The truss was constructed using a combination of ASTM A36 (Fy=36ksi), A572 (Fy=50ksi), and A514 (Fy=100ksi) steels. This combination of steel grades provides the large range of member capacities required in a truss without depth variation, thus avoiding inefficient and hence uneconomical sections. The A36 steel is used near the end supports and in the chord members near the points of inflection, as well as those diagonals subjected to small forces. Conversely, the A514 steel is used for chord members at the center of the main span and at the center supports and for some highly stressed diagonals near the center supports. The remaining members are composed of A572 steel. This is illustrated in Figure 5.1.

fig 5.2

A summary of the final design and construction requirements and material specifications is provided in HNTB's General Notes. The observant reader will notice that a revised earthquake acceleration factor (0.15g) was used in the final design. An interesting account of the seismic study for this bridge has been published in the Proceedings of the Third U.S. National Conference on Earthquake Engineering, held in Charleston in 1986, 100 years after the "big one." (ref. 6)

The compression members of the truss are closed-off box sections, fully welded at the steel fabricator, with diaphragm plates at the ends to seal out moisture, as shown in Figure 3.1. Tension members are welded H-sections, as shown in Figure 3.2. See Table 2a for the design loads used for the chords. Table 2b gives the member make up and cross sectional properties of the chords. See Table 3a for the design loads used for the diagonals. Table 3b gives the member make up and cross sectional properties of the diagonals. The use of such simple sections leads to clean lines and economies in fabrication and maintenance when compared with conventional truss bridges which feature built-up sections, pin connections, and lacing bars. The members are joined in the field with bolted connections. The lateral bracing system employs moment-resisting connections which reduce member sizes and further add to the neat appearance of the structure.

fig 3.1

fig 3.2

The concrete deck was designed to be fully continuous over the 1600 foot span by eliminating longitudinal expansion joints. This was accomplished by: (1) Fixing the two center stringers to the floorbeams and allowing the others to slide freely on teflon bearings; (2) Accommodating the secondary horizontal bending stresses in the floorbeams. To clarify this last point, see Figure 5.4 Assume the floorbeam spacing and panel point length along the bottom chord is L. As the truss deforms under load, the panel point length changes to L'. Measured at the floorbeam ends, the floorbeam spacing is now L', but measured at the floorbeam centerline, where the stringers are fixed, the spacing is still L. This deformation is accompanied by bending about a vertical axis, and it is these horizontal bending stresses that need to be accommodated.

fig 5.4

Elimination of expansion joints lessens the problem of corrosion of the steel supporting members and reduces maintenance costs. Epoxy-coated steel reinforcing bars are used in the deck slab to address the problem of rapid deterioration that has been a problem historically in bridge decks.

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