Review of Previous Failures 93

Compression shear failures of the type observed in the NCHRP 12-56 testing program were not reported in the reviewed literature as presented in Chapter 3. In this section, the type of experiments and failures observed by Ma and Tadros [7-1, 7-2], Bruce, Russell, and Roller [7-3, 7-4], and Shahawy and Batchelor [7-5] are reviewed. Several key differences exist between the tests that were completed for NCHRP 12-56 and tests that were completed by the

aforementioned researchers. Key elements of the NCHRP 12-56 testing program are presented and compared to the key elements of the work of others.

7.1.1 Loading

The bridge girders tested for NCHRP 12-56 were subjected to uniformly distributed loading applied by a series of 44 hydraulic cylinders over the central 44 feet of a 50 foot span. Most other previously reported work on full size prestressed concrete members have utilized concentrated loads that were applied at relatively short shear span-to-depth ratios in order to generate shear failures prior to flexural failures. The proximity of the load application point to the support significantly influences the amount of shear a member is able to resist as the direct compressive strut that forms between load and support can support a large portion of the shear force while placing relatively little demand on the transverse reinforcement. In addition, it can be shown from elastic analyses that loading via a point load would not produce the level of

combined high shear and compression at the base of the web that can be expected in the field and that can lead to the types of failures observed in the NCHRP 12-56 testing program.

Ma, Tadros, and Baishya completed four shear tests on NU1100 I-girders with an overall height of 51 inches including a 7.5-inch thick concrete composite deck. Members were simply supported on a span that varied between 61 and 76 feet and the loading was applied using 3 hydraulic cylinders located at distances of 4.5, 8.5 and 12.5 feet from the midpoint of the support.

Bruce, Russell, and Roller carried out 6 shear tests on 73-inch bulb-tee bridge girders that were topped with an 8-inch thick concrete composite deck. The members were simply supported

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on a span that ranged from 43 to 46 feet. Load was applied using three pairs of hydraulic cylinders located 10, 13 and 16 feet from the midpoint of the support.

Shahawy and Batchelor performed tests on 33 AASHTO type II girders that had an overall height of 44 inches that included an 8 inch concrete composite deck. Two tests were performed on each girder; the span for the first test was 40 feet while the span length for the second test varied according to the level of damage that was sustained during the first test. The tests were completed by applying load with a single hydraulic actuator near the support with shear span-to-depth ratios that ranged from 1.3 to 4.8.

7.1.2 Web Dimensions

The NCHRP 12-56 tests were carried out on full-scale 63-inch bulb-tee girders. Bulb-tee girders have larger web depths than AASHTO type girders. It is hypothesized that the aspect ratio of the web (width-to-depth) impacts the flow of forces near the support and may influence the failure mode. The web width-to-depth ratios for all of the aforementioned girder types are presented in Table 7.1. Values range from 0.111 to 0.4 and well represent the range of values of most precast bridge girders used in the field.

Table 7.1 Web Width-to-Depth Ratios

Girder Type Web Depth_(inches) Web Width_(inches) Width _Depth

BT 63 45 6 0.133

BT 72 54 6 0.111

AASHTO type II 15 6 0.4

NU1100 15.2 5.9 0.388

7.1.3 Level of Prestressing

The level of prestressing applied to the NCHRP 12-56 girders was significantly higher than what was applied to girders in the other experimental programs. The NCHRP 12-56 girders were over designed in flexure by a factor of between 1.1 and 1.4 and intended to fail in shear under a uniformly distributed load. The application of shear through a point load located near the support as used in generating the shear failures in most other experimental programs reduced the need for increasing the flexural resistance. Table 7.2 shows the maximum area of

prestressing steel that was placed in the bottom bulb of a girder for each of the 4 testing

programs. All stands were stressed to a level of 0.75 fpu for fabrication; actual values of fpe are

not listed because they were not reported for all members.

Table 7.2 Maximum Area of Prestressing Steel

Research Program _Strands# of Strand Diameter_(inches) Total Area _(in2₎

NCHRP 12-56 44 0.6 9.55

Ma, Tadros, and Baishya 38 0.5 5.81 Bruce, Russell, and Roller 24 0.6 5.21 Shahawy and Batchelor 16 0.5 2.45

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7.1.4 Concrete Strength

The NCHRP 12-56 tests were completed to examine the shear behavior of members cast with high-strength concrete. The members were also designed to take advantage of the higher shear stress ratios permitted in the LRFD provisions. The combination of these two effects resulted in test beams that needed to be designed to support very high shear design stress levels. Table 7.3 shows the range of maximum compressive strength of concrete used in the all four experimental programs.

Table 7.3 Maximum Concrete Compressive Strength Research Program

Maximum Concrete Compressive Strength

(ksi) NCHRP 12-56 17.8 Ma, Tadros, and Baishya 10.8 Bruce and Russell 12.7 Shahawy and Batchelor 7.2