Minimum Reinforcement Requirements Research Articles

Bending resistance mechanism of prestressed ultra-high performance concrete - reinforced concrete beam based on a full-scale experiment

Ultra-High Performance Concrete (UHPC) is a new type of engineering material with high compressive strength, high tensile strength, and high fracture toughness. Its bending failure mechanism is different from that of traditional concrete beams, which requires a new computational model to describe the bending failure phenomena of the prestressed ultra-high performance concrete - reinforced concrete (UHPC-RC) beam without web reinforcement. Therefore, this paper, through full-scale tests on a 30m prestressed UHPC-RC beam without web reinforcement, captures unique bending failure phenomena, including initial cracking, development of local cracks, and rupture of prestressed steel strands. Considering the tension-compression constitutive relationship of UHPC material, an innovative computational model for bending bearing capacity is proposed. Based on this model, a study on the minimum reinforcement ratio of full prestressed-ordinary steel bars is conducted. The results show that in the bending failure of the prestressed UHPC-RC beam without web reinforcement, excessive tensile strain of steel strands will occur at the local crack location. At this time, the structure does not satisfy the assumption of plane sections, and the introduction of the calculation model of the limit state of external prestressed tendons can effectively match this model, which is highly consistent with the experimental results. The minimum reinforcement ratio of full prestressed-ordinary steel bars is revised to the auxiliary reinforcement ratio of full prestressed-ordinary steel bars, quantifying the minimum reinforcement requirements of ordinary steel bars. The research results of this paper can provide reference for the next step of theoretical research.

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.

Evaluation of minimum flexural reinforcement for precast prestressed concrete NEXT beam bridges

The minimum flexural reinforcement requirement has been used in the current bridge design specifications to protect the member from brittle failure after the formation of the first flexural cracks. Several variables have been reported to affect this requirement, such as concrete strength, amount of prestressing in the member, and type of cross section. Recently, the Precast/Prestressed Concrete Institute (PCI) developed a new type of beam section (NEXT beam) to accelerate bridge construction and enhance the sustainability of bridges. As a newly developed beam section, no research on the minimum flexural reinforcement has been reported for NEXT beam bridges. This paper aimed to examine the minimum flexural reinforcement requirements in the current AASHTO LRFD Bridge Design Specifications for NEXT beam bridges. A comprehensive parametric study was analytically conducted with various parameters, including bridge section, beam section, concrete strength, and span length. The results from this study showed that the current minimum flexural reinforcement requirements were met for all bridges examined herein; the concrete strength, beam cross section, and span length could affect the levels of safety against brittle failure after first flexural cracks for NEXT beam bridges.

Behavioral response of reinforced concrete beams with ultra-ductile fiber-reinforced cementitious composite layers – Experiments, models and reinforcement limits

Ultra-ductile fiber-reinforced cementitious composite-reinforced concrete beams (RC-UDFRC) lack well-established bar-reinforcement limits and accurate behavioral modeling. Based on the experimental results obtained in this study, analytical formulae have been presented for calculating bar reinforcement limits for RC-UDFRC beams. A comprehensive numerical method has also been proposed that combines the use of equilibrium conditions, constitutive material models, and finite layers for predicting the entire flexural behavior of RC-UDFRC beams. Results of the study showed that introducing UDFRC into RC beam construction significantly impacted the minimum bar reinforcement requirements. Further, the simplified analytical approach for the ultimate moment capacity of RC-UDFRC beams exhibited reasonable predictability. The proposed model produced relatively accurate curvatures at cracking and ultimate loading with mean error values of 15 and 7%, respectively. Additionally, the proposed numerical method had rational cracking, yielding, and ultimate loading calculations with the mean of the tested-predicted values of 1.08, 1.11, and 0.98, respectively.

A unified approach for determining the strength of FRC beams subjected to torsion–Part II: Analytical modeling

AbstractThe addition of fibers to structural concrete is an accepted means in academia and practice to traverse tension across cracks. Several international codes of practice contain provisions, which allow a designer to rely on the fibers to resist flexure and shear. However, despite the wider community appreciating that the presence of fibers can significantly increase the capacity of reinforced concrete in resisting torsion, design guidelines are currently unavailable to the practicing engineer. As a result of minimum reinforcement requirements prescribed by codes to prevent excessive torsional cracking in reinforced concrete, a significant underutilization of the fibers is often encountered and this can lead to costly and exorbitant designs. In the accompanying paper, benchmark experimental data were presented on 18 large‐scale fiber‐reinforced concrete (FRC) members subjected to torsion, which clearly identified the beneficial effects of the fibers at all stages of loading. In this study, simplified and advanced mechanically consistent analytical models are developed within the so‐called level of approximation approach (within the framework of current Model Code) to describe the strength of FRC beams loaded in torsion.

Analysis of influencing factors on shear behavior of the reinforced concrete deep beams

A database including 833 specimens on reinforced concrete (RC) deep beams with shear failure is presented in this paper. Comprehensive influences of web reinforcement ratio, shear span-to-depth ratio, depth-to-width ratio, and compressive strength of concrete on the shear behavior of deep beams are discussed. It is found that the normalized shear strength of deep beams decreases with an increase of the shear span-to-depth ratio and this trend is affected significantly by the concrete strength, depth-to-width ratio, and web reinforcement ratio. Besides, the restrictions on minimum web reinforcement and minimum sectional size are investigated. It is found that the restrictions on minimum web reinforcement and the minimum sectional size are significantly affected by the web reinforcement ratio and are closely related to the reinforcement strength and concrete strength. However, the effect of the strength ratio of concrete to shear reinforcements on the limit of the amount of web reinforcements has been ignored in current codes. The characteristic value of web reinforcement, considering the web reinforcement ratio and the strength ratio of concrete to shear reinforcements, is proposed. For the horizontal and vertical reinforcements, the minimum characteristic values of 0.54 and 0.25 are suggested as the restrictions on the minimum amount of web reinforcement, respectively. The maximum characteristic values of 1.5 and 1.7 are proposed as the restriction on the minimum sectional size. Moreover, an equation for the ultimate shear strength of deep beams corresponding to the minimum sectional size is also suggested. Compared with the relevant provisions of the current codes, the suggestions on minimum web reinforcement requirements and minimum sectional restrictions are better.

Comparison of Reliability Based Design (RBD) With Eurocode 2 for A Singly Reinforced Concrete Beam

This research compares the Reliability-Based Design (RBD) method with the Eurocode 2 (EC2) for a singly reinforced concrete beam. Advanced First Order Second Moment (AFOSM) was used and the equation for the ultimate limit state design consideration for Moment of resistance and Applied moment was applied. The parameters which were all functions of the Ultimate load, G were replaced respectively with variables and their derivatives G(x) obtained with being the characteristic strengths of concrete and the yield stress of steel respectively. Given the original vector space and the transformed reduced vector space, the Taylor series was used to expand the reduced vector space and thus reliability index ßx expressed. The procedure was then coded using JAVA to produce the Reliability Index and Probability of failure for respective steel ratio. The required probability index of 3.9 corresponds to 0.048% of the steel ratio. Typical analysis of a singly reinforced beam according to EC2 for b=300mm, h=500mm, d=447mm, cover of 40 mm, gave a steel ratio of 1.31 %. Comparing this to the reliability-based design of 0.048%, the code provision exceeded the RBD output by 2000 times and 63 times the minimum reinforcement requirement by the code.

QUICK ASSESSMENT PROCEDURES FOR TWO STORIES BUILDING BASED ON NUMERICAL SIMULATION RESULTS

Quick assessment is one of an important step in emergency response following an earthquake. This step is mainly purposed to estimate the financial lost based on the building damage criteria. The result of a quick assessment is also needed to determine whether the damaged building can be repaired and reused or not. There are three damage levels which are classified based on the building conditions after the earthquake. Based on the experience, there is a time constraint in conducting the building assessment. Then a quickcorrect method is very important to accomplish the task in the limited time. Base on the series of numerical simulation, the quick assessment procedure is proposed in this study. The structural elements that need to be checked in the quick assessment are including column, beam, plate, and foundation. The numerical result was conducted for some two stories buildings with minimum reinforcement on those structural members. The minimum reinforcement requirements are base on Indonesia code SNI-1736-2012. The critical point of every structural component to be checked, especially columns, is discussed here.

ANALISIS KOMPARASI RASIO KAPASITAS KOLOM GEDUNG BERTINGKAT RENDAH PADA 23 KABUPATEN DI PROVINSI ACEH BERDASARKAN SNI 03-1726-2002 DAN SNI 03-1726-2012

In 2002, through the national standardization agency, the Indonesian government issued SNI 03-1726-2002 on Earthquake Resilience Planning Standard for Building Structure to anticipate the effects of earthquake on building. However, after the standard has been issued there have been several other major earthquakes that rendered it necessary to revise the existing standards. Therefore, in 2012 the government issued SNI 03-1726-2012 on Procedures for Earthquake Resistance Planning for Building and Non Building Structures. When viewed from both these standards, it can be seen that Aceh province is one of the provinces with high earthquake intensity so it is necessary to do a research to observe the effect of changes of both standards to the ratio of column capacity and minimum reinforcement requirements for buildings constructed in Aceh province. Based on the results of the research, the average column capacity ratio against the load in SNI 03-1726-2012 has increased by 45.02% and the average minimum reinforcement requirement available is only 54.5%. The increase is due to an increase in primacy factor from 1.0 to 1.5, causing increased structural load. In addition, there was an increase in the combined burden in the form of 30% additional redundant factors as well as the addition of earthquake load due to an increase in rock bed acceleration factors in some regencies. Thus it can be concluded that the building under review has not fulfilled the requirements of SNI 03-1726-2012 and is recommended for planning by adding a shear wall to reduce the structural load.

Seismic design and performance of dry-assembled precast structures with adaptable joints

Previous research projects and post-earthquake field observation showed that dry-assembled precast frame structures with hinged beams and cantilever columns restrained at their base, if correctly designed and detailed, can attain good seismic performance, mainly due to their flexibility and robustness. Their seismic design is often conditioned by the need of reducing their flexibility by increasing the cross-section of the columns, which, due to minimum reinforcement requirements, results in their over-strengthening. High flexibility may also induce displacement compatibility issues with non-structural elements. The paper concerns the proposal of an innovative enhanced structural frame system, based on the adaptation of hinged beam-column joints into rigid through the activation of special mechanical connection devices performed after the installation of the slab. While keeping all the benefits of the dry prefabrication, the resulting moment-resisting frame is provided with enhanced redundancy and stiffness. A design comparison among three precast frames with similar geometries and different static schemes shows how the joint adaptation can be exploited to optimise the structure by modifying the distribution of bending moment. The results of dynamic non-linear analyses on a three-storey precast structure with adaptable joints tested as a part of the SAFECAST research programme show the seismic performance of this system through different static schemes, and the comparison with the experimental results provides information about the validity of the models and the effectiveness of the technological solutions employed.

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.

Minimum Longitudinal Reinforcement Requirements for Boundary Elements of Limited Ductile Walls for AS 3600

Observations of poor performance of reinforced concrete walls in recent earthquake events have been associated with a light amount of longitudinal reinforcement. In particular, single-crack failures have been observed for reinforced concrete walls that have an insufficient amount of longitudinal reinforcement to allow secondary cracking. It has been proposed that the next revision of the Concrete Structures code in Australia (AS 3600) increase the current minimum longitudinal reinforcement required for limited ductile reinforced concrete walls to mitigate against this type of failure in the event of a large earthquake. This research investigates the current and proposed longitudinal reinforcement requirements of AS 3600. A reinforced concrete wall is analysed using a state-of-the-art finite element modelling program for a range of different longitudinal reinforcement configurations. The wall detailed to the minimum longitudinal reinforcement requirements proposed for the next revision of AS 3600, which requires boundary elements, was found to allow secondary cracking and using less bars than a wall with distributed reinforcement. The displacement capacities of the walls that formed secondary cracks are found to be limited unless transverse reinforcement is used to confine the longitudinal reinforcement in the boundary ends of the wall.

Minimum shear reinforcement ratio of prestressed concrete members for safe design

Design codes have specified the minimum shear reinforcement requirement for reinforced concrete (RC) and prestressed concrete (PSC) members to prevent brittle and premature shear failure. They are, however, very different from one another, and particularly, ACI318 code allows the required minimum shear reinforcement to be reduced in PSC members, compared to that in RC members, by specifying the additional equation for PSC members whose basis is not clear. In this paper, the minimum shear reinforcement ratio for PSC members was proposed, which can provide a sufficient reserved shear strength and deformation capacity. The proposed equation was also verified by the test results of PSC specimens lightly reinforced in shear, comparing to design codes and other proposed equations from previous studies.

Web-shear capacity of prestressed hollow-core slab unit with consideration on the minimum shear reinforcement requirement

Prestressed hollow-core slabs (HCS) are widely used for modern lightweight precast floor structures because they are cost-efficient by reducing materials, and have excellent flexural strength and stiffness by using prestressing tendons, compared to reinforced concrete (RC) floor system. According to the recently revised ACI318-08, the web-shear capacity of HCS members exceeding 315 mm in depth without the minimum shear reinforcement should be reduced by half. It is, however, difficult to provide shear reinforcement in HCS members produced by the extrusion method due to their unique concrete casting methods, and thus, their shear design is significantly affected by the minimum shear reinforcement provision in ACI318-08. In this study, a large number of shear test data on HCS members has been collected and analyzed to examine their web-shear capacity with consideration on the minimum shear reinforcement requirement in ACI318-08. The analysis results indicates that the minimum shear reinforcement requirement for deep HCS members are too severe, and that the web-shear strength equation in ACI318-08 does not provide good estimation of shear strengths for HCS members. Thus, in this paper, a rational web-shear strength equation for HCS members was derived in a simple manner, which provides a consistent margin of safety on shear strength for the HCS members up to 500 mm deep. More shear test data would be required to apply the proposed shear strength equation for the HCS members over 500 mm in depth though.

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.

Proposed Minimum Steel Provisions for Prestressed and Nonprestressed Reinforced Sections

The current ACI code includes two separate provisions for minimum steel reinforcement: one for nonprestressed reinforced sections, and another for prestressed sections. For nonprestressed reinforced sections, the current minimum steel requirement is written in terms of geometric and material properties of the section. For prestressed concrete, the current provision is written in terms of the cracking moment of the section. Prestressed concrete only cracks when the applied flexural tensile stress exceeds both the tensile strength of the concrete and the net compressive stress from the prestressing force in the steel. Consequently, when bonded prestressing steel quantities are increased, the cracking moment of the section increases. Depending on the shape of the cross section and ultimate strength requirements, in certain instances it is possible that a section can contain a large volume of bonded prestressing steel and yet not meet the minimum reinforcement requirement. A parametric study of several cross sections was performed to investigate this behavior. This paper describes an exact and unified solution approach for specifying minimum reinforcement for both nonprestressed and prestressed sections. A second parametric study to validate the proposed minimum steel provisions is also presented.

안전한 설계를 위한 철근콘크리트 부재의 최소비틀림철근비

현행 설계기준들에서는 비틀림모멘트를 받는 철근콘크리트 부재의 취성적인 파괴를 방지하기 위하여 최소비틀림철근비를 규정하고 있다. 그러나, 국내 현행기준 및 ACI318-11에서 규정하고 있는 최소비틀림철근비 산정식은 종방향 최소철근비, 공간트러스모델의 역학적 평형관계 및 여유강도 확보 등의 측면에서 불합리한 문제점들을 내포하고 있다. 따라서, 이 연구에서는 이러한 문제점을 극복하기 위하여, 보다 합리적이고 충분한 강도여유율을 확보할 수 있는 최소비틀림철근비 산정식을 제안하였다. 또한, 제안식을 기존실험 결과와 비교하여 검증하였으며, 제안모델이 모든 대상실험체들의 최소비틀림철근비를 안전측으로 평가하는 것을 확인하였다.

Review of Transverse Reinforcement Requirements for Precast Load-Bearing Wall Panels with Large Openings under Compression

AbstractPrecast concrete load-bearing wall panels have been used for decades with unparalleled success due to their architectural versatility, thermal insulation, ease of production and erection, and structural performance. According to the current building code, structural walls do not need transverse ties if the vertical reinforcement ratio is less than 1% or when vertical reinforcement is not required as compression reinforcement. Also, minimum reinforcement requirements do not apply to structural walls with an average effective prestress of 1,551 kPa (225 psi) or greater. Elimination of transverse ties in structural precast walls simplifies the production of the panels. The presence of large openings in precast walls is very common in the design of modern buildings. This results in a significant reduction of the panel cross section. Most designers tend to design the remaining portion of the panel cross section as a column and not a wall, which requires minimum transverse ties according to the current ...

Efficient Shear Reinforcement Design Limits for Prestressed Concrete Beams

This study evaluates the appropriateness of the current design specifications for minimum shear reinforcement and maximum nominal shear strength for prestressed concrete members. The evaluation uses the University of Texas’ Prestressed Concrete Shear Database (UTPCSDB). The UTPCSDB contains data from 30 references published between 1954-2008, with test results from 506 tests conducted on beams with a variety of shapes. The test data indicated that none of the current minimum shear reinforcement requirements showed a clear separation between desirable and undesirable residual shear strength for prestressed concrete beams. New design provisions for minimum shear reinforcement and maximum nominal shear strength are proposed based on the empirical evidence from the database.

Effect of Detailing of Web Reinforcement on the Behavior of Bottle-shaped Struts

The effect of detailing of web reinforcement as per current design codes on the strength and serviceability behavior of bottle-shaped struts has been experimentally investigated, particularly in the context of codal minimum reinforcement requirements, by testing eleven scaled deep beams. The experimentally obtained efficiency factors of the bottle-shaped struts besides being significantly higher than the values given in the ACI 318-08 were also found to be sensitive to the amount of web reinforcement. The minimum web reinforcement requirements specified in current design codes were found to be adequate for controlling service load crack widths though the unreinforced bottle-shaped struts allowed in the ACI code were observed to violate the maximum crack width requirements. It has been shown that the transformation equation in the ACI code does not accurately represent the relationship between the strut efficiency factor and the amount of transverse reinforcement whereas a corrected version of the equation shows a clear dependence of the former on the latter.