Bridge Engineering Volume 1 Author:John Alexander Low Waddell This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1916 edition. Excerpt: ...equipment. The force necessary to overcome the effect of the unbalanced cables is F3 = R, where R is the weight of the un... more »balanced cables at any point in the travel of the span. It must be remembered that for the first half of the operation in either direction this unbalanced load acts against the force tending to move the span, whereas in the latter half thereof it acts with that force and against the braking action. For normal operation a wind load of two (2) pounds per square foot shall be assumed as acting against the exposed area of the span as it is seen in vertical projection. The friction on the guides due to this wind load must be overcome by the operating ropes. This friction shall be taken as fifteen (15) per cent of the said wind load. For normal operation of from one (1) to one and one-half (1J) minutes the operating equipment must be capable of overcoming the above forces. It must also be capable of moving the span for all wind loads of less than fifteen (15) pounds per square foot, although the time of operation under such a condition shall be increased accordingly. The span-locks for lift bridges shall, as a rule, be operated by hand, when the operator is located in the machinery-house. However, when mechanical operation is required therefor, it shall be designed to meet the case in hand. 89. Bascule Spans For bascule bridges the power equipment will depend on the type of bascule used; and, in general, it will be governed by the preceding specifications for lift bridges. For rolling bascules the coefficient of rolling friction shall be taken at eight (8) per cent. The operating equipment on all types must be capable of holding the span in any position for a wind load of fifteen (15) pounds per square foot on the exposed...« less