THK Fl'LL BBCTIOS Mi: I HOI) 



159 



crown bare, and this is done ly pulling them ahead from their 

 original position over thr masonry of the completed section of 

 the roof arch. It will be understood that the crown bars are 

 not pulled ahead their full length at one operation, but are 

 advanced by successive short movements as the excavation 

 progresses, their outer ends being supported by temporary 

 posts until the transverse framework is built at the face of the 

 excavation. 



Centers, Two standard forms of centers are employed in 

 the English method of tunneling, as shown by Figs. 84 and 85. 

 Both consist of an outer portion, constructed much like a 

 typical plank center, which is strengthened against distortion 

 by an interior truss framework. The elemental members of 



Pioft. 84 and 85. Sketches of Typloal Timber Koof-Arch Center*. Knglinti Method. 



this truss framework take the form of a queen-jM^st truss, HA is 

 shown more particularly by Fig. 84. In Fig. 85 the queen- 

 post truss construction is less easily distinguished, owing to 

 the cutting of the bottom tir-U-am and other modifications, but 

 it can still be observed. The possibility of cutting the tie-beam 

 as shown in Fig. 85, without danger, is due to the fact that 

 the lateral pressures on the haunches of the center counteract 

 the tendency of the center to flatten under load, which is 

 usually counteracted by the tie-l>eam alone. The object of 

 rutting the tie-beam is to afford room for the props running 

 from tin- completed masonry to the transverse framework of 

 the strutting as shown by Fig. 83. 



Generally four or five centers are used for each length of 

 areh built They are set up so that the tie-beams rest on 



