Crushing strength, bed 11,505 pounds, edge 17,181 pounds per square inch. Weight, 150 pounds per cubic foot.

Effects of Freezing on Mortar.-Both cement and limemortar, mixed with salt, have been used in freezing weather without any bad effects. (See American Architect. vol. xxi., p. 263.)

Rule for the proportion of salt said to have been used in the works at Woolwich Arsenal: "Dissolve one pound of rock-salt in eighteen gallons of water when the temperature is at 32 degrees Fahr., and add three ounces of salt for every three degrees of lower temperature."

Durability of Hoop Iron Bond.-I believe that, embedded in lime-mortar at such depth as to protect it from the air, hoop iron bond is indestructible. In cement mortar containing salts of potash and soda, I doubt its lasting 1,500 years uncorroded. -M. C. MEIGS, May 17, 1887.

Grouting.*

It is contended by persons having large experience in building that masonry carefully grouted, when the temperature is not lower than 40° Fahr., will give the most efficient result.

The following buildings in New York City have grouted walls : Metropolitan Opera House.

Produce and Cotton Exchanges.

Mortimer and Mills Buildings.

Equitable and Mutual Life Insurance Buildings.

Standard Oil Building.

Astor Building.

The Eden Musée.

The Navarro Buildings.

Manhattan Bank Building.

The Presbyterian, German, St. Vincent, and Woman's Hospitals. etc.; also, the Mersey Docks and Warehouses at Liverpool, England, one of the greatest pieces of masonry in the world, have been grouted throughout. It should be stated, however, that there are many engineers and others who do not believe in grouting, claiming that there is a tendency of the materials to separate and form layers.

*See American Architect, July 21, 1687, p. 11.

Architectural Terra-Cotta-Weight and Strength.

The lightness of terra-cotta, combined with its enormous resisting strength, and taken in connection also with its durability and absolute indestructibility by fire, water, frost, etc., renders it specially desirable for use in the construction of all large edifices.

Terra-cotta for building purposes, whether plain or ornamental, is generally made of hollow blocks formed with webs inside, so as to give extra strength and keep the work true while drying. This is necessitated because good, well-burned terra-cotta cannot safely be made of more than about 11⁄2 inches in thickness, whereas, when required to bond with brick-work, it must be at least four inches thick. When extra strength is needed, these hollow spaces are filled with concrete or brick-work, which greatly increases the crushing strength of terra-cotta, although alone it is able to bear a very heavy weight. "A solid block of terra-cotta of one foot cube has borne a crushing strain of 500 tons and over."

Some exhaustive experiments, made by the Royal Institute of British Architects, give the following results as the crushing strength of terra-cotta blocks :

3. Hollow block of terra-cotta, slightly made and unfilled. 80

Tests of terra-cotta manufactured by the New York Company, which were made at the Stevens Institute of Technology in April, 1888, gave the following results :

From these results, the writer would place the safe working strength of terra-cotta blocks in the wall at 5 tons per square foot when unfilled, and 10 tons per square foot when filled solid with brick-work or concrete.

The weight of terra-cotta in solid blocks is 122 pounds. When

made in hollow blocks 14 inches thick, the weight varies from 65 to 85 pounds per cubic foot, the smaller pieces weighing the most. For pieces 12′′ x 18′′ or larger on the face, 70 pounds per cubic foot will probably be a fair average.

For the exterior facing of fire-proof buildings, terra-cotta is now considered as the most suitable material available.

CHAPTER VII.

STABILITY OF PIERS AND BUTTRESSES.

A PIER or buttress may be considered stable when the forces acting upon it do not cause it to rotate or "tip over," or any course of stones or brick to slide on its bed. When a pier has to sustain only a vertical load, it is evident that the pier must be stable, although it may not have sufficient strength.

It is only when the pier receives a thrust such as that from a rafter or an arch, that its stability must be considered.

In order to resist rotation, we must have the condition that the moment of the thrust of the pier about any point in the outside of the pier shall not exceed the moment of the weight of the pier about the same point.

To illustrate, let us take the pier shown in Fig. 1.

Let us suppose that this pier receives the foot of a rafter, which exerts a thrust 7 in the direction AB. The tendency of this thrust will be to cause the pier to rotate about the outer edge b; and the moment of the thrust about this point will be T× a,b1, a,b, being the arm. Now, that the pier shall be just in equilibrium, the moment of the weight of the pier about the same edge must just equal T× ab1. The weight of the pier will, of course, act through the centre of gravity of the pier (which in this case is at the centre), and in a vertical direction; and its arm will be b1c, or one-half the thickness of the pier.

Hence, to have equilibrium, we must have the equation,

Tx a1b1 = W × b1c.

But under this condition the least additional thrust, or the crushing off of the outer edge, would cause the pier to rotate: hence, to have the pier in safe equilibrium, we must use some factor of safety.

This is generally done by making the moment of the weight equal to that of the thrust when referred to a point in the bottom of the pier, a certain distance in from the outer edge.

This distance for piers or buttresses should not be less than onefourth of the thickness of the pier.

Representing this point in the figure by b, we have the necessary equation for the safe stability of the pier,

Tx ab = W × it,

t denoting the width of the pier.

We cannot from this equation determine the dimensions of a pier to resist a given thrust; because we have the distance ab, 1, and W, all unknown quantities. Hence, we must first guess at the size of the pier, then find the length of the line ab, and see if the moment of the pier is equal to that of the thrust. If it is not, we must guess again.

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Graphic Method of determining the Stability of a Pier or Buttress. When it is desired to determine if a given pier or buttress is capable of resisting a given thrust, the problem can easily be solved graphically in the following manner.

Let ABCD (Fig. 2) represent a pier which sustains a given thrust Tat B.

To determine whether the pier will safely sustain this thrust, we proceed as follows.

Draw the indefinite line BX in the direction of the thrust. Through the centre of gravity of the pier (which in this case is at the centre of the pier) draw a vertical line until it intersects the line of the thrust at e. As a force may be considered to act anywhere in its line of direction, we may consider the thrust and th weight to act at the point e; and the resultant of these two forces can be obtained by laying off the thrust T from e on eX, and the weight of the pier W, from e on the line eY, both to the same scale (pounds to the inch), completing the parallelogram, and drawing the diagonal. If this diagonal prolonged cuts the base of the pier at less than one-fourth of the width of the base from the outer edge, the pier will be unstable, and its dimensions must be changed. The stability of a pier may be increased by adding to its weight