222 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



Rise of Gates. An important element as regards the plan of 

 gates is the rise. This is the length of the perpendicular line 

 drawn from the meeting-point on the centre line of the passage 

 to a transverse line passing through the centre of the heel-posts. 

 The amount of rise greatly influences the compressive strains 

 that is, the strains vary inversely as the rise. 



Sir F. Bramwell 1 Ins pointed out that the most economical 

 form generally is that which, when the gates are closed, produces 

 a continuous circular arc, subtending an angle of 1 33*56 at the 

 centre of the circle of which the arc forms a part. The rise of 

 the gates is thus equal to the width from centre to centre of the 

 heel-posts, multiplied by "32958, or, in round numbers, one-third 

 of the span. 



The most economical is not, however, necessarily always the 

 most advisable form to adopt. A large rise increases the length 

 of the gate-leaf, and therefore affects the effective length of 

 the entrance. This, in the case of locks and dry docks, more 

 especially, might prove a serious matter. 



A large camber in the gates requires a greater depth of 

 recess, which is inconvenient, is not always admissible, and is 

 generally to be avoided if possible. 2 



The greater the rise 3 the more oblique is the thrust on the 

 hollow quoin with respect to the side walls, and therefore more 

 difficult to meet ; particularly 4 when the gates are required to 

 be near the outer side of the entrance; for in such a case it may 

 be undesirable to put a great amount of masonry in front of the 

 gates ; and, therefore, necessary to direct the thrust, if possible, 

 across the wall, where an abutment can be obtained by placing 

 heavy counterfoils at the rear. 



The longer the gate-leaf 5 the weaker it is to withstand the 

 strain of opening and shutting, and any blows which it in.-iy 

 receive when home in the recess. 



For small and medium-sized gates, say up to GO feet clt-nr 

 span, Mr. Brown, in his paper on " Dock Gates," 6 suggested as a 

 convenient form that shown in Fig. 200. In this figure, the 

 lino of thrust is a continuous arc, A 15, |.,-issing through the centre 

 of the heel-posts and of the meeting faces of the mitre-posts, and 

 having its centre on the central line of the entrance, the rise or 



1 M.P.LC.E., vol. Iviii. p. 202. 



1 Ibid., vol. xxxi. p. 331; and vol. Iviii. p. L'n-J Ibid., vol. xxxi. p. 334. 



4 1 lid., p. 318. 5 Ibid., vol. xxxi. p. 314. lli.l, v..l. Iviii. p. 203. 



