bi directional loading). Prototype ten story

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1 NEESR SG: Behavior, Analysis and Design of Complex Wall Systems The laboratory testing presented here was conducted as part of a larger effort that employed laboratory testing and numerical simulation to advance understanding of the earthquake behavior, analysis and design of concrete walls. The research effort was sponsored by NSF through the NEES Research Program (Grant No ) with supplemental funding provided by the Charles Pankow Foundation. The following provides an over view of the experimental test program; details of the experimental test program are provided in the documents archived on NEEShub. Overview of the Test Program To investigate earthquake behavior and develop data to advance simulation and design, eight RC wall specimens were tested using the NEES laboratory at the University of Illinois ( Test specimens were onethird scale and represented the bottom three stories of a ten story prototype planar, planarcoupled or c shaped wall. Figure 1 shows the experimental test matrix, which was developed to provide understanding of the impact on performance of design details (splices and reinforcement distribution) and loading (shear demand, axial load resulting from coupling, and Figure 1: Experimental test matrix; rows and columns identify bi directional loading). Prototype ten story design parameters investigated through experimental testing. walls were designed using current codes (ACI and ASCE 7 10) and standard practice; test specimens represented the bottom three stories of the 10 story prototype walls. Figures 2 4 show specimen designs. 3" (TYP.) HOOKS OVERLAP TIE 2 (TYP.) Detail B Scale: Not to Scale #2 2" o.c. (TYP.) Notes: 1) boundary elements have 3.5% longitudinal reinforcement ratio, 2) longitudinal reinforcement is spliced at the base of the wall, 3) horizontal and vertical reinforcement ratios for the interior of the wall are 0.27%. Figure 2: Reference planar wall specimen (PW1). Additional planar walls specimen designs were identical (PW2), employed uniformly distributed longitudinal reinforcement (PW3), and employed longitudinal reinforcement that was not spliced by continuous from the wall to the foundation (PW4).

2 Figure 3: Planar coupled wall specimen (CW5) Figure 4: C shaped wall specimen design (UW6, UW7 and UW8). All specimens were nominally identical with the exception that the transverse reinforcement shown in boundary element A (above) included a crosstie for UW7 and UW8. Loading Protocols The advanced testing capabilities of the UIUC NEES laboratory were employed to develop a load distribution in the wall specimens representative of that which could be expected to develop in the

3 bottom stories of the prototype ten story walled building. Loads were applied using two of the loadand boundary condition boxes (LBCBs) available at the UIUC NEES laboratory. Each LBCBs can be used to apply load in six degrees of freedom. Loads were applied to the wall specimens using mixed mode control, with some degrees of freedom loaded to achieve a target displacement history and others loaded to achieve a target load history. All specimens were subjected to quasi static cyclic lateral loading and axial loading as follows: Planar rectangular wall specimens were subjected to a constant axial load (0.1f c A g ). Moment and shear were applied to the top of the specimen and shear loads were applied at the top of the first and second stories to achieve a prescribed cyclic lateral displacement history at the top of the wall and to maintain a moment shear ratio at the base of the wall that was consistent with either a uniform lateral load distribution (PW2 4) or the ASCE 7 05 equivalent lateral force (ELF) distribution for a building with uniform stiffness and mass (PW1). For the planar coupled wall specimen (CW5), moment, shear and axial were applied to the top of each wall pier to i) achieve a prescribed average cyclic lateral displacement history for the two piers, ii) maintain a constant axial load at the base of the wall, iii) achieve a moment shear ratio at the base of the wall that was consistent with the ASCE 7 05 ELF distribution, and iv) achieved a prescribed degree of coupling (DOC) in which 80% of the base moment was due to the tension compression forces developed in the wall piers. The target DOC as well as the shear and moment applied to the individual piers was established from nonlinear analysis of the prototype 10 story wall. C shaped wall specimen UW6 was subjected to a constant axial load (0.05f c A g ) and lateral loading parallel to the web of the wall, inducing strong axis bending. Moment and shear were applied to the top of the specimen to achieve a prescribed cyclic lateral displacement history at the top of the wall and to maintain a moment shear ratio at the base of the wall that was consistent with the ASCE 7 05 equivalent lateral force (ELF) for a building with uniform stiffness and mass. The top of the wall was restrained to prevent out of plane displacement and torsional rotation. C shaped wall specimen UW7 was subjected to a constant axial load (0.05f c A g ) and bidirectional lateral loading. Moment and shear were applied to the top of the specimen to achieve a prescribed cyclic lateral displacement history at the top of the wall and to maintain a momentshear ratio at the base of the wall that was consistent with the ASCE 7 05 equivalent lateral force (ELF) for a building with uniform stiffness and mass. Initially, the prescribed displacement history was cruciform in shape, with the wall loaded to a peak displacement demand in one direction while maintaining zero lateral displacement in the orthogonal direction). Towards the end of the test, the wall was subjected to displacement cycles parallel to the web of the wall while the displacement in the orthogonal direction was held at a constant, non zero, value. C shaped wall specimen UW8 was subjected to loading to develop load patterns within the wall representative of those which would develop in a coupled core wall system subjected to earthquake loading. Loads were applied to achieve a cruciform shaped lateral displacement history. For loading parallel to the web of the wall, which would not induce coupling action in a core wall system, the wall was subjected to a constant axial load (0.05f c A g ) and moment and shear were applied to maintain a moment shear ratio at the base of the wall that was consistent with the ASCE 7 05 equivalent lateral force (ELF) distribution. For loading perpendicular to the web of the wall, the specimen was subjected to different axial, moment and shear demands depending on whether the loading direction was such that the wall was acting as the tension or compression pier of the coupled system. Different axial, moment and shear load patterns were used for different phases of the test to simulate the impact of changes in wall stiffness and coupling action; these different load patterns were established through nonlinear analyses of a

4 core wall system. For most of the test, for each load cycle, the specimen was first loaded such that it represented the compression pier and the axial load, moment and shear demands at the peak displacement demand were recorded. These demands were then used to establish force targets, based on satisfying equilibrium for the entire core wall systems, for loading in the opposite direction, when the specimen represented the tension pier. Figures 5 7 show selected wall specimen in the UIUC NEES laboratory. Figure 5: Planar wall specimen PW2 in the UIUC NEES laboratory. Two blue and orange LBCBs are used to apply loads to the top of the specimen. For specimens PW2 PW3 and PW4, ancillary actuators were used to apply shear loads at the top of the first and second stories. Figure 6: Coupled planar wall specimen CW5 in the UIUC NEES laboratory. Blue and orange LBCB is used to apply loads to the top of each wall pier. Instrumentation and cameras are mounted on white and red steel sections. Figure 7: C shaped wall specimen UW6 in the UIUC NEES laboratory. Two blue and orange LBCBs are used to apply loads to the top of the specimen.

5 Experimental Data A large volume of experimental data was collected using a range of instrumentation system: Applied load. The loads and moments applied in all six DOF for each LBCB were recorded. The load applied by each of the six actuators in each of the LBCBs was also recorded. However, given that the orientation of each actuator changes at each step of the load history; LBCB resultant load histories are typically more useful than individual actuator loads. Absolute displacement field data. Two systems were used to generate displacement field data from portions of each wall specimen. The first system is the Nikon Metrology/Krypton Optical CMM system. This system employs LED targets that were attached to the specimen with a grid spacing of 8 to 12 inches, a three camera imaging system and proprietary software to determine target displacement in three dimensions. The second system is a close range digital photogrammetry system that employed high contrast paper targets attached to the specimen, Nikon digital cameras, and the PhotoModeler software to determine target displacement in two dimensions (i.e. out of plan displacements were not computed). For most specimens, displacement field data were generated for only a portion of the specimen surface. Displacement field data were used by the research team to generate strain field data as well as to computed relative rotation and shear and axial deformation for regions of the wall. Absolute displacement data. String potentiometers (string pots.) and linear voltage displacement transducers (LVDTs) were used to monitor the absolute displacement of various points on the specimen, including various points on the loading block at the top of the specimen, the three story wall region, and the foundation block. Relative displacement data. Steel rods were embedded in the specimen at various locations and LVDTs mounted on these rods were used to measure the relative movement of the rods. These data were used by the research team to compute the relative rotation and the shear and axial deformation of regions of the wall. Local strain data. Strain gages were attached to the surface of the specimen and to the surface of reinforcement embedded within the concrete specimen. These gages were used to monitor strains at critical locations. Concrete crack data. The orientation, length and width of a few representative and all critical cracks was measured at peaks in the displacement history and zero load points. Image data. Video and still cameras were used to collect image data throughout the test. Image data were used by the research team to characterize the damage state of the structure at various points in the load history. Additionally, for the planar wall specimens, concrete crack patterns determined from photographic image were combined with displacement field data to generate average crack width data. In addition to the above experimental data, detailed notes were taken during the experimental tests. These notes describe specimen response including the onset of various damage states. Observations about the Earthquake Behavior of Walls Data from the experimental tests combined with data from previous experimental tests conducted by others and with results from numerical simulation support a number of observations about the earthquake behavior of walls. The most significant of these are Slender walls typically achieve nominal flexural strength as defined by ACI ; but do not exhibit significant hardening. Modern slender concrete walls have the potential to exhibit compression controlled flexural failure, characterized by rapid strength loss, at relatively low drift demands. The potential for compression controlled flexural failure is exacerbated by increased compression demand

6 resulting from high axial loads (rare), coupling of two walls (common in mid to high rise buildings with multiple elevators) or an asymmetric wall cross section (common in low to midrise buildings). The potential for compression controlled flexural failure is exacerbated also by increase shear demand. The potential for compression controlled flexural failure is reduced in symmetric flanged configurations loaded such that the entire flange carries compression. Slender walls that exhibit significant loss of lateral load carrying capacity typically maintain moderate axial load carrying capacity. The location of a lap splice in the wall longitudinal reinforcement may determine the critical section on which damage accumulated and inelastic action initiates. If reinforcement is spliced at the base of the wall and the base of the wall is the point of maximum moment; flexural yielding will likely occur above the splice. For the planar walls tested as part of this study, in the post yield regime, total drift was due to rotation at the wall foundation interface (approx. 40%), flexural deformation elsewhere within the wall (approx. 20%), and shear deformation (approx. 40%). The planar coupled wall tested as part of this study exhibited rapid loss of lateral load carrying capacity due to simultaneous crushing of concrete and buckling of reinforcement in the compression pier. At the point of loss of lateral load carrying capacity, coupling beams had yielded but exhibited moderate damage (spalling of cover concrete). For the planar coupled wall, in the post yield regime, total drift was due to rotation at the wallfoundation interface (approx. 20% on average for the tension and compression piers), flexural deformation elsewhere within the wall (approx. 70% on average for the tension and compression piers), and shear deformation (approx. 10% on average for the tension and compression piers). For the tension pier, shear deformation exceed the average by 10% and flexural deformation trailed the average by 10%; this was reversed for the compression pier. For the c shaped walls tested as part of this study, i) loss of lateral load carrying capacity resulted from buckling and subsequent fracture of longitudinal reinforcement, ii) yielding and rupture of web reinforcement resulted in significant sliding at the wall foundation interface, and iii) bi directional loading resulted in reduced unloading reloading stiffness for strong axis bending.