38 
BULLETIN 1259, U. S. DEPARTMENT OF AGEICULTUEE. 
joints '^'^^^"^^"'^ '^^ provide for expansion and contraction movement, 
suitable floor expansion joints shall be provided at the 
expansion ends of all spans and at other points where 
they may be required. 
Apron plates, when used, shall be designed to properly 
bridge the joint and to prevent, as far as possible, the 
deposit of roadway debris upon the bridge seats. 
BRACING. 
bracing 
58. Design of Lateral, longitudinal, and transverse bracing shall be 
racing. composcd of angles or other shapes ofl^ering resistance to 
deformation when subjected to compressive stress, and 
shall have riveted connections. 
In general, bracing shall consist of a double system of 
diagonal tension members with transverse compression 
members. The diagonals in each system shall be pro- 
portioned to carry the total lateral stress in tension, 
the transverse struts (or floor beams) acting as compres- 
sion members for both systems. 
All intersections of lateral and sway bracing shall be 
riveted to add rigidity and prevent deformations. 
59. L a t e r a 1 Bottom lateral bracing shall be provided in all bridges 
except I-beam spans, from which it may be omitted. 
Bottom laterals preferably shall be supported by rigid 
hangers at the intersections. 
Top lateral bracing shall be provided in deck spans 
and in thi'ough spans having sufficient head room. 
Lateral bracing for compression chords shall 
consist of either two-angle or four-angle latticed sections; 
and so designed as to engage effectively both flanges of 
the chords. 
Lateral bracing shall have concentric connections to 
chords at end joints, and preferably throughout. The 
connections between the lateral bracing and the chords 
shall be designed to avoid, as far as possible, any bending 
stress in the truss members. 
Portal and Tlirough truss spaus shall have portal bracing, prefer- 
ably of the two plane or box type, rigidly connectea to the 
end post and top-chord flanges, and constructed as deep 
as the minimum clearance will allow. When a single 
plane portal is used it preferably shall be located in the 
central transverse plane of the end posts, with diaphragms 
between the webs of the posts to provide for a proper 
distribution of the portal stresses. The portal bracing 
shall be designed to take the full end reaction of the top 
chord lateral system and the end posts shall be designed 
to transfer this reaction to the truss bearings. 
Deck truss spans shall have adequate sway bracing 
at the ends and at all intermediate panel points. This 
bracing shall occupy the full depth of the trusses below 
the floor system. The bracing shall be proportioned to 
transfer the end reaction of the top lateral system to the 
substructure. 
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sway bracing 
