickwork to be 112 pounds per cubic foot, and that it would crush under 450 pounds per square inch, then a vertical uniform column 580 feet high would crush at its base under its own weight. Average sandstones at 145 pounds per cubic foot would require a column 5950 feet high to crush it; and average granite at 165 pounds per cubic foot would require a column 10,470 feet high. The Merchants' shot-tower at Baltimore is 246 feet high, and its base sustains a pressure of six tons and a half (of 2240 pounds) per square foot. The base of the granite pier of Saltash Bridge (by Brunel) of solid masonry to the height of 96 feet, and supporting the ends of two iron spans of 455 feet each, sustains nine tons and a half per square foot. The highest pier of Rocquefavour stone aqueduct, Marseilles, is 305 feet, and sustains a pressure at the base of thirteen tons and a half per square foot. Working-Strength of Masonry. — The working-strength of masonry is generally taken at from one-sixth to one-tenth of the crushing-load for piers, columns, etc., and in the case of arches a factor of safety of twenty is often recommended for computing the resistance of the voussoirs to crushing. Mr. Trautwine states that it cannot be considered safe to expose even first-class pressed brickwork in cement to more than thirteen or sixteen tons' pressure per square foot, or good hand-moulded brick to more than two-thirds as much. (See page 181.) Sheet lead is sometimes placed at the joints of stone columns with a view to equalize the pressure, and thus increase the strength of the column. Experiments, however, seem to show that the effect is directly the reverse, and that the column is materially weakened thereby.1 Piers. Masonry that is so heavily loaded that it is necessary to proportion it in regard to its strength to resist crushing, is, as a general rule, in the form of piers, either of brick or stone. As 1 Trautwine's Pocket-book, p. 176. these piers are often in places where it is desirable that they should occupy as little space as possible, they are often loaded to the full limit of safety. The material generally used for building piers is brick: block or cut stone is sometimes used, for the sake of appearance; but rubblework should never be used for piers which are to sustain posts, pillars, or columns. Brick piers more than six feet in height should never be less than twelve inches square, and should have properly proportioned footing courses of stone not less than a foot thick. The brick in piers should be hard and well burned, and should be laid in cement, or cement mortar at least, and be well wet before being laid, as the strength of a pier depends very much upon the mortar or cement with which it is laid: those piers which have to sustain very heavy loads should be built up with the best of the Rosendale cements. The size of the pier should be determined by calculating the greatest load which it may ever have to sustain, and dividing the load by the safe resistance of one square inch or foot of that kind of masonry to crushing. EXAMPLE. In a large storehouse the floors are supported by a girder running lengthwise through the centre of the building. The girders are supported every twelve feet by columns, and the lowest row of columns is supported on brick piers in the basement. The load which may possibly come upon one pier is found to be 65,000 pounds. What should be the size of the pier ? Ans. The masonry being of good quality, and laid in cement mortar, we will assume 12 tons per square foot, or 166 lbs. per square inch (see p. 181), for the safe working load. Dividing 65,000 lbs. by 166, we have 391 square inches for the size of the pier. This would require a pier 20 x 20 inches. It is the custom with many architects to specify bond stones in brick piers (the full size of the section of the pier) every three or four feet in the height of the pier. These bond stones are generally about four inches thick. The object in using them is to distribute the pressure on the pier equally through the whole mass. Many first-class builders, however, consider that the piers are stronger without the bond stone; and it is the opinion of the writer that a pier will be just as strong if they are not used. Section 3 of the Building Laws of the city of New York requires that every isolated pier less than ten superficial feet at the base, and all piers supporting a wall built of rubble-stone or brick, or under any iron beam or arch-girder, or arch on which a wall rests, or lintel supporting a wall, shall, at intervals of not less than thirty inches in height, have built into it a bond stone not less than four inches thick, of a diameter each way equal to the diameter of the pier, except that in piers on the street front, above the curb, the bond stone may be four inches less than the pier in diameter. Piers which support columns, posts, or pillars, should have the top covered by a plate of stone or iron, to distribute the pressure over the whole cross-section of the pier. In Boston, it is required that "all piers shall be built of good, hard, well-burned brick, and laid in clear cement, and all bricks used in piers shall be of the hardest quality, and be well wet when laid. "Isolated brick piers under all lintels, girders, iron or other columns, shall have a cap-iron at least two inches thick, or a granite cap-stone at least twelve inches thick, the full size of the pier. "Piers or columns supporting walls of masonry shall have for a footing course a broad leveller, or levellers, of block stone not less than sixteen inches thick, and with a bearing surface equal in area to the square of the width of the footing course plus one foot required for a wall of the same thickness and extent as that borne by the pier or column." For the Strength of Masonry Walls, see Chap. III. The following tables give the results of some tests on brick, brick piers, and stone, made under the direction of the author, in behalf of the Massachusetts Charitable Mechanics Association. The specimens were tested in the government testing-machine at Watertown, Mass., and great care was exercised to make the tests as perfect as possible. As the parallel plates between which the brick and stone were crushed are fixed in one position, it is necessary that the specimen tested should have perfectly parallel faces. The bricks which were tested were rubbed on a revolving bed until the top and bottom faces were perfectly true and parallel. The preparation of the bricks in this way required a great deal of time and expense; and it was so difficult to prepare some of the harder brick, that they had to be broken, and only one-half of the brick prepared at a time. TABLE Showing the Ultimate and Cracking Strength of the Brick, the Size and Area of Face. The Philadelphia Brick used in these tests were obtained from a Boston dealer, and were fair samples of what is known in Boston as Philadelphia Face Brick. They were a very soft brick. The Cambridge Brick were the common brick, such as is made around Boston. They are about the same as the Eastern Brick. The Boston Terra-Cotta Company's Brick were manufactured of a rather fine clay, and were such as are often used for face brick. The New-England Pressed Brick were hydraulic pressed brick, and were almost as hard as iron. From tests made on the same machine by the United States Government in 1884, the average strength of three (M. W. Sands) Cambridge, Mass., face brick was 13,925 pounds, and of his common brick, 18,337 pounds per square inch, one brick developing the enormous strength of 22,351 pounds per square inch. This was a very hard-burnt brick. Three brick of the Bay State (Mass.) manufacture showed an average strength of 11,400 pounds per square inch. The New England brick are among the hardest and strongest brick in the country, those in many parts of the West not having one-fourth of the strength given above, so that in heavy buildings, |