146. Cost of Dressing Stone. Although, as explained above, the cost of dressing stone should properly be estimated by the square foot of surface dressed, most figures which are obtainable give the cost per cubic yard of masonry, which practically means that the figures are applicable only to stones of the average size used in that work. A few figures are here quoted from Gillette's "Handbook of Cost Data:" (a) HAND DRESSING—Wages, 50 cents per hour. Soft, 25 to 30 cents; medium, 40 to 45 cents; hard, 75 to SO cents, per square foot of surface dressed.
(b) HAND DRESSING—Wages, $3 per day. Limestone, bush-ham mered, 25 cents per square foot.
(c) HAND DRESSING LIMESTONE —36 square feet of beds and joints per 9-hour day (or 4 square feet per hour); wages, 40 cents per hour, or 10 cents per square foot.
(d) HAND DRESSING GRANITE—For i-inch joints, 26 cents per square foot.
(e) SAWING SLABS BY MACHINE RY —Costs approximately 17 cents per square foot.
147. Constructive Features—Bonding. It is a fundamental principle of masonry construction, that vertical joints (either longi tudinal or lateral) should not be continuous for any great distance.
Masonry walls (except those of concrete blocks) are seldom or never constructed entirely of single blocks which extend clear through the wall. The wall is essentially a double wall which is frequently con nected by headers. These break up the continuity of the longitu dinal vertical joints. The continuity of the lateral vertical joints is broken up by placing the stones of an upper course over the joints in the course below. Since the headers are made of the same quality of stone (or brick) as the face masonry, while the backing is of com paratively inferior quality, it costs more to put in numerous headers, although strength is sacrificed by neglect to- do so. For the best work, stretchers and headers should alternate. This would usually mean that about one-third of the face area would consist of headers. One-fourth or one-fifth is a more usual ratio. Cramps and dowels are merely devices to obtain a more efficient bonding. An inspector must guard against the use of blind headers, which are short blocks of stone (or brick), which have the same external appearance on the finished wall, but which furnish no bond. After an upper course has been laid, it is almost impossible to detect them.
should be thoroughly flushed with mortar. All vertical joints should likewise be tightly filled with mortar.
148. Allowable Unit-Pressures. In estimating such quantities, the following considerations must be kept in mind : (a) The accuracy of the dressing of the stone, particularly the bed joints, has a very great influence.
(b) The strength is largely dependent on that of the mortar.
(c) The strength is so little dependent on that of the stone itself that the strength of the stone cannot be considered a guide to the strength of the masonry. For example, masonry has been known to fail under a load not more than five per cent of the ultimate crushing strength of the stone itself.
(d) The strength of a miniature or small-scale prism of masonry is evidently no guide to the strength of large prisms, The ultimate strength of these is beyond the capacity of testing machines.
(e) So much depends on the workmanship, that in any structure where the unit-stresses are so great as to raise any question concerning the strength, the best workmanship must be required.
Judging from the computed pressures now carried by noted structures, and also from the pressures sustained by piers, etc., which have shown distress and have been removed, it is evident that, as suming good workmanship, we may depend on masonry as follows: Granite Ashlar up to 400 pounds per sq. inch Limestone or Sandstone Ashlar " " :300 " Squared Stone " " 250 " Rubble " " 100 " It 4: It is interesting to note• that, although concrete has been con sidered inferior even to rubble, unit-stresses of 400 pounds per square inch are now being freely employed for concrete.
140. Cost of Stone Masonry. The total cost is a combination of several very variable items as follows: 1. Value of quarry privilege; 2. Cost of stripping superincumbent earth or disintegrated rock; 3. Cost of quarrying; 4. Cost of dressing; 5. Cost of transportation (teaming, railroad, etc.) from quarry to site of work; 6. Cost of mortar; 7. Cost of centering, scaffolding, derricks, etc.; S. Cost of laying; 9. Interest and depreciation on plant; 10. Superintendence.
Some of the above items may be practically nothing, in cases. The cost of some of the items has already been discussed. The cost of many items is so dependent on local conditions and prices that the quotation of the cost of definite jobs would have but little value and might even be deceptive. The following very general values may be useful to give a broad idea of the cost: Cost of Stone Masonry Rubble Masonry in Mortar 83.00 to $ 5.00 per cubic yard.
Squared-Stone Masonry 6.00 to 10.00 " " Dimension Stone, Granite Ashlar. up to 60.00 " " "