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

MANUFACTURERS' FURNITURE EXCHANGE BUILDING, CHICAGO, ILL.

MANUFACTURERS' FURNITURE EXCHANGE BUILDING, CHICAGO, ILL.

Wm. Ernest Walker, Architect; Condron & Sinks Co. Engineers, Chicago

Reinforced Concrete Construction, Brick Facing. Concreting of First Story Started October 10, 1906; Concrete Roof Completed December 27, 1906. Building is 70 feet by 170 feet.

MANUFACTURERS' FURNITURE EXCHANGE BUILDING, CHICAGO, ILL., SHOWING METHOD OF REINFORCED CONCRETE CONSTRUCTION.

MANUFACTURERS' FURNITURE EXCHANGE BUILDING, CHICAGO, ILL., SHOWING METHOD OF REINFORCED CONCRETE CONSTRUCTION.

Column Centers are 14 feet and 16 feet 19 inches. Columns Consist of 4 Angles Latticed, Filled in with Concrete, with the Angles Acting as Reinforcement for Same. Floor Construction. 14-Foot-Span Slabs on Reinforced Concrete Beams. Live Loads Figured on: First Floor, 150 lbs.; Second to Eighth Floors, 100 lbs.; Roof, 40 lbs. Test was made over an Area of 12 by 15 feet, with a Total Load of 55,780 lbs. plus Dead Load, making a Load of 310 lbs. per Square Foot plus Dead Load. The Deflection was Less than 1/16 Inch.

Mortar per Cubic Yard of Masonry

Grade of Masonry

Volume of Mortar per Cubic Yard of Masonry

Ashlar

1 to 2

cubic

feet

Squared-Stone

4.5 to 7

,,

,,

Rubble

5.5 to 9

,,

,,

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, assuming good workmanship, we may depend on masonry as follows:

Allowable Pressures on Masonry

Granite Ashlar .....

up to 400 pounds per sq. inch

Limestone or Sandstone Ashlar..

,,

,,

300

,,

,,

,,

,,

Squared Stone....

,,

,,

250

,,

,,

,,

,,

Rubble....

,,

,,

100

,,

,,

,,

,,

It is interesting to note that, although concrete has been considered inferior even to rubble, unit-stresses of 400 pounds per square inch are now being freely employed for concrete.