H.J. Pam

Influence of transverse steel configuration on post-elastic behaviour of high-strength reinforced concrete columns

The influence of transverse steel configuration on the inelastic behaviour of high-strength reinforced concrete (HSRC) columns that were designed complying with the shear strength of BS 8110 (BS columns), the authors’ proposed equation (NEW columns) and the provisions of Design Guidance for High Strength Concrete (BS’ columns) has been studied experimentaily. Totally ten (five pairs) HSRC columns having concrete cube strength from 57 to 111 MPa were fabricated and tested under various levels of compressive axial load as well as reversed cyclic inelastic displacement excursions. Each pair of the columns contained almost identical cross-sectional properties, including concrete strength, applied compressive axial load level, content of longitudin’al steel and transverse steel, except the configuration of transverse steel. In the BS columns (two pairs) as well as the NEW columns (two pairs), one of the columns in each pair contained Intermediate cross ties along with single closed square hoops, which formed the transverse steel of the other column of the same pair. Ail end hooks of the transverse steel were bent 91l° in the BS columns, but 135° (forming 45° angle) in the NEW columns. In the BS’ columns, both of them contained intermediate cross ties in addition to single closed square hoops, but their end hooks were bent 91l° in one of the columns and 135° in the other column. It was evident from the results that: (1) the NEW columns with cross ties had ultimate deformability superior than their counterparts without crass ties, (2) adverse effects in flexural strength and ductility were found in the BS columns containing cross ties with moderate amount of longitudinal steel due to large transverse steel spacing, (3) 45° end hooks effectively delayed the inelastic buckling of longitudinal steel, and (4) all the NEW columns behaved in a limited ductile manner.

Effects of concrete grade and steel yield strength on flexural ductility of reinforced concrete beams

Abstract A previously established theoretical equation correlating the flexural ductility of a reinforced concrete beam section to the steel ratios and the concrete strength has been extended to account for the effects of the tension and compression steel yield strengths by means of a parametric study. Through the study, it was revealed that at a fixed degree of the beam section being under-or over-reinforced, the flexural ductility decreases slightly with the tension steel yield strength as well as the concrete strength but increases slightly with the compression steel yield strength. From charts plotting the flexural strength and flexural ductility that could be simultaneously achieved by a beam section, it was evident that the use of a higher concrete strength could increase either the flexural strength or the flexural ductility or both. On the other hand, at a given flexural strength, a higher tension steel yield strength would lead to lower flexural ductility while a higher compression steel yield strength would lead to higher flexural ductility. Lastly, a simple method of designing beam sections to meet any specified flexural ductility requirement has been proposed.

Effects of Steel Lap Splice Locations on Strength and Ductility of Reinforced Concrete Columns

Longitudinal steel lap splice is always required in reinforced concrete (RC) columns. Normally, in countries having high seismic risk, lap splices of longitudinal steel must be located around mid-height of the storey. However, in regions of low-medium seismic risk, including Hong Kong, lap splices of longitudinal steel begin right above the beam-column interface to facilitate ease of construction. Such splicing method would undesirably cause the column critical region to move away from the beam-column interface under inelastic deformation. In this paper, the effects of different lap splice locations of longitudinal steel on flexural strength and ductility of RC columns with concrete cube strength around 100 MPa are studied. Four RC column specimens, which contained no lap splice, all lap splices within and outside critical region, as well as lap splices in staggered manner, were tested under simultaneous compressive axial load and reversed cyclic inelastic displacement. It is evident from the results that the column containing lap splices within its critical region had the largest strength but the poorest ductility performance. On the contrary, the column containing lap splices outside its critical region had strength and ductility comparable to those of the column without lap splice. Based on these observations, a recommendation is proposed for positioning longitudinal steel lap splices in RC columns.

Extent of Critical Region and Limited Ductility Design of High-strength Reinforced Concrete Columns for Hong Kong Practice

Extent of critical region, which is usually referred to as plastic hinge length, of high-strength reinforced concrete (HSRC) columns, where extensive inelastic damages might occur under seismic loading, was experimentally investigated. A total of 6 HSRC columns that can be grouped into 2 series or 3 pairs, were tested under various axial load levels as well as reversed cyclic inelastic displacement excursions simulating seismic loading. The first series of columns contained transverse steel designed according to the shear resistance requirement of BS 8110, while the second series, representing limited ductile columns, to the authors’ proposed equation. From test observation, progressive development and formation of the critical region in these columns were discussed. The extents of these critical regions were subsequently evaluated rigorously using direct (by physical observation or measured column curvature profiles) and indirect (by back calculation using an idealised column curvature profile) methods. From the results, it was evident that: (1) the critical region developed starting from the point of maximum bending moment over a finite length along the concrete spalling area, (2) the extent of critical region was largely influenced by volumetric ratio of transverse steel, concrete strength and axial load level, (3) the extents of critical region evaluated using both methods were in good agreement, (4) the HSRC columns containing transverse steel calculated using the authors’ proposed equation proved to behave in a limited ductile manner, and the extent of their critical regions was smaller than that of the counterpart columns designed complying with BS 8110. Finally, some guidelines for the design of limited ductile HSRC columns were proposed.

Effects of Using High-strength Concrete on Flexural Ductility of Reinforced Concrete Beams

The complete moment-curvature behavior and flexural ductility of reinforced concrete beams made of normal- or high-strength concrete have been analyzed by a newly developed theoretical method that uses the actual stress-strain curves of the materials and takes into account the strain reversal of the tension reinforcement in the analysis. It was found that as expected the flexural ductility decreases with the tension steel ratio and increases with the compression steel ratio. However, the variation of the flexural ductility with the concrete grade is quite complicated; at fixed tension and compression steel ratios, the flexural ductility increases as the concrete grade increases but at a given degree of being under- or over-reinforced, the flexural ductility decreases as the concrete grade increases. In order to better reveal the combined effects of the steel ratios and the concrete grade, the flexural ductility has been plotted against the flexural strength for different steel ratios and concrete grades. From the graphs plotted, it can be clearly seen that the use of a higher grade concrete could increase flexural ductility at same flexural strength, increase flexural strength at same flexural ductility or increase both flexural strength and flexural ductility.