HIGHWAY BKIDGE LOCATION 
29 
carried during 15 of the 24 hours, the traffic-carrying capacity of the 
structure is reduced 20 per cent. The results of the introduction of 
a movable span on a busy bridge, especially if the waterway traffic is 
such as to make the bridge active, is obviously a congestion of traffic 
back of each gate. This congestion may extend back across lateral 
streets, blocking them and creating at times a rather chaotic condi- 
tion, especially during the handling of exceptionally heavy traffic. 
From the standpoint of cost, in the majority of cases the low-level 
structure, even with the expensive movable span, machinery, power 
equipment, etc., will be cheaper in first cost than the high-level struc- 
ture with its long approaches and high piers or towers. For high- 
level structures in cities and towns, it is also rather difficult to avoid 
heavy property damage to the property fronting on the street between 
the harbor line or river front and the point at which the high-level 
approaches come to grade. There is also the question of transriver 
traffic originating or terminating on the river front which will be com- 
pelled to travel a more devious path with a high-level crossing than 
with one whose grade coincides with the grade of the river-front 
streets. 
As regards maintenance and operation costs, the high-level struc- 
ture has a marked advantage. The following tabulation of operation 
cost for three typical months for the Morrison Street bridge over the 
Willamette river, at Portland, Oreg., will be of interest in this connec- 
tion. 
Month 
Num- 
ber of 
open- 
ings 
Total 
time 
open 
Cost of 
operation 
(labor 
only) 
Januarv, 1920 
614 
880 
748 
h. m. 
38 59 
52 25 
46 23 
$1, 247. 50 
1,826 21 
July, 1920 
December, 1920 . 
1, 360. 00 
Average . ._ 
747 
45 56 
1, 477. 90 
It will be observed from the above that not only does this draw 
span obstruct traffic on the highway about one and one-half hours 
per day, but the labor cost of operation will capitalize at 5 per cent to 
$354,696. At a conservative estimate, therefore, a high-level struc- 
ture is worth $300,000 more at this point from a standpoint of econ- 
omy alone. 
If a movable span is to be used there are several points which 
deserve careful study when the location is made, among which may 
be listed the following: 
DIRECTION AND VELOCITY OF CURRENTS, CROSS CURRENTS, AND TIDAL FLOWS AS 
AFFECTING THE HORIZONTAL CLEARANCE REQUIRED 
In Figure 26 the general direction of the river is along the line C C 
so that from casual inspection either of the locations shown crosses 
the waterway at nearly a right angle. Tidal currents have deter- 
mined a boating channel, however, which hugs the north bank at the 
crossing of line A and is rather pronouncedly skewed with this loca- 
tion. At this crossing the movable span (which is always the most 
