Longitudinal strutting consists of a timber structure so arranged as to have all the principal members supporting the poling-boards parallel to the axis of the tunnel. This system of strutting is peculiar to the English method of tunneling. The longitudinal timbers rest on this finished masonry at one end, and are carried on a cross frame or by props at the other end. At intermediate points the longitudinals are braced apart by struts in planes transverse to the tunnel axis. This construction makes a very strong strutting framework, since the transverse struts act as arch ribs to stiffen the longitudinals; but the use of transverse poling-boards requires the excavation of a larger cross-section than is necessary when longitudinal poling-boards are employed, and this increases the cost both for the amount of earth excavated and the greater quantity of filling required. In polygonal strutting the main members are in a plane normal to the axis of the tunnel. They form a polygon whose sides follow closely the sectional profile of the excavation. These polygonal frames are placed at more or less short intervals apart, and are braced together by short longitudinal struts lying close to the sides of the excavation, and running from one frame to the next, and also by longer longitudinal members which extend over several frames. The polygonal system of strutting is peculiar to the Austrian method of tunneling, and is fully described in a succeeding chapter. One of its distinctive characteristics is that the poling-boards are inserted parallel to the tunnel axis. Polygonal strutting is generally held to be stronger than longitudinal strutting under uniform loads, but it is more liable to distortion when the loads are unsymmetrical. [Illustration: FIG. 25.--Shaft with Single Transverse Strutting.] [Illustration: FIG. 26.--Rectangular Frame Strutting for Shafts.] [Illustration: FIG. 27.--Reinforced Rectangular Frame Strutting for Shafts in Treacherous Materials.]