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Application of the proposed methodology to girder tests completed by other researchers is made in order to further access the ability of the method to evaluate the likelihood of shear compression failures. Unfortunately, detailed test information is not available from these experimental programs and assumptions must be made to complete the evaluation.

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7.4.1 Ma, Tadros, and Baishya – NU1100 Members

Two failure modes were observed in the experimental work completed by Ma, Tadros, and Baishya. The first failure mode was described as a web crushing failure which resembles the shear compression failures observed in the NCHRP 12-56 testing program; the second observed failure mode was referred to as a shear-bond failure. Photographs of both failure modes are shown in Figure 7.13.

(a) Web Crushing Failure (b) Shear Bond Failure Figure 7.13 Failure Modes from Shear Tests on NU1100 Members

(taken from Reference 7-1)

In order to obtain the geometric details for calculating the web-flange interface shear stress ratio, assumptions needed to be made regarding the dimensions and angle of the strut funneling load into the support. An angle of 43.3 degrees was assumed for the angle of the strut directly over the support. This value is based on geometry and the location of the load

application nearest to the support and assumes that a direct strut forms between the first load application point and the support. The load closest to the support was applied 54 inches from the midpoint of the bearing pad while the overall height of the member was 51 inches. A bearing plate that was 12 inches in length along the longitudinal direction of the girder was placed at the end of the girder. The distance that the girder extends past the bearing pad is assumed to be zero based on photographic figures in reference 3-6. End blocks were cast after the fabrication of the members to provide additional strand anchorage for the bent-up strands. The distance that the end block extended past the bearing pad was not included in the determination of length of the strut along the web-flange interface because it was not cast integral with the member and it would not significantly contribute to the shear stress that could be resisted at the interface.

Following the procedure detailed in Section 7.3.1, the normal and shear stresses at the web-flange interface over the support are evaluated. For the tests completed on the NU1100 members the geometry in the end region can be described as follows: α = 43.3 degrees, le = 0

inches, lb = 12 inches, df = 16.5 inches, and tw = 5.9 inches. These values were used in

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Table 7.7 Summary of Shear Tests on NU1100 Members Specimen Designation Failure Mode R (kips) Concrete Strength (ksi) σn / f’c vwfi / f’c AR05908X web crush 629.5 8.1 0.45 0.47 AVW14408X web crush 593.0 8.1 0.42 0.45 BVW20408X web crush 589.8 10.8 0.31 0.33 BOW20408X did not fail 820.3 10.8 0.44 0.46 AVW14608Y shear-bond 459.9 8.1 0.33 0.35 AR05908X AVW14408X BVW20408X BOW20408X AVW14608Y 0.00 0.10 0.20 0.30 0.40 0.50 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 σn / f'c vwf i / f 'c α=43.3º, K=0.84 Eq. 7-11

Figure 7.14 Plot of Normalized Values of vwfi and σn for NU1100 members

Table 7.7 provides basic information detailing the failure modes of the members that were tested. Figure 7.14 is a plot of the normalized values of vwfi and σn for the members test by Ma, Tadros,

and Baishya. As expected, members with high normalized shear stresses at the web-flange interface failed due to shear-compression or web-crushing type failures. Member BOW20408X exhibited a high normalized horizontal shear stress in the end region but was not taken to failure because the capacity of the test equipment was reached. It should be noted that member

BOW20408X had a grid of welded wire that provided reinforcement in the transverse and longitudinal directions. As discussed previously, the failure mode of members with normalized horizontal shear stress ratios near 0.30 is dependent on the tensile capacity of the longitudinal reinforcement located below the web-flange interface. Table 7.7 indicates that member

AVW14608Y experienced a shear-bond failure while member BVW20408X failed due to web crushing.

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It would be beneficial to follow the procedures detailed in Sections 7.3.2 and 7.3.3 to further investigate members AVW14608Y and BVW20408X but this is exceedingly difficult due to a lack of experimental information including the measured yield strength of the reinforcement and measured values for the level of prestress in the strands after losses. Additionally, the external strand anchorage makes it difficult to assess the longitudinal tensile capacity for

member BVW20408X and almost ensures that it will not fail due to a loss of longitudinal tension capacity.

7.4.2 Bruce, Russell, and Roller – LTRC 72-inch Bulb-Tees

Results reported by Bruce, Russell, and Roller indicate that two types of failures were observed. The first was described as a strand slip failure while the second type was related to distress at the web-bulb interface immediately in front of the support. The interface failures are relatively localized and do not display a significant amount of concrete crushing as was observed in shear-compression failures on the NU1100 members or in the NCHRP 12-56 tests. A

photograph of the failure for specimen BT8-Dead is shown in Figure 7.15.

Figure 7.15 Interface Failure from a Shear Test on Specimen BT8-Dead (taken from Reference 7-3)

Again, it was necessary to make assumptions about the geometry associated with the end conditions of the members. The angle, α, was assumed to be 40 degrees since the geometry of the 72-inch bulb-tee, with the exception of overall height, is identical to that of the 63-inch bulb- tee used in the 12-56 experiments. Additionally, the first load point was located 120 inches from the midpoint of the support. At this distance only a portion of the load would be transmitted to the support via direct strut. Exact dimensions of the conditions of at the support were not

provided in the reports. It was however indicated that the mid-point of the support was located 6 inches from the end of the member. The length of the bearing plate aligned with the longitudinal axis of the girder is assumed to be 12 inches for the purpose of accessing the stresses at the support.

Once again, and following the procedure detailed in Section 7.3.1, the normal and shear stresses at the web-flange interface over the support are calculated. For the tests completed on the LTRC 72-inch bulb-tees, the geometry in the end region can be described as follows: α = 40

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degrees, le = 0 inches, lb = 12 inches, df = 10 inches, and tw = 6 inches. These values were used

in conjunction with equations 7-12 through 7-13 to generate Table 7.8 and Figure 7.16. Table 7.8 Summary of Shear Tests on LTRC 72-inch Bulb-Tees

Specimen Designation Failure Mode R (kips) Concrete Strength (ksi) σn / f’c vwfi / f’c BT6 live strand slip 592 11.8 0.35 0.42 BT6 dead strand slip 557 11.6 0.33 0.40 BT7 live did not fail 614 12.4 0.35 0.41 BT7 dead interface failure 605 12.7 0.33 0.39 BT8 live did not fail 599 11.9 0.35 0.42 BT8 dead interface failure 564 11.3 0.35 0.41

Table 7.8 provides a basic summary of the failure modes and support reactions of the members that were tested. Figure 7.16 is a plot of the normalized values of vwfi and σn for the members

test by Bruce, Russell, and Roller. Normalized horizontal shear stress values ranged between 0.39 and 0.42 for all six members that were tested and are well within the range where the predicted failure mode is based on the capacity of the longitudinal tensile reinforcement.

BT6-Live BT6-Dead BT7-Live BT7-Dead BT8-Live BT8-Dead 0.00 0.10 0.20 0.30 0.40 0.50 0.00 0.10 0.20 0.30 0.40 0.50 σn / f'c vwfi / f'c α=40º, K=0.84 Eq. 7-11

Figure 7.16 Plot of Normalized Values of vwfi and σn for LRTC 72-inch Bulb-Tees

However, it is worth noting that the assumption of the bearing plate dimensions has a direct impact on the calculation of these values and if the bearing plate is larger than was assumed the stresses would decrease accordingly.

Equations 7-15 and 7-16 are applied to the LTRC members as described in Section 7.3.2. Table 7.9 presents the details of the calculations. In all cases the horizontal shear demand

exceeds the longitudinal tensile capacity of the reinforcement at the support. This indicates that all of the LTRC members are expected to fail in a manner characterized by insufficient

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Table 7.9 Horizontal Shear Demand and Longitudinal Tensile Capacity of LTRC Members Specimen Designation R (kips) Apsf (in2) fse (ksi) Ast (in2) fy (ksi) T (kips) Vwfi (kips) T-Vwfi (kips) BT6 live 592 6.944 143.2 - - 559.5 705.5 -146.1 BT6 dead 557 5.642 143.2 - - 559.5 663.8 -104.4 BT7 live 614 6.944 142.4 1.76 65.5 671.4 731.7 -60.4 BT7 dead 605 7.812 142.4 1.76 65.5 671.4 721.0 -49.6 BT8 live 599 7.812 142.4 1.76 65.5 671.4 713.9 -42.5 BT8 dead 564 7.812 142.4 1.76 65.5 671.4 672.1 -0.8

longitudinal tensile capacity which is consistent with the reported failure modes observed in the experimental program. BT7-Dead and BT8-Dead both displayed minor interface distress at failure but as mentioned before it was not as extensive as the shear compression interface failures displayed by the NU1100 members and the 12-56 tests.