222 APPLIED MECHANICS 
depend on the nature of the material on which the girder rests, and is 
usually limited in the case of stone to from 12 to 20 tons per square 
. foot. The safe bearing pressure between the stone and brickwork set in 
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Fie. 324. oi 325. 
cement may be taken at from 6 to 10 tons per square foot, and between 
stone and brickwork in mortar at from 4 to 5 tons per square foot. 
One end, and sometimes both ends, of a girder are left free to slide, 
so that a certain amount of expansion or contraction can take place with 
changes of temperature. 
For large spans, say, of 50 feet and upwards, cast-iron bed plates are 
provided, on which the ends slide. These bed plates are usually sunk 
into the stone templet a small distance, as shown in Fig. 326. These 
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Fig. 326. 
bed plates are bolted down to the stone templet. At (2), in Fig. 326, the 
holding down bolts for the bed plate are shown passing through the 
bolster plate, the bolt holes in the bolster plate being elongated to perme 
of the girder sliding a small amount. 
For spans of over 80 feet, bearings similar to those shown in Figa 
368 and 369 would be used. 
201. Connection of Cross Girders to Main Girders.—Figs. 327, 
328, and 329 show methods of attachment of cross giftlers to main 
