Drift Concentration Research Articles

Reduced Streamflow From Water Diversion Alters Stream Ecology and Fish Behavior

ABSTRACTUnderstanding invertebrate and fish responses to altered streamflows is important for water management that balances ecological function and societal needs. We studied the impacts of a significant water diversion on a stream food web, including benthic biofilms, benthic macroinvertebrates (BMI), and drifting macroinvertebrates. In the dry season, drift flux (number of aquatic individuals or biomass/s) and drift concentration (individual/m3) were lower below the diversion than above. Drift concentration (biomass/m3) was slightly higher below the diversion, due to the presence of large Trichopterans. All measures of drift dropped to nearly zero at the end of the dry season below the diversion, when streamflows were reduced by > 90% below the diversion relative to above, representing a significant loss of food resources for trout. Simultaneously, native Rainbow Trout (Oncorhynchus mykiss) switched from drift to benthic foraging 1 month earlier below the diversion as drift and water velocities declined. The ecology of the benthos was not as impacted by the diversion: BMI densities were higher below the diversion, while benthic biofilm densities were similar at the two sites. The taxonomic composition of BMI was also similar between the sites, but fewer Ephemeroptera entered the drift below the diversion. Overall, the late season decrease in drift below the diversion despite the similar or higher benthic production suggests that streamflows influence food availability for trout even in the absence of benthic responses. Reduced streamflows, due to water diversions as studied here, or due to climate change, will likely reduce food resources and growth opportunities for fishes.

Mitigating inter-story drift concentration in seismic-resistant self-centering braced frames by using strong backup systems

Seismic-resistant self-centering concentrically braced frames (SC-CBFs) are susceptible to the concentration of inter-story drifts during earthquakes owing to the relatively low energy dissipation ability of braces. To address this limitation, this study proposed a novel solution by designing a strong backup (SB) system to mitigate inter-story deformation concentration in “weak” stories. The proposed SB system consisting of truss members can be attached to the existing SC-CBF through pin connections, forming a system, termed strong backup SC-CBF (SC-CBF-SB), to promote a more uniform distribution of inter-story drifts along the height of the frame and mitigate the weak story behavior. A six-story chevron-braced frame is adopted to investigate the seismic performance of SC-CBF and SC-CBF-SB. Finite element models of SC-CBF and SC-CBF-SB are built. The mechanical characteristics and dynamic responses of the SC-CBF-SB are examined. To comprehensively evaluate the performance of both SC-CBF and SC-CBF-SB, static pushover analyses and nonlinear time-history analyses are conducted. Additionally, incremental dynamic analysis (IDA) is performed to evaluate the responses (particularly drift concentration) of both frame types subjected to increasing seismic intensity levels. Numerical results show that the maximum value of the drift concentration factor (DCF) is around 1.3 and 1.8 for SC-CBF-SB and SC-CBF, respectively, indicating that SC-CBF-SB can effectively mitigate inter-story drift concentration of SC-CBF. Meanwhile, the proposed SB system has a minimal negative impact on the favorable SC ability of the frame.

Nonlinear modeling and time-history analysis of dry-assembled precast concrete frames with buckling-restrained braces

A combination of dry-assembled precast concrete frames and buckling-restrained braces (BRB) is a viable choice for achieving the full potential of building industrialization and promoting the popularization of precast structures in earthquake-prone zones. A recent study has tested the seismic performances of a dry-assembled precast concrete frame with BRBs (BRB-DPCFs) under cyclic quasi-static loading. To study the comparative seismic behaviors of BRB-DPCFs and the monolithic concrete frames with BRBs (BRB-MCFs) under ground motion records, buildings between 3 and 12 stories in height were designed based on GB50010-2016, Chinese code for design of concrete structures, and investigated numerically. This investigation includes two-dimensional nonlinear time-history analyses of BRB-MCFs and BRB-DPCFs under two seismic hazard levels. BRB-DPCF demonstrated a larger maximum story drift compared to BRB-MCF, while the maximum residual story drift of the former is negligible in comparison to the latter at a higher building height or under a higher seismic intensity. The two systems show similar drift concentration factors, which was observed irrespective of the hazard level and the specific height of the building.

Cooling Tower Plume and Drift Concentration in presence of Urban Canyon

This paper presents a computational analysis of the cooling tower plume and drift emitted by mechanical draught cooling towers situated in urban areas. The data from the Wang X (2019) and Briggs plume lift height formula are used to prove results obtained from computational Modeling of cooling tower drift and it was found to be effective. The simulation setup uses a multiphase mixture model for plume analysis and DPM for analysis of drift in the Ansys fluent. The CFD simulation results are accurate within 50 to 100 m with the Briggs plume lift height formula. The effect of wind coming from urban canyons on the cooling tower results in decreased plume lift height and increase drift concentration and deposition rate on the ground.

Theoretical model to predict the seismic drift concentration effect for self-centering-braced frame

This study is motivated to predict such drift concentration effects for SCB frames with the perfect flag-shape hysteretic behavior. Nonlinear time history analyses are conducted to compare and elucidate different deformation mechanisms between BRB and SCB frames. An elastic-plastic static model is established to predict the static drift concentration factor (DCFS) for the SCB frame under static lateral loads, where influential parameters include the column-to-frame stiffness ratios, α, and the SCB's post-yielding stiffness ratio, βk,2. To capture the dynamic loading effect, a regressive formula is further developed as the dynamic amplification factor by normalizing the dynamic drift concentration factor (DCFD) with the DCFS under static loads. The results show that unlike BRB frames with drift concentrated in the bottom story, SCB frames often feature peak story drifts in the top story. As opposed to BRB frames, the equivalent lateral loads do not feature an inverted triangle distribution but a parabolic distribution along the building height. The drift concentration effect for the SCB frame is reliably predicted by multiplying static DCFS with the corresponding closed-form formula for the dynamic amplification factor. The predicted DCFD is also utilized to identify a design range of βk,2 (i.e., 0.2–0.4) to effectively control the peak story drift and drift concentration for SCB frames.

Post-earthquake functionality assessment of MRFs enhanced by resiliently lateral-resistant and bottom systems

The accumulated stiffness and strength degeneration of rigid bottom joints and uneven lateral deformation mode of the moment-resisting frame (MRF) during earthquakes can result in disastrous structural failures. Damage accumulation, excessive residual inter-story drift, and the lack of self-centering behavior and energy-dissipation capacity in structures are major challenges in controlling losses after earthquakes. This paper aims to propose a self-centering energy-dissipation (SCED) system that enhances the seismic performance of MRFs. This system, called the resilience-enhanced MRF (ReMRF), utilizes a rocking truss as a lateral-resistant structure and incorporates two bottom joints with SCED capabilities into the MRF design. These bottom joints, referred to as the Buckling Restraint Steel Plate (BRSP) bottom joint and the Light Self-Centering (LSC) bottom joint, are composed of key components such as BRSPs and post-tensioned (PT) strands, respectively. Structural design methods and parameters for the rocking truss and bottom joints are discussed to determine optimal values for balancing stiffness demand, SCED capacity, and construction convenience. Special attention is paid to the working stability of the bottom joints during earthquakes and the effectiveness of the SCED system. Results indicate that the performance of the bottom joints and the SCED system is in line with expectations. The findings from both pushover and time history analyses have substantiated that ReMRF outperforms MRF in terms of achieving higher seismic performance objectives. It is intuitively demonstrated by several quantitative metrics derived from static and dynamic analyses, including the overstrength factor, ductility factor, drift concentration factor, peak inter-story drift, and residual inter-story drift.

A design method for seismic retrofit of reinforced concrete frame buildings using aluminum shear panels

Despite significant progress in research and development of aluminum shear panels in recent decades, their implementation for seismic retrofit of existing reinforced concrete (RC) buildings can still be significantly extended. Their application is limited by the general lack of relatively simple and effective design criteria and proper guidelines. This paper develops a design method for the seismic retrofit of reinforced concrete buildings using aluminum multi-stiffened shear panels as dampers. Both the nonlinearity in the structure and the dampers-structure interaction are considered to give an optimal distribution of the shear panels over the height of the building. The analytical laws refer to dissipative aluminum shear panels recently tested and analyzed by the authors. The proposed procedure has been described in detail. Its applicability has been demonstrated by analyzing two typical RC buildings having drift capacity-to-demand ratios ranging from 0.505 to 0.624. The design value of the panel-to-frame stiffness ratio has been found to range from 0.594 to 1.432 as a function of the lateral stiffness of the existing building. The verification of the proposed procedure has been carried out by checking the validity of the design assumptions. The first one (i.e., the mode shapes remain the same before and after retrofit) has been checked using the modal assurance criterion that gives values ranging from 0.992 to 0.998. The second one (i.e., uniform yield drift distribution over the building height) has been checked by comparing the yield drifts with their average value giving a standard deviation ranging from about 11 to 15%. The effectiveness of the design method has been finally validated through nonlinear time-history analysis for different seismic accelerograms and hysteresis models. The results show that the seismic retrofit design procedure is effective in significantly reducing inter-story drift (maximum inter-story drift ratio demands ranging from 1.04 to 2.07%) thus satisfying the acceptance criteria of the building, and avoiding drift concentration and consequential weak story collapse.

Improved seismic resilience of existing steel moment frame by built-in continuous columns

A seismic resilience strategy of retrofitting the existing columns of frame structures into built-in continuous columns (BCCs) hinged to the main frame is presented in this paper. The continuous columns provide vertical continuous stiffness to adjust the structural deformation distribution achieving collapse control. This study retrofits three 5-, 7- and 9-story prototype steel moment frames (SMFs) through external continuous columns (ECCs) or BCCs. In addition, effects of strength and stiffness deterioration on the seismic response of the SMFs are also considered. Results from pushover and incremental dynamic analysis show that the ECCs and BCCs can both mitigate structural displacement concentration. With a performance target of plasticity hinge ratio up to 50% for the main frame, when the stiffness demand is satisfied, the inter-story drift concentration factor of the BCC frame is always smaller than that of the ECC one. It indicates that the inter-story deformation distribution of the BCC frame is more uniform under the premise of satisfying the target collapse resistance performance.

Seismic design and parametric study of steel modular frames with distributed seismic resistance

Structures in modular buildings typically have some unique characteristics as compared with conventional structures, e.g., discrete connection of modules through inter-module connections, discontinuous floor diaphragms. The behavior of steel modular structures under earthquake excitations has not been fully understood, and no seismic design method specifically tailored for modular building structures is available. Moreover, although various inter-module connections with different rotational connectivity have been proposed, their suitability for seismic application is questionable. In this paper, the distributed seismic design method, which makes use of the lateral resistance inherent in all modules, was proposed for modular buildings with steel frames. A numerical parametric study was conducted on a 9-story prototype building. The effect of three parameters, i.e., the rotational stiffness of inter-module connections, the seismic design force level, and the height-wise distribution of the design base shear, were studied. The results show that the rotational stiffness of inter-module connections has limited impact on the elastic lateral stiffness and the fundamental period of modular steel frames. However, in the inelastic range, the increase in the rotational stiffness will lead to less plastic drift concentration and better collapse prevention performance. Increasing the seismic design force may not result in enhanced collapse prevention performance, as it is also dependent on the height-wise distribution of the design base shear and if significant higher-mode response is involved in the total response of the structure.

Seismic application of steel rocking column bases with replaceable cover plates in steel MRFs

This paper presented unidirectional and bidirectional cyclic loading test results of a novel steel rocking column base. Feasibility of the column base in steel moment-resisting frames was verified through nonlinear static and time history analysis. It was found that the column base was capable of dissipating seismic energy through plastic bending deformation of the replaceable cover plates and flag-shaped hysteretic curves can be observed. Nevertheless, it is not expected to provide much energy dissipation for the whole structural system, since the plastic bending deformation of each cover plate is only in one direction. A three-dimensional simplified numerical model of the column base was developed and validated against the test results. Following that, nonlinear static and time-history analysis were conducted on prototype steel moment-resisting frames with and without the rocking column bases. In general, replacing some column bases with the rocking ones (i.e., 4/24 of all in this study) did not cause much reduction in lateral resisting stiffness and capacity. Moreover, this reduction was even slighter when the loading direction was perpendicular to the weak axis of the rocking column section. On the other hand, replacing all fixed column base with the proposed rocking column base may cause drift concentration in the ground storey, which is too radical for the prototype building. The seismic responses of modified frames were generally very close to that of the original frame with all fixed column bases. Even though the maximum drifts in the ground storey of these frames were larger than that of the original frames with fixed bases, the maximum drifts of the whole structures were not expected to be amplified. • Bidirectional cyclic loading tests on a novel steel rocking column base were conducted. • The column base showed flag-shaped hysteretic cures and dissipated seismic energy through plastic deformation of the replaceable cover plates. • Feasibility of the column base in steel moment-resisting frames was verified through nonlinear static and time history analysis. • Replacing some column bases with the rocking ones did not cause much reduction in lateral stiffness and capacity.

Application of self-centring hybrid rocking columns in steel frames

This study proposed a novel rocking component, namely self-centring hybrid rocking columns (SCR). Cyclic loading tests are conducted on a substructure consisting of the SCR and a frame beam-to-SCR connection to investigate its hysteretic performance. A numerical model of the SCR substructure is built and validated against the test results. Finally, the seismic performance of steel frames equipped with the SCR is demonstrated through the nonlinear time history analysis. It shows that the proposed self-centring hybrid rocking column has acceptable lateral resisting, energy dissipation and self-centring capability. The simplified numerical model of the SCR can reasonably well capture the hysteretic behaviour of the SCR substructure no matter with or without the steel damper and tension braces. The system-level analysis of steel frames with the SCRs verifies the effectiveness of the proposed SCR in mitigating maximum and residual drift responses of steel frames under earthquakes. After equipping the SCRs, the inter-storey drift concentration is also mitigated under earthquakes.

Seismic retrofitting of SMRFs using varied yielding cross-section damper: A companion paper

This paper is a companion paper to our recent research titled “Seismic Performance of a New S-shaped Mild Steel Damper with Varied Yielding Cross-sections”. In this study, the newly developed varied yielding cross-section damper is employed for seismic retrofitting of steel moment-resisting frames (SMRFs) and a retrofitting design method without performing nonlinear time-history analysis is proposed. The damper has the feature of preventing energy dissipation plate from plasticity concentration-induced failure and of being conveniently replaced post-earthquake. Its hysteretic behavior has been verified through a series of cyclic loading tests. Three prototypes respectively with 3, 9 and 20 stories are adopted as typical low-, mid- and high-rise SMRFs to be damper-retrofitted. And the dampers are designed using the proposed retrofitting procedure in which performance objective of different seismic hazard level and story drift concentration are incorporated. Subsequently, experimentally validated numerical modelling approach for the damper is provided, and detailed numerical models for the bare frames (BFs) and damper-retrofitted frames (DFs) are established. In order to analyze the effectiveness of the developed dampers and the retrofitting design method, seismic performance of the BFs and DFs is assessed by conducting pushover and nonlinear dynamic analyses. The results indicate that the retrofitting strategy can significantly enhance the bearing capacity against strong earthquakes and mitigate the structural damages by reducing inter-story drift ratio and residual inter-story drift ratio; the proposed retrofitting design procedure has ability to achieve the expected seismic performance level and mitigate the story drift concentration.

Novel hysteretic damper to improve the distribution of story drifts and energy dissipation along the height of braced frames

A novel hysteretic damper is designed for deployment in Chevron-braced frames of buildings to improve their seismic performance. The new damper, Torsional Hysteretic Damper for Frames (THDF), is a device that utilize torsional yielding of energy dissipating units in the shape of cylinders, which are made of ductile steel, to dissipate the imposed energy through seismic movements. The damper is distinguished by a variable, displacement-dependent post-elastic stiffness. This feature of the damper is intended to improve the height-wise distribution of story drifts and reduce the drift concentration in multi-story buildings. Following the conceptual development of the damper, derivation of force–displacement equations and other analytical studies necessary to implement the damper in practice are performed. Next, numerical studies are conducted to explore the effect of the THDF on the height-wise distribution of story drifts. The THDF was found to improve the distribution of story drifts along the height of braced frames. In the experimental phase of the research, a 1/3rd–scale Chevron-braced frame equipped with the newly-developed damper was subjected to cyclic tests, in order to verify the performance of the THDF and its force–displacement characteristics. Experimental force–displacement loops of the THDF was compared with analytical predictions and a good agreement was observed.

Application of seismic resilient energy-dissipative rocking columns with HSS tension braces in steel frames

Steel moment resisting frames are widely used as seismic resisting systems of buildings in earthquake regions. Since their satisfactory performance is achieved by developing plastic deformation in critical regions, it usually leads to inter-storey drift concentration during a strong earthquake and significant residual deformation after a strong earthquake. To mitigate seismic damage concentration and excessive residual deformation, this study proposed a high post-yield stiffness rocking system, namely energy-dissipative rocking columns with high strength steel tension braces (EDR-TB). The study commenced with lateral cyclic loading tests on the proposed EDR-TB column. Subsequently, numerical model of the EDR-TB was built and validated against test results. System-level nonlinear static and time-history analysis were then conducted on the steel frame with various layouts of the EDR-TB columns. Compared with a conventional steel frame, the frame with EDR-TB columns exhibited much reduced maximum and residual drifts. In addition, the frame with EDR-TB columns exhibited more uniform distribution of drifts. The results also reveal that an appropriate arrangement of lateral resisting capacity of the EDR-TB columns over the structural height may achieve an enhanced seismic performance of the whole structures.

Experimental and numerical study of beam-through energy-dissipative rocking columns for mitigating seismic responses

The Energy Dissipative Rocking Column is a novel seismic mitigation device that could effectively mitigate maximum inter-storey drift and drift concentration of low-rise buildings under earthquakes. This study proposed a beam-through configuration for the connections between steel frames and Energy Dissipative Rocking Columns (EDRCs) to reduce the necessary workload in the application of EDRCs. The study commenced with a cyclic loading test on a substructure composed of the beam-through EDRC and frame beam-to-EDRC connections. Following the cyclic loading test, a simplified numerical model of the beam-through EDRC was built and validated against the test results. Finally, parametric studies were conducted to determine the seismic demand on the beam-to-EDRC connections. The results showed that the beam-through EDRCs have acceptable lateral resisting and energy dissipation capacity. The simplified numerical model could reasonably well predict the hysteretic behaviour of the specimens with and without the steel damper. The parametric study results, based on hundreds of nonlinear time history analyses, indicated that increasing connection stiffness in the range of 0 to 1.5 is most effective in mitigating both maximum drift and drift concentration. Moreover, the beam-through EDRC has very close effectiveness with the conventional column-through EDRC when the connection stiffness ratio is close to or larger than 4.

Seismic response of dual concentrically braced steel frames with various bracing configurations

Steel dual structures incorporating special concentrically braced frames (SCBFs) and special moment frames (SMFs) are, in general, effective for their high lateral stiffness and ductility. As such, ductile dual CBFs are often treated as a remedy for mitigating permanent drift demands along with the potential drift concentration in isolated stories. However, despite the benefits and interest, the effect of the bracing configuration on the seismic response of the dual concentrically braced systems, designed following the current capacity design approach stipulated in AISC Seismic Provisions, in particular, has not been thoroughly examined to date. The present study aims to shed light on the impact of the bracing scheme on the seismic behavior of dual SCBFs coupled with SMFs as backup frames. For this purpose, 4- and 10-story dual SCBFs with the commonly used chevron, split X, V, and cross X type bracing configurations are designed according to ASCE 7 and AISC 341. Then, the frames are subjected to a suite of ground motions. The nonlinear time-history analysis results are contrasted and discussed in terms of the peak and permanent drift demands, horizontal floor acceleration demands along with brace ductility demands.

Performance-based seismic design method for retrofitting steel moment-resisting frames with self-centering energy-absorbing dual rocking core system

This research intends to propose an upgraded strategy for steel moment-resisting frames (SMRFs) by implementing Self-Centering Energy-absorbing Dual Rocking Core (SEDRC) systems to achieve enhanced seismic performance, improved seismic resilience, and avoid inter-story drift concentration. A performance-based method is developed to design the SEDRC system for upgrading the existing SMRF. The maximum inter-story drift and drift concentration factor are adopted as the design performance objectives. Two prototype SMRFs with three and nine stories are introduced to validate the proposed performance-based design procedure. Nonlinear static analyses were first performed for the original SMRFs and the updated ones (denoted as SEDRC-MRFs) to investigate the efficiency of the proposed retrofit strategy. The pushover curves of the analytical structures confirm that the SEDRC systems can increase the stiffness and strength of SMRFs, and SEDRC-MRFs upgraded through the proposed design procedure show no strength deterioration before the fracture of the brace in the SEDRC system. A set of 18 ground motions were chosen to study the seismic performance of the designed SEDRC-MRFs. The analysis results indicate that SEDRC-MRFs can obtain the desired performance objectives. The SEDRC system could reduce the maximum and residual inter-story drifts of SMRFs and promote uniform inter-story drift distribution. Moreover, the designed SEDRC-MRFs can be used immediately without the need for repairing structural members after the design basis earthquakes and can be repaired economically after the maximum considered earthquakes.

Proportioning of self‐centering energy dissipative braces

AbstractSelf‐centering energy dissipative (SCED) braces are designed to limit the maximum story drifts in buildings during earthquakes and to nearly eliminate residual drifts. The key properties of conventional braces (eg, strength, stiffness) can be determined with current seismic design procedures, but additional criteria are needed to select the hysteretic properties of SCED braces (eg, prestressing level, fuse activating deformation). Past investigations of the seismic performance of the SCED braced frames selected brace characteristics to match the hysteretic characteristics of buckling restrained braces (BRB). However, those characteristics may not be achieved because the performance of the SCED system is constrained by the elongation capacity of the post‐tensioned tendons. To develop recommendations for proportioning SCED braces, the researchers conducted a parametric study on the properties of two typical SCED systems, using a nine‐story steel braced frame office building. The seismic performances of the SCED systems were then compared with that of a BRB system including an analysis without property uncertainty and an analysis with uncertainty using Monte Carlo simulation. Based on the cost and seismic performances of SCED systems designed using various characteristics, recommendations are proposed for proportioning the SCED systems. Besides having much smaller residual drifts, the SCED systems had a smaller likelihood of exceeding a maximum story drift of 3% and less drift concentration compared with the BRB system, though they generated larger floor accelerations and forces in connections and adjacent members. The property uncertainty increased the residual drift of the SCED systems.

Seismic performance assessment of steel frame structures equipped with buckling-restrained slotted steel plate shear walls

The near-fault ground motion records may impose high seismic energy input to the building structures. The steel plate shear walls (SPSW) are one of the robust and efficient lateral-force resisting and energy-dissipating components and SPSWs are increasingly equipped in steel moment frame to form a dual structural system. A modified type of buckling-restrained SPSW with inclined slots has been proposed and applied in the steel frame. This paper presents the seismic performance of steel frame-SPSW system subjected to near-fault ground motions. Four multi-story (5-, 10-, 15- and 20-story) steel frame-SPSW structures were selected and designed. The accuracy of the SPSW models' predictions was assessed using previous experimental results. Nonlinear time-history analysis assuming two groups of 15 near-fault ground motions and 15 far-fault ground motions was used to predict the structures' interstory drift ratios, floor accelerations, residual interstory drift ratios and drift concentration factor. Incremental dynamic analysis was used to predict collapse probabilities using the same groups of ground motions. The analytical results can provide significant insights to the seismic behavior of steel frame-SPSW structures when subjected to near-fault ground motions.

Foraging modes and movements of Oncorhynchus mykiss as flow and invertebrate drift recede in a California stream

Salmonids frequently adapt their feeding and movement strategies to cope with seasonally fluctuating stream environments. Oncorhynchus mykiss tend to drift-forage in higher velocity habitat than other salmonids, yet their presence in streams with seasonally low velocity and drift suggests behavioral flexibility. We combined 3D videogrammetry with measurements of invertebrate drift and stream hydraulics to investigate the drivers of O. mykiss foraging mode and movement during the seasonal recession in a California stream. From May to July (2016), foraging movement rate increased as prey concentration and velocity declined; however, movement decreased in August as pools became low and still. In May, 80% of O. mykiss were drift-foraging, while by July, over 70% used search or benthic-foraging modes. Velocity and riffle crest depth were significant predictors of foraging mode, while drift concentration was a poor univariate predictor. However, top-ranked additive models included both hydraulic variables and drift concentration. A drift-foraging bioenergetic model was a poor predictor of foraging mode. We suggest that infall and benthic prey, as well as risk aversion, may influence late-summer foraging decisions.