Journal of Structural Engineering

The local buckling restraining behavior of thin-walled circular concrete-filled tubular (CFT) columns is examined under seismic loads by conducting a bidirectional cyclic loading and a bidirectional shaking table test. According to the responses of the deformations and strains measured in the bidirectional cyclic loading test, the axial compressive force acting on the buckled part of a steel tube is reduced with the increase in its local buckling deformation because most of the compressive axial force is transferred from the steel tube to the in-filled concrete. The reduction in the compressive axial force in the steel tube slows the progress of the buckling deformation. In addition, under a cyclic load applied after the occurrence of local buckling, the opening and closing of major horizontal cracks and dilation occur in the in-filled concrete. As a result, a predominant tensile axial force acts repeatedly on the buckled part of the outer steel tube. This tensile force restrains or restores the local buckling deformations by stretching them. The magnitude of the tensile force is enhanced further if a diaphragm is installed on the steel tube at the upper surface of the in-filled concrete. The shaking table test confirms that the local buckling restraining behavior is similar under seismic accelerations. The shaking table test together with the numerical analysis illustrates that the ratio between the residual sway displacement ${\delta }_r$ and the maximum response sway displacement ${\delta }_m$, defined as ${\delta }_r/{\delta }_m$ for CFT columns, is generally much smaller than that for hollow columns because of the enhanced strength and ductility of CFT columns.

A nonlinear finite element (FE) technique is used to study the bond mechanism at the steel/concrete interface in reinforced concrete (RC) tie members. The accuracy of three different FE bond models, namely, the perfect bond, the diagonal link element, and the bond zone, is examined against the experimental data from two different sources. The bond zone model is shown to be reasonably accurate in predicting both bond shear and normal stresses. Based on the results of the FE analysis, a multilinear curve is proposed to approximate the distribution of the bond shear stresses. The peak bond shear stresses and the average splitting tensile stresses caused by the bond normal stresses are determined through FE parametric studies using the bond zone approach. The results of this study can be used as a guideline for the determination of bond-related parameters that are essential in the crack prediction of one-way and two-way members.

Capacity models to assess structural safety are primarily based on experimental outcomes typical of new constructions. In order to analyze deformation and strength mechanisms of members that do not comply to current seismic codes and practices, an experimental investigation was undertaken on full-scale columns reinforced with deformed or plain rebars. The experiments involved eight tests under monotonic or cyclic actions on square or rectangular reinforced concrete columns designed according to provisions, construction practice, and material properties enforced between the 1940s and 1970s. Experimental outcomes allowed pointing out global and local behavior of RC columns typically found in existing constructions, in which both plain and deformed bars can be found. Theoretical plastic rotation capacities provided by the proposed supplement to the current ASCE standard are discussed in this paper and compared to those experimentally obtained. The results indicate that predicted plastic rotations corresponding to significant loss of lateral-force capacity are very conservative if compared with experimental outcomes. This seems especially clear in the case of columns reinforced with plain rebars. For these columns, the contribution of the base rotation on the global deformation mechanism is considerable.

Much of past research in the civilian area on the response of civil structures to explosive loading has focused on large detonations in the far field that result in relatively uniform pressure distribution over the structure and specific structural elements. A paucity of research has been conducted that investigates the effect of explosive loading in close proximity to key structural elements. The studies that have been conducted focused primarily on loading perpendicular to the strong axis of bending that result in global deformation, but no rupture or loss of material. Through experimental testing and finite-element simulation, the present study investigates the effect of blast loading on wide flange columns loaded perpendicular to the weak axis of bending. This loading scenario is critical for such columns because the near field shock wave can rupture the web, and in some cases, lead to material loss; both conditions can potentially jeopardize the axial load carrying capacity of the column as a result of increased demands on flanges and possible local buckling of the unrestrained flanges. Therefore, this critical scenario needs to be considered for developing blast resistant measures or assessing the remaining axial and bending capacity of the column. Finite-element simulation can be used for this purpose; the analyses conducted as part of this study replicate, with reasonable accuracy, the experimentally obtained localized deformation, ruptures, and loss of material as a result of blast load, although the finite-element simulation is less successful at replicating the global deformation of the column.

The drift spectrum has recently been introduced as a new measure for estimating the interstory drift demands in buildings under near-field earthquakes. In this study, a method for estimating the drift demands in buildings with different structural systems is presented. Unlike previous studies that use a continuous beam, a stick model is employed to estimate the seismic-induced interstory drift demands. The proposed method can overcome the deficiencies of the previously introduced models. It considers the effect of heightwise stiffness reduction on drift demands, which is shown to be very important, and makes it possible to construct the drift spectra for low and medium-rise as well as high-rise buildings. The suggested method is simple because it uses the well-established modal analysis technique of lumped mass beams. The drift spectra for shear building models are constructed using near-field strong ground motions. The effects of the ratio of the lateral stiffness of top story to that of first story, heightwise variation of structural stiffness, number of stories, damping ratios, and higher modes on drift demands are investigated. A method is presented for using these spectra in estimating the drift demands in moment resisting frame structures, and the accuracy of the method is verified using a number of building models.

Full-scale static test to failure was conducted on a 21.9-m-long 65-kW wind turbine tower to evaluate the flexural strength and failure mode. The test was conducted in a displacement-control mode. The flexural limit state in the form of local buckling near the tower base was developed. The buckling occurred in the elastic range because of a high diameter-thickness ratio of the section. Although local buckling caused slight strength degradation, the reduction was not very significant and the recorded response showed a good amount of post-buckling ductility. The local buckling flexural strengths predicted from established standards correlated very well with the measured buckling strength. A parametric finite-element analysis was also conducted. By properly modeling the boundary condition and a local dent on the tower wall, a good correlation of the structural response and failure mode was also achieved. It was found that the ultimate strength and failure mode were sensitive to the boundary condition considered in the model. The presence of a local dent would affect both the location and type of local buckling, although the failure load is less sensitive to local dent.

To quantify the safety margin of shear design provisions for ultrahigh performance fiber-reinforced concrete (UHPFRC), an experimental campaign has been performed. In a four-point bending configuration, shear tests have been conducted on 11 3-m long and 0.38-m high I-shaped girders with varied types of shear reinforcement (stirrups and/or fibers, or neither), combined with longitudinal prestressing or mild steel reinforcing bars. These shear tests have been analyzed in conjunction with a complete materials characterization. To identify the contribution of the fibers to the shear response, prisms have been extracted horizontally, vertically, at 30 and 45° in both undamaged ends of the beams to determine the effective orientation factor. Through this unique combination of data, detailed in the writers’ paper, design provisions and models have been developed, as detailed in a companion paper.

The safety margin of shear design provisions for ultrahigh performance fiber reinforced concrete (UHPFRC) requires quantification prior to widespread use. To this aim, the results of a dedicated experimental campaign described in a companion paper and available data from the literature have been used. Different models have been tested for predicting the shear-cracking strength and the ultimate shear capacity of UHPFRC beams. The lack of characterization of the tensile UHPFRC behavior often impairs the quality of data obtained from shear tests on UHPFRC beams. This issue has been partially overcome by approximating the UHPFRC postcracking strength under tension from fibers and matrix properties in using the variable engagement model. The safety of the French recommendations for UHPFRC has been confirmed, and improved models for serviceability-limit states prediction and realistic accounting of critical shear-cracking have been developed.

To overcome the drawbacks in the traditional interval perturbation method due to the unpredictable effect of neglecting the higher-order terms of Neumann series, a modified interval perturbation method is presented and used to determine the static response interval of the structure with interval parameters. In the proposed method, the inverse of the interval structural stiffness matrix is approximated by a modified Neumann expansion in which the higher-order terms of Neumann series are considered. For the uncertain structure with large uncertainty levels, the subinterval analysis technique is introduced into the proposed method and the approach to determine the number of subintervals is proposed. The modified interval and subinterval perturbation methods are implemented in the finite-element framework. Numerical results on a six-bar space truss structure and a shell structure with interval parameters verify the accuracy and efficiency of the proposed methods. Hence, the modified interval and subinterval perturbation methods can be successfully applied to the nonprobabilistic structural model, and in the future have wide application prospects.

Steel gravity framing systems, one of the most commonly used structural systems in the United States, have an unknown resistance to collapse when a column suffers damage that compromises its ability to carry gravity loads. One potential mechanism for these flexible systems to arrest collapse is through the development of an alternate load path in a sustained tensile configuration resulting from large vertical deflections. The ability of the system to develop such an alternate load path is partly dependent on the ability of the gravity connections to remain intact after undergoing extreme local deformations. This study experimentally evaluates the resistance of steel gravity connection subassemblages to loading consistent with the removal of an interior column. Characteristic connection behaviors are identified and peak resistance values and connection demands are reported for several different connection configurations. An approach to determine the deformations of fibers, used to discretize the connections, is also proposed that can predict fiber deformations from system displacement. Here, the approach is used to determine the fiber displacements at connection failure.

This paper aims to predict ductile fracture of mild steel under monotonic loading only from the test results of notchless tensile coupons. A simple fracture model based on the concept of a damage index with only one model parameter is proposed to predict ductile fracture of structural steels. The model is based on an idea of a combination of the void growth model and Miner’s rule in incremental form. Moreover, a new method to modify the true stress-true strain data after necking initiates is proposed, and it is found that the hardening modulus of several structural steels after necking initiates is approximately the same. Finally, ductile fracture of smooth and notched steel specimens is numerically simulated for three types of structural steels, which proves simplicity and acceptable accuracy of the fracture model and the modification method of the true stress-true strain.

The design of reinforced masonry (RM) structural walls subject to high inelastic curvature demands can often be hindered by prescriptive requirements in design codes regarding the ratio between compression zone depth ($c$) and wall length (${\ell }_w$). A possible solution to this would be the detailing of a confined boundary element at the critical wall toes to increase the curvature ductility capacity of the plastic hinge region. The Masonry Standards Joint Committee (MSJC) code currently prescribes the possible use of such boundary elements for the special reinforced masonry shear walls category. However, the MSJC provides no relevant prescriptive detailing guidance, leaving the onus on the designer to specify appropriate detailing to ensure the desired performance. The results of an experimental program designed to develop a confinement scheme based on conventional masonry construction practices are reported in this paper. Five half-scale RM walls, detailed with confined boundary elements containing a double layer of vertical reinforcement enclosed by stirrups, were tested under reversed cycles of quasi-static loading. The walls, detailed to represent typical low- to medium-rise construction, varied by their height, length, and aspect and vertical reinforcement ratios. Test results are reported in terms of force-based seismic design parameters, such as the wall strength, stiffness and ductility characteristics, as well as displacement and performance-based seismic design parameters for the next-generation of seismic codes in North America. Overall, the performance of the walls indicates that the selected boundary element detailing is a designer-friendly means of addressing the need for lower $c/{\ell }_w$ ratios and higher curvature levels as indicated by the MSJC.

Because of its potentially beneficial properties, there has been an increased interest in recent years on performance of self-compacting concrete (SCC) in structural members. The capability of SCC in flowing through and filling in even the most congested areas makes it ideal for use in congested reinforced concrete (RC) structural members such as beam-column joints (BCJ). However, members of tall multistory structures impose high capacity requirements where implementing normal-strength self-compacting concrete (NSSCC) is not preferable. In the present study, six beam-column joint specimens were designed following the guidelines of the New Zealand concrete standards; namely, three high-strength self-compacting concrete (HSSCC), one conventionally vibrated high-strength concrete (CVHSC), one conventionally vibrated concrete (CVC), and one CVC with HSSCC in its joint region. Factors such as the concrete type (HSSCC, CVHSC, and CVC), amount of joint shear stirrups, axial load ratio (1% and 10% of section capacity), and direction of casting were considered as variables in designing these specimens. All BCJs were tested under a displacement-controlled quasi-static reversed cyclic loading regime. Seismically important features such as shear force, drift ratio, member deformations, strains, crack pattern, and crack width were measured; and damping, stiffness, joint shear stress, contribution of different components, and elongation of plastic hinge zone were calculated. Joint shear stresses were varied from 6 to 7 MPa; yet within the maximum code limit of about 10 MPa. It was found that not only none of the seismically important features were compromised by using HSSCC, but also the quality of material and ease of construction boosted the performance of beam-column subassemblies.

Physical simulations of 914 mm (36 in.) deep girder-cross frame subassemblies subjected to cyclic loading were carried out to study the effects of distortion-induced fatigue and to evaluate the effectiveness of a new retrofit measure. The new repair method consists of adding steel angles connecting the girder web to the cross frame transverse connection plate (CP), and a steel plate on the opposite side of the girder web. Parameters of the experimental study included the retrofit configuration and length of the fatigue cracks. Test results showed that the new angles-with-plate retrofit measure was effective in preventing fatigue crack reinitiation for both horseshoe-shaped cracks around the CP-to-web weld and cracks along the flange-to-web weld. When the retrofit measure was implemented, specimens were able to exceed the number of cycles equivalent to infinite fatigue life for AASHTO Fatigue Category A details without any measureable fatigue crack growth, regardless of crack configuration. A test trial was also carried out to measure fatigue crack reinitiation life of the web-gap region with undersized crack-arrest holes. Experimental results showed that, for the stress range imposed at the web-gap region of the specimens and the crack-arrest hole diameter evaluated, fatigue cracks reinitiated at 40,000 cycles, which was below the limit for AASHTO Fatigue Category ${\mathrm{E}}^{\prime }$ details.

This paper presents the results from computer simulations of 914 mm (36 in.) deep girder-cross frame subassemblies to study the effects of distortion-induced fatigue and to evaluate the effectiveness of a newly proposed cost-effective retrofit measure. The proposed retrofit measure consists of adding steel angles connecting the girder web and the transverse connection plate (CP), and a steel plate on the back side of the girder web. The retrofit measure is intended to reduce stress demand at the welds, to restrain the web-gap region from deforming in the out-of-plane direction, and to distribute lateral forces transferred by cross-frames over a wider region of the web. Parametric studies were carried out to determine the optimal configuration to prevent growth of fatigue cracks of various lengths in the web-gap region. It was found that the proposed retrofit measure reduced peak stress demands with respect to the unretrofitted configuration by 50% or more, making it likely that it will prevent fatigue crack reinitiation. The parametric studies showed that the proposed retrofit measure became more effective in reducing the peak stress demand as the stiffness of the elements increased, with the lowest average stress demand occurring when the angles and backing plate were assigned thicknesses equal to 2.67 times the thickness of the web. Experimental verification of this study is presented in a companion paper.

Modern tall buildings are characterized by their slenderness and sensitivity to resonant wind effects. This is especially true considering the acceleration-based motion perception criteria under which they must be designed. In light of the significance of the resonant response, damping plays an important role in the design of tall buildings. Unfortunately, unlike other mechanical characteristics of structures, damping is far more difficult to estimate. This is due to the inherent complexity and high number of mechanisms responsible for damping. For this reason, the experimental determination of damping levels for tall buildings from full-scale data collected during monitoring programs has obtained a tremendous amount of interest over the past years. This paper firstly reviews the predictive damping models that are available in the literature highlighting their merits and shortcomings in light of the extensive experimental damping data collected over the past few years. A novel amplitude-dependent data-driven model is then proposed based on a fully probabilistic description of the mechanisms that are hypothesized to generate the majority of damping in tall buildings. Finally, the proposed model is calibrated to a number of specific buildings demonstrating its robustness.

Tsunamis can generate a considerable amount of flow velocity on land. The associated hydrodynamic effects coupled with the plethora of unrestrained objects and frangible structures produce significant debris that can travel similar velocities as the flow. Design of structures to resist the tsunami-driven debris requires a conservative estimation of the forces generated at impact. To quantify the forces generated, an experimental study was conducted on a full-scale wood utility pole, steel tube, and ISO shipping container subjected to in-air axial impacts. The impact force is found to vary linearly with the impact velocity and the impact duration remains constant for elastic response of the debris. For inelastic axial impact of the debris, the duration of the impact event increases and the impact force demands reach a limit. The results are compared with a simplified method, which is found to provide an accurate estimate of the impact demands. The model presented in this paper is developed for use in design guidelines to define debris impact forces and durations for design.

A wind tunnel study was performed to examine wind loads on canopies attached to the walls of low-rise buildings. A model of a building with an attached canopy of geometric scale of $1:100$ was constructed and tested in a simulated open terrain exposure. The attached canopy model was equipped with pressure taps at both upper and lower surfaces to allow for simultaneous monitoring of wind pressures and evaluation of the overall load. A total of 63 different building/attached canopy configurations were tested for 28 wind directions. Pressure and correlation coefficients were generated to provide a better understanding of how the wind-loading patterns at upper and lower surfaces of the attached canopy contribute to the net loading effect. Current design guidelines and building code and standard provisions are assessed and compared with the experimental results of the present study. The influence of the geometry of each configuration on the experimental net pressure coefficients was assessed and recommendations for design wind load standards and codes of practice are made.

Premature failure of wood roof sheathing under wind loading has primarily been blamed on poor nail installation resulting in reduced nail withdrawal strengths. However, this increased vulnerability could also result from environmental effects and aging of the materials, but little information exists on the in situ withdrawal strengths of nails in actual wood buildings. To address this knowledge gap, the authors evaluated the withdrawal strengths of nails from 17 existing Florida homes, and they observed relatively low in situ nail withdrawal capacities in the tests. Further, laboratory tests compared the withdrawal strengths of nails determined using an industry-standard test protocol against an in situ nail test procedure. The study documented the effects of (1) through-sheathing nail installation, (2) nail withdrawal rate, and (3) sheathing removal methods on nail withdrawal capacity. The in situ nail test procedure consistently produced lower nail withdrawal capacities than did the industry-standard test protocol. Nail withdrawal capacity is reduced by combination of local sheathing removal methods to access the nail head, and by driving nails through sheathing. The results suggest empirically determined nail withdrawal capacity for nails provide nonconservative results.

The primary purpose of the present study was to investigate the effect of incident flows (i.e., variations in terrain roughness) on wind loads and wind load combination effects on a target low-rise building. Three area densities—6, 11, and 25%—were considered, and three boundary-layer flows—a low-turbulent flow, and a grid-generated flow—were simulated. Results show that pressure coefficients for the isolated model differ significantly depending on the incident flows, which is a well-known fact, showing large absolute pressure coefficients for a flow with a larger power-law exponent (or high turbulence intensity). For the target model, which has other buildings in its immediate surroundings, the pressure coefficients seem to be less affected by the various incident flows, showing similar variation trends. The effects of boundary-layer flows on interference factor, zoning interference factor, and interference factor of combination factor, are small. Thus, it is concluded that terrain variations can be used to determine interference factors in other terrains, within the range in which most buildings are placed. However, those factors obtained from the grid-generated flow differ significantly from those from other incident flows.

This technical note investigates the dynamic response and stability of a rocking frame that consists of two identical free-standing slender columns capped with a freely supported rigid beam. Part of the motivation for this study is the emerging seismic design concept of allowing framing systems to uplift and rock along their plane in order to limit bending moments and shear forces— together with the need to stress that the rocking frame is more stable the more heavy is its cap-beam, a finding that may have significant implications in the prefabricated bridge technology. In this technical note, a direct approach is followed after taking dynamic force and moment equilibrium of the components of the rocking frame, and the remarkable results obtained in the past with a variational formulation (by the same authors) is confirmed—that the dynamics response of the rocking frame is identical to the rocking response of a solitary, free-standing column with the same slenderness, yet with larger size, which produces a more stable configuration. The motivation for reworking this problem by following a direct approach is to show, in the simplest possible way, that the heavier the freely supported cap beam, the more stable is the rocking frame, regardless of the rise of the center of gravity of the cap beam. The conclusion is that top-heavy rocking frames are more stable that when they are top-light.