Essentials in the theory of framed structures . n of the train for maximumnegative shear, in any panel, is the same as for maximum bend-ing moment at Z/ in a beam having the length NQ. 105. Impact.—In computing live load stresses in a bridge,the train is considered as a static load, gently placed upon thestructure in the required position for maximum stress in agiven member. In order to provide for the increased stresscaused by dynamic effect of the train in motion, an additionalstress known as impact is combined with each live load amount of impact to be added is determined arbitra
Essentials in the theory of framed structures . n of the train for maximumnegative shear, in any panel, is the same as for maximum bend-ing moment at Z/ in a beam having the length NQ. 105. Impact.—In computing live load stresses in a bridge,the train is considered as a static load, gently placed upon thestructure in the required position for maximum stress in agiven member. In order to provide for the increased stresscaused by dynamic effect of the train in motion, an additionalstress known as impact is combined with each live load amount of impact to be added is determined arbitrarilyfrom an empirical formula. The one most frequently used is 300 + I i6o THEORY OF FRAMED STRUCTURES Chap. IV in which I = impact stress L = live load stress I = length of train causing the live load example, the live load stress in UiUz (Fig. io6), as deter-mined in Article 98, is The impact stress is X 300 compressive 300 +100The live load tension stress in UiLi. (Fig. 108), as determined U, Ug U3 U4 Us U6 Ut. Fig. III. in Article 103 is The impact stress is X —T—r- = S°-2 tensile 300 + 63 The live load compression stress is 18. i, and the impact is X ^00 - -^ = compressive 300 + 33 The live load stress in UiLi is The length of the traincausing this stress extends over two panels only, hence theimpact stress is 75-6 X 300 300 + 43 106. Stresses in a 200-ft. Pratt Truss.—(Fig. m). Theassumed dead loads are as follows: Track (rails and ties) 450 Floor (floor beams and stringers) 550 Two trusses and bracing i, 200 2, 200 lb. per linear foot The live load is a Coopers E-40 train. The impact will betaken in accordance with the formula of Article 105. Dead-load Stresses.—The dead load supported by each trussis 1,100 lb. per linear foot, or per panel. The stresses Sec. Ill BRIDGES l6l may be determined by placing a load of at the seven jointsLi to Lr and drawing a stress diagram; but in the case ofpa
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectstructu, bookyear1922
