22 
BULLETIN 1486, TJ. S. DEPAKTMENT OP AGKICULTTJKE 
counteract them. In contrast to these truly artistic creations, a day's 
drive in any section of the United States will disclose examples of 
expensive and otherwise handsome concrete-bridge construction, 
seriously marred, if not completely ruined in appearance, by unfor- 
tunate combinations in alignment and grade-line treatment. Plate 6 
illustrates a correct grade-line treatment and Plate 4, C illustrates the 
bad effect produced by the use of a concave grade line. 
The following circular letter, issued from the writer's office in 1921 
covers the essential requirements as regards camber and grade-line 
treatment. Inasmuch as the grade line chosen has an important 
bearing upon the alignment, this information should be available and 
should be carefully studied at the time the preliminary location for the 
structure is run in on the ground. 
To All Resident Bridge Engineers: 
In order that practice in regard to camber and grade lines on bridge structures 
may be uniform and in order that grade lines over and adjacent to bridge cross- 
ings may be laid to conform and harmonize with camber requirements, the follow- 
ing general instructions are issued herewith. These instructions are for the guid- 
ance of locating engineers, division engineers, designers, draftsmen, and resident 
engineers on construction. 
Camber is placed on bridge structures in order to facilitate the removal of sur- 
face water from the deck (see pi. 5, C) and for appearance. For wooden struc- 
Fig. 18.— Treatment of grade line on bridge for a convex intersection of grades 
tures, camber has a further value in that it affords a certain amount of leeway so 
that shrinkage of the timber and seating of the joints will not cause a sag or water 
pocket in the deck. 
There are three dangers which must be avoided in the grade line over any 
bridge, as follows: 
1. A concave grade line. 
Experience has shown that it is impossible to properly drain the structure or to 
produce a satisfactory appearance on a bridge rail where any portion of it is con- 
cave. Where necessary to adopt a concave vertical curve, as shown in Figure 22, 
such curve must always be placed on the approach fills and not on any portion of 
the structure. 
2. Too much camber on flat grades. 
The camber should be sufficient to provide adequate drainage and to remove 
any flat or hollow appearance in the rail, but not so marked as to cause the struc- 
ture to appear humped. 
3. Insufficient amount of camber on timber truss spans. 
The camber for this type of structure must be sufficient to take up all shrinkage 
and settlement at the joints. 
The following grade-line treatments are recommended : 
CASE 1. CONVEX INTERSECTION 
This arrangement of grade lines as shown in Figure 18 is the most 
desirable arrangement possible. The ascending grades should be 
equal or as nearly equal as possible and the vertex should lie over the 
