247 —— 
Pratt 
busts 
•〜 One of the two portions of an angle iron separated from each other by the bend, 
^ever Arm. — The perpendicular from the centre of moments to the line of action of a 
Ce, The lever arm of a couple is the perpendicular distance between the lines of action of the 
贫 0 ef l Ua ^ and parallel forces. 
1401 id. — The moving* or passing load upon a structure. 
Mnviiio Tru** (also called ' Double Quadrangular/* “Whipple.” and a Double System 
truss). 一 A. truss with vertical posts and diagonal ties spanning two panels. It is the 
^presented on Plate I. 
hock Nut. — See check nut. 
r 40011 Ky e . 一 An eye on the end of a rod or square bar, elongated into the form of a loop, 
S shown ⑽ Plate II,, Figs. 4 and 8. 
howor Pal»« work. — The falsework below the level of the lower chords, 
th n, »«ono l. — A tension member of a truss, sloping upward towards the nearer end of 
span. Main diagonals in iron bridges are not adjustable, 
foment. 一 The product of a force by its lever arm. 
加 1 * meut «f inertia. — Represented by the equation, 1 -- Ap = 2 r2dA, where A is the 
the section considered, p the radiu? of gyration, and r the distance of any point from au 
0 f ecl 】 i ne lying either in the surface or outside of it : in other words, the moment of inertia 
surface about, any axis is the product of the area by the square of the radius of gyration : or 
e summation of the products of each differontial of the area by the square of its distance 
in . • 6 aX ^ S, 灯 the axis lie in the surface, the moment of inertia is called a surface moment of 
a > while, if the axis be perpendicular to the surface, the moment of inertia is called a polar 
〜ntof inertia.. 
donkey Wr«»ncii. 一 A wrench capable of being adjusted so as to fit nuts of different 
(Plate XU., Fig. 9.) 
句 zes. 
W. 
Moving L oa< j. — y ee ]i ve l oa d. 
hid f Iud " Si11, — A. timber, usually from G" by 6’’ to 12" by 】 2", at the bottom of a bent. It is 
or izontally in a trench, and the posts of the bent rest upon it. 
Ui ^ a，Ue Plate . 一 A plate of iron placed in a conspicuous position on a bridge, containing 
如边 0 of the maker or designer of the structure. 
Negative Rotation. 一 Rotation in a direction opposite to that of the hands of a watch. 
^ et Secf , °». — See effective area. 
jj 0r ^ eutral s «i*face. — That part of a member subjected to bending, wliich is neither extended 
ttl P re ssed. In symmetrical wrought-iron beams, with equal or nearly equal flanges, it is 
11 be at the centre line of the web. 
rn* ^ Sma ^ piöue of iron with a threaded core to fit on the screw end of a bolt, rod, or 
(Plate II., pig. 7> ) 
Wer Bui . — a form of bill used in ordering material from the manufacturers. 
e » pn , na，n «'ntai work. — Fancy work at the portals of a bridge to give it architectural 
. (Plates I. and IX.) 
Us Uall ea<, BracI «»S» 一 The upper lateral or sway bracing in through bridges. The term is 
a Pp]ied to the vertical sway bracing, if there be any j if not, to the upper lateral bracing. 
^ a «king. 一 gee chord packing. 
Ca Ued a ^ el * That portion of a truss between adjacent posts or struts in Pratt truss bridges ； 
p ttnel t4en 8*h. — The distance between two adjacent panel points of the same chord. 
— g ee a p ex 
^e«le*tal. — The foot of a batter brace or end post. (Plate II., Fig. 12.) 
to s ^ res?? m，lÄent Set - — The alteration in length of a piece of material which has been subjected 
’ remaining after the stress has been removed. 
— See column. 
，〇 r Pin Pilot. 一 A nut, one end of which, is a truncated cone, used to protect the 
