Minimum Vertical Reinforcement Research Articles

Sectional response of non-rectangular concrete walls with minimum vertical reinforcement

Past research that investigated the behaviour of rectangular lightly reinforced concrete walls resulted in revisions to minimum vertical reinforcement provisions in concrete design standards in both New Zealand (NZS 3101:2006) and the United States (ACI 318-19). However, the minimum vertical reinforcement provisions developed for rectangular wall sections may not be suitable for non-rectangular walls due to the influence of flanges on the nominal flexural and cracking section capacities. A parametric study confirmed that non-rectangular wall sections with minimum vertical reinforcement in accordance with current NZS 3101 design provisions exhibit a lower margin between cracking and nominal flexural strength than comparable rectangular wall sections. The ratio of the sectional nominal flexural strength to cracking strength (Mn/Mcr ) was less than 1.0 for non-rectangular sections with long flange lengths and low axial loads. The model results indicated that current vertical reinforcement requirements are insufficient to prevent a sudden and potentially unstable strength drop when cracking occurs in non-rectangular walls. A theoretical equation to calculate the required minimum vertical reinforcement was proposed for the typical I-shaped wall sections, including the impact of concrete strength and flange to web ratio. The proposed equation highlighted the need for an increase in the minimum vertical reinforcement limits for non-rectangular wall sections compared to the existing minimum vertical reinforcement requirement applicable to rectangular wall sections.

Shear behavior of concrete panels reinforced with GFRP bars under cyclic diagonal tension tests

Glass Fiber Reinforced Polymer (GFRP) bars aims to improve the properties of concrete conventional reinforcement and to reduce the weaknesses of steel rebars in terms of corrosion and temperature changes. In this study, GFRP bars were evaluated as a replacement of steel bars in shear walls. Diagonal tension test under unreversed cyclic quasi-static load on Reinforced Concrete (RC) panels was used. Twenty-one concrete panels with a square section, 600 mm on each side and 100 mm thick, reinforced with distinct types and reinforcement ratios were evaluated. Two types of GFRP bars with different surface coating were used: epoxy Helical wrap (GFH), and Sand coated (GFS). For comparison purposes, RC panels with steel Welded Wire Mesh (WWM) were also assessed. The minimum vertical and horizontal reinforcement required by ACI 318 (0.25%) and half of it (0.125%) were considered. The research analyzed the hysteresis curves in terms of shear stress-strain (τ-γ) and the parameters associated with shear strength, shear strain, crack width, stiffness, and toughness. Results showed that the stiffness and toughness of panels reinforced with GFH bars is similar to that of RC panels with WWM, and the toughness of panels reinforced with GFS bars is between 25% and 38% higher than those with WWM. Different models are proposed to estimate the stiffness, toughness, and crack width of the tested panels.

Data Set for Cyclic Tests of Eleven Lightly Reinforced Concrete Walls

A series of reinforced concrete wall tests were conducted at the University of Auckland to address the lack of experimental data on flexure-dominant lightly reinforced concrete walls that are common in multistorey buildings in regions of low or moderate seismicity. The experimental program comprised 11 rectangular reinforced concrete walls that were subjected to pseudo-static cyclic loading. The tests were used to investigate minimum vertical reinforcement provisions for reinforced concrete walls and formed the basis for revisions to the New Zealand Concrete Structures Standard NZS 3101 and the US Building Code Requirements for Structural Concrete ACI 318. The recorded documentation and data collected throughout the test program have provided a high-quality dataset that is a valuable resource to researchers investigating the seismic behavior of reinforced concrete walls. The dataset is published and publicly available on DesignSafe-CI with the project number PRJ-1648. The methodology of the data collection is described, and a road map for navigating the dataset is presented to support future use of the archived dataset.

Discussion on “Minimum vertical reinforcement in RC walls: Theoretical requirements for low and high ductility demands”

Discussion on “Minimum vertical reinforcement in RC walls: Theoretical requirements for low and high ductility demands”

Experimental Validation of Minimum Vertical Reinforcement Requirements for Ductile Concrete Walls

Experimental Validation of Minimum Vertical Reinforcement Requirements for Ductile Concrete Walls

Comparison of Minimum Vertical Reinforcement Requirements for Reinforced Concrete Walls

Comparison of Minimum Vertical Reinforcement Requirements for Reinforced Concrete Walls

Minimum vertical reinforcement in RC walls

Recent earthquakes and research have shown that the minimum vertical reinforcement requirements in current concrete standards are insufficient to ensure well distributed cracking occurs in ductile reinforced concrete (RC) walls. To address the deficiencies of existing requirements, new theory was proposed to calculate the minimum distributed and end zone vertical reinforcement required for RC walls to meet current performance expectations. The distributed vertical reinforcement requirement was intended to prevent non-ductile behaviour for walls with low ductility demands, and was derived based on the requirement that nominal flexural strength must exceed the cracking moment capacity. The vertical reinforcement required in the ends of the wall was intended to ensure that well distributed secondary cracks form in the plastic hinge region of walls with high ductility demands, and was derived to ensure that the concrete tensile strength could be overcome by the tensile demands imposed when the vertical reinforcement in the ends of the wall yields. The proposed requirements considered the key parameters that influence the behaviour of walls with minimum vertical reinforcement. In addition, the proposed formulas were compared with current minimum vertical reinforcement limits from different concrete design standards by considering the margin of safety between cracking and nominal flexural strength and the secondary cracking behaviour. The deficiencies of the existing requirements were demonstrated and the proposed requirements were proved to be superior for walls with both low and high ductility demands.

Numerical modelling of reinforced concrete walls with minimum vertical reinforcement

Recent experimental results have shown that current minimum vertical reinforcement limits in many concrete design standards are insufficient to ensure that large ductility can be achieved during earthquakes. A detailed finite element model was developed in VecTor2 to provide a tool for further investigating the seismic behaviour of lightly reinforced concrete (RC) walls. The model was verified using experimental data from recent RC wall tests with minimum vertical reinforcement, and was shown to accurately capture both the overall response and local response parameters with good accuracy such as the cyclic hysteresis response, crack pattern, and vertical reinforcement strains. The model could also be used to estimate the drifts at which reinforcement buckling initiated and when reinforcement fractured occurred. The results from additional analyses showed that a potential size effect exists when considering the failure of lightly reinforced concrete walls. When keeping the reinforcement ratio and shear span ratio constant, the lateral drift capacity decreased significantly as the wall length increased. Using reinforcement with higher strength and lower ductility did not significantly impact the crack pattern, but did decrease the lateral drift capacity of the walls. Furthermore, reducing the strain hardening ratio of the reinforcement, or increasing the concrete strength, both resulted in a reduction in secondary cracking in the plastic hinge region and a reduced lateral drift capacity. It is recommended that wall length and average material properties should be accounted for when assessing the seismic behaviour of lightly reinforced concrete walls or when developing design standard requirements.

Cyclic Testing of Reinforced Concrete Walls with Distributed Minimum Vertical Reinforcement

AbstractDuring the 2010/2011 Canterbury earthquakes in New Zealand, several reinforced concrete (RC) walls in multistory buildings formed only a limited number of cracks at the wall base with a fra...

Effect of reinforcing steel bond on the cracking behaviour of lightly reinforced concrete members

The ductility of lightly reinforced concrete (RC) members is dependent on the distribution of cracks, as well as the strain penetration of reinforcement at each crack. A series of experimental tests were conducted to investigate how the bond characteristics of reinforcing steel would influence the strain penetration and crack distribution in lightly reinforced concrete members. To vary the bond characteristics, reinforcement with three different deformation patterns were investigated, including a standard deformation pattern and two modified bars with either half the rib height or double the rib spacing of a standard bar. Pull-out tests were conducted to quantify the bond strength of the reinforcement with different deformations patterns, followed by direct tension prism tests that represented the end region of an RC wall with minimum vertical reinforcement. The pull-out tests indicated that the standard deformation pattern achieved the highest initial bond stiffness, whereas the double rib spaced deformation pattern achieved the highest ultimate bond strength. The direct tension prism tests showed similar crack distributions for the two modified bar types, but increased secondary cracking for the standard bar due to higher initial bond strength. However, only the half rib height bar displayed a higher ductility than the standard bar, with significantly greater yield penetration at each crack causing larger ultimate crack widths prior to bar fracture.

Experimental evaluation of the seismic performance of reinforced concrete structural walls with different end configurations

The Canterbury Earthquake Royal Commission report (2013) showed that cantilever reinforced concrete (RC) walls failed at a lower ductility capacity than expected due to a plasticity concentration region within a very limited height near the location of the primary cracks at the base of the walls. The New Zealand Standards (NZS 3101) (2006) [2] and the Canadian design standards (CSA A23.3-14) (2014), adopt the same capacity design approaches for RC walls design, with both standards specifying a minimum vertical reinforcement ratios (ρv%) of 0.25% for RC walls. Subsequently, the current study was conducted to study the seismic performance of RC walls with different vertical reinforcement ratios and cross sectional configurations. In this paper, six half-scaled RC structural walls were constructed and tested under quasi-static displacement controlled cyclic loading. The walls had three different cross sectional configurations; rectangular, flanged and boundary elements and were tested with specific design characteristics selected to evaluate and compare the wall ductility capabilities. In this respect, wall ductility can be defined as the ability of the walls to undergo inelastic deformations with no/low strength degradation, which is essential in Seismic Force Resisting Systems (SFRS) as it is not economically feasible to design SFRS to behave elastically under seismic loadings. So the ductility quantification of the structural walls used were ductility ratio between the intended displacements with the yield displacement. Based on the test results, the ultimate drift at 20% ultimate strength degradation varied between 0.9% and 1.6% and the ultimate level displacement ductility (μΔ0.8u), ranged approximately between 4.0 and 6.0. Although the flanged walls and the walls with boundary elements were designed to develop almost the same capacity as that of the rectangular walls, the seismic performance of the former wall type was found to be superior to that of their rectangular counterparts with respect to both the ultimate displacement capacity and ductility level. Moreover, using the flanges and the boundary elements walls resulted in approximately 30% reduction of the vertical reinforcement compared to that of the rectangular walls when designed to resist the same lateral loads while carrying identical gravity loads. In addition to gaining insights on the response of walls with boundary elements, the results indicated that structural walls with low vertical reinforcement ratio can experience reduced ductility as indicated in the Canterbury Commission Report.

Strength and drift capacity of squat recycled concrete shear walls under cyclic loading

In order to provide an improved understanding of the behavior of squat reinforced concrete shear walls and promote the application of recycled concrete in structures, six rectangular squat recycled concrete wall specimens were tested under in-plane cyclic loading. The specimens were designed based on Chinese code for design of concrete structures GB 50010-2010, which specified minimum horizontal and vertical reinforcement ratios of 0.25% in web, and vertical reinforcement ratio of 1.0% in boundary element. The main parameters investigated are axial load level and the amount of vertical and horizontal web reinforcement. This research presents the experimental results which include test observation, lateral load versus drift response, and measured strain distribution of vertical and horizontal reinforcement, measured strength and drift capacity of wall specimens. It was found that increasing of axial load level resulted in a higher peak load but less ultimate drift capacity, and increasing of horizontal web reinforcement had small effect on peak load but could improve the drift capacity. In this study, a mixed flexure and diagonal compression mechanism was proposed to reflect the lateral load resisting behavior of squat walls. Particularly, a simplified analytical method was developed to predict the peak loads of squat walls failed in flexure or a mixed flexural–diagonal compression mode, which was proved to accurately predict the peak loads of specimens.

Understanding Poor Seismic Performance of Concrete Walls and Design Implications

The 2010–2011 Canterbury earthquakes in New Zealand revealed (1) improved structural response resulting from historical design advancements, (2) poor structural performance due to previously identified shortcomings that had been insufficiently addressed in design practice, and (3) new deficiencies that were not previously recognized because of premature failure resulting from other design flaws. This paper summarizes damage to concrete walls observed in the February 2011 Christchurch earthquake, proposes links between the observed response and specific design concerns, and offers suggestions for improving seismic design of walls in the following areas: amount of longitudinal reinforcement in wall end regions, suitable wall thickness to minimize the potential for out-of-plane buckling, and minimum vertical reinforcement requirements.

Assessment of minimum vertical reinforcement limits for RC walls

During the 2010/2011 Canterbury earthquakes, several reinforced concrete (RC) walls in multi-storey buildings formed a single crack in the plastic hinge region as opposed to distributed cracking. In several cases the crack width that was required to accommodate the inelastic displacement of the building resulted in fracture of the vertical reinforcing steel. This type of failure is characteristic of RC members with low reinforcement contents, where the area of reinforcing steel is insufficient to develop the tension force required to form secondary cracks in the surrounding concrete. The minimum vertical reinforcement in RC walls was increased in NZS 3101:2006 with the equation for the minimum vertical reinforcement in beams also adopted for walls, despite differences in reinforcement arrangement and loading. A series of moment-curvature analyses were conducted for an example RC wall based on the Gallery Apartments building in Christchurch. The analysis results indicated that even when the NZS 3101:2006 minimum vertical reinforcement limit was satisfied for a known concrete strength, the wall was still susceptible to sudden failure unless a significant axial load was applied. Additionally, current equations for minimum reinforcement based on a sectional analysis approach do not adequately address the issues related to crack control and distribution of inelastic deformations in ductile walls.

Precast Concrete, L-Shaped Spandrels Revisited: Full-Scale Tests

This paper presents results from full-scale testing conducted on 4 precast, prestressed concrete L-shaped spandrels. The 4 L-shaped spandrels were each loaded through 12-ft-long, prestressed double tees that rested on the spandrel ledge at one end and on an independent support at the other. None of the beams were constructed with closed stirrups of mild-steel reinforcement. Rather, different arrangements of transverse L-shaped bars, welded-wire reinforcement, and longitudinal bars were provided to resist the shear and torsion induced in the spandrels. Shear and torsion forces were created by the double-tee reaction forces that were loaded eccentrically to the spandrels. The transverse and longitudinal reinforcement resisted the combined effects of vertical shear and out-of-plane bending of the web and satisfied minimum vertical hanger reinforcement requirements for ledge-to-web attachment. All beams sustained loads in excess of their factored design loads. Eliminating the need for closed reinforcement in slender spandrels would be of significant benefit to the precast concrete industry. This design approach would enhance constructability of slender members, which could increase plant productivity and reduce overall costs. Behavior of all 4 spandrels at various limit states is presented, including crack patterns and failure modes. Researchers used these test results to better understand the fundamental mechanism developed in the L-shaped spandrels to resist shear and torsion.