Characterization of Wall Assemblies

This task consists of a targeted experimental investigation focused on the three-dimensional web crushing behavior of structural wall assemblies. The assembly will be in the form of a rectangular wall system, or assembly, analogous to a hollow pier. A major objective of the pier tests is to evaluate the accuracy of the analytical models and the three-dimensional behavior of such wall assemblies, particularly when, under diagonal loading, the compression zone extends beyond the corner-confined element and into the structural walls. Therefore, a hybrid prototype pier that can be tested at a reasonable scale and that can guarantee to obtain elastic and inelastic web-crushing failures under cyclic loading will be created. The prototype is chosen after the Skyway piers of the new San Francisco-Oakland Bay Bridge but with equal longitudinal and transverse aspect rations, i.e., a square section. A 1/4-scale test unit is estimated to be within the capabilities of MAST.

The prototype cross-section of the pier test units is as shown in Figure 5. Both units will feature the same geometry but will use different concrete strengths, namely 34 and 137 MPa (5 and 20 ksi). Both units will be designed to fail in flexure-shear web crushing, with the difference that the first unit is to reach this failure limit at low ductility levels (mD ≈ 2), while the second unit is to reach this failure limit at moderate to high ductility levels, i.e., mD ≈ 6. Reinforcement details will be based on the findings of Task 1 and the assessment tools from Task 2.

Figure 5. Wall-Assembly Test Unit Cross-Section, Preliminary Dimensions and Reinforcement

The pier test unit will have an effective height of 3 m (10 ft). Testing of the unit will require provision of a heavily reinforced concrete footing, estimated to be approximately 2.4m x 2.4m x 1.2m (8 ft x 8 ft x 4 ft), and a reinforced concrete loading stub, with anticipated dimensions of 1.3m x 1.3 m x 0.61m (4.3 ft x 4.3 ft x 2 ft). The test unit footing will be post-tensioned to the strong floor of the MAST laboratory and the MAST cross-head will attach to the unit loading stub. Longitudinal and transverse loads will be applied by the horizontal actuators of the MAST system, while a constant axial load equal to 0.10f’cAg based on f’c = 34 MPa (5 ksi) will be applied with the vertical actuators attached to the MAST cross-head. A rendering of the test setup at the MAST laboratory is shown in Figure 6.

Figure 6. Rendering of Wall-Assembly Test Unit Setup in MAST Laboratory

The loading protocol taken for the SFOBB diagonal test will serve as a starting point and its applicability will be determined for the current project based on results from Task 1. The loading protocol was developed based on non-linear time-history analyses of the SFOBB Skyway for six different earthquake time histories, which showed that the piers were deformed along their principal and diagonal (bi-directional) axes and in a sweeping motion from one principal axis to another (Figure 7). The bi-axial loading protocol allows for comparison of damage and performance at specified displacement ductility levels for four major axes of rotation.

Figure 7. Preliminary Biaxial Loading Protocol for Wall-Assembly Tests

Instrumentation will consist of approximately 30 to 40 displacement transducers for curvature, shear deformation, and main displacement measurements. Only a minimal amount of strain gages will be used to evaluate the spread of plasticity within the section and identify the onset of yield. This focused instrumentation approach will permit faster construction of the test units and will produce information that is both compact and useful. In addition to the use of conventional instrumentation, deformation and strain measurement of the wall will also be done by means of digital image correlation (DIC) photogrametry.