Laying Th Brick

Conclusions from Preceding Tests. The preceding tests show (1) that the strength of piers built of the same brick is closely proportional to the strength of the mortar; and (2) that the strength of piers built with the same mortar varies as the crushing strength of the brick. The only exceptions to these conclusions are abnormal cases where an unusually strong mortar is used with a very weak brick, or where a very weak mortar is used with a very strong brick.

Since brick masonry gives evidence of distress when the load is about half the ultimate strength (1 618), the factor of safety should be based upon this value rather than upon the load producing com plete 3ollapse. The nominal pressure that may be safely allowed upon brick masonry depends upon (1) the quality of the materials employed; (2) the degree of care with which the work is executed; whether it is for a temporary or permanent, an important or unim portant structure; and, (3) the care with which the nominal maxi mum load is estimated.

Pressure Allowed in Practice.

The pressure allowed on brick masonry was considerably smaller formerly, when the bricks were soft and were usually laid in lime mortar, than at present, when the ordinary brick is much better than the best formerly and when brick masonry is usually laid in cement mortar where great strength is required.

The pressure at the base of a brick shot-tower in Baltimore, 246 feet high, is estimated at 61 tons per sq. ft. (about 90 lb. per sq. in.). The pressure at the base of a brick chimney at Glasgow, Scotland, 468 ft. high, is estimated at 9 tons per sq. ft. (about 125 lb. per sq. in.); and in heavy gales this is increased to 15 tons per sq. ft. (210 lb. per sq. in.) on the leeward side. Twenty years ago the leading architects of Chicago were counted as good authorities in such matters, and did not consider it safe to allow more than 10 tons per square foot (139 lb. per sq. in.) on the best brick laid in 1:2 portland-cement mortar; but now this value is frequently greatly exceeded (§ 629).

A large committee of leading architects and engineers of Chicago in June, 1908, recommended the following values for in corporation in the building laws of that city.

In view of the values obtained in experiments, the above recom mendations seem quite conservative; but it should be remembered that it is likely that a better grade of work will be obtained in making test specimens than in actual work, particularly if the work is let to the lowest bidder and is not subject to rigid inspection during con struction.