Editor’s Note

A two-part paper by Berman and Bruneau examines the performance of “Tubular Links for Eccentrically Braced Frames.” The first paper describes the development and results of a finite element parametric study of eccentrically braced frame links with hollow rectangular cross sections. A wide range of compactness ratios and link lengths are considered 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. Analysis results indicate that if the compactness ratio recommendations proposed by the writers are satisfied, tubular links will be able to achieve the maximum rotation levels in the AISC (American Institute of Steel Construction) Seismic Provisions for links with wide-flange (WF) cross sections. The second paper presents the results of an experimental study to verify proposed design requirements. Twelve primary link specimens and two supplementary links that have a different end connection are tested. 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 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.

Qian et al. present the results of an experimental investigation of typical extended end-plate steel beam-to-column joints at elevated temperatures in “Behavior of Steel Beam-to-Column Joints at Elevated Temperature: Experimental Investigation.” Three tests were conducted at temperatures of $400°\mathrm{C}$ , $550°\mathrm{C}$ , and $700°\mathrm{C}$ , respectively, and another three tests were carried out on specimens at $700°\mathrm{C}$ with different axial compression forces applied to the beams. Moment-rotation-temperature characteristics are summarized to investigate the degradation of the behavior of this type of steel joint at elevated temperatures.

A series of column tests were conducted by Young and Chen to investigate the adequacy of current standards for “Design of Cold-Formed Steel Built-Up Closed Sections with Intermediate Stiffeners.” 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. 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 are generally conservative.

New equations, in terms of curvature demand, for “Confinement Reinforcement Design for Reinforced Concrete Columns” are proposed by Paultre and Légeron, and are derived from a parametric study of a large number of columns to reach a certain level of sectional ductility. They 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.

Sezen and Shamsai investigate a new steel reinforcement system in “High-Strength Concrete Columns Reinforced with Prefabricated Cage System.” 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. The axial strength, confinement, and displacement capacity of 15 small-scale column specimens reinforced with PCS and conventional rebar are experimentally investigated. 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 show that the axial load carrying capacity of specimens reinforced with PCS was similar to, or better than that of reinforced concrete specimens.

In “Experimental In-Plane Strength Investigation of Reinforced Concrete Masonry Walls with Openings,” Voon and Ingham present 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 is shown that the current New Zealand nonspecific masonry design standard over-predicts the strength capacity of the walls with small openings and that it is increasingly conservative as the depth of openings increased.

The “Relevance of Eighteen Flutter Derivatives in Wind Response of a Long-Span Cable-Stayed Bridge” is investigated by Mishra et al. Flutter conditions are obtained from complex eigenvalues of the modal state-space equation of motion. The importance of lateral flutter derivatives and lateral modes in the flutter phenomenon are highlighted. Flutter critical wind speeds determined from the theoretical derivatives and from Selberg’s formula were examined and found to be much higher than experimentally observed values. A frequency-domain “Analysis of Alongwind Tall Building Response to Transient Nonstationary Winds” is presented by Chen. The transient wind fluctuations and associated wind loads are modeled as the sum of the deterministic time varying mean and evolutionary random fluctuating components. The alongwind loads are determined from approaching winds using strip theory and considering 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 (RMS) values of building response. The influence of time varying mean wind speed, mean wind speed vertical profile, and spatial correlation of wind fluctuations on building response is discussed.

In “Bending Strength of Timber Beams Rehabilitated with Reinforced Epoxy Mortar Plates,” Duarte et al. discuss a rehabilitation system for timber beams, consisting of the replacement of decayed parts with new ones of nearly the same material, connected to the sound parts by reinforced epoxy mortar plates. The use of the proposed technique allows for lighter and less intrusive rehabilitation. The experimental analysis undertaken focused on the flexural bonding behavior, with stainless steel, reinforcement steel, and GFRP (Glass Fiber Reinforced Plastic) bars being used as alternatives. In spite of a reduction of strength when compared to the original elements, the safety level of the rehabilitated elements complies with Eurocode 5 requirements. Deam et al. investigate the stiffness and strength of composite laminated veneer lumber (LVL) and concrete beams intended for use in long-span flooring systems in their paper “Experimental Behavior of Prestressed LVL-Concrete Composite Beams.” Quasi-static bending tests and impact tests were conducted on four specimens: a plain LVL member with a strong shear connection and a concrete slab; a proprietary, lightweight non-structural concrete panel as a slab; a straight prestressing tendon and a strong shear connection; and a draped prestressing tendon and a weak shear connection. The stiffness of timber-concrete composite section is shown to be mostly related to the stiffness of the shear connection. 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.

A series of bi-directional pseudo-dynamic tests were carried out by Di Ludovico et al. to investigate the “Seismic Behavior of a Full-Scale RC Structure Retrofitted Using GFRP Laminates.” Tests were conducted on the ‘as-built’ structure which was designed only for gravity loads, and also on the glass fiber reinforced polymer (GFRP) retrofitted configuration. The experimental results show that the GFRP retrofit allowed the structure to withstand a level of excitation, in both directions, 1.5 times larger than that applied to the ‘as built’ structure. The GFRP laminates provided considerable enhancement in global deformation capacity without significantly affecting its strength. The shear retrofit of the exterior joints and wall-type column prevented brittle failure mechanisms, thereby improving the energy dissipating capacity of the structure.

A methodology for “Estimation of Cumulative Deformation Capacity of Buckling Restrained Braces” is proposed by Takeuchi et al. Past experiments on buckling restrained braces (BRBs) are revisited 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.

In “Damage Identification of Shear Connectors with Wavelet Packet Energy: Laboratory Test Study,” Ren et al. assess different damage scenarios on a 1:3 scaled bridge model which was constructed with removable anchors linking the slab and girders as shear 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 loosening shear connectors were effectively detected. 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. Data from the intact (undamaged) bridge is not required in the proposed method.

An “Assessment of ASCE 7 Standard Wind Load Factors for Tall Building Response Estimates” is presented by Gabbai et al. The writers contend that the wind load factors incorporated in ASCE 7 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, rather than the power two, as is the case for rigid buildings. Second, 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 wind load factors for the design of tall flexible buildings results in safety levels that can be significantly lower those typical of common, rigid structures. Findings from wind tunnel tests on a 1:50 scale one, two, and three story hip and gable roof buildings are reported by Vickery in “Component and Cladding Wind Loads for Soffits.” The tests were conducted in open and suburban terrain conditions, with and without surrounding buildings. Results show that the soffit pressures are nearly fully correlated with nearby wall pressures, and a simple approach to prescribing component and cladding pressures for the design of soffits is proposed.

Welch et al. propose a new methodology to predict the capacity of laterally edge restrained reinforced concrete (LERRC) thin and thick slabs in their paper “Specializing Park’s Compressive Membrane Theory.” The suggested methodology employs the point of peak thrust, rather than a midspan deflection estimate (used in Park and Gamble’s compressive membrane theory), to select the peak compressive membrane load capacity for LERRC one-way slabs. Comparison of averaged values of normalized load capacities selected using the experimentally measured peak capacity deflection or the calculated peak thrust 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. In “Unbalanced Moment Resistance in Slab-Column Joints: Analytical Assessment,” Gayed and Ghali evaluate a specific requirement in the American Concrete Institute Building Code ACI-318 regarding the unbalanced moment transferred between column and slab. Using nonlinear finite element simulations and supporting experimental results, the writers argue that a provision in the code that permits a reduction in the fractional constant determining the proportions of the unbalanced moment to be resisted by shear and flexure should be removed.