266 W. P. Troiohrldge — Design for a Bridge. 



2. If the lower chord be uniformly loaded^ the same principles 

 and reasonino- .i})ply. If loaded at separate points, it will be observed 

 that the strains arising from any load will be transmitted through the 

 stay-rods nearest it directly to the anchox"ing-rods in the pier ; and 

 thus it will be impossible for several loads to concentrate their effects 

 upon any one, two, or three sets of rods. This condition will give 

 stiffness, or freedom from vertical vibration, under moving loads. The 

 only vertical oscillation that can arise imder these circumstances will 

 occur from the stretching of the rods under the tensions brought upon 

 them. This will be so small in amount as to be inapprecialjle. In some 

 suspension bridges this elasticity of stay-rods is a dangerous element, 

 however, because the shorter rods may be stretched beyond their limits 

 of elasticity, from the greater extension of the longer rods. This cir- 

 cumstance has not usually been taken into account in suspension bridges, 

 and frequent disasters have occurred from the unaccountable giving 

 way of the stay-rods. Long rods will stretch more than short rods, 

 of the same diameter, in the exact proportion to their greater length, 

 and even more on account of their additional Aveight ; and if two such 

 rods of greatly unequal lengths support equal loads, this element of 

 elasticity shoidd be taken into account. There are two modes of 

 doing this ; one is to increase the diameter of the longer rods with 

 especial reference to this stretching, and the other to permit the plat- 

 form of the bridge to yield to accommodate itself to the increased 

 length of the rods. This plan is adopted in the construction under 

 consideration. The stay-rods being all parallel, and not being all 

 brought from the top of the tower, the stretching of the rods under 

 passing loads will increase from the pier, where it is nothing, out- 

 ward to the point A, and this end being unattached, all the points 

 of suspension from the pier outward may move in proportion to the 

 stretching of the rods, in small arcs of circles having a common cen- 

 ter at the pier. Thus the movement of the platform so adjusts itself 

 that the limits of elasticity will be reached at the same instant in all 

 the stay-rods ; and no injurious bending or shearing strain can be 

 thrown upon the platform near the j^ier. 



3. Action under change of Temperature. — This is one of the most 

 important considerations in all iron bridges of long span. In this 

 structure it is evident that the only effect of change of temperature 

 will be to cause an outward or inward movement of tlie points A and B, 

 and an upward or downward movement of the point T, without dis- 

 turbino- the lines of dii'ection or causing a movement of sliding hori- 

 zontallv on the ])ier, the structure A T B being homogeneous and inde- 



