Page 189 - Bridge and Highway Structure Rehabilitation and Repair

164 SECTION 2 STRENGTHENING AND REPAIR WORK

Table 4.5 Selection of HS-20 and alternate HL-93 tandem truck moments and forces for smaller increments in span.
Max. Truck Moments Max. Truck Shear/Reactions
Span †
Author’s Formula* Tandem Governing Truck Author's
Length HS-20 Two 25 Kip axles Moments Formula** Tandem Two 25 Governing Truck
(Feet) (Kip-Ft) (Kip-Ft) (Kip-Ft) HS-20 (Kips) Kip Axles (Kips) Shear (Kips)
20 28’distance between 200 moments 200 38.4 45 45
outer axles> 20’
formula N/A
22 133.85 225 225 41.45 45.45 45.45 Tandem governs
24 168.38 250 250 44 45.83 45.83
26 203.12 275 275 46.15 46.15 46.15
Small spans
28 238.05 300 300 Tandem governs 48 46.43 48
30 273.12 325 325 49.6 46.67 49.6
32 308.31 350 350 51 46.88 51
34 343.59 375 375 52.24 47.06 52.24
36 378.95 400 400 53.33 47.22 53.33
38 414.38 425 425 54.32 47.37 54.32
40 449.87 450 450 55.2 47.5 55.2 HS-20 governs
42 485.40 475 485.4 56 47.62 56
45 538.78 512.5 538.78 57.07 47.78 57.07
50 627.91 575 627.91 58.56 48 58.56
55 Medium spans 717.20 637.5 717.20 HS-20 governs 59.78 48.18 59.78
60 806.61 700 806.61 60.8 48.33 60.8
65 896.11 762.5 896.11 61.66 48.46 61.66
70 985.68 825 985.68 62.4 48.57 62.4
75 1075.31 887.5 1075.31 63.04 48.67 63.04
* M max 3 18 (L 6 1.55)(L 6 14)/Lkip-ft
** V max 3 24(3L 6 28)/L kips

Advantages of rolled sections are thicker webs, and stocky, compact sections. Ready
availability due to mass production is the reason for their widespread use for smaller spans.
Due to thicker webs, the use of transverse web stiffeners is avoided. Weathering steel helps
in maintenance costs through minimal painting.
The disadvantages are using a uniform fl ange thickness along the span which is uneco-
nomical since bending moment is maximum at midspan but reduces along the length.
2. Rolled sections with cover plates.
With single or two layers of plates, the application of rolled sections can be extended to
spans of about 80 feet. It has been a practice in the past to weld cover plates in regions of
peak positive and negative bending moments. This resulted in economical design compared
to using rolled sections only. However, tension welds at midspan and at supports are sub-
jected to fatigue and need to be checked. Inspection of welds using NDT techniques and
maintenance of fatigue prone details for older bridges are expensive. Current practice is to
avoid this approach in favor of fabricated girders.