Current Design Code Requirements Research Articles

Performance evaluation of composite concrete-filled steel tube columns by steel fibers and different cross-section shapes: Experimental and numerical investigations

Increasing the load-bearing capacity of columns without increasing their dimensions has become an essential issue for engineers. Concrete-filled steel tube (CFST) column is one of the common techniques used in different constructions. This study aims to assess the influence of steel fibers (SF) on the uniaxial performance of CFST columns to improve their behavior. 27 specimens are produced and tested under a uniaxial compressive load. Specimens are made with three cross-section shapes: circular, rectangular, and octagonal. Also, steel fibers are added at the three-volume percentages of 0%, 1%, and 2%. Low, normal, and high-strength concrete is used to manufacture concrete columns to consider the influence of compressive strength. The bearing capacity, load-axial shortening curves, and axial-hoop strain performance of columns are measured. Also, the obtained outcomes are compared with the requirements of current design codes and previous models. Based on the obtained results, new models are developed to predict the uniaxial bearing capacity of CFST columns with different concrete core compressive strengths, cross-section shapes, and SF contents. The results indicated that adding SF led to changing the softening behavior of the column to the hardening behavior due to raising the axial compressive strength of the concrete core. Nonetheless, this impact is more significant in CFST columns with octagonal and square cross-section shapes. Additionally, the proposed formulas in this study, with high accuracy, could be employed to predict the uniaxial bearing capacity of CFST columns with different concrete core compressive strengths, cross-section shapes, and SF contents.

Tensile and shear behavior of box connector for precast concrete shear walls

As precast concrete (PC) shear walls have been widely used in housing industrialization, reliable connections are important to ensure structural integrity of prefabricated shear walls. In the present study, a pair of innovative steel sleeve and box connections were proposed for rapid installation of fully assembled PC shear walls. Pull-out tests were performed on six PC specimens with sleeve and box connectors. Further, monotonic and cyclic lateral loading tests were performed on eight PC wall-ground beam joints. The test parameters were the configuration of connectors in the pull-out tests, and screw bolt strength, axial compression ratio (ACR), and loading type in the lateral loading tests. The test results showed that early pull-out failure occurred in the embedded sleeve due to flexural deformation of the horizontal pin rod, while fracture of the screw bolt occurred in the box component. Shear strength of the wall joint was developed by adhesive/interlock action, dowel action, and shear friction action. As screw bolt strength and ACR increased, the shear strength increased. To investigate the characteristic failure behavior of the connector and wall joint specimens, nonlinear finite element analysis (FEA) was conducted using ABAQUS program. A parametric study was performed to investigate the effect of design parameters on the pull-out and shear strength of the wall joint. Further, the test results were compared with the current design code requirements. Finally, an improved connection was suggested to increase the structural performance of the wall joint.

Research on axial behaviour of concrete columns retrofitted with pre-stressed steel strips

In many cases today, concrete columns do not meet the requirements of current design codes due to a number of deficiencies, including low concrete strength, inadequate transverse rebar and low construction quality. This paper presents the results from an experimental study of a strapping method for retrofitting the axial performance of concrete columns. In the method explored herein, the high-strength steel strips surrounding the concrete columns are pre-stressed using an air pump to provide additional transverse confinement. To analyse the axial performance of cylindrical plain concrete columns retrofitted with pre-stressed steel strips, a static loading experiment was performed on 18 large-scale specimens. Failure mode, strain development and ultimate bearing capacity were thoroughly examined, and the effects of volumetric ratio, number of layers and spacing of steel strips were also considered. Test results show that the use of pre-stressed steel strips provides significant improvement in strength and deformability, and that the number of layers and spacing of the steel strips both play important roles in this regard. Finally, the calculations, which are based on the Newman model and regression analysis for predicting the retrofitted columns’ axial compressive strengths, are presented.

Bond and ductility: a theoretical study on the impact of construction details - part 2: structure-specific features

The first part of this two-part paper discussed some basic considerations on bond strength and its effect on strain localization and plastic deformation capacity of cracked structural concrete, and analytically evaluated the impacts of the hardening behavior of reinforcing steel and concrete quality on the basis of the Tension Chord Model. This second part assesses the impacts of the most frequently encountered construction details of existing concrete structures which may not satisfy current design code requirements: bar ribbing, bar spacing, and concrete cover thickness. It further evaluates the impacts of the additional structure-specific features bar diameter and crack spacing. It concludes with some considerations on the application of the findings in practice and an outlook on future research needs.

A pre-assembly method of steel reinforcement to improve the constructability of pier coping

This study proposes a pre-assembly method of steel reinforcement for pier coping to improve constructability. In the proposed pre-assembly method, the reinforcement arrangement has been modified to minimize interference between reinforcement while satisfying the current design code requirements. To validate the structural safety of the proposed construction method, half-scale models were tested and analyzed using a nonlinear finite element method. From test and analysis, it was found that the proposed pre-assembly method yields considerable ease of construction, and shows almost the same behavior as that of conventional pier coping.

Analysis and design of rehabilitated built-up hybrid steel compression members

Compression members in steel bridges designed before 1960 may be deficient according to current design code requirements and so require strengthening. This paper explores the response of steel wide-flange columns reinforced with new steel flange cover plates, accounting for: residual and locked-in dead-load stresses; different yield strengths of the original W shape and the new cover plates; initial out-of-straightness; and load eccentricity at the member ends. A refined numerical analysis model is formulated and validated that computes the compressive resistance from principles of equilibrium, compatibility, and force–deformation relationships. Parametric studies conducted indicate that the capacity of the reinforced column is relatively insensitive to the magnitude of the locked-in dead-load stress in the original member, and that the strong axis capacity is insensitive to the magnitude of the residual stresses in the original member. A preliminary approach for designing strengthening for these members is presented and illustrated with an example calculation.

Rehabilitation of RC frame joints using local steel bracing

Reinforced concrete (RC) structural frames that were built prior to the 1970s generally do not meet current design code requirements and may behave in a non-ductile manner. The lateral load carrying capacity of these structures is often insufficient due to non-ductile reinforcement detailing, which includes insufficient or no beam-column joint transverse reinforcement and inadequate anchorage for the beam's bottom reinforcement. Experience shows that such frames are prone to earthquake damage and often suffer shear and bond slip non-ductile modes of failure. Recent earthquakes have demonstrated that the beam-column joint safety is an important factor in keeping the integrity of the entire structure. Concentric steel bracing systems have been used for the rehabilitation of non-ductile RC buildings in a number of countries. However, the application of eccentric (local) steel bracing in the rehabilitation of RC structures still lags behind due to the limited research and information on the design, modelling and behaviour of such a combined concrete-steel system. In this paper a new beam-column joint rehabilitation technique using local steel brace members is proposed. Two full-scale specimens representing a standard joint and a rehabilitated joint were made and tested under reversed cyclic load. Their behaviour was compared to that of a non-rehabilitated specimen. It is shown that the rehabilitation technique was successful in enhancing the overall performance of the deficient joint and upgrading it towards a close to current standard performance.

Use of FRP for RC Frames in Seismic Zones: Part I. Evaluation of FRP Beam-Column Joint Rehabilitation Techniques

Multi-storey reinforced concrete frames that were built prior to the 1970's generally do not meet current seismic design code requirements. The lateral load carrying capacity of these structures is often insufficient due to non-ductile reinforcement detailing, which includes either insufficient or no beam-column joint transverse reinforcement. It was observed during recent earthquakes that deficient beam-column joints can jeopardise the integrity of entire structures. Thus, several beam-column joint rehabilitation techniques have emerged to upgrade such substandard joints. It is essential to evaluate the standings of joints rehabilitated with such techniques based on current design code requirements.

On the development of a robust element for second-order `non-linear integrated design and analysis (nida)'

This paper describes and applies a theory for practical second-order analysis and design of steel frames. The practicality, convenience and high precision of the method of `Non-linear Integrated Analysis and Design (NIDA)' that allows for the second-order effects for design of steel frames and trusses is demonstrated. The need for using two or more elements per member to accurately simulate the axial load effect on the element stiffness and to locate the maximum moment along an element is eliminated. Unlike previous work for `Advanced Analysis' which is difficult to use in practice as it ignores many important practical features such as element imperfection, the proposed method meets the current design code requirements and is based on the first-plastic-hinge concept which is currently adopted by engineers. Furthermore, the element by itself can predict the beam-column load carrying capacity using a single element per member to model the P- δ effect and, therefore, it can be used for simultaneous analysis and design. The use of several elements per member still possesses an error of idealising a physically smoothly curved member by a series of segments of straight elements. Conceptually, the suggested method designs a structure by modelling accurately its true behaviour, instead of making use of empirical formulae for individual member checks. It is readily available for design application of realistic structures and it is envisaged that the NIDA will initiate a revolution in the practical design of steel structures.

BEHAVIOUR OF A WEB PLATE IN SHEAR WITH AN INTERMEDIATE STIFFENER.

The behaviour of a simply supported plate with a single stiffener, subject to in-plane shear forces, is studied using elastic critical buckling analyses and non-linear elasto-plastic finite-element analyses. Particular attention is paid to factors relevant to design of the stiffener. Variations in stiffener rigidity, panel side ratio, panel slenderness, and imperfection shapes are considered. A primary conclusion for the panel slendernesses considered is that the stiffener behaviour depends primarily on moments caused by transverse shear forces in the panels on each side of the stiffener, and that the axial force in the stiffener has a relatively small effect. This contrasts with current design methods which are based primarily on axial forces resulting from tension field action. Stiffener requirements based on the analyses are compared with the requirements of current design codes, and rather large differences are found. Large differences between codes are also found. A subsequent paper will use the results presented in developing and validating a phenomenologically based model for stiffener design. (A)

Toriconical heads: A parametric study of elastic stresses and implications on design

A thin shell finite element program is used to investigate the linear elastic behaviour of pressurised toriconical end closures. In particular, a series of design curves is presented which enables maximum stress concentration factors to be predicted for a wide range of geometries. A simple approximate formula is also developed for predicting peak stress levels. The implications of the work in terms of current design code requirements are discussed and recommendations are made.