. Theory of structures and strength of materials. CE N —n-OE^ ^2+7 w I n — iN r N- {N. n)n\~^ ^nk{N — n) n r + \{N- 7i)?i\~^ N-n (4) Hence, when the load moves from A towards C, eq. (2)gives the diagonal stress when « is even, and eq. (4) gives thestress when 11 is odd If the load moves from C towards A, the stresses are re-versed in kind, so that the braces have to be designed to actboth as struts and ties. 626 THEORY OF STRUCTURES. Note.—By inverting Fig. 391, a bowstring girder is obtainedwith the horizontal chord in compression and the bow intension. 14. Bowstring Suspension Bridge {Lentic
. Theory of structures and strength of materials. CE N —n-OE^ ^2+7 w I n — iN r N- {N. n)n\~^ ^nk{N — n) n r + \{N- 7i)?i\~^ N-n (4) Hence, when the load moves from A towards C, eq. (2)gives the diagonal stress when « is even, and eq. (4) gives thestress when 11 is odd If the load moves from C towards A, the stresses are re-versed in kind, so that the braces have to be designed to actboth as struts and ties. 626 THEORY OF STRUCTURES. Note.—By inverting Fig. 391, a bowstring girder is obtainedwith the horizontal chord in compression and the bow intension. 14. Bowstring Suspension Bridge {Lenticular Truss).—This bridge is a combination of the ordinary and invertedbowstrings. The most important example is that erected atSaltash, Cornwall, which has a clear span of 445 feet. Thebow is a wrought-iron tube of an elliptical section stiffenedat intervals by diaphragms, and the tie is a pair of chains. A girder of this class may be made to resist the action of apassing load either by the stiffness of the bow or by In Fig. 392, let BD = k, BD = k. Let Hhc the horizontal thrust at B, and T the horizontal pullat B, when the live load covers the whole of the girder. Then H = w P 8 k + kFirst, let k = k. Then 16 k 1= T. H= T = which is one half of the corresponding stress in a bowstring^girder of span / and depth k. One half of the total load is supported by the bow and onehalf is transmitted through the verticals to the tie. Hence, /the stress in each vertical = -irA^ + iv), w being the portion of the dead weight per lineal foot borneby the verticals, and iVthe number of panels. CANTILEVER TRUSSES. 62/ The diagonals are strained only under a passing load. Let PP be a vertical through E, the point of intersectionof any two diagonals in the same panel, and let the load movefrom A towards O. By drawing the tangent at P and proceeding as in Art. 13,the expression for the diagonal stress in QS becomes, as before, w n{n — \)l — 2x ^ 2 N r^x ^°^^^ ^ (^
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