Strain Profiles and Girders Rotation
Based on the deformations measured by the PI gages, strain profiles were determined using the measured strain at the extreme top and bottom fibers of each bridge deck. It should be noted that all the strain profiles are plotted for the final loading cycle only, and therefore residual strains are shown at zero load. The strain profile obtained from the two PI gages located in the right span at 14 in. (356 mm) from the centerline of the deck (T6 and B10 in Figure 4) is depicted in Figure 16. The strain profile indicates that the top surface of the concrete at the vicinity of the punching area exceeded the limiting compressive strain value. The limiting compressive strain at a distance у from the edge of the loading plate reported by Kinnunen and Nylander (1960) was 0.0019, Marzouk and Hussein (1991) was 0.00215, and Mufti and Newhook (1998) was 0.002, where у is the distance from the bottom surface of the slab to the root of the shear crack at failure. The strain profile obtained from
Fig. 16. Strain profile from T6 and B10 PI gages for the first and third bridge decks.
Fig. 17. Strain profile from T8 and B12 PI gages for the three bridge decks.
the PI gages at the edge of the right span (T8 and B12 in Figure 4) is shown in Figure 17. The strain profile shows that the strain values were very small, which is another indication that the length of the bridge deck is effective and representative to the behavior of typical bridges.
Rotation of the three supporting girders was monitored throughout the test of the three bridge decks as shown in Figure 18. For all three decks the two outside girders exhibited larger rotations in comparison to the middle girder due to the unbalanced moment effect
Fig. 18. Rotation of supporting girders.
The predicted shear strengths for the three bridge decks according to the different design codes are given in Figure 19 as well as the experimental values. The design codes included are: American Association of State Highway and Transportation Officials (AASHTO), American Concrete Institute (ACI 318-02), and Ontario Highway Bridge Design Code (OBBDC, 1991). The equations used are as follows:
OHBDC: Vc = [0.6fr + 0.25fpc]b0d + 0.9Vp; units: N & mm,
where Vc = punching shear capacity of bridge deck; fic = ratio of long side to short side of loading plate; fc = concrete compressive strength; b0 = perimeter of critical section at a distance of d/2 from loading plate; d = effective section depth; as = constant; fr = concrete tensile strength; fpc = compressive stress in concrete due to prestressing; and Vp = component of effective prestressing force in direction of applied shear.
It is clearly seen from Figure 19 that the predicted values according to the AASHTO and ACI design codes predict very well the measured values for the bridge decks using MMFX and Grade 60 steel.