Residual Load-bearing Capacity Research Articles

Prediction of the residual load-bearing capacity of naturally corroded beams using the variability of tension behaviour of corroded steel bars

This paper shows the residual behaviour of a highly corroded beam stored under sustained load in a chloride environment for 28 years. A non-corroded beam of the same age was also kept under sustained loading but in a non-aggressive environment as a control beam. Both beams were subjected to a three-point bending experiment until failure. The lower tensile reinforcement was extracted so that the tension properties of the corroded bars could be investigated. The yield strength, ultimate strength and ultimate strain of the corroded bars followed a normal distribution. Both the average value and the characteristic value of the yield strength, ultimate strength and ultimate strain were used to predict the residual mechanical performance of the beams. The changes in yield capacity and ultimate capacity of the corroded beam in comparison with the non-corroded beam are discussed in relation to the damage to the steel reinforcing bars. According to the variability of the residual behaviour of steel bars in tension experiment, it is possible to predict the variability in the load-bearing capacity of the corroded beams which fail due to the rupture of the tension bars.

Mechanical behavior of concrete-filled square steel tube with FRP-confined concrete core subjected to axial compression

A novel column consisting of steel, concrete and fiber-reinforced polymer (FRP) is presented and assessed through experimental study. The sectional form of steel–concrete–FRP–concrete (SCFC) columns has a square steel tube as the outer layer and a circular filament-wound FRP tube as the inner layer, and concrete is filled between these two layers and within the FRP tube. Thus the column can be regarded as a concrete-filled square steel tube (CFST) with a FRP-confined concrete core (FCCC). In comparison to existing composite column sections, several advantages are achieved such as ease of connection to beams, the stronger confinement of core concrete, ductile response, and residual load capacity. 18 SCFC stub columns are tested with different concrete strengths, FRP tube thickness and steel tube thickness in comparison to four traditional composite columns. The distinctive behaviors of SCFC columns, including significant post-yield stiffness, ductile failure, and stable residual bearing capacity are found. Test results show that the strengths of concrete, FRP, and steel are effectively utilized at different stages, including the initial linear stage, the secondary linear stage (hardening stage), and the post-peak stage. The influences of key design parameters on the mechanical characteristics of initial compressive stiffness, peak strain, residual load-bearing capacity, and ductility are also clarified.

Post-fire behaviour of high strength steel endplate connections — Part 1: Experimental study

In practice, high strength steels have been more and more popularly employed in significant structures and landmark constructions. However, in current literatures of civil engineering the very limited reports on behaviour of high strength steel structures cannot sufficiently guide the design of high strength steel structures, especially for their fire safety. The post-fire performance of steel structures is of significance in the fire-resistance design and post-fire evaluation, since the residual forces and deformations redevelop in steel structures after fire, which might be more dangerous than the fire condition. In order to reveal more information and better understanding on the behaviour and failure mechanisms of high strength steel endplate connections after fire, an experimental study has been conducted and presented in this paper. Tests on seven endplate connections were carried out after cooling down from fire temperature of 550°C. The experimental behaviour of high strength steel endplate connections after fire was compared with that of mild steel endplate connections as well as with their original behaviour at ambient temperature without fire exposure. Moreover, the provisions of Eurocode 3 were validated with post-fire test results of high strength steel endplate connections. This experimental study indicates that a proper design of endplate connection employing a thinner high strength steel endplate can achieve the same failure mode, similar residual load-bearing capacity and comparable or even higher rotation capacity after fire, in comparison with a connection with thicker mild steel endplate.

Residual mechanical properties of calcareous and granite aggregate concretes after fire

The residual mechanical properties of concrete after a fire depend on the high temperatures to which the material was subjected and the nature of the fire extinguishing process used. Knowledge of these properties is of the utmost importance in an evaluation of the residual load-bearing capacity of reinforced concrete elements and their repair after fire. This paper presents the results of an assessment of the residual mechanical properties of ordinary concrete after fire. Concretes comprising granite and calcareous aggregates were studied as they are commonly used around the world in civil engineering construction. The study considered different loading levels (0·3fcd and 0·7fcd), different cooling processes (cooling in air and by water jet) and varying maximum temperatures of concrete exposure (20, 300, 500 and 700°C). The mechanical properties of the concretes (residual compressive strength, direct and splitting tensile and flexural strengths and modulus of elasticity) after being subjected to high temperatures were studied. High temperatures and sudden cooling of the concrete showed a negative effect on its residual mechanical properties. This effect was more significant for the residual compressive strength than for the other mechanical properties.

Post-fire behaviour of reinforced concrete stub columns confined by circular steel tubes

Experimental and numerical studies into the structural behaviour of reinforced concrete columns confined by circular steel tubes after exposure to standard fire conditions are presented. These elements differ from conventional concrete-filled steel tubular (CFST) columns in that breaks in continuity are present at the member ends, which limit the longitudinal stresses in the steel and maximise the level of confinement afforded to the concrete through the generation of hoop stresses in the tube. The temperature distributions in the specimens were measured during the heating and cooling phases, while the load–displacement relationships and longitudinal and transverse strains in the steel tube were recorded during the subsequent compressive tests. 3D finite element (FE) models were also developed using the program ABAQUS to investigate the post-fire performance of circular steel tube confined reinforced concrete (CSTCRC) columns, including both heat transfer and stress analyses. The FE models were used to identify the influences of key parameters on the residual capacity of CSTCRC columns, following exposure to the ISO-834 standard fire. The considered parameters included heating time, cross-sectional dimensions, strength of the materials, steel tube to concrete area ratio and ratio of reinforcement. Finally, a design method was proposed for predicting the residual load bearing capacity and compressive stiffness of CSTCRC columns after standard fire exposure.

Loading Tests of Experimental Models of an Integral Bridge Loaded by the Effects of Water Pressure during Floods

This paper presents an experimental and computer analysis of an integral bridge structure exposed to a load caused by the horizontal pressure of water during floods. The paper focuses on the residual load-bearing capacity of an integral bridge structure that was able to withstand the effects of flooding. ​​Three models of integral bridges built to a scale of 1:5 were made for the purpose of testing. The frames have a span of 2.6 m, the thickness of the supports is 0.2 m, and the thickness of the deck in the middle is 0.125 m. The frames are loaded by horizontal forces representing the water pressure during floods. Each frame is securely anchored at the base and is gradually loaded by two horizontal forces situated in the corners of the frame. Loading is interrupted when cracks of 0.3 mm appear. Subsequently, the frame is loaded by a pair of vertical forces acting on the top surface of the frame, up to the load-bearing capacity. This procedure was established in order to find the residual load-bearing capacity of an integral bridge that managed to withstand the effects of flooding. . The paper includes a set of computer simulations.

Failure mechanism of compressed short brick masonry columns confined with FRP strips

The article addresses the issues of strengthening and stabilisation of compressed masonry structures by inorganic FRP fabrics. The confinement of a masonry column with fabrics of carbon fibres with a high modulus of elasticity and high tensile strength relieves the transverse tensile stresses in the masonry and increases the ultimate deformation and the ultimate load. The experimental research of brick masonry columns under concentric load performed to-date has pointed out the necessity of adopting a different approach to assessing the load-bearing capacity or residual load-bearing capacity, which takes into account the different failure modes of reinforced and unreinforced brick masonry.

Stabilization and Strengthening of Historic Buildings' Stone Masonry Columns

The experimental research of failure mechanism of stone columns made of coursed masonry of regular sandstone blocks and coursed masonry of irregular (freestone) blocks under concentric compression and the research of the performance of non-reinforced as well as CFRP-reinforced stone columns completed to-date pointed out the necessity of a different approach to the assessment of the load-bearing capacity, or residual load-bearing capacity, of masonry composed of stone blocks.

Comparative experimental analysis of the effect caused by artificial and real induced damage in composite laminates

This paper presents the results of an extensive experimental campaign aimed to examine the effect upon the vibration response and on the residual load-bearing capacity caused by both: isolated artificially induced interlaminar damage and low-velocity impact induced damage in composite laminates. The experimental programme included modal testing, drop-weight impact testing, ultrasonic inspection, transverse quasi-static loading testing and compression testing conducted on a set of 72 carbon fibre-reinforced composite laminated coupons. Both types of damage caused measurable changes in laminate performance, however marked divergent trends were observed. The results allowed for conclusions to be drawn regarding the adequacy of the artificial damage approach and highlighted the importance and role of other forms of degradation upon damage tolerance of laminated composites containing damage.

Failure mechanism of compressed reinforced and non-reinforced stone columns

The article deals with the issues of the strengthening and stabilisation of stone masonry columns reinforced by fabrics based on inorganic fibres under concentric compression. These structures represent a frequent design solution of load-bearing structures of historic buildings. High-strength carbon fibres fabric placed along the perimeter of a masonry column—in thirds of the column height—prevents the undesirable redistribution of compressive normal stresses along the compressed element’s cross section in the phase of crack propagation and crack development. In this phase, despite growing horizontal and vertical deformations (mainly in the central part), a masonry element is able to transfer the growing compressive load. In this case, a gradual exhaustion of the ultimate strength of the individual masonry components occurs as a consequence of wrapping in a composite based on high-strength fibres. The experimental research of the failure mechanism of stone columns made of coursed masonry of regular sandstone blocks and coursed masonry of irregular (freestone) blocks under concentric compression and the research of the performance of non-reinforced as well as CFRP-reinforced stone columns completed to-date reveals the necessity of a different approach to the assessment of the load-bearing capacity, or residual load-bearing capacity, of masonry composed of stone blocks.

Assessment of Structural Reliability of Bridge Beams Based on Measured Symptoms

The evaluation of the safety and the estimation of the remaining service life of existing structures need a thorough knowledge of the effective operational conditions, achievable implementing a health monitoring system. The measured quantities, if opportunely chosen, allow to follow the evolution of the structural behavior in the lifetime and to recognize the occurrence of damages. Hence the collected data can be considered as symptoms and used to assess the reliability and the prognosis of the monitored structure. This paper presents the application of symptom-based reliability formulation to a real case study: a set of precast bonded post-tensioned concrete bridge beams, recently dismounted after a service life of 50 years. Both static and dynamic tests were carried out at the aim of evaluating the residual load-bearing capacity and investigating the effects of deterioration on their dynamic properties. The experimental results of test campaigns have been used as symptoms in order to estimate the reliability function of the beams and the evolution of their resistance in time.

Fracture behavior of carbon nanotube/carbon microfiber hybrid polymer composites

Growing carbon nanotubes (CNTs) on the surface of fibers has the potential to modify fiber–matrix interfacial adhesion, enhance composite delamination resistance, and possibly improve toughness. In the present study, aligned CNTs were grown upon carbon fabric via chemical vapor deposition. Continuously monitored single-fiber composite fragmentation tests were performed on pristine and CNT-grafted fibers embedded in epoxy, and single-laminate compact-tension specimens were tested for fracture behavior. A significant increase (up to 20 %) was observed in the interfacial adhesion, at the cost of a decrease in the fiber tensile strength. As a result, the maximum load of the composite was decreased, but its residual load-bearing capacity more than doubled. The likely sources of these effects are discussed, as well as their implications.

Vibration tests on dismounted bridge beams and effects of deterioration

This paper aims at describing the tests campaign carried out on five precast bonded post-tensioned concrete bridge beams, recently dismounted after a service life of 50 years. The girders were part of the deck of a recently dismounted viaduct of an Italian motorway. The beams showed different deterioration levels, mainly due to the different exposure to corrosive agents. The test campaign were designed for evaluating the residual load bearing capacity of the members. Dynamic measurements were acquired before and after the static tests by using different excitation sources. This experimental research highlights that the natural frequencies of the beams can be used as a predictor of the ultimate bending moment of an existing structure.

On the orientation of fibres in structural members fabricated with self compacting fibre reinforced concrete

The incorporation of fibres into concrete produces important benefits, mainly on the residual load-bearing capacity. These improvements depend on the type, content and orientation of the fibres, being a strong relationship between the number of fibres in the fracture surfaces and the post peak parameters. Although the fibres could be homogeneously distributed after mixing, the casting and compaction processes can significantly affect the fibre distribution and orientation, and consequently the mechanical performance of the material. In the case of Fibre Reinforced Self Compacting Concrete (FR-SCC) the existence of significant flow and wall effects may influence fibre orientation. This paper analyzes the fibre orientation in thin structural elements cast with FR-SCC and its effects on the residual mechanical properties. A slab of 0.90 × 1.80 × 0.09 m, a wall of 0.50 × 2.00 × 0.08 m, and a beam of 0.15 × 0.15 × 2.50 m were selected as representative elements where different concrete flow conditions take place. A strong heterogeneity in the orientation of the fibres was found. The fibre orientation varied with the flow rate and with the wall effect; the thickness of the elements or the proximity to the bottom of the moulds appeared as important variables. It was demonstrated that in thin elements the residual mechanical properties can be quite different when diverse zones and/or directions of the structural elements are considered.

The Study of Post-Fire Load Bearing Capacity of Steel Column under Axial Compression Exposure to one-Side Fire

The section of steel column under axial compression presents heterogeneous material and its load condition changes after experiencing fire damage of one side. Taking H-section steel column under axial compression commonly used in engineering for study object in this thesis in order to study post-fire residual load bearing capacity, the post-fire eccentric effect of steel column under axial compression exposure to one-side fire is analyzed using the method of equivalent section which considering the change of the material strength after high temperature, the influence of non-uniform temperature field on material properties and the stiffness degradation of member section. Calculation methods of the eccentricity and additional moment are proposed and further given a formula of residual load bearing capacity.

Experimental Study on the Mechanical Properties of Reinforced Concrete Columns after Exposure to Fire

This paper provided three test data pertaining to the mechanical properties of reinforced concrete columns after exposure to ISO834 standard fire and three comparative test data pertaining to the mechanical properties of reinforced concrete columns at room temperature, mainly concerning the influence of fire on failure mode, distortion performance and ultimate load bearing capacity of reinforced concrete columns under axial and eccentric compression. Test results show that the failure mode of reinforced concrete columns after exposure to fire is basically same with that at room temperature. With the same concrete strength and heating condition, the bearing capacity of specimens reduces as the eccentricity increases. Strain along the section height of eccentric columns after fire basically agree with the plane section supposition while the flexural rigidity and ultimate load bearing capacity decreases obviously. The residual load bearing capacity of reinforced concrete columns after exposure to fire is only about 25% to 37% of that at room temperature.

Time-Dependent Load Bearing Capacity of Corroded R.C. Columns in Atmosphere Environment

As a supporting member of concrete structures, the reliability of the columns is of critical importance. To have models targeted to estimating the residual load-bearing capacity of corroded reinforced concrete (R.C.) columns so that inspection procedures and maintenance interventions can be well conducted, a time-dependent capacity model for corroded R.C. column is presented. The model was based on equilibrium equations on which load-bearing capacity of R.C. column at given time (year) can be predicted considering the overall deteriorations of rebar, concrete section, mechanical properties and bond behavior between corroded reinforcement and concrete. The model was verified by published literature test results of corroded R.C. columns served in atmosphere environment for years. The comparison of the model predictions with the test results shows the validity of the model. In the end, considerations for use of the model were suggested.

Mechanical behavior of tubular T-joint after fire

This paper presents experimental results of the post-fire behavior of tubular T-joints. The research aims at the failure modes and the residual strengths of the T-joints after fire. Three tests of full-scale tubular T-joints are conducted. The first one is carried out to study the mechanical behavior of T-joints under ambient temperature. The other two tests are performed to study the influence of pre-load, heating and cooling phase on residual load-bearing capacity of the tubular T-joints. The test results show that the sustained axial load on the brace has remarkable influence on the residual deflection of the T-joints which is cooled down to room temperature. The results of the experiments also indicate that the axial load level and heating and cooling history have more significant effects on the compressive stiffness of the T-joints than the residual strength. In the numerical study, the result of finite element model agrees well with the test result. The work in this paper provides a basis for further parametric analysis and theoretical study on the structural evaluation after fire.

Experimental Study on Residual Load Bearing Capacity of SRC Eccentric Columns after Exposure to Fire

This paper provided three test data pertaining to the mechanical properties of steel reinforced concrete (SRC) eccentric columns after exposure to fire and one comparative test data pertaining to the mechanical properties of steel reinforced concrete columns at room temperature. The influence of eccentricity on failure mode, distortion performance and ultimate load bearing capacity are mainly studied. Test results show that the failure modes of steel reinforced concrete eccentric columns after exposure to fire are similar to that at room temperature. Strain along the section height at mid-span section of eccentric columns before loaded to 90% ultimate load bearing capacity is linearly distributed and well agree with the plane section supposition. After exposure to fire, the flexural rigidity and load bearing capacity of specimens are all declined compared with that at normal temperature. In various loading stages from the initial loading to 80% ultimate strength, the ratio of flexural rigidity of SRC eccentric columns after exposure to fire and at normal temperature is ranged from 0.30 to 0.59. With the same concrete strength and heating condition, the ultimate strength of specimens decreases with the increasing of eccentricity. The ultimate bearing capacity of all specimens at normal room temperature is calculated on the method proposed by Chinese regulation JGJ 138-2001. The compared results of experimental values and calculating values show that the residual load bearing capacity of SRC eccentric columns after exposure to fire is about 69% to 81% of that at room temperature.

Investigation on Toughness of Fibre Cocktail Reinforced Self Consolidating Concrete after High Temperature

The effect of different fibres on the residual load-bearing capacity and the failure pattern of high-performance self consolidating concrete (HPSCC) after exposure to high temperature hass been studied in this work. The polypropylene fibers mitigate the spalling of HPSCC element clearly, but did not show clear effect on the mechanic properties of concrete. The macro steel fiber reinforced HPSCC showed higher flexural toughness and ultimate load before and after high temperatures. The mechanical properties of hybrid fibre reinforced HPSCC (HFHPSCC) after heating were better than that of mono-fibre reinforced HPSCC. The failure mode changed from pull-out of steel fibers at lower temperature to broken down of steel fibers at higher temperature. The use of hybrid fibre can be effective in providing the residual strength and failure pattern, and improving the toughness of HPSCC after high temperature.