Hemlock.
Respecting the merits of hemlock, the Minneapolis Lumberman has a good word in its last issue. It quotes from a correspondent at Williamsport, Pa., regarding the experience with the wood there as a foundation for paving blocks; and in regard to a stretch of Nicholson pavement there which had been down sixteen years, goes on to say:
“The blocks had been placed on two thicknesses of one inch hemlock, the boards coated both sides with coal tar. When the pavement was taken up, the boards were found to be in good condition—so perfectly sound that they were put back again for possibly another sixteen years. The correspondent says that out of a mile of sixty foot street, less than 10,000 feet of the old planking was condemned. There seems to be no question as to the superlative merit of hemlock for paving purposes.”
This is valuable testimony, and better evidence of the lasting qualities of hemlock under paving blocks than any Western experience has produced, for the reason that it has nowhere in the West been so long as that in use. It has been recognized, however, as a wood excellently suited to this purpose, and is employed almost exclusively wherever the cedar block pavement is freely used, which includes a good many of the large cities in the United States, and practically all Western towns in which any paving is done. It is apparent that lumbermen are beginning to take a strong interest in hemlock, and evidences of its growth in favor are becoming rather plentiful. It occasionally gets a setback from some local dealer, who has come off second best in an encounter with its slivers, but it is bound to come more and more in use in spite of the strong objection that is made to it by some users. Its light weight and great strength for many building purposes are factors that tell.—Timberman.
Collapse of Walls of Burning Buildings.
Mr. Alex. Black, writing to the Building News, says: The expansion of brick by fire heat may be estimated at rather above half that of wrought iron; and of mortar at about one‐fourth more than that of wrought iron. The mortar joints in the wall may occupy, say, one‐fifth to one‐sixth of the height of the wall. There is no accurate data as to the maximum heat developed at Whiteley’s fire. It, however, depends on the nature of the contents, etc., consumed and the accumulation of draught currents. If there is free lime, etc., in brick or mortar, there is added to the expansion more or less disintegration, which would become not the least potent cause of collapse.
We may assume that the interior surface of the brickwork exposed to the fire expanded, say, 1 in. in 8 ft. or 10 ft., vertically and horizontally, which would produce a distortion by buckling, or curving inward, dish‐like, of the inside half thickness of the wall, both vertically and horizontally, leaving for a time the outside half thickness (say, for convenience of description) not much disturbed; in the meanwhile, air gets in between these inside and outside half thicknesses, or slices, and it may become gradually expanded by heat and help to force them further apart until the whole wall collapses.
In setting iron girders, the usual practice appears to be to build the ends solidly in the wall to act as a tie; but by having cross flanges at ends the wall may be built close to these on inside toward interior of building, and space left for expansion on their outside; but this space is of no use without the ends of the girders are set upon rollers or rockers, as is done for bridge girders, because the rigidity of wall would not be sufficient to withstand undisturbed the expansive pushing out, or horizontal thrust, of loaded girder end if resting upon a rough bearing plate or block.
If building timber joists into walls, it is a safe method to bevel off the upper corner equal to the bearing of the end in the wall, which allows the projecting portion of joist, if broken accidentally, to drop down without disturbing the wall, by the leverage which it would exert if built in the wall in the usual way.