TRANSVERSE STRENGTH OF BRION MASONRY. Occasion ally the transverse strength of brick-work is of importance. For example, if a wall is to be built upon a beam spanning an opening, it is necessary to know the load that will come upon the beam; or again, if an opening is to be cut through an old wall, it is important to know whether the wall will be self-supporting over the opening. Since the adhesion of mortar is much less than the tensile strength of either the mortar or the brick (see § 256), the transverse strength of brick-work is dependent upon the adhesion of the mortar. While experiments show that the adhesion of any kind of mortar to either brick or stone is small in comparison with its cohesive or tensile strength, experience in demolishing old walls shows that ordinary masonry has a considerable transverse strength.
Not many experiments have been made to determine the trans verse strength of brick masonry, the following being all the records of tests that can be found: Engineer and Architect's Journal, Vol. i, p. 30, 45, 102, 135 (1837) ; Vol. xi, p. 294 (1848) ; Vol. xiv, p. 510 (1851). Engineering, Vol: xiv, p. 1 (1872) ; Indian Engineering, Jan. 9, 1892, or Railroad Gazette, Feb. 26, 1892. Several of the above experiments were made to determine the effect of hoop-iron bonding straps, and the remainder give no information as to the quality of the mortar; and hence none of the results are applicable to plain brick masonry, and the details of the experiments are so meagre that the experiments are of no practical value.
Five experiments under the direction of the author* gave a mean transverse strength of 120 lb. per sq. in. for a good-quality soft-mud building brick laid in a poor 1 : 2 natural-cement mortar, tested when 50 days old. The results of eleven tests of this series seemed to indicate that brick beams bonded as regular masonry have a modulus of rupture equal to about twice the tensile strength of the mortar when built with ordinary care, and about three times when built with great care. When the beams are constructed as piers, i.e., with no interlocking action, the modulus of rupture is about equal to the tensile strength of the mortar.
Four tests under the direction of the University of Illinois En gineering Experiment Station t gave a mean modulus of rupture of 89.5 lb. per sq. in. for under-burned soft-mud building brick laid in 1 :3 portland-cement mortar, when 76 days old; and 298 lb. per sq. in. for vitrified shale building brick laid in 1 : 3 portland-cement mortar when 76 days old.
it was also shown that in which = the height of the wall, in feet, producing a maximum load on the lintel. Notice that = which shows that the maximum stress on the lintel occurs when the height of the wall is half of its self-supporting height, at which time one half of the wall will be self-supporting and one half will require extraneous support; or in other words, the maximum moment on the lintel is one quarter of that due to the self-supporting height.
By the use of equation 1 and the values of the modulus of rupture of brick masonry stated in the preceding section, it is possible to compute whether or not, if a given opening is cut through a brick wall, the masonry will be self-supporting. To determine whether the lintel can resist the maximum moment which will come upon it, determine by equation 1 the self-supporting height of the wall, and then the moment on the lintel will be that due to one quarter of the self-supporting wall considered as uniformly distributed. If the lintel can not safely carry the load that will come upon it, the girder must be supported temporarily, or time must be given for the mortar to set, or a stronger or at least a quicker-setting mortar must be used.
The substantial correctness of this method of computing the stress on a lintel is proved by the fact that large openings are fre quently cut through walls without providing any extraneous sup port; and also by the fact that walls are frequently supported over openings on timbers entirely inadequate to carry the load if the masonry did not have considerable strength as a beam.
Custom differs as to the manner of estimating the pressure on a girder due to a superincumbent mass of masonry. One extreme consists in assuming the masonry to he a fluid, and taking the load on the lintel as the weight of all the masonry above the opening; but as the wall is likely to be several days in building, the masonry first laid attains considerable strength before the wall is completed; and hence, owing to the cohesion of the mortar, the final weight on the girder can not be equal to, or be compared with, any fluid volume. This method always gives a result that is too great.
The other extreme consists in assuming the pressure to be the weight of the masonry included in a triangle of which the opening is the base and whose sides make 45° with this line. This method gives a load S times that which takes account of the transverse strength of the brick-work. If R is relatively large and S is small, this fraction will be more than unity, under which conditions this method is safe; but if R is small and S is large, then this fraction is less than one, which shows that under these conditions this method is unsafe.