LOCK GATES AND VALVES. 181 



Unit Stresses. Wood. In a wooden gate it will generally be found that only tensile 

 stress need be considered, as if the pieces are made strong enough in that respect the 

 section will be ample for all stresses of compression and of shear. Those parts of a gate 

 which usually sustain a full head, such as the beams above the lower pool surface, may be 

 designed for the minimum unit stresses given below, while those parts which are rarely 

 subject to maximum loading, such as the beams of the lower gate below the lower pool, 

 which can support the greatest head only when the lower pool is drawn down, may be 

 designed for the maximum unit stresses given. 



A safe working tensile stress for gates of white pine is 900 Ibs. per square inch, 

 with a maximum of 1200 Ibs. In a few cases the first unit stress has been made 

 as high as 1200 Ibs., but where the timber was subject to constant exposure the 

 strains induced were found to be somewhat high. For oak and yellow pine a tensile 

 stress of 1200 Ibs. per square inch may be used, with a maximum of 1600 Ibs. 



Steel. The unit stresses used for steel gates have varied considerably. Thus on the 

 gates of the St. Mary's Falls Canal, Mich., (1895) 9000 Ibs. per square inch was used for 

 the general framing, with a maximum of 10,000 Ibs. One authority recommends 10,000 

 Ibs. per square inch for compression and 12,000 Ibs. per square inch for tension, while in 

 several examples of gates for locks in rivers a maximum of 12,000 Ibs. per square inch 

 has been used for compression and 16,000 Ibs. for tension, the beams or girders in these 

 cases being assumed to receive no additional strength from the sheathing plates. On 

 the gates for the proposed locks of the Deep Waterways from the American Lakes to 

 the Atlantic (1900), the flange stress of the main girders was limited to 10,000 Ibs. per 

 square inch, part of the sheathing being assumed to act as a flange-plate. The pres- 

 sure between the wooden quoin-posts and the masonry was limited to 400 Ibs. per 

 square inch. 



As a gate is rarely subject to greater loading than that of static water pressure, 

 it would appear safe to use moderately high unit stresses, making allowance as far as 

 practicable for deterioration by rust 



WOODEN GATES. 



Vertical vs. Horizontal Framing. Vertically framed wooden gates are practically 

 obsolete in America, and for all river locks of ordinary size the simplest and best 

 method of construction consists of horizontal beams extending in one length from the 

 toe to the heel, well bolted and strapped together, their ends being shaped to fit the 

 miter and the hollow quoin respectively. This gives a solid timber from end to end, 

 and avoids the weakness of beams jointed into vertical heel- and toe-posts, such as arc- 

 found in the older styles of gates. 



Spacing of Beams. In wooden gates, except where they are very small, the upper 

 portion of the gate is usually paneled, that is, the beams are placed at varying dis 

 tances, and the spaces between are closed with 2 -inch or 3 -inch plank. This is an 



