A plate girder span is a series of steel plates and angles riveted together to form a rigid beam like a long shoe box with both ends removed. You could comare a plate girder span to a log bridge with the solid interior gouged out to save weight. The triangulated trusses are the key to further reducing the dead loadwithout sacrificing strength or rigidity. You can think of a truss as the side panel of a plate girder span with all the inessential parts removed. The loss of these materials would leave holes, called meshes.
Some chords are in tension, pulling on the material where their ends are attached.
Some chords are in compression, pushing on the material where their ends are attached. Normally, the chords form a series of adjoining triangles. The triangle is an especially stable geometric shape because each side of a triangle is braced by the other two. Unlike a spuare with can deform into a diamond shape without any of its sides getting longer or shorter, the trianlge can contort only if the lengths of the sides change. The triangulation priciple can be seen in the guy wires that keep television antenna on your roof from blowing over in a storm or in a shelf support on a wall.