Advances in Structural Engineering

In cold-formed steel structures, such as trusses, wall frames, and portal frames, the use of back-to-back built-up cold-formed steel channel sections for the column members is becoming increasingly popular. In such an arrangement, intermediate fasteners at discrete points along the length prevent the individual channel sections from buckling independently. Current guidance by the American Iron and Steel Institute and the Australian and New Zealand Standards for built-up sections describes a modified slenderness approach, to take into account the spacing of the screws. Limited experimental tests or finite element analyses, however, have been reported in the literature for such sections to understand the effect of screw spacing. This issue is addressed herein. The results of 30 experimental tests are reported, conducted on back-to-back built-up cold-formed steel channel sections covering stub columns to slender columns. A finite element model is then described which shows good agreement with the experimental test results. The finite element model is then used for the purposes of a parametric study comprising 144 models. It is shown that while the modified slenderness approach is in general conservative, for stub columns it can be unconservative by around 10%.

This paper presents a comprehensive study of residual displacements of the bilinear single degree of freedom (SDOF) systems under the near-fault ground motions (NFGMs). Five sets of NFGMs were constructed in this study, in which the natural ones as well as the synthesized ones were both considered. By way of the nonlinear time history analyses, three different residual displacement spectrums were obtained and analyzed in detail. Utilizing the calculated data, a back propagation (BP) neural network was established to predict the residual displacements of the bilinear SDOF systems under the NFGMs. The results show that the structural parameters, including the strength reduction factor and the post-yield strength ratio, have significant and relatively consistent impacts on the residual displacement spectrum. However, the ground motion characteristics, including the fault type, the closest distance from the site to the fault rupture, the earthquake magnitude, and the site soil condition, exhibit more complex effects on the residual displacement spectrum. In addition, the proposed BP neural network can fully incorporate the parameters affecting the residual displacements of the bilinear SDOF systems under the NFGMs, while having a fairly good accuracy in predicting the residual displacements.

Based on elastic-plastic time-history earthquake analysis of the single degree of freedom (SDOF) system, the inelastic displacement amplification factor C_d2 of the elastic-perfectly plastic (EPP) model with the same yield strength reduction was investigated for different normalized periods T/T_gR, site types, strength reduction factors R, damping ratios, post yield stiffness ratios and second order ( P-Δ) effects. The factor C_d2 can be used to estimate the maximum inelastic response displacement from the elastic displacement under the design seismic conditions. It is found that the probability distribution of C_d2 satisfies the lognormal distribution approximately. The factor C_d2 decreases with decreasing damping (in the medium-long period range) and with increasing post yield stiffness. The existence of the P-Δ effect will increase C_d2 significantly and needs to be considered in future seismic design. Three other hysteretic models: the modified-Clough (MC), shear-slipped (SSP) and bilinear elastic (BIL) models, were analyzed to check the effect of the hysteretic model on C_d2 spectra The results were compared to those of the EPP model and it was found that the C_d2 of the EPP model is the smallest among the four hysteretic models.

Based on the elastic-plastic time-history analysis of SDOF(single degree of freedom) systems for the Modified-Clough(MC) model and the elastic perfectly-plastic (EPP) model, this paper carried out a study of the seismic force modification factors R_μ (SFMF) both when the P-Δ effects are not allowed for ( R_μ) and when they are allowed for ( R'_μ). 370 earthquake records from 4 site & soil conditions, 74 to 106 records for each site, were used to calculate the structural response. The structural periods varied from 0.1 to 6sec and the ductilities were taken to be 2,3,4,5 and 6. The stability index θ were varied from 0.025 to 0.1. The ratio spectra S of R_μ to R'_μ were constructed to analyze the influence of P-Δ effects on the SFMF. It is found that the spectral values of S depend on the ductility, the stability index and the normalized period T / T_gR. The results indicate that the influence of P-Δ effect is different for the two hysteretic models. The increase in strength demand to maintain constant ductility in the presence of P-Δ effects using the EPP model is greater than that given by the modified- Clough model.

This paper presents a method for the service-load analysis of continuous composite beams. Both short-term and time-dependent analyses are carried out, in which cracking, creep and shrinkage of the concrete slab are considered. The time-dependent response of individual cross-sections is modelled using the Age-Adjusted Effective Modulus Method coupled with a relaxation procedure, and the lengthwise or longitudinal analysis of the member makes use of the force method of structural analysis. Both propped and unpropped construction may be modelled. The numerical solution requires iteration, but is suited to straightforward spreadsheet or conventional programming on a personal computer, on which the analysis is performed rapidly. The scope of the method is demonstrated in a simple example of the behaviour of a two-span beam with the same sustained loading that is cast propped or unpropped.

The increasing interest in timber as a sustainable construction material has led to the development of a new type of structures referred to as ‘hybrid fibre-reinforced polymer–timber thin-walled structures’. In these structures, thin layers of fibre-reinforced polymer are combined with timber veneers to create high-performance, lightweight and easy-to-construct structural members. This new type of structural members harnesses the orthotropic properties of both timber and fibre-reinforced polymer by appropriately orientating material fibre directions for optimal composite properties as well as efficient thin-walled cross-sectional shapes. Hybrid fibre-reinforced polymer–timber thin-walled members can be used in many applications such as load-bearing walls, roofs, floor panels and bridge decks. This article describes several novel hybrid fibre-reinforced polymer–timber structural member forms and presents results from a preliminary experimental investigation into the compressive behaviour of hybrid fibre-reinforced polymer–timber wall panels. A comparison of behaviour between a hybrid fibre-reinforced polymer–timber wall panel and a pure timber wall panel is presented to show that the hybrid fibre-reinforced polymer–timber system significantly outperforms the pure timber system in terms of both load resistance and axial strain at failure.

This article tries to contribute to quantify the amount of steel corrosion at the cracking of concrete cover. The amount of steel corrosion when the concrete cover cracks, is a key factor in the life of concrete structures. During the time between steel depassivation and concrete cover cracking, the process of steel corrosion develops in three stages: free expansion of the corrosion products, stress initiation of the concrete cover, and cracking of the concrete cover. A concrete cracking model is presented here to estimate the total amount of steel corrosion at the onset of the cracking of the concrete cover. This model is applied to some test results reported by other researchers. The amount of steel corrosion predicted by the proposed model when cracking of the concrete cover occurs is in agreement with the experimentally observed results.

The estimation of the initial stiffness of columns subjected to seismic loadings has long been a matter of considerable uncertainty. This paper reports a study that is devoted to addressing this uncertainty by developing a rational method to determine the initial stiffness of RC columns when subjected to seismic loads. A comprehensive parametric study based on a proposed method is initially carried out to investigate the influences of several critical parameters. A simple equation is then proposed to estimate the initial stiffness of RC columns. The applicability and accuracy of the proposed method and equation are then verified with the experimental data obtained from literature studies.

A new method of upgrading seismic performance for underground structures is proposed in this paper. Both single-story and double-story underground subway stations are studied. Using the famous single-story Daikai Station model, by which the reliability of the numerical model is verified, seismic efficiency of Shear Panel Damper (SPD) in underground structures is proven. Then, in order to design appropriate and efficient SPDs for underground structures, strength ratio, one of the design parameters, is employed to investigate the seismic performance of structures. The recommended optimum range of strength ratio is given. Afterwards, typical double-story three-span stations with different SPDs layout forms are analyzed to figure out the optimal placement of SPDs. And some interesting conclusions are obtained, which may provide a convenient way to design SPDs in multi-story underground structures.

For the development of underground structures toward large-scale, long-span, and complex structural styles, comprehensive seismic mitigation and controlling measures that consider reducing internal forces together with controlling lateral structural deformation and upgrading energy consumption are significant for improving seismic performance and enhancing resilience of underground structure. For this purpose, a self-centering energy-dissipation column base, which originated from the concept of earthquake resilient structures in aboveground space, is proposed for the framed underground structures in this study. To verify the effectiveness of self-centering energy-dissipation column base, three-dimensional time history analyses are conducted on a single-story double-span subway station. The analysis results show that the self-centering energy-dissipation column base effectively decreases the internal forces of central column and the peak and residual values of story drift and column drift are also minimized about 4%–5%. Meanwhile, it is found that a cyclic opening–closing exists at the column base during an earthquake and the uplift of column returns to zero at the end of the earthquake. It means the self-centering effect of the column base is achieved as expected. Moreover, replaceable energy-dissipating devices provide supplementary energy dissipation to relieve the development of structural plasticity and the uplift behavior of column base avoids the occurrence of plastic hinge. As a result, the structural damages are effectively reduced after the earthquake.