LOCK GATES AND VALVES. 193 



require a deeper recess in the walls. For these reasons the girder section was found 

 to be more economical and suitable.* 



All the plating was to be of single sheathing, flat plates from f to J inch thick 

 being used. The framing throughout was of the horizontal type of equal spacing, 

 only two vertical diaphragms being put in each leaf, and these were employed solely for 

 the sake of stiffness. The quoin and toe posts were of wood, except for gates for locks 

 80 feet wide, with heads of over 30 feet, in which cases steel had to be used, as wood 

 was unequal to the pressure. 



In order to overcome the upward pressure under the gate, as was necessary in 

 certain cases, the arrangement was adopted of using a narrow vertical plate for the 

 bottom panel, stiffened with angles and attached to a main frame just above by cast 

 brackets. When the gate was closing the main frame would pass over the sill and be 

 stopped by shoulders on the brackets which bore against the sill. These shoulders 

 were to be about a foot wide, up and down stream, the spaces between them being 

 closed by a horizontal plate stiffened with angles. By this means the water would 

 press upward only on the horizontal plate, instead of on the whole width of the gate. 

 The sill was to be trimmed to fit this special construction. 



Cost of Gates. The cost of gates varies considerably, as will be seen by the fol- 

 lowing figures, which give the cost of several wooden gates built by the United States 

 Government by hired labor. 



1896. Two pairs of gates of 12" timbers, for a lock 27 feet wide and of 9 

 feet lift. 



Cost of timber (13,000 feet B. M.) $325.00 



:< cast iron (14,000 Ibs.) 423 . 50 



" " wrought iron (9,400 Ibs.) 553 . 50 



$1302.00 



Framing, placing, and miscellaneous 1 204 . oo 



(about $92.50 per M.) 



Total $2506.00 



About $193 per M., or $2.20 per square foot. 

 (This includes cost of operating spars, butterfly valves, etc.) 



* The comparative ability of the arched and of the straight girder type to withstand shocks may 

 be illustrated by supposing a boat to strike, with sufficient impact to force the leaf open, the lower 

 side of a girder supporting a head of water. In an arched girder, which depends for its equilibrium 

 tinder a load on a fixed direction of the resultant pressures at the toe, the framing is suddenly 

 submitted to intermediate bending stresses for which it was not designed, and which, if the pressure 

 is great, will distort it seriously. In a straight girder, which acts as a beam, the point of support is 

 merely changed from the toe to a point a few feet distant (as the point of impact when a boat strikes a 

 gate is almost invariably near the miter), and the supporting power of the girder remains practically 

 unimpaired. 



