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bridges is very great and includes nearly all the important industrial plants in Pittsburgh as well as all the freight stations and the principal warehouses, retail stores and other commercial establishments of Pittsburgh and Allegheny. Any considerable increase of gradient on these bridges means a reduction in average size of load hauled by vehicles of this important class, and a corresponding increase in the number of trips and in the number of teams required to do the work, making for increased cost and greater congestion of traffic. For all horse-drawn vehicles an increase of gradient on the bridges, beyond a certain limit, means, especially in wet or snowy or frosty weather, more slipping and falling, more stalling of all bridge traffic by such accidents, more wear and tear on horse flesh, and a resultant increased burden on the people. To raise the gradient of the bridges from those now existing to those indicated in the plans of the local United States Engineers' office would more than double the traction effort required in hauling over these bridges.

It must be borne in mind that, as the gradients increase, the cost of teaming and the wear and tear on teams increases much more rapidly than the theoretical effective horse power, because of the increased difficulty of foothold. It is impossible to measure the effect of any given increase of grade with precision, but a comparison of the existing conditions with those resulting from various possible bridge heights will give a good general idea of the effect as shown by the following tables:

TABLE SHOWING BRIDGE GRADES INVOLVED BY THE ADOPTION OF VARIOUS CLEARANCE HEIGHTS

Elevation in feet above pool level of under side of bridge over 360' channel	Present	37'	42'	47'	Present	37'	42'	47'
	Maximum gradients**				Amount of rise in feet above Duquesne Way
Sixth Street bridge	2.3%	3.2%	4.5%	5.8%*	7.5	10.4	15.4	20.4
Seventh Street bridge	3.0%	3.7%	5%*	6.3%	10.0	14.2	19.2	24.2
Ninth Street bridge	2.8%	3.5%	5%	6.5%*	10.1	13.3	18.3	23.3

*Gradients for this clearance height are greater than those shown on United States Engineers' plans because of greater width of channel. If United States Engineers' plans were adopted the maximum gradients would be as follows: Sixth Street, 4.35%; Seventh Street, 4.93% and Ninth Street, 4.98%.

**The maximum gradients here given assume the improvement of the short pitches now existing on some of the bridge approaches.

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Last modified on 22-Dec 1999
Design format: copyright 1997-1999 Bruce S. Cridlebaugh
Original document: Frederick Law Olmsted, 1910