Journal of Structural Engineering

This paper describes the development and results of a finite element parametric study of eccentrically braced frame links having hollow rectangular cross sections (i.e., tubular links). The parametric study involves over 200 combinations of geometries and properties and is divided into two parts. Part A considers a wide range of compactness ratios and link lengths to determine appropriate compactness ratio limits such that links with tubular cross sections can achieve desired rotation levels prior to significant strength degradation from local buckling. Part B of the study involves models developed with flange compactness ratios, web compactness ratios, and stiffener spacings near the proposed design limits from Part A, and also examines links having webs and flanges with different yield stresses (i.e., hybrid links). Results of the parametric study are also used to investigate the adequacy of a method for approximating link overstrength. A companion paper describes the experimental verification of the proposed design requirements.

This paper describes the results of an experimental study to verify proposed design requirements for eccentrically braced frame links with hollow rectangular (i.e., tubular) cross sections. Twelve primary link specimens and two supplementary links that have different end connections are tested. Three cross sections and four normalized link lengths are considered. Two of the cross sections have web and flange compactness ratios near the proposed limits while the third cross section has compactness ratios significantly less than the proposed limits. Shear, intermediate, and flexural, link lengths are tested, including some at the critical transition length from shear-to-intermediate link behavior. Results indicated that tubular links satisfying the proposed compactness and stiffener requirements can achieve the target plastic rotations for wide-flange links when subjected to the loading protocol specified in the 2005 AISC Seismic Provisions. The predominant failure mode is fracture of the link flange near the connection to the end plates used for testing. A method for calculating the ultimate link shear, proposed by others for wide-flange links, is also investigated and modified to agree with the results of the tubular link tests.

Although beam-to-column joints are a critical part of steel and composite framed structures, very few tests have been conducted under fire conditions. Of the tests conducted, only a limited range of joint types has been studied. The main reason for this is the high cost associated with elevated-temperature tests. This paper presents the results of an experimental investigation of typical steel extended end-plate beam-to-column joints at elevated temperatures. Six beam-to-column joints were tested. These included three tests conducted at 400, 550, and $700°\mathrm{C}$ , and another three tests on specimens at $700°\mathrm{C}$ with different axial compression forces applied to the beams to simulate restraint effects. Moment-rotation-temperature characteristics are summarized in order to investigate the degradation of this type of steel joint at elevated temperatures.

A series of column tests on cold-formed steel built-up closed sections with intermediate stiffeners is presented in this paper. The test specimens were first brake pressed from structural steel sheets to form open sections with intermediate web stiffeners, then two of the open sections were connected at their flanges using self-tapping screws to form the built-up closed sections. The high strength structural steel sheets had the measured 0.2% proof stress up to $586\mathrm{MPa}$ . Initial and overall geometric imperfections as well as material properties and residual stresses of the test specimens were measured. Tests were performed over a range of lengths such that column curves could be obtained. The test strengths are compared with the design strengths calculated using the direct strength method in the North American Specification and Australian/New Zealand Standard for cold-formed steel structures. Three different methods were used to obtain the local and distortional buckling stresses for the calculation of the direct strength method. Reliability analysis was performed to assess the reliability of the direct strength method on cold-formed steel built-up closed section columns. It is shown that the direct strength method using single section to obtain the buckling stresses is generally conservative.

This paper presents new equations for the design of confinement reinforcement for ductile earthquake-resistant rectangular and circular columns based on performance measured in terms of curvature demand. These equations are developed from a parametric study of a large number of columns to reach a certain level of sectional ductility and account for the influence of concrete strength, transverse reinforcement yield strength, axial load level, and transverse confinement reinforcement spatial distribution. Simplification of these equations, while retaining the main controlling parameters, leads to design equations appropriate for design codes. These equations are then validated against a large set of experimental results. Their implementation in the Canadian Standard for Design of Concrete Structures is explained.

A new steel reinforcement system is introduced to be used in concrete columns. This new reinforcement, named Prefabricated Cage System (PCS), is an alternative to the rebar cage used in traditional reinforced concrete for faster, easier, and more reliable construction. PCS reinforcement is prefabricated off-site and then placed inside the formwork eliminating the time-consuming and costly labor associated with cutting, bending, and tying steel bars in traditional rebar construction. The axial strength, confinement, and displacement capacity of 15 small-scale column specimens reinforced with PCS and conventional rebar are experimentally investigated. The behavior of PCS specimens is evaluated and compared with that of similar rebar reinforced concrete columns. The effect of several parameters, such as steel tube thickness, opening dimensions, number and spacing of longitudinal and transverse steel, on the strength and displacement capacity is also investigated. Test results have shown that the axial load carrying capacity of specimens reinforced with PCS was similar to or better than that of reinforced concrete specimens. Axial load–displacement relations for the test columns are also predicted and compared with the measured response.

This paper presents test results of eight partially grout-filled perforated concrete masonry walls that were subjected to cyclic lateral loading. Test results obtained from this research indicated that the size of openings and the length of trimming reinforcement significantly affected the lateral strength of perforated masonry walls. It was shown that the current New Zealand nonspecific masonry design standard NZS 4229 unsafely overpredicts the strength capacity of concrete masonry walls with small openings, and an amendment is proposed to rectify this matter. It was also shown that NZS 4229 is increasingly conservative as the height of openings increased. Diagonal cracking patterns that formed during testing were observed to align well with the load paths by which lateral shear force was assumed to be transferred to the foundation when using strut-and-tie analysis. This observation supports the use of the strut-and-tie technique as a viable tool to evaluate the flexural strength of walls of this type.

Prediction of flutter critical wind velocity is of major concern in the wind resistant design of long-span cable-supported bridges. A full set of 18 experimental flutter derivatives in the past has not been used in the flutter analysis of such bridges. In particular, lateral flutter derivatives have been ignored or used from quasisteady theory. In this paper, all experimentally determined flutter derivatives have been used to analyze the flutter behavior of a long-span cable-stayed bridge. Flutter conditions have been obtained from the complex eigenvalue of modal state-space equation of motion. The importance of lateral flutter derivatives and lateral modes in the flutter phenomenon has been highlighted. Flutter critical wind speeds from the application of the theoretical flutter derivatives and the Selberg’s formula were also examined and were found to be exceptionally higher than that obtained using all experimental flutter derivatives for a long-span cable-stayed bridge.

Transient nonstationary extreme winds such as thunderstorm downbursts are responsible for significant structural damage and failures. This study deals with the frequency domain analysis of alongwind tall building response to transient nonstationary winds based on nonstationary random vibration theory. The transient wind fluctuations and associated wind loads are modeled as the sum of deterministic time varying mean and evolutionary random fluctuating components. The alongwind loads on buildings are determined through the approaching winds by using strip theory and taking into account the unsteady force characteristics in terms of aerodynamic admittance and joint acceptance functions. An analysis framework is developed to quantify the time varying mean, evolutionary spectrum, and time varying root-mean-square values of building response. The traditional analysis framework concerning stationary boundary layer winds serves as a special case of this novel framework. Applications of this general framework are addressed to the cases where the mean wind speed is characterized by a time-invariant vertical profile and a single time varying function that also serves as the modulation function for the wind fluctuations. The influence of time varying mean wind speed, mean wind speed vertical profile, and spatial correlation of wind fluctuations on building response is discussed using tall building examples.

The need for restoration of the built heritage increases the demand for new strengthening and rehabilitation techniques of structures. This paper discusses a rehabilitation system for timber beams, consisting of the replacement of the decayed parts by new ones of nearly the same material, connected to the sound parts by reinforced epoxy mortar plates. The use of this technique allows for lighter and less intrusive works, compatible with the normal building occupation during the rehabilitation process. The experimental analysis undertaken focused on the flexural bonding behavior with stainless steel, reinforcement steel, and glass fiber reinforced plastic bars being used as alternatives. A simple analytical model, based on the usual assumptions of the bending theory, is proposed, leading to a reasonable agreement with the experimental results. In spite of a reduction of strength when compared to the original elements, the safety level of the rehabilitated elements is high enough to comply with the Eurocode 5 requirements and is, therefore, suitable for actual works.

The paper investigates the stiffness and strength of composite laminated veneer lumber (LVL) and concrete beams intended for use in long-span flooring systems. Quasi-static bending tests and impact tests were conducted on four, $6\mathrm{m}$ long specimens to observe the failure mechanisms and to estimate the static and dynamic properties of the systems. One specimen, with a plain LVL member, a strong shear connection, and a concrete slab, was used as a control. The other three specimens investigated the use of (i) a proprietary, lightweight nonstructural concrete panel as a slab; (ii) a straight prestressing tendon and a strong shear connection; and (iii) a draped prestressing tendon and a weaker shear connection. All four specimens had a T-shaped cross section, with the slab supported by either one or two LVL beams. The stiffness of control beam, which is mostly related to the stiffness of the shear connection, is shown to be almost three times that of a bare LVL beam, but the beam is only 74% stronger than the bare beam. The prestressing tendons are shown to have little effect on the stiffness and strength, but reduce the deflection due to permanent load, particularly when they are draped. The proprietary lightweight concrete panels are shown to provide little structural benefit.

In the framework of the seismic performance assessment and rehabilitation research project, a series of bidirectional pseudodynamic tests were performed on a full-scale torsionally unbalanced three-story reinforced concrete (RC) frame structure. The structure was designed only for gravity loads and was subjected to two rounds of tests both on the “as-built” and glass fiber reinforced polymer (GFRP) retrofitted configuration at the ELSA Laboratory of the Joint Research Centre of the European Commission. The aim of the GFRP repair was to enhance the global performance of the structure in terms of ductility and energy dissipation by providing columns with more deformation capacity and preventing the development of brittle failure modes. The driving principles for the design of the GFRP retrofit and the experimental results in terms of global and local performance are presented and compared in order to validate the proposed retrofit technique. The experimental results represent a reference database for the development of design criteria for the seismic retrofit of RC structures using composite materials.

Recently, passive vibration control design has gained popularity. The aim of this design is to absorb most of the seismic energy by using additional dampers while maintaining the elasticity of the main structure. Buckling restrained braces (BRBs) are used as one of the elastoplastic dampers in this design and, therefore, the cumulative deformation capacity of BRBs to the point of fatigue fracture is an important benchmark for their performance. The estimation of this value under a random amplitude is complicated however, as this value is affected by the loading history. In this study, past experiments on BRBs are revisited in order to investigate the relation between the cumulative deformation capacity and the applied loading history. Based on these analyses, a simple method is proposed for predicting the cumulative deformation and energy absorption capacities of BRBs under random amplitudes. In contrast to Miner’s method, the proposed method does not require analyses of the individual amplitudes, and the values of interest are directly determined from the response indexes.

Shear connectors are widely used to link the slab and girders together in slab-on-girder structures. Assessment of the integrity of shear connectors is an important issue in the maintenance and health monitoring of such structures. In this study, a 1:3 scaled bridge model was constructed with removable anchors linking the slab and girders as shear connectors. Each anchor consisted of a threaded bar penetrating through the soffit of a girder and anchored in the slab by an embedded nut cap with welded steel bar. Different damage scenarios were simulated by removing some of these connectors. A signal-based damage detection method in which the damage feature is characterized by the wavelet packet energy changes was applied to the damage identification of the shear connectors. Using measurements of hammer impact responses, the locations of the loosening of the shear connectors were effectively detected. This paper demonstrates that the wavelet packet-based energy index is a sensitive local damage indicator. The results also show that comparison of the relative vibration between the slab and the girders is a more effective technique for evaluating the shear connector damage than comparison of the vibration of the slab before and after shear connector damage. This finding is particularly interesting because data from the intact (undamaged) bridge is not required. The suitability and efficiency of the approach is expected to be applied in situ to assess the integrity of shear connectors in existing structures.

Wind load factors incorporated in the ASCE 7-05 standard are based on rough approximations of wind effects and the uncertainties inherent in them. These factors are routinely applied to tall building design, even though the original calculations on which they are based disregard two important facts characterizing tall building response to wind. First, wind effects on flexible structures are proportional to the wind speeds raised to powers larger than two, instead of the power two, as is the case for rigid buildings. Second, dynamic response parameters, that is, the natural frequencies of vibration and the damping ratios affecting the response, exhibit significant uncertainties. It is shown that, for these reasons, the use of ASCE 7-05 wind load factors for the design of tall flexible buildings results in safety levels that can be significantly lower than safety levels typical of common, rigid structures.

The wind induced failure of soffits of low rise structures is a failure mechanism commonly observed in post hurricane damage investigations. The failure of the soffits has the potential to allow both wind and water to enter the attic space of the building. The current US national wind loading standard (ASCE 7) provides no guidance as to the wind load requirements for the design of soffits. In order to address this deficiency, wind tunnel tests on 1:50 scale models of one-, two-, and three-story hip and gable roof buildings were performed where measurements of the wind induced pressures and suctions on the soffits, roofs, and walls were obtained. The wind tunnel tests were performed in open and suburban terrain conditions, with and without surrounding buildings in place. The results of the tests clearly show that the soffit pressures are nearly fully correlated with nearby wall pressures, and a simple and accurate solution to the soffit loading deficiency in ASCE 7 is to prescribe that the component and cladding pressures for use in the design of soffits be identical to the component and cladding loads used for the design of wall components.

Park’s compressive membrane theory has been used for many years to predict the capacity of laterally edge restrained reinforced concrete (LERRC) thin slabs. This note examines his compressive membrane theory and presents a methodology which proved to be successful in predicting, in a different manner, the capacity of LERRC thin and thick slabs. The suggested methodology employs the point of peak thrust, rather than a midspan deflection estimate, to select the peak compressive membrane load capacity for LERRC one-way slabs. Comparison of averaged values of normalized load capacities $(w_{\mathrm{theory}}∕w_{\mathrm{test}})$ selected using the experimentally measured peak capacity deflection (i.e., $w_{\mathrm{theory}}∕w_{\mathrm{test}}=0.91$ ) or the calculated peak thrust (i.e., $w_{\mathrm{theory}}∕w_{\mathrm{test}}=0.85$ ) at peak compressive membrane capacity showed that using the peak thrust to select the load capacity was almost as good as using the experimentally measured deflections, especially for thick slabs used in protective construction. The calculation of generated thrust is already inherent within Park’s compressive membrane theory, whereas additional testing is required to accurately predict the midspan deflection at peak compressive membrane load capacity for an acceptable range of span-to-thickness ratios. The limited experimental data set for LERRC one-way slabs ranged from a span-to-thickness ratio of 2.7 to 28.3.

ACI 318-05 considers that unbalanced moment, $M_u$ , transferred between column and slab is resisted in part $\left({\gamma }_{\nu }{M}_u\right)$ by shear and the remaining part, $\left[(1-{\gamma }_{\nu })M_u\right]$ by flexure. The code specifies the fraction ${\gamma }_{\nu }$ and permits, in Section 13.5.3.3, that the fraction can be decreased when two requirements for the shearing force, $V_u$ , and for the flexural reinforcement ratio, ${\rho }_{\mathrm{flex}}$ , are satisfied. Nonlinear finite-element analyses for interior, edge, and corner connections are employed to calculate the value of ${\gamma }_{\nu }$ at all load stages up to failure, indicating that the ${\gamma }_{\nu }$ reduction as permitted by 13.5.3.3 is not justified. This statement is supported by experimental interaction graph.