Low Axial Load Research Articles

The behavior of ultra-high-strength reinforced concrete columns under axial and cyclic lateral loads

In general Ultra High Strength Concrete (UHSC) is a new class of concrete that has been developed in recent decades. UHSC is characterized by extraordinary mechanical and durability properties. The UHSC-Matrix is very brittle material behavior. In this research an experimental program consists of twelve square UHSC columns is being carried out to study the behavior of UHSC columns subjected to constant axial load combined with cyclic lateral loading in order to simulate the case of seismic action. The main parameters of this program were: longitudinal reinforcement ratio, percentage of steel fiber, stirrups ratio, axial load level and concrete compressive strength. In this experimental program each specimen represents a column extending on both sides from the beam-column connection to the location of the point of inflection. Particular attention is paid to the effect of each variable on the strength enhancement, stiffness degradation, energy dissipation capacity, curvature ductility and displacement ductility of the tested columns. Valuable conclusions were obtained from the research results. By increasing the concrete compressive strength the column capacity increases accompanied by a decreasing in the ductility aspects, increasing the longitudinal steel ratio from 2% to (3.6% and 4.5%) leading to an increase in the column capacity and ductility aspect of the tested columns. Using steel fiber between (1.33–2.67)% is recommended in UHSC columns in seismic zones. The Egyptian concrete code of practice limits for the columns stirrups is suitable in seismic zones for columns subjected to a low axial load level.

Fragility Assessment of Slab-Column Connections

The development of fragility relations is an important step in the application of performance-based earthquake engineering. Data from 69 tests were collected and reviewed to develop fragility relations for new slab-column frame construction. Damage states were defined and fragility relations were developed for reinforced and post-tensioned concrete connections, both with and without shear reinforcement. Test results were reviewed to determine the likelihood and type of repair for various damage states. Derived empirical fragilities were incorporated into the ATC-58 loss estimation software Performance Assessment Calculation Tool (PACT). Nonlinear pushover analyses were conducted on test specimens and findings were compared to test results to assess bias in common modeling approaches. Comparisons indicate that the values of median model to test ratios vary between 0.9 and 1.25 for displacements corresponding to yield and for models with column stiffness based on ASCE 41-06 median stiffness at low axial loads of 0.2 EIgross.

Identification of suitable process parameters for friction surfacing of mild steel with AA6063 aluminium alloy

The present study is aimed at identifying suitable process parameters for friction surfacing of IS2062 mild steel with AA6063 aluminium alloy and obtain relationships between the input parameters and the process response that can be used for sound coating with adequate strength and ductility. Factorial experimental design technique was used to investigate and select the parameter combination to achieve adequate strength and ductility. Mechatrode rotational speed, axial load and substrate traverse speed were observed to be the most significant factors on the process response. Based on the results, it was observed that (i) lower axial load and traverse speed with high rotational speed and (ii) higher axial load and traverse speed with lower rotational speed produced sound coatings. It has also been observed that sound coatings could be obtained in a narrow set of parameter range as the substrate-coating materials are metallurgically incompatible and have propensity to form brittle intermetallics.

Ultrasonic evaluation of friction stud welded AA 6063/AISI 1030 steel joints

Friction stud welding is a promising technique in many applications related to oil and gas industries. It is used to attach grating to offshore oil platforms in areas where arc welding is not permitted because of the risk of causing a fire or explosion. Attachment of anodes inside seawater discharge pipelines in a gas processing plant is performed by this process. This solid state joining process permits metal combinations such as welding of aluminum studs to steel which would be problematic with arc welding because of the formation of thick and brittle inter-metallic compounds. In the present work, AA 6063 is joined to AISI 1030 steel using friction stud welding machine. Properties that are of interest to manufacturing applications such as Young’s modulus, longitudinal velocity, bulk modulus and shear modulus are evaluated by means of an ultrasonic flaw detector. At the interface of the joint, there is an increase of 4.4%, 1.8%, 1.15% and 4.42% is observed for the properties Young’s modulus, longitudinal velocity, bulk modulus and shear modulus respectively. This is due to the formation of intermetallic compound and increase in hardness at the interfacial region. Energy Dispersive X-ray analysis confirms the presence of FeAl as the intermetallic compound. Scanning Electron Microscope evaluation shows the presence of an unbound zone at the center of the inner region which is due to the minimum rotational speed and low axial load experienced at that point. In the unbound zone, there is an incomplete bond between dissimilar metals and it is detrimental to joint strength. Optimum value of friction time and usage of pure aluminum interlayer during the friction stud welding process hinders the formation of unbound zone and enhances the quality of AA 6063/AISI 1030 joints. The tensile strength of AA 6063/AISI 1030 joint was found to be 36.64% and 78.15% lower than that of AA 6063 and AISI 1030 respectively. Reasons for lower joint strength are attributed to low frictional pressure, non-optimal rotational speed and friction time, improper mechanical mixing, presence of oxide layer and formation of brittle intermetallic compound at the joint interface.

The Effect of Lap Splice Length on the Cyclic Lateral Load Behavior of RC Members with Low-Strength Concrete and Plain Bars

Many existing reinforced concrete buildings in developing countries located in seismic areas do not possess sufficient strength and ductility characteristics to resist the effects of severe earthquakes. Among other deficiencies, improper detailing of reinforcement and poor quality of materials are major causes of poor seismic performance. More specifically, inadequate lap splice lengths provided at floor levels on plain column longitudinal bars, particularly when concrete strength is low, is a widespread deficiency that has not been investigated in detail to date. Information on the behavior of such columns under earthquake actions is extremely important for reliable assessment of the seismic safety of many existing structures with detailing deficiencies and poor construction quality. In this study, the effect of lap splice length on the cyclic lateral load behavior of low-strength RC columns with plain longitudinal bars (14 mm diameter) was investigated experimentally. The specimens were designed to represent columns with low axial loads. The geometric ratio of the longitudinal reinforcement and the volumetric ratio of the lateral reinforcement of the columns are 1% and 0.8%, respectively. The test program included five RC columns with lap splice lengths of 25, 35, 44 and 55 times longitudinal bar diameter, as well as a reference specimen with continuous longitudinal bars. Test results clearly demonstrated that presence of 180-degree hooks at the ends of the lap splice reduces the negative influence of the inadequate lap splice length on RC member performance, even in the case of low-strength concrete. All specimens reached their flexural strengths and did not experience considerable strength degradation until large drift levels. Test observations were supported with findings of a fiber-based analytical model, which also demonstrated the influence of hooks on improving the bond slip behavior along inadequate lap splices.

Experimental study on the mechanical behaviour of a heat exchanger pile using physical modelling

This study aims to provide knowledge on the thermo-mechanical behaviour of heat exchanger piles, through a laboratory scale model. The model pile (20 mm in external diameter) was embedded in dry sand. The behaviour of the axially loaded pile under thermal cycles was investigated. After applying the axial load on the pile head, the pile temperature was varied between 5 and 30 °C. Seven tests, corresponding to various axial loads ranging from 0 to 70 % of the pile estimated bearing capacity, were performed. The results on pile head displacement show that heating under low axial load induced heave and cooling induced settlement; the pile temperature-displacement curve was found to be reversible and compatible with the thermal expansion curve of the pile. However, at higher axial loads, irreversible settlement of the pile head was observed after a few thermal cycles. The axial load profile measured by the strain gauges evidenced that the pile head load was mainly transferred to the pile toe. Nevertheless, thermal cycles modified significantly the mobilised skin friction along the pile. The total pressure measured at various locations in the soil mass was also slightly influenced by the thermal cycles.

Experimental study of the seismic performance of L-shaped columns with 500 MPa steel bars.

Based on tests on six L-shaped RC columns with 500 MPa steel bars, the effect of axial compression ratios and stirrup spacing on failure mode, bearing capacity, displacement, and curvature ductility of the specimens is investigated. Test results show that specimens with lower axial load and large stirrup characteristic value (larger than about 0.35) are better at ductility and seismic performance, while specimens under high axial load or with a small stirrup characteristic value (less than about 0.35) are poorer at ductility; L-shaped columns with 500 MPa steel bars show better bearing capacity and ductility in comparison with specimens with HRB400 steel bars.

Fatigue Damage in Diameter-enlargement Part Formed by Cyclic Bending and Axial Compressive Load

We proposed a new cold processing method to partially enlarge the diameter of a shaft with a combination of a cyclic bending load and an axial compressive load that is lower than the yield stress of sample material. We call this cold processing method a diameter-enlargement working method, and call this processed part a diameter-enlargement part. Key features of the processing method are chiefly as follows: the diameter enlargement deformation progresses easily under a low axial compressive load at room temperature; the processed part has little temperature increase, although the processing causes large plastic deformation. We have tested the influence of the processing conditions and the mechanical properties of the sample materials in terms of diameter enlargement deformation behavior. However, little is known about the fatigue damage condition in a diameter- enlargement part. Therefore, we carried out processing experiments to clarify crack generation conditions, and simulated working processing using finite element method in order to investigate behaviors of stress and strain during the processing. Furthermore, we evaluated low cycle fatigue damage in the processed part by using the Manson-Coffin expression. This paper clarified the mechanism of crack generation during the processing and evaluates the fatigue strength of the processed part.

Studies on the Provisions of Confining Reinforcement for High-strength Concrete Column

It is general knowledge that the design of earthquake-resistant structures for high-strength concrete columns requires confining-reinforcement with a relatively high volumetric ratio to ensure the ductility of the structure. This implies that the mechanical behavior of high-strength concrete differs significantly from the behavior of normal-strength concrete. However, the provisions on the minimum volumetric ratio of confining-reinforcement contained in the Indonesian Concrete Code (SNI 2847-2013) is essentially derived from the test results for normal-strength concrete. This paper studies the confining-reinforcement provisions used in several standards, i.e., SNI 2847-2013, ACI-2011, NZS-2006 and CSA-2004, to determine the ductility of the concrete columns. The case study is based on the analysis of the cross-section of high-strength concrete columns, the parameters that affect the strength, and by evaluating the value of the column's cross-section curvature ductility. The study results showed that the equation for confining-reinforcement adopted in the SNI 2847-2013 is very conservative compared to other codes when applied to low axial load levels (≤0.2), but is relatively less conservative if the axial load level is greater than 0.3.

Experimental and Numerical Investigation on Reinforced Concrete Beam-Column Joints

The elasto-plastic analysis of reinforced concrete square columns was introduced to study the relationship between lateral force and curvature by using the open source program OpenSees. A pseudo static test on the inverted "T" shape reinforced concrete square column was conducted for the comparative analysis. The results indicated that the lateral force calculated by program agree with experimental data with an axial load ratio of 0.33. The calculated yield lateral force was 29.7 kN, the error was lower than 7% compared with experimental results. In addition, the cross-section curvature were obtained, which were difficult to obtain through the traditional experimental study, the calculated yield curvature was 1.825×10-5. The calculated results with different axial load ratios were presented in this paper, which showed that both the yield lateral force and curvature of reinforced concrete square columns were increased with low axial load ratio, and at the same time it could reduce some experimental work by using computer simulation.

Buckling and Postbuckling Finite-Element Analysis of Pultruded FRP Profiles under Pure Compression

Results of nonlinear finite-element analyses of commercial pultruded fiber-reinforced plastic (PFRP) I-section profiles subjected to pure compression are presented. One narrow flange and four wide flange shapes are analyzed for 10 values of the column slenderness, involving 50 finite-element (FE) models. The imperfection sensitivity is investigated with reference to different imperfection shapes. For wide flange columns, if the imperfection amplitudes are identified with the limiting amplitudes reported by the pultruders, extremely low ultimate axial loads are obtained, associated with unrealistic failure mechanisms and strong interaction between local and global buckling modes. In contrast, reduced imperfection amplitudes reported in the literature lead to well documented failure mechanisms and to a small influence of the buckling mode interaction. As is known, the local buckling of wide flange shapes is triggered by the flange instability. Conversely, the local buckling of narrow flange shapes generally is triggered by the web instability. Because of this property, the paper shows that at equal cross section area, the narrow flange profiles may exhibit higher ultimate loads with respect to the wide flange profiles in a broad range of column slenderness.

Preliminary study on the mechanical behaviour of heat exchanger pile in physical model

The effects of temperature changes on the mechanical behaviour of heat exchanger piles need to be taken into account in geotechnical design. In this preliminary study, the behaviour of an axially loaded pile under thermal cycles was investigated using a physical model. After applying the axial load on the pile head, the pile was heated from 25°C to 50°C and subsequently cooled to 25°C. Four tests (corresponding to four values of axial load) were performed and two temperature cycles were undertaken in each test. The pile temperature and the pile head displacement were monitored. When low axial loads were applied, the heating induced heave and cooling induced settlement of the pile head. In this case, the displacement–temperature curve was found to be reversible and similar to the thermal expansion curve of the pile. However, in the case of higher axial loads, the heave of the pile head, obtained during heating, was lower than the thermal expansion of the pile, suggesting settlement of the pile toe. Irreversible settlement of the pile head was observed after these thermal cycles.

Experimental Study on Seismic of High-Strength Spiral Stirrups Prefabricated Column Encased by Steel Plate Hoop and Bolt Reinforcement

The assembled monolithic compound spiral reinforced concrete column in this paper is a kind of concrete column reinforced by high-strength compound spiral stirrups and jointed by local steel tubular and bolt reinforcement. Experiments of four specimens under low cyclic loading and different axial compressive loads are carried out, and the result testifies that the anti-seismic property of these measures is not worse than the integral compound spiral hoop columns under the same load.

Performance limits for structural walls: An analytical perspective

Recently proposed changes to modeling and acceptance criteria in seismic regulations for both flexure and shear dominated reinforced concrete structural walls suggest that a comprehensive examination is required for improved limit state definitions and their corresponding values. This study utilizes nonlinear finite element analysis to investigate the deformation measures defined in terms of plastic rotations and local concrete and steel strains at the extreme fiber of rectangular structural walls. Response of finite elements models were calculated by pushover analysis. We compare requirements in ASCE/SEI 41, Eurocode 8 (EC8-3) and the Turkish Seismic Code (TSC-07). It is concluded that the performance limits must be refined by introducing additional parameters. ASCE/SEI 41 limits are observed to be the most accurate yielding conservative results at all levels except low axial load levels. It is shown that neither EC8-3 nor TSC-07 specifies consistent deformation limits. TSC-07 suggests unconservative limits at all performance levels, and it appears to fall short of capturing the variation reflected in the calculated values. Likewise EC8-3 seems to fail to represent the variation in plastic rotation in contrast to several parameters employed in the calculation. More accurate plastic rotation limits are proposed.

Research of Assembled Monolithic Compound Spiral Reinforced Concrete Column Encased by Steel Plate Hoop and Jointed by Bolts

The assembled monolithic compound spiral reinforced concrete column in this paper is consisted of two concrete columns reinforced by high-strength compound spiral hoop and jointed by encased steel and bolts. Experiments of four specimens under low cyclic loading and different axial compressive loads are carried out, and the result testifies that the anti-seismic property of this measure is not worse than the integral compound spiral hoop columns under the same load.

Effective Stiffness of Rectangular Confined Concrete Columns with Continuous Compound Spiral Hoop

Existing models appropriate for design applications tend to overestimate the measured effective stiffness and are unacceptably inaccurate, because they generally neglect the influence of anchorage slip on the effective stiffness of the column. A three-component model that explicitly accounts for deformations due to flexure, anchorage-slip, and shear is shown to provide a more accurate estimate of the measured effective stiffness for the database columns. This model is simplified by neglecting small terms and approximating the results of moment-curvature analysis to obtain an accurate and rational effective stiffness model appropriate for design applications. Comparison the measured effective stiffness of reinforced concrete columns from the PEER Structural Performance Database with stiffness calculated following the FEMA 356. The FEMA 356 procedure substantially overestimates the stiffness of columns with low axial loads, in which there can be significant bar slip in the beam-column joints or footings. This model provides practical recommendations for improving estimates of effective stiffness.

Reconsidering the use of high-strength reinforcement in concrete columns

Current US building codes impose restrictions on the yield stress of steel bars used as longitudinal reinforcement in concrete columns. This paper revisits the origins of these restrictions and investigates the feasibility of using yield stress in excess of 80ksi (550MPa) to reduce the amounts of reinforcement required in structures to resist seismic loads. Analytical data from hypothetical cross sections and experimental data from tests are used to demonstrate that for columns with axial loads below the balanced point, the reduction in longitudinal reinforcement is not associated with reductions in strength and limiting curvature. The analytical and experimental results indicate that the reinforcement in a column can be decreased in direct proportion to an increase in yield stress.The evidence presented supports the hypothesis that for columns with low axial load, the use of high-strength bars can lead to reductions in congestion without critical consequences in performance.

Effects of strain hardening of reinforcement on flexural strength and ductility of reinforced concrete columns

AbstractIn the evaluation of flexural strength of reinforced concrete (RC) columns, the elastic–perfectly plastic constitutive model is generally used for steel reinforcement, which ignores the strain hardening effect. While some engineers believe that the flexural strength so obtained is on the safe side, others are concerned that underestimation of member strength could lead to inaccurate prediction of overall structural behaviour especially under extreme events. In any case, better understanding of the possible over‐strength and its effects on flexural ductility and failure mechanism is necessary. In this paper, the effects of strain hardening of reinforcement on the flexural strength and ductility of reinforced normal‐ and high‐strength concrete columns are studied based on rigorous full‐range moment–curvature analysis. The study has identified if and how various parameters affect the strain hardening effect, which include axial load ratio, concrete strength, confining stress, reinforcement ratio and the tensile‐strength‐to‐yield‐stress ratio of steel. The effects of strain hardening can be quite significant for RC columns under relatively low axial load and relatively high confining stress. Copyright © 2009 John Wiley & Sons, Ltd.

Experimentally Measured Permeability of Uncracked and Cracked Concrete Components

In this paper, the relationship between concrete damage and air flow is evaluated using model-scale reinforced concrete walls. The focus is on the walls behavior under uniaxial and biaxial loading conditions. The method for evaluating the damage-flow rate relationship includes structural testing of scaled specimens, damage identification, and air flow rate experiments. Concrete damage is characterized on a local and global level, via consideration of crack characteristics (length and width) and drift ratio, respectively. Nine model uniaxial specimens and 13 model biaxial specimens were tested, with variations in geometry, material, and loading details. All specimens had a well-defined region of interest for damage identification and air flow testing. Air flow tests, in the form of pressure decay tests, were used to measure the permeability of the concrete at different loading stages. Results indicate that the largest cracked concrete permeability occurs in specimens with low concrete strength, low reinforcement ratio, higher cycle count loading protocols, low axial load, and low aspect ratio. In contrast, the lowest cracked concrete permeability values are associated with high strength concrete, high reinforcement ratio, and high axial load.

Seismic performance of steel reinforced ultra high-strength concrete composite frame joints

To investigate the seismic performance of a composite frame comprised of steel reinforced ultra high-strength concrete (SRUHSC) columns and steel reinforced concrete (SRC) beams, six interior frame joint specimens were designed and tested under low cyclically lateral load. The effects of the axial load ratio and volumetric stirrup ratio were studied on the characteristics of the frame joint performance including crack pattern, failure mode, ductility, energy dissipation capacity, strength degradation and rigidity degradation. It was found that all joint specimens behaved in a ductile manner with flexural-shear failure in the joint core region while plastic hinges appeared at the beam ends. The ductility and energy absorption capacity of joints increased as the axial load ratio decreased and the volumetric stirrup ratio increased. The displacement ductility coefficient and equivalent damping coefficient of the joints fell between the corresponding coefficients of the steel reinforced concrete (SRC) frame joint and RC frame joint. The axial load ratio and volumetric stirrup ratio have less influence on the strength degradation and more influence on the stiffness degradation. The stiffness of the joint degrades more significantly for a low volumetric stirrup ratio and high axial load ratio. The characteristics obtained from the SRUHSC composite frame joint specimens with better seismic performance may be a useful reference in future engineering applications.