Browsing by Author "Robertson, Ian Nicol"

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    Seismic response of connections in indeterminate flat-slab subassemblies
    (1990) Robertson, Ian Nicol; Durrani, Ahmad J.
    Recent earthquakes have shown the vulnerability of flat-slab structures to severe ground motion. The failure in such structures typically initiates at the slab-column connections in the form of a punching failure. This investigation was carried out to evaluate the adequacy of current procedures for the design of slab-column connections. The variables studied included the intensity of gravity loading, slab shear reinforcement at the column line, slab overhang and stiff edge beam at exterior connections, and the indeterminacy of the connection subassembly. Nine half scale slab-column subassemblies were tested under simulated earthquake loading. Seven of the subassemblies simulated a single floor of a two-bay flat-plate structure. Each subassembly consisted of one interior and two exterior slab-column connections. The remaining two specimens were individual interior and exterior connections. All specimens were subjected to the same predefined displacement routine which consisted of twenty cycles of incremental displacements increasing to a maximum of seven percent drift. Increased slab gravity load significantly reduced the drift capacity of both interior and exterior connections. To achieve a lateral drift level of 1.5 percent prior to failure, the ultimate direct shear on flat-plate connections must be limited to $V\sb{u} \leq C\sb{d}\sqrt{f\sb{c}\sp\prime}b\sb{o}d$, where $C\sb{d}$ = 2.0 for exterior connections and $C\sb{d}$ = 1.4 for interior connections. The ACI code design approach for interior connections was unconservative for higher gravity load levels. For the range of shear stress levels studied in these tests, the ACI Committee 352 recommendations that moment and shear be treated independently for design of exterior connections appears reasonable. The ultimate direct shear capacity of the exterior connections in this study was $V\sb{u} = 2\sqrt{f\sb{c}\sp\prime}b\sb{o}d$. Closed hoop stirrups enclosing the slab flexural reinforcement passing through the interior connection prevented punching shear failure and increased the ductility of the connection. A stiff edge beam or slab overhang at the exterior connections increased both the strength and ductility of these connections. The behavior of the combined specimens was similar to a summation of the individual connections especially at drift levels less than 1.5 percent.
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    Shear strength of prestressed concrete t-beams with welded wire fabric as shear reinforcement
    (1985) Robertson, Ian Nicol; Durrani, Ahmad J.; Austin, Walter J.; Lutes, Loren D.
    In thin webbed prestressed concrete T-beams, the placement of conventional stirrups is difficult and time consuming. Welded wire fabric appears to provide a relatively economical alternative. In this study, the shear strength of prestressed T-beams reinforced with welded wire fabric as shear reinforcement is investigated. Thirteen large-scale bonded and unbonded prestressed T-beams were tested. The test variables included the type, amount and anchorage conditions of the shear reinforcement. These beams were tested under static third-point loading over a span of eleven feet. In all beams, shear failures occured along shallow diagonal tension cracks at loads in excess of the code predicted values. Anchorage of the welded wire fabric by means of two horizontal wires at top and bottom of the vertical web wires as recommended in the code performed well. However, the quality of the welded wire fabric was found to be critical for satisfactory performance.