# -A structural detail in a high

-A structural detail in a highway bridge that has been in service for 20 years fits AWS category E. When the bridge was designed, a weight limit on trucks of 30 tons was in effect, where 1 ton = 2000 lb = 8.896 kN. Traffic monitoring when the bridge was one year old indicated

a traffic volume averaging 800 trucks a day, with the numbers and weights of trucks allowing cyclic stress ranges to be determined for the detail, as listed in Table P10.44. (Automobiles and other light vehicles cause insignificant stresses.) (a) Calculate the equivalent constant level of cyclic stress
Sq that is expected to cause the same life as the given stresses if applied for the same number of cycles. (b) For the given stresses, estimate the life of the detail in years. Given that the usual design life for bridges is 75 years, what are the safety factors in life and in stress? (c) The traffic volume on the bridge has gradually increased and at the present time is twice the early value—that is, 1600 trucks a day. Moreover, the legislature in the state involved is being lobbied to raise the weight limit on bridges of this type to 40 tons. Estimate the remaining life of the bridge if this change is made. (Suggestion: Assume that the new loading corresponds to increasing all of the stresses in Table P10.44 by the ratio of 40/30 = 1.333.)

(d) You are an engineer who lives in this state. Draft a letter to your state legislator, explaining the expected effect of the weight limit change on the life of the bridge, and giving your opinion as to what the legislature should do.

38- Examine a crank shaft in internal combustion engine. (a) Draw a sketch of it, and point out any design features that are beneficial in avoiding fatigue failure. (b) Suggest any design changes that might improve fatigue resistance. (c) Of what material does this part appear to be made? Try to explain the choice of material from the viewpoints of function, cost, and resistance to fatigue, wear, and other possible failure causes. Examine the helical springs in the axle suspension system of a small boat trailer, and then answer (a), (b), and (c), as in Prob. 10.45. Examine a cotter joint with sleeve, and then answer (a), (b), and (c) as in Prob. 10.45

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