Design of Steel Structure
The portal bracing in a truss bridge is used to

Keep the rectangular shape of the bridge cross-section
Prevent the sides-way buckling of top chord
Transfer load from top of end posts to bearings
Stiffen the structure laterally

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Design of Steel Structure
The mechanism method and the statical method give

lower and upper bounds respectively on the strength of structure
upper and lower bounds respectively on the strength of structure
lower bound on the strength of structure
upper bound on the strength of structure

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