Behaviour of precast reinforced concrete columns in moderate seismic conditions

© 2011 Dr. Bidur KafleThis research project contributes towards fulfilling a long-term objective of appropriate seismic evaluation of building structures in regions of low to moderate seismicity such as Australia. Structures such as unreinforced masonry (URM) walls, soft-storey buildings, gravity structures and non-structural components which include free-standing objects are well known to be non-ductile and yet commonly found in regions of low-moderate seismicity. Potential significant degradation in strength in these structural systems in projected earthquake scenarios has been a cause for concern. As the structure, or component, is excited into large displacement (and experiences “rocking” behaviour pertaining to overturning) its effective natural period value is expected to be lengthened into the high period range well above the initial natural period value (as a result of strength and/or stiffness degradation). Consequently, the risk of collapse of the structure is essentially controlled by response spectral parameters in the high period range. Therefore, displacement-controlled behaviour phenomenon applicable to conditions of low to moderate seismic regions has formed the basis for performance evaluation of the structures in this thesis. When the conditions of displacement controlled behaviour are reached the seismic displacement demand on the structure, or component, can become insensitive to any further increase in the natural period of the structure. Seismic assessment is hence much simpler and direct, particularly when the natural period of the structure is variable or difficult to estimate. According to this phenomenon, structures are deemed seismically safe if their displacement capacity exceeds the imposed displacement demand irrespective of their strength and energy dissipation capacity. Results of the tests and complementary analytical simulations of rigid body objects experiencing rocking behaviour revealed that earthquake induced overturning hazards were best represented by the peak displacement demand (PDD) parameter. The increase of probability of overturning with decreasing size of the object has been clearly established from developed fragility curves, importantly demonstrating size effect phenomenon. A conservative model based on the worst scenario of magnitude 7 earthquake and class D site for 5% probability of overturning has also been developed. The understanding of rocking behaviour of rigid objects as observed from the shaking table experiments and analytical simulations was extended to precast reinforced concrete (RC) columns supporting soft-storey system which are nominally connected to the rest of the structure. Field tests conducted on a four-storey soft-storey building supported by precast RC columns in Melbourne, Australia revealed that the columns maintained their gravity load carrying capacity up to a drift of about 8% under quasi-static conditions. The force-displacement relationships of the columns as estimated by a numerical model in Ruaumoko (Carr, 2008) have been found to be in good agreement with those observed from the field tests. It has been found from the developed fragility curves that the probability of failure of precast columns decreases with increasing size of the column, demonstrating the size effect phenomenon. A comparative evaluation study of one low rise (4 storey above ground level) and a high rise (20 storey above ground level) buildings, each with a soft-storey at ground floor level, was conducted which involved assessing existing building stocks using both the conventional force-based procedure as stipulated by current codes of practice (AS1170.4, 2007; AS 3600, 2009) and the more realistic displacement-based procedure which involved non-linear THA of SDOF systems. The risks of collapse of the twenty-storey building were found from the dynamic analyses to be much lower than that of the four-storey building. However, both buildings failed to comply with conventional force-based design requirements. The displacement based approach has highlighted the importance of the effect of size upon the vulnerability to failure of precast RC columns which has not been well captured by conventional force based method

Influence of Hybrid Basalt Fibres’ Length on Fresh and Mechanical Properties of Self-Compacted Ambient-Cured Geopolymer Concrete

Recently, short basalt fibres (BFs) have been gaining considerable attention in the building materials industry because of their excellent mechanical properties and lower production cost than their counterparts. Reinforcing geopolymer composites with small volumes of fibres has been proven an efficient technique to enhance concrete’s mechanical properties and durability. However, to date, no study has investigated the effect of basalt fibers’ various lengths and volume content on self-compacted geopolymer concrete’s fresh and mechanical properties (SCGC). SCGC is prepared by mixing fly ash, slag, and micro fly ash as the binder with a solid alkali-activator compound named anhydrous sodium metasilicate (Na₂SiO₃). In the present study, a hybrid length of long and short basalt fibres with different weight contents were investigated to reap the benefits of multi-scale characteristics of a single fibre type. A total of 10 mixtures were developed incorporating a single length and a hybrid mix of long (30) mm and short (12) mm basalt fibres, with a weight of 1%, 1.5% and 2% of the total binder content, respectively. The fresh and mechanical properties of SCGC incorporating a hybrid mix of long and short basalt fibres were compared to plain SCGC and SCGC containing a single fibres length. The results indicate that the hybridization of long and short fibres in SCGC mixture yields better mechanical properties than single-length BF-reinforced SCGC. A hybrid fibre coefficient equation will be validated against the mechanical properties results obtained from the current experimental investigation on SCGC to assess its applicability for different concrete mixes

Influence of Hybrid Basalt Fibres’ Length on Fresh and Mechanical Properties of Self-Compacted Ambient-Cured Geopolymer Concrete

Recently, short basalt fibres (BFs) have been gaining considerable attention in the building materials industry because of their excellent mechanical properties and lower production cost than their counterparts. Reinforcing geopolymer composites with small volumes of fibres has been proven an efficient technique to enhance concrete’s mechanical properties and durability. However, to date, no study has investigated the effect of basalt fibers’ various lengths and volume content on self-compacted geopolymer concrete’s fresh and mechanical properties (SCGC). SCGC is prepared by mixing fly ash, slag, and micro fly ash as the binder with a solid alkali-activator compound named anhydrous sodium metasilicate (Na₂SiO₃). In the present study, a hybrid length of long and short basalt fibres with different weight contents were investigated to reap the benefits of multi-scale characteristics of a single fibre type. A total of 10 mixtures were developed incorporating a single length and a hybrid mix of long (30) mm and short (12) mm basalt fibres, with a weight of 1%, 1.5% and 2% of the total binder content, respectively. The fresh and mechanical properties of SCGC incorporating a hybrid mix of long and short basalt fibres were compared to plain SCGC and SCGC containing a single fibres length. The results indicate that the hybridization of long and short fibres in SCGC mixture yields better mechanical properties than single-length BF-reinforced SCGC. A hybrid fibre coefficient equation will be validated against the mechanical properties results obtained from the current experimental investigation on SCGC to assess its applicability for different concrete mixes

Effect of Fibre Orientation on the Bond Properties of Softwood and Hardwood Interfaces

Increasing concerns regarding carbon emissions and climate change are prompting a shift toward the use of sustainable materials in the construction industry. Engineered timber products are gaining attention in the construction industry due to advancements in lamination techniques and adhesives as well as the renewable characteristics of wood. Bond properties play a significant role in engineered timber products. In Australia, Radiata Pine (RP, softwood) and Shining Gum (SG, hardwood) share a large proportion of local and native plantation forest resources. The present paper investigates the bond behaviours of Australian softwoods (RP–RP), hardwoods (SG–SG) and hybrid-wood (RP–SG) combinations in both parallel (PAL) and perpendicular (PER) bonding directions using one-component polyurethane adhesives. The results indicate that most of the softwood samples were subjected to wood-side (timber) failure, whereas hardwood samples failed due to delamination but exhibited higher strength and stiffness regardless of bond direction. In contrast, bond direction had a significant effect on the bond characteristics of hybrid configurations. Improved bond properties were observed when bonded in PAL directions; however, negative effects were seen when bonded in PER directions. Obtained characteristic (5th percentile) shear bond strengths for RP–RP–PAL, RP–SG–PAL and SG–SG–PAL samples were 3.88 MPa, 6.19 MPa and 8.34 MPa, whilst those for RP–RP–PER, RP–SG–PER and SG–SG–PER samples were 3.45 MPa, 2.96 MPa and 7.83 MPa, respectively

Appendix B: A report on the visit to the region stricken by the Wenchuan Earthquake

Earthquake ground shaking is one of the environmental actions to be considered in structural design. In regions of high seismicity, the design of structural elements is usually governed by load combinations which include seismic loading. The effects of an earthquake on non-structural elements such as damage to facades, glass, plasters, veneers, gas and water pipes, etc. can also be a significant issue even in the regions with low-to-moderate seismic activity. The Wenchuan Earthquake with the moment magnitude of 7.9 (reported by USGS) occurred in Sichuan Province in China on May 12, 2008 at local time 14:28. As of July 2 2008, 69,195 people have been killed, 373,606 injured, 18,389 missing, 5 million homeless, and 79,852 rescued (Ref. 3). To investigate damage to infrastructure caused by this earthquake, a group of 8 researchers (2 from The University of Melbourne, 1 from Swinburne University of Technology and 5 from The University of Hong Kong) was sent to Sichuan, China in late June-early July 2008 (about 50 days after the event) for visiting the city of Chengdu, Dujiangyang, YingXiu and Mianyang cities which have suffered different degrees of damage. This report is mainly on the observations of the damaged area. The aim herein is to classify damage based on building types, modes of failure, distance from the epicentre and relevant ground motion parameters

A Critical Review of Cold-Formed Steel Built-Up Composite Columns with Geopolymer Concrete Infill

Concrete-filled built-up cold-formed steel (CFS) columns offer enhanced load-carrying capacity, improved strength-to-weight ratios, and delayed buckling through providing internal resistance and stiffness due to the concrete infill. Integrating sustainable alternatives like self-compacting geopolymer concrete (SCGC) with low carbon emissions is increasingly favoured for addressing environmental concerns in construction. This review aims to explore the current knowledge regarding CFS built-up composite columns and the performance of SCGC within them. While research on geopolymer concrete-filled steel tubes (GPCFSTs) under various loads has demonstrated high strength and ductility, investigations into built-up sections remain limited. The literature suggests that geopolymer concrete’s superior compressive strength, fire resistance, and minimal shrinkage render it highly compatible with steel tubular columns, providing robust load-bearing capacity and gradual post-ultimate strength, attributed to the confinement effect of the outer steel tubes, thereby preventing brittle failure. Additionally, in built-up sections, connector penetration depth and spacing, particularly at the ends, enhances structural performance through composite action in CFS structures. Consequently, understanding the importance of using a sustainable and superior infill like SCGC, the cross-sectional efficiency of CFS sections, and optimal shear connections in built-up CFS columns is crucial. Moreover, there is a potential for developing environmentally sustainable built-up CFS composite columns using SCGC cured at ambient temperatures as infill