| Struts to reënforce the frame | | 6 | 8 | 8 | 8 | | Distance apart of the strutting | | | | | | | (in feet) | 6 | 4.5 | 4 | 3 | 2.6 | +---------------------------------+-----+-----+--------+------+------+ IRON STRUTTING. In 1862 Mr. Rziha employed old iron railway rails for strutting the Naensen tunnel, and his example was successfully followed in several tunnels built later where timber was scarce and expensive. The advantages which iron strutting is claimed to possess over the more common wooden structure are: its greater strength; the smaller amount of space which it takes up; and the fact that it does not wear out, and may, therefore, be used over and over again. [Illustration: FIG. 28.--Strutting of Timber Posts and Railway Rail Caps.] [Illustration: FIG. 29.--Strutting made entirely of Railway Rails.] =Iron Strutting in Headings.=--In strutting the headings the cross frames have a crown bar consisting of a section of old railway rail carried either by wood or iron side posts. When wooden side posts are used their upper ends have a dovetail mortise, and are bound with an iron band, as shown by Fig. 28. The base of the rail crown bar is set into the dovetail mortise and fastened by wedges. When iron side posts are employed they usually consist of sections of railway rails, and the crown bar is attached to them by fish-plate connections, as shown by Fig. 29. The iron cross frames are set up as the heading advances, and carry the plank lagging or poling-boards, exactly in the same manner as the timber cross frames previously described. [Illustration: FIG. 30.--Rziha’s Combined Strutting and Centering of Cast Iron.] [Illustration: FIG. 31.--Cast-Iron Segment of Rziha’s Strutting and Centering.] =Full Section Iron Strutting.=--The iron strutting devised by Mr. Rziha for full section work is shown by Fig. 30. Briefly described, it consists of voussoir-shaped cast-iron segments, which are built up in arch form. Fig. 31 shows the construction of one of the segments, all of which are alike, with the exception of the crown segment, which has a mortise and tenon joint which is kept open by filling the mortise with sand. The segments are bolted together by means of suitable bolt-holes in the vertical flanges, and when fully connected form an arch rib of cast iron. This arch rib, A, Fig. 30, carries a series of angle or T-iron frames bent into approximately voussoir shape, as shown at B, Fig. 30. Above these frames are inserted the poling-boards, running longitudinally, and spanning the distance between consecutive arch ribs. By removing the bent iron frames the cast-iron rib forms a center upon which to construct the masonry. Finally, to remove the cast-iron rib itself, the sand is drawn out of the mortise and tenon joint in the crown segment, which allows the joint to close, and loosen the segments so that they are easily unbutted.