CIVIL & STRUCTURAL ENGINEERING a shear failure (eg as shown Figure 1c) or yielding in flexure occurs (eg as shown in Figure 3c). Thus, the sort of shear damage depicted in Figure 1c starts at the support and moves over a few milliseconds towards the centre of the column until its rigid body motion is stopped. In a blast environment, RC columns are unlikely to act similarly to beams in their performance and the failure modes of most concern are likely to be quite different from those of concern for RC beams. Moreover, the key metric pertaining to the degradation of a column resulting from a blast loading is loss of axial capacity, which usually cannot be inferred from the lateral deflection of the column, as is implied by the continued use of ductility as damage criteria (Table 1). Furthermore, many column responses to blast are not well addressed with bending theory (eg as shown in Figures 3c/d/e), especially so for columns having large cross sections (Figure 3d).
COLUMN RESPONSE AND FAILURE MODES
A clear picture of RC column behaviours has emerged from the data garnered from the extensive series of tests described above, which were conducted by K&C over the last 20 years [9-17] to assess RC column performance in severe blast environments (ie more than 50 tests of full scale columns). RC COLUMN TEST RESULTS The wide array of data collected by K&C pertaining to the behaviours observed in tests of RC columns, and the verification of these behaviours in analytic studies conducted on HFPB FE models, provide a comprehensive and detailed picture of the kinds of behaviours likely to be encountered by RC columns subjected to blast effects loads. This data also depicts the complexities and types of modelling that need to be incorporated into the analyses that might be used to assess a column’s performance under a blast load, or to develop a design for a col-
umn for a specific blast resistance. All of these tests were performed on full-scale column specimens that included specimens that had both close and wide tie spacing, were bare (ie non-retrofitted) and retrofitted with FRP and steel jackets, and employed cross sections ranging in size from 1 foot to 3 feet. In many of these tests, the axial capacity of the column was measured after the test to definitively assess the level of degradation incurred (eg for a column like the one shown in Figure 3d, it is not obvious that it can still carry a large load). Of particular importance in these tests, was the use of full-scale specimens and boundary conditions reflective of the column’s placement in an actual structural system. These are key aspects of the test protocol and are crucial to the ability to obtain results with sufficient fidelity to the actual behaviours of columns when hit with VBIEDs or PBIEDs. The data pertaining to column behaviours, which were alluded to in the Introduction, represents results from four kinds of tests (Figures 1 to 6): • Building tests (Figures 1 and 2): These RC column tests were conducted using ground floor columns of an actual building (Figure 1a) which was designed by K&C to mimic a typical nonseismic flat-slab framing system. These tests are unique in that the columns were par t of an actual framing system (ie as compared to the component tests cited below). This data provided assurance that the setup used for the component tests (Figure 3) truly reflects the response, were the column embedded in an actual structural framing system. • CTRS tests (Figure 3):The column test reaction structure (CTRS) was designed [14-16] to provide a way to perform blast tests on RC columns at a much lower cost [ie as compared to the tests conducted when the column was
par t of the framing system (Figure 2)]. As such, CTRS provided a device to restrain the lateral and rotational motions at the top and base of the column, while still allowing an axial load to be applied so that the constraints and forces on the column would mimic those, had the column been par t of an actual building. • Powell Lab tests (Figure 4): Quasi-static tests were conducted in the lab using the same form of column specimen (Figure 5a) as used in the CTRS tests [11]. Here, the interest was in directly measuring the lateral load-resistance behaviour of the column and the effect of the inplane force impar ted to the column by the lateral deformation which is shown in Figure 5b. The lateral loads were applied using three hydraulic rams that were controlled in such a way as to apply the same force as would occur in a blast test, which was intended to mimic the uniform manner in which a blast might load a column. The column was constrained in rotation and laterally at its top and bottom suppor ts. An initial axial load of 100 kips was applied to the top of the column, and then its axial motion was constrained so as not to change during the test. Test parameters and results are listed in Table 2. • Tests on columns subjected to PBIED blast loads (Figure 6): Several forms of PBIED tests were conducted. In some situations, the same kind of full height column specimen, as used in the VBIED tests (Figure 3), was used, while in other tests, half-height columns were tested. The CTRS was used to hold many of columns in place, as shown in Figure 6. Documentation for these tests may be found in K&C reports [14-16]. The data from these tests was crucial in obtaining an understanding
August 2016 THE SINGAPORE ENGINEER
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