Journal of Structural Engineering

Postfabricated beams, that is, newly constructed beams in an existing structural system, provide an effective remedial solution to the serviceability and strength problems in an existing floor system and the discontinuities and irregularities in the seismic-force-resisting system of a structure, mostly caused by design and construction errors or intended omissions of certain members. The purpose of this study is to develop new techniques for postfabrication of these beams and connecting the postfabricated beams to the existing columns and slab. In this respect, an experimental study was planned and a total of 16 half-scale specimens, including 3 monolithic (slab, columns, and beams cast monolithically) specimens and 13 specimens with postfabricated beams, were tested under monotonic vertical loads. The longitudinal reinforcement ratio of the postfabricated beam and the connection techniques of the postfabricated beam to the surrounding columns and slab constituted the test parameters. The tests indicated that the specimens with postfabricated beams were able to reach the strength and behavior of the monolithic specimens.

The performance of post-tensioned rocking cross-laminated timber (CLT) panels depends on the ability to maintain the post-tensioned force in the strand. This post-tensioned force may change over time due to the creep behavior of wood, which, in turn, is a function of time and moisture content in the CLT panels. In this study, a numerical moisture content diffusion model was developed to predict moisture content migration through CLT panels when the ambient relative humidity changes. Fick’s second law and the moisture content diffusion coefficients were applied to derive the differential diffusion equation for use in a numerical model. A four-element creep model was included to estimate the creep deformation of CLT panels over time under an axial load with changing environmental conditions. Data from a series of moisture content and creep tests under different configurations and environmental conditions were used to calibrate the proposed moisture content diffusion and creep model for CLT panels. The moisture content diffusion model was calibrated for two relative humidity steps, 50%–70% and 70%–90%. Then, a new creep model at material level that considers the change of moisture content in CLT panels was introduced. The viscoelastic parameters and mechano-sorptive constants were recommended for the creep model based on creep test data. Axial strain in CLT panels varied under 2% when ambient relative humidity switched between 50% and 70%. The axial strains in CLT panels with three layers were more sensitive to variations in surrounding relative humidity than those of CLT panels with five layers.

This paper deals with the experimental determination of ductility of ultra-high-performance fiber-reinforced concrete (UHPFRC) columns with longitudinal passive reinforcement, tested under combined axial and alternate flexural load. The influence of axial load has been studied, as well as the presence or not of a reduced shear reinforcement. One companion column made of ordinary concrete and with a high shear reinforcement ratio has also been tested for comparison. The ductility has been assessed with energy dissipation and ductility coefficient calculations. In addition, the maximum bending moment has been compared with theoretical calculation, using the material characteristics determined from characterization tests. The paper concludes that UHPFRC columns with a longitudinal reinforcement which have not been specifically designed for seismic resistance do display a ductility coefficient higher than 2 and a noticeable ability to protect the rebars from buckling, but have a rather high decrease of bending moment under the succession of series of alternate flexural load cycles.

This paper aims to accurately calculate the time-varying standard deviation of nonstationary wind speed by modeling wind speed as a time-varying mean wind speed plus a uniformly modulated nonstationary process and applying the autoregressive moving average–generalized autoregressive conditional heteroscedasticity (ARMA-GARCH) model to analyze the time-varying standard deviation of the wind speed. This paper also proposes a convenient and practical first-order difference GARCH method for calculating the time-varying standard deviation of uniformly modulated nonstationary processes that can decrease the computation time. The applicability of the ARMA–GARCH model, first-order difference GARCH method, and existing common methods for calculating the time-varying standard deviation of nonstationary wind speed are verified and analyzed using numerical simulations. The results show that the ARMA-GARCH model and first-order difference GARCH method are superior to the existing common methods. Finally, with the combination of the ARMA-GARCH model and first-order difference GARCH method, the nonstationary wind characteristics (considering the nonstationarities of mean wind speed and standard deviation) of Typhoon Chan-hom are investigated and compared with the results only taking into account time-varying mean wind speed.

Coastal structures are subjected to multihazard events such as hurricanes which consist of hurricane-induced surge and waves as well as winds. Hurricanes are a common natural hazard in the United States and cause considerable damage every year, with resulting annualized losses in the United States in the tens of billions of dollars. Although improvements in construction practices have been notable over time for individual hazards, there is still a dearth of risk and damage prediction methods in the area of multiple hazards that are based on principles of mechanics. In this study, a methodology to develop multihazard damage fragilities is summarized and illustrated for a wood-frame residential-building archetype subjected to hurricane winds, storm surge, and waves. The National Flood Insurance Program (NFIP) requires new buildings along the US coastline to be constructed with the first finished floor set at an elevation that exceeds a minimum necessary elevation. Therefore, two different elevations are considered for the lowest horizontal structural member of the archetype to also examine its effect on damage fragilities. The developed multihazard fragilities are used to calculate the time-dependent probability of each damage state at a given location over the timeframe of an event, i.e., hurricane. In this regard, the spatial and temporal data of wind speeds, flood depths, and significant wave heights for Hurricane Ike are simulated by the ADCIRC + SWAN model (a tightly coupled version of the ADvanced CIRCulation model and the Simulating WAves Nearshore model for simulating the propagation of storm surge and waves from deep water to the coastal region). The performance of nonelevated and elevated archetypes is examined at different locations in southeast Texas for Hurricane Ike and a scenario of damage states predicted for this area for the elevated archetype.

Fatigue cracking has caused a number of high-mast lighting tower (HMLT) failures throughout the United States. Both buffeting and vortex shedding have been shown to contribute to the failures observed. To increase HMLT safety, an investigation of the loading spectrum and response was conducted. The study included multiple goals; the primary deliverable was the development of a rational wind load for the fatigue design of HMLTs. Long-term remote monitoring was used to collect wind and strain data from 11 HMLTs distributed throughout the United States over the course of 2 years in order to investigate the effects of buffeting, vortex shedding, and associated dynamic oscillations. Data obtained from the field monitoring led to the development of a notional load model for HMLT fatigue design. The combined wind effect concept for fatigue design, which accounts for the effects of a variety of wind-induced loads, is introduced. Results from the research have been implemented into current design practice.

This paper presents the results of an experimental investigation of D-type self-centering eccentrically braced frames (SCEBF) subjected to cyclic loading. Analytical formulation of the SCEBF module frame’s load-displacement relationship is first presented. A total of five one-bay one-story SCEBF module frame specimens were experimentally tested under cyclic loading to investigate their seismic behavior. Finite-element simulation study of Specimen SCEBF2 was also conducted to assist with specimen design. The replaceable hysteretic damping (RHD) devices made of low-yield steel were installed at the corners of a rocking link beam to use the amplified motion due to the rocking link beam’s rotation and gap opening for energy dissipation. The experimental test results revealed that all the SCEBF specimens were capable of recentering and that the damage was confined to the RHD devices. It was also found that after severe cyclic loading, the SCEBF module frames with replaced RHD devices exhibited almost identical stiffness, strength, recentering ability, and energy dissipation capacity to those of the original structure.

This paper presents ultralow cycle fatigue tests and the calibration of different fracture models for duplex stainless-steel devices of high seismic performance braced frames. Two different geometries of the devices were tested in full scale under 14 cyclic loading protocols up to fracture. The imposed protocols consisted of standard, constant-amplitude, and randomly generated loading histories. The test results show that the devices have stable hysteresis, high postyield stiffness, and large energy-dissipation and fracture capacities. Following the tests, two micromechanics-based models, i.e., the cyclic void growth model and the built-in ABAQUS ductile fracture model, were calibrated using monotonic and cyclic tests on circumferentially notched coupons and complementary finite-element simulations. In addition, Coffin-Manson-like relationships were fitted to the results of the constant-amplitude tests of the devices, and the Palmgren-Miner’s rule was used to predict fracture of the devices under the randomly generated loading protocols. Comparisons of the experimental and numerical results show that the calibrated models can predict ductile fracture of the devices due to ultralow cycle fatigue with acceptable accuracy.

Modern structural design relies heavily on accurate numerical simulations of materials and structures. For concrete and RC, however, available computational models, although successful for many applications, fail to a large extent to correctly capture complex failure mechanisms. This is the case, for example, for failures occurring in regions in which the assumptions of classical structural theories do not apply and for situations characterized by extensive fracture and size effect. To overcome this issue, this paper investigates the use of a discrete mesoscale model, the so-called lattice discrete particle model (LDPM), for simulation of the flexural behavior of RC structural elements. LDPM captures naturally complex fracture phenomena in a variety of loading conditions because it simulates material heterogeneity. This is obtained by replacing the actual concrete internal structure with a system of polyhedral cells interacting through nonlinear and fracturing lattice struts. The results presented in this paper show that LDPM can be used to predict with great accuracy the ultimate flexural behavior of RC beams with a wide range of main and secondary reinforcements. LDPM predicts very well the transition from ductile to brittle behavior for increasing reinforcement ratios in slender and stocky beams, and, most importantly, predicts the quasi-brittle characteristics of failure and the associated size effect.

This paper presents a wind performance–based evaluation of an existing 47-story steel moment frame high-rise building designed and built in the early 1970s according to the Houston Building Code in effect at the time with limited wind design criteria (strength requirement under factored wind loads, but no serviceability requirements considered) and no seismic design criteria. The building survived Hurricane Alicia (a Category-3 storm, 1983) without significant structural damage. The nonlinear dynamic response of the building to different wind hazard levels was evaluated by developing a 3D nonlinear finite-element model and using an incremental dynamic analysis (IDA) approach. Using results obtained from the IDA study and using wind performance criteria currently in practice, a wind performance assessment was carried out by evaluating the estimated performance levels as a function of basic wind speed. The results of this study indicated a surprisingly robust lateral capacity associated with the structural nonlinearity that occurred, but confirmed the existence of unacceptable serviceability performance under various levels of wind loading.

Loadbearing concrete block masonry construction in North America is generally designed by engineers as a 50% running bond pattern. However, in some situations an architect opts for a stack pattern (or stack bond) unit coursing, not realizing that there are structural implications to this choice. The lack of research in the area of the structural effects of a unit bonding pattern has led to inconsistent and sometimes very punitive requirements for loadbearing masonry design in North America. This paper presents the results of an experimental study on 12 concrete block masonry beams with depths of two, three, and four courses to study the structural effects of a unit bonding pattern. Beam stiffness, crack patterns, deflection profiles, and ultimate strengths are presented and discussed. Using a stretcher unit with approximately 50% of the web height removed to permit grout continuity demonstrated no significant difference between running bond and stack pattern unit coursing. This evidence supports changes to current Canadian design requirements to include stack pattern masonry beams with this unit configuration. This evidence may also be considered to support the adoption of more stringent recommendations in current American design requirements.

A novel model based on the three-parameter Weibull function is proposed to describe the three-stage fatigue deformation behavior of plain and fiber-reinforced concrete. The fatigue strain at a particular stress between zero and the maximum fatigue stress can be modeled using the proposed model, and all the model parameters have clear physical meanings. This model is validated via comparison of its results with previously reported results of compressive, tensile, and flexural fatigue tests. Cases of application of the model to plain concrete and fiber-reinforced concrete with high ductility are examined in order to investigate the variation of the model parameters. Additionally, a deformation-based method for prediction of the fatigue life of concrete is presented, and the prediction results demonstrate that the proposed model can be successfully applied to the estimation of the fatigue life of concrete materials.

In this paper, static and dynamic experiments on reinforced concrete beam-column frames under single-column-removal scenario applying a multipoint loading method were conducted. One of the objectives was to investigate structural behavior compared with the single-point loading method, which has been popularly used in previous studies. By equally applying point loads at four locations of the double-span beam structure, the test setup successfully simulated the uniformly distributed loading condition, which is generally applicable for gravity loads in practice. Compared with the previously conducted static tests based on concentrated loading method, structural response from the static tests under multipoint loading condition differed not only on load-bearing capacity but also on failure sequence and displacement profile. On the basis of the test results, the analytical relationship between behaviors of the two loading methods was developed and verified. Compared with the static tests, the dynamic tests highlighted dynamic effects created by the column loss event and confirmed the structural behavior and failure modes observed in the static environment. The dynamic tests also verified the correctness and conservatism of the dynamic assessment framework using a previously proposed energy-based approach. Most important, both the static and the dynamic tests of structures under distributed loads showed less development of catenary action against progressive collapse compared with concentrated loading tests.

This paper presents a numerical investigation of the influence of varying three parameters of the cyclic analysis of shear walls in wood-frame structures (CASHEW) 10-parameter wood-frame shear wall hysteretic model that have distinctly different effects on the shape of the backbone curve. The purpose is to assess the sensitivity of wood-frame shear wall collapse performance to changes from the baseline assumptions used to validate the FEMA P695 collapse performance methodology. The variations in backbone curve shape are intended to reflect observed variations in experimental response due to test methods, test boundary conditions, and shear wall aspect ratio. The paper also investigates the influence of including P-delta effects and the use of assumed equivalent viscous damping of 5% of critical. The baseline assumptions exclude P-delta effects and use 1% damping. A total of 126 unique analyses are conducted for six of the FEMA P695 wood-frame shear wall index models ranging from two to five stories. The results show that improved collapse performance (increased adjusted collapse margin ratio) occurs for hysteretic model assumptions that reduce (less negative) postpeak stiffness, increase displacement at peak strength, and increase peak strength intercept. These hysteretic model changes produce improved collapse performance for cases modeled with and without P-delta and for cases modeled with 1% and 5% damping. The inclusion of P-delta effects reduces adjusted collapse margin ratio by an average of 10% relative to the without-P-delta baseline. Use of 5% damping in these models improves adjusted collapse margin ratio by an average of 13% relative to the 1% damping baseline.

The development of cross-laminated timber (CLT) has created new opportunities for the use of lumber from low-grade and low-density species in structural applications. In order to ensure the structural viability of CLT panels with such material, hybrid layups have been proposed, in which the low-grade and low-density lumber was placed in core layers, while high-grade (#2 or better) and high-density lumber was used in the face layers. One major criterion in the evaluation of hybrid CLT for structural uses is their performance with metal connectors, which plays a major role in seismic behavior of CLT structures. Previous studies have shown that when properly designed and detailed, CLT buildings can perform well during seismic loading, and they emphasized the role of connections in structural design, modeling, and analysis of CLT buildings. In the United States, the existing design standards, however, do not account for the variation of density in wood when estimating connection performance characteristics. This paper presents the results of a testing program developed to estimate the hysteretic response of selected CLT connection systems applied on hybrid CLT panel layups. The cyclic performance of three connection systems readily available in the North American market was determined using a modified Consortium of Universities for Research in Earthquake Engineering (CUREE) cyclic loading protocol. Results from the testing program indicate that a statistically significant difference exists between results of the connections systems on hybrid and conventional panel layups only when the failure mode involved fastener damage that penetrated the core of the hybrid CLT panels. Analytical results are also provided for cyclic force-deformation response of the connections for which modeling parameters have been calibrated using a proposed semiautomated calibration algorithm.

Stress fields around structural discontinuities such as cracks usually cause complex but distinct strain contours/maps when structures are subjected to load. Hence, mechanical strain on structural surfaces can provide useful information on the condition of materials, including damage location and severity. The phenomena of stress concentration or strain concentration around discontinuities (such as holes and cracks) can be used to perform nondestructive evaluation (NDE) of structural components. Among analytical computation, numerical simulation, and experimental studies to investigate the stress/strain field around a structural discontinuity, experiments are the most accurate in revealing the actual complex strain conditions. In this paper, a strain-sensing smart skin (${\mathrm{S}}^4$) film sensor was used to study the strain distribution in metallic plates near different structural discontinuities. ${\mathrm{S}}^4$ is a newly developed, noncontact, full-field strain technology that utilizes the strain-sensitive photoluminescent properties of single-walled carbon nanotubes (SWCNTs). Aluminum bars in tension were studied in two cases—with a central hole and with an edge notch. In both cases, ${\mathrm{S}}^4$ film sensors measured the residual strain contours near structural discontinuities under large axial loading at room temperature. Linear elastic fracture mechanics (LEFM) was used to compute the closed-form solution of strain fields. Finite-element elastoplastic nonlinear models were also constructed and validated by using strain gauge data from the experiments. The FE analysis results were found to match the strain distribution obtained from ${\mathrm{S}}^4$ measurements. ${\mathrm{S}}^4$ technology can be usefully applied in the realms of nondestructive evaluation, experimental mechanics, and structural health monitoring.

Management of bridge assets requires a life-cycle approach that balances design and construction of new bridges, and maintenance of existing bridge stock. There has been a growing need for inspection, maintenance, repair, and strengthening of existing bridge structures due to aging transportation infrastructure. Condition assessment of post-tensioning systems could allow bridge owners to take timely, proactive actions to mitigate or prevent further deterioration and unanticipated failure. An experimental study to evaluate various nondestructive evaluation (NDE) techniques for the condition assessment of grout defects in external post-tensioning systems is presented. To evaluate the NDE technologies, a full-scale post-tensioned girder specimen was constructed at the Texas A&M University RELLIS Campus. Several grout defects including voids, water infiltration, and compromised grout were fabricated and placed in the specimen for the evaluation of the NDE methods. The research team, in collaboration with several industry partners, used various NDE technologies, such as sounding, impact echo, low-frequency ultrasound, ground-penetrating radar, infrared thermography, and electrical capacitance tomography, to evaluate the grout condition of the external tendons of the post-tensioned girder specimen. Comparison of the test results with the actual defects revealed the performance of each method in detecting the location and the severity of the various grout defects in the external tendons of the post-tensioning system.