Lateral Load-resisting Elements Research Articles

Seismic Response of Stiffness Irregularity at Ground Floor with and without Shear Walls

In today's scenario, one of the greatest challenges for structural engineers is designing and constructing seismic-resistant structures. Seismic activity, or earthquakes, poses a significant threat to civil engineering structures, and ensuring that buildings can withstand the forces generated during an earthquake is crucial for public safety and infrastructure resilience. Irregular configurations, whether in the building's floor plan or its elevation, are widely acknowledged as significant contributors to failure during seismic events. These irregularities can lead to uneven distribution of forces and stresses, compromising the building's ability to withstand the seismic forces and potentially resulting in structural failure. Soft storeys, typically located at ground levels for various functional purposes, pose challenges in seismic regions due to their lack of sufficient lateral load-resisting elements resulting in excessive lateral deformation and collapse during intensive earthquakes. Hence, the present study investigated the seismic response of irregular reinforced concrete structures possessing stiffness irregularity at ground floor with and without shear walls. A ten-storey regular frame is modified by incorporating vertical irregularity in elevation by increasing the height of the ground floor. The complete structural analysis and modeling are carried out by using the software ETABS 2020. The Time History method is applied, and the study is focused on seismic zones V in India. The performance of structures are compared based on criteria such as storey displacement, storey drift, storey shear and overturning moment. The results lead to the conclusion that a building structure exhibiting stiffness irregularity is prone to instability which is indicated by higher displacement and drift values. Structures incorporating shear walls have demonstrated greater stability compared to structures without shear wall as they exhibited higher base shear values and experienced a reduction in lateral displacement by more than 40%. The presence of shear walls also has enhanced the stability and strength of the structure, showing a linear response during critical earthquakes.

3D FE modeling of a complex multi-stage foundation underpinning for a high-rise building

ABSTRACT The piled foundation of an existing high-rise building in Cairo was assessed as inadequate. The building foundation consists of 370 piles arranged below the columns and walls. The building was constructed in 1977, and up to the start of this investigation in 2014, it was just a concrete skeleton without finishing. The existing foundation was subjected to underpinning in 1985 using micropiles to support new lateral load-resisting elements proposed by the project structural consultant. According to the new Egyptian codes of practice, underpinned foundations are still considered inadequate and it was proposed to add additional micropiles with a raft connecting the different pile groups. This paper presents the 3D FE modeling for this complex multi-stage foundation underpinning considering the construction sequence and the different interactions between the original piles, existing and proposed groups of micropiles, supporting soil and the capping raft. The load transferred by direct contact between the raft and the soil is also investigated. Available detailed geotechnical investigation and evaluation of the available micropiles loading tests were used to estimate the parameters of soil and pile−soil interface. The advanced 3D FE model is used to ensure that the proposed number of additional micropiles satisfies the required safety and stability. The outcome of the analysis proved to conservatively satisfy safety requirements. The raft transferred a considerable part of the total load directly to the soil, hence increasing the capacity of the combined piled raft foundation system.

Influence of Post-Elastic Range Bidirectional Interaction for Various Angles of Incidence of Ground Motions on One-Story Asymmetric Structures

ABSTRACT The effect of the angle of incidence of ground motion on an asymmetric structure is recognized to be significant. However, the previous studies considered only shear wall-type elements for analyses with in-plane stiffness. These models did not capture the bidirectional structural response exhibited by column-type elements, especially in the inelastic range. A detailed investigation has been undertaken to evaluate the effect of angle of incidence on asymmetric structures considering bidirectional interaction arising in the post-yield behaviour of such lateral load-resisting elements. The study shows that bidirectional interaction is a critical design consideration for asymmetric structures.

Rational decisions for seismic collapse-prevention design of low-rise asymmetric RC buildings considering deterioration properties

This research points to an essential need to develop a well-defined design method considering the deterioration properties of structural elements. Although the seismic design provisions have changed over the past decades, they ignore the deterioration effects and the collapse probability of frames designed to the latest specifications still exceed the expected norms. The issue becomes more significant for asymmetric buildings where the lateral load-resisting elements behave differently. To this end, several sensitivity analyses are performed using a new index to show the importance of deterioration sources on the seismic performance of analysis models. The evaluation showed that the deterioration effects depend on the torsional flexibility of asymmetric models. The results recommended not to adopt a unique design decision for all resisting frames of an asymmetric building. Simple and efficient design strategies were presented to improve the seismic behavior of plan-irregular buildings up to the collapse state, while keeping their seismic performance the least sensitive to the deterioration effects. Among the proposed approaches, those strategies that vary the plastic rotation capacity of elements have a large effect on the deteriorating behavior.

Semi-mechanism-based hysteretic model for traditional heavy timber frames with forked column foot joints and various wood panel infill

Frames are essential lateral load-resisting elements in traditional Chinese timber structures. This paper presents a semi-mechanism-based hysteretic model for the lateral load-displacement relationship of traditional heavy timber frames with forked column foot joints and various wood panel infill. The backbone curve of the proposed model was determined based on the rocking and the racking mechanisms of the frames. Key points of the backbone curve, including the elastic limit point and the peak load point, and the corresponding tangent stiffness were expressed by the geometrical and physical parameters of the frames. Reloading and unloading paths were defined based on the elastic limit load and the elastic stiffness. Experimental results of typical frames were utilized to verify the proposed model regarding the backbone curve, hysteretic behavior and energy dissipation. Good agreement was achieved. Model-based parametric study was further conducted. The influence of infilled wood panels and vertical compression load was investigated. It is found that the lateral resistance contribution of the wood panels with a door opening is slightly greater than that of wood panels with a window opening, and the performance of frames under higher vertical compression load levels (without causing compression failure of the columns, which is rare) are superior to those under lower levels, e.g., a reduction of 50% in the vertical load can lead to a drop in the peak load and the cumulative dissipated energy of the frame of 13% and 30%, respectively.

Interpretable XGBoost-SHAP Machine-Learning Model for Shear Strength Prediction of Squat RC Walls

AbstractRC shear walls are commonly used as lateral load-resisting elements in seismic regions, and the estimation of their shear strengths can become simultaneously design-critical and complex whe...

Quantitative assessment of nonlinear macro-models for global behavior and design of planar RC walls

Performance-Based Seismic Design (PBSD) procedures require adequate estimation of the global cyclic force–displacement (F-D) behaviors of the primary lateral load-resisting elements. As reinforced concrete (RC) walls constitute one of the most widely used lateral load elements in building construction, several nonlinear analytical modeling approaches have been developed over the past several decades to numerically simulate the hysteretic behavior of these structures. However, comparisons between these models have often provided few details on the model input parameters, have not used consistent model discretization, and have assessed hysteretic predictions more qualitatively than quantitatively. Accordingly, the objectives of this paper are to: 1) provide detailed information on the modeling parameters used to simulate the cyclic lateral F-D behavior of a set of planar RC walls with flexure-dominant as well as shear-dominant behaviors, 2) evaluate the sensitivity of the F-D curves to different wall model discretizations applied consistently between different models, and 3) propose and use a new quantitative approach to assess the hysteretic behaviors predicted by the numerical models. The considered models are PERFORM 3D, MVLEM, SFI-MVLEM, and BTM. To represent the information that is typically known during the design of a structure, the nonlinear modeling parameters were limited to the wall geometry, reinforcing layout, concrete compression strength, and reinforcing steel yield strength, with no calibration of the material behaviors to the reported test results. A total of 2,300 analyses were performed to simulate four slender and four squat previously-tested RC walls. Results of the cyclic lateral F-D predictions were evaluated based on commonly used measures of effective stiffness, maximum strength, and ultimate displacement. Additionally, the hysteretic response of the numerical simulations was assessed by proposing, validating, and using the Modified Nash-Sutcliffe Efficiency (NSEm) metric. Comparisons between the computing times for the models were also made, as well as comparisons of the local nonlinear behavior of a slender wall. Results show that the discretization used in the PERFORM 3D, MVLEM, and SFI-MVLEM simulations did not significantly affect the predicted global response, whereas the discretization did affect the F-D curves from the BTM model. The values of NSEm agreed with qualitative evaluations of the F-D curves and were able to properly evaluate complex cyclic behaviors of RC walls including pinching and stiffness deterioration. The results of this investigation are expected to be used as modeling guidelines of RC walls when conducting PBSD.

Modeling Hysteretic Behavior of Lateral Load-Resisting Elements in Traditional Chinese Timber Structures

AbstractDou-gong connections and mortise-tenon jointed traditional timber frames are widely used in traditional Chinese timber structures to resist lateral loads. Such elements exhibit strong nonli...

Experimental evaluation of the lateral load distribution in the elastic-plastic phase of timber-steel hybrid structures with a novel light timber-steel diaphragm

A cyclic loading experiment involving a timber-steel hybrid structure consisting of a steel frame and a novel light timber-steel diaphragm is presented to quantify the flexibility of the diaphragm and its ability to distribute lateral loads in the elastic-plastic phase of the structure. A lateral load-distribution factor was proposed, and its relationship to the ratio of the stiffness of the diaphragm to that of the lateral load-resisting elements was investigated. The diaphragm was classified based on these variables. The results indicated that the failure modes of the structure were associated with the forms of damage experienced by the lateral load-resisting elements, whereas little damage was observed for the diaphragm. The diaphragm exhibited the ability to continuously adjust the distribution of lateral loads to each lateral load-resisting element; accordingly, each lateral load-resisting element had approximately the same shear force, the same lateral stiffness, and the same lateral displacement during the loading process. As the lateral displacement increased, the stiffness ratio and load-distribution factor both gradually increased, and the diaphragm correspondingly changed from semi-rigid to rigid. At times, as the lateral displacement increased, the diaphragm rapidly became rigid, and it was unnecessarily rigid during the initial loading phase when the in-plane stiffness reached a certain threshold.

Diaphragm Connections in Structures with Rocking Timber Walls

Tall timber structures require lateral load-resisting elements which are very stiff and strong without complex connections. Post-tensioned timber (Pres-Lam) walls provide such properties and can also limit damage in case of extreme seismic events. Low damage can only be guaranteed if the connections between the walls and the diaphragm can transfer the horizontal loads and accommodate the displacement incompatibilities from the seismic rocking of the walls. This paper describes an experimental program on a post-tensioned timber wall specimen with ten different wall-to-diaphragm connections. The test results show that the degree of decoupling between rotational and translational deformations can significantly affect the strength and stiffness of the overall wall–beam system. A cost analysis showed that the higher the degree of decoupling, the more expensive the connection. If the wall-to-diaphragm interaction is not eliminated by decoupling at the connection, both the strength and stiffness of the wall–beam system are increased. The level of enhancement can be determined by an iterative procedure which is based on equilibrium equations and the stiffness of the floor beams and the connection.

Effect of Unreinforced Masonry Infill on Floor Amplification and Other Parameters in Frame-Wall Buildings

Frame-shear wall buildings or dual systems are a common form of reinforced concrete building structures. In such systems, shear walls form the primary lateral load-resisting elements intended to carry about 75% of lateral loads. The beam-column frame elements carry the vertical loads and the balance amount of lateral loads. Unreinforced masonry infill panels are common in frame-wall buildings which are incorporated to serve functional purposes. Infill panels are generally not modeled while designing the buildings. However, the influence of infill on the behavior of the building is considerable. In the present study, the response parameters in terms of average floor spectra and roof displacements have been studied in dual system buildings designed as per Unified Performance-Based Design (UPBD) method, with and without masonry infill struts. The UPBD method satisfies both target design drift and performance level simultaneously. Floor spectra are useful in the design of Operational and Functional Components (OFCs) in buildings which are mounted on floors. For the design of OFCs, parameters such as Floor Amplification factor (FAF), Component Amplification Factor (CAF), and Total Amplification Factor (TAF) are important and have been studied here. Frame-wall buildings with two plans and various heights with and without infill struts have been considered for specified target drift and performance levels. It has been observed that floor response diminishes with the incorporation of infill struts. Some important conclusions have been drawn on FEMA provisions.

Special Issue on Performance of Timber and Hybrid Structures

Special Issue on Performance of Timber and Hybrid Structures

Load Distribution in Timber Structures Consisting of Multiple Lateral Load Resisting Elements with Different Stiffnesses

It is well known that the stiffness properties of diaphragms and lateral load resisting elements (LLREs) influence the load distribution between LLREs under lateral load induced by earthquake or wind. Where a more sophisticated method of calculating the load distribution in a lateral load resisting system is used, often it is based on the concept of a beam on an elastic foundation. This approach could be tedious to apply in design when there are more than a few LLREs. A multiple-spring model (MSM), whereby the translational springs are used to model the diaphragm stiffnesses and the stiffnesses of the LLREs, is proposed. The model was validated with test and finite-element results of a particular benchmark building. The lateral load distribution between LLREs with various stiffness ratios of diaphragm to LLRE was also investigated. The results show that, contrary to common belief, the forces transferred by a semirigid diaphragm to supporting LLREs may be higher than those predicted by flexible and rigid-diaphragm assumptions. Therefore, using the envelope force approach may lead to underestimation of the design forces in the shear walls.

Parametric Identification of Nondegrading Hysteresis in a Laterally and Torsionally Coupled Building Using an Unscented Kalman Filter

AbstractA parametric approach is proposed for identifying the inelastic response of individual elements, which provide resistance against lateral loads in torsionally coupled buildings. Depending on the severity of the ground shaking in each lateral direction and the amount of eccentricity, different lateral load resisting elements (LLREs) will experience different levels of permanent deformation and energy dissipation based on their location and orientation. The proposed method utilizes the well-known Bouc-Wen model for representing the hysteretic response of each LLRE. An unscented Kalman filtering approach is implemented for identifying the Bouc-Wen model parameters, and for direct estimation of the force-displacement loops experienced by each LLRE during damaging seismic events. The simulated response of a single-story building to ground motion is used for demonstrating the utility of the proposed method, which may also be applied to multistory frames in a straightforward fashion, if certain instrumen...

Modelling of seismically induced storey-drift in buildings

This paper contains detailed descriptions of a dynamic time-history modal analysis to calculate deflection, inter-storey drift and storey shear demand in single-storey and multi-storey buildings using an EXCEL spreadsheet. The developed spreadsheets can be used to obtain estimates of the dynamic response parameters with minimum input information, and is therefore ideal for supporting the conceptual design of tall building structures, or any other structures, in the early stages of the design process. No commercial packages, when customised, could compete with spreadsheets in terms of simplicity, portability, versatility and transparency. An innovative method for developing the stiffness matrix for the lateral load resistant elements in medium-rise and high-rise buildings is also introduced. The method involves minimal use of memory space and computational time, and yet allows for variations in the sectional properties of the lateral load resisting elements up the height of the building and the coupling of moment frames with structural walls by diaphragm action. Numerical examples are used throughout the paper to illustrate the development and use of the spreadsheet programs.

Inelastic seismic demand of low-rise buildings with soil-flexibility

Relying on the ductile behaviour of structures during earthquake, building codes introduce response reduction factors ( R) to reduce design forces in earthquake resistant design. However, applicability of such factors has not been systematically explored for low-rise buildings with stiff periods. Present study is an attempt to address this issue. Both elasto-plastic and degrading hysteresis behaviour for lateral load-resisting structural elements are considered herein, while sub-soil is idealized as linear and elasto-plastic in parallel. The study recognizes that inelastic response for short period systems is very sensitive to R and may be phenomenally amplified even for small R due to soil–structure interaction implying restrictive applicability of dual-design philosophy. Limited study on the plan-asymmetric low-rise buildings depicts that inelastic response of the asymmetric structure relative to its symmetric counterpart is not appreciably influenced due to soil–structure interaction (SSI). The study also confirms that equivalent single story model characterized by the lowest period rather than the fundamental one of the real system tends to yield conservative estimation of inelastic demand at least for the short-period systems.

Shear Strength of Lightly Reinforced Wall Piers and Spandrels

Between the 1950s and 1970s, a significant number of buildings were constructed using lightly reinforced perimeter walls with openings. Evaluation and rehabilitation of such buildings requires accurate assessment of the expected shear strength, stiffness, and ductility of the wall segments (wall piers and spandrels) that comprise the primary lateral load-resisting elements. Assessing wall shear strength is complicated by factors such as use of a single curtain of distributed reinforcement, lack of hooks, and use of weakened plane joints, which are all common in older construction. To address these issues, a database of existing test results was assembled and reviewed; and tests were conducted on lightly reinforced wall piers and spandrels to address significant gaps in the available test data. Observations indicate that the amount of boundary reinforcement provided, presence of axial load, and the location of a weakened plane joint on the wall are the most important factors in the assessment of nominal shear strength.

Design for forces induced by seismic torsion

Eccentricities between the centres of rigidity and centres of mass in a building cause torsional motion during an earthquake. Seismic torsion leads to increased displacement at the extremes of the building and may cause distress in the lateral load-resisting elements located at the edges, particularly in buildings that are torsionally flexible. For an equivalent static load method of design against torsion, the 1995 National Building Code of Canada specifies values of the eccentricity of points through which the inertia forces of an earthquake should be applied. In general, the code requirements are quite conservative. They do not place any restriction on the torsional flexibility, however. New proposals for 2005 edition of the code which simplify the design eccentricity expressions and remove some of the unnecessary conservatism are described. The new proposals will require that a dynamic analysis method of design be used when the torsional flexibility of the building is large. Results of analytical studies, which show that the new proposals would lead to satisfactory design, are presented.Key words: torsional response to earthquake, natural torsion, accidental torsion, design for torsion, National Building Code of Canada, interdependence of strength and stiffness.

Inelastic seismic response of code-designed reinforced concrete asymmetric buildings with strength degradation

The study presents the inelastic seismic response of code-designed asymmetric buildings idealized as asymmetric one storey systems. The strength eccentricity of the code-designed buildings are studied and accordingly, the strength of various load-resisting elements of the idealized asymmetric systems are adjusted in such a way that these systems have the strength eccentricities similar to those of the code designed buildings. Two categories of systems are studied. The first category has lateral load-resisting structural elements with ideal elasto-plastic hysteresis behaviour. The load-resisting elements of the other category have a strength-deteriorating characteristics representing the deterioration in yield strength of structural elements, which are known to arise in reinforced concrete (RC) members under successive yield excursions due to seismic loading. The study of the first category of systems shows that code-designed buildings may exhibit only a moderate increase in displacement and ductility demand if the load-resisting elements have a purely elasto-plastic behaviour. On the contrary, the study of the second category of systems reveals that the shifting of resistance centres due to asymmetrically localized yielding of load-resisting element at one edge under coupled lateral-torsional motion, and the progressive deterioration of strength of this edge element, may cause a displacement and ductility demand several times those of the similar symmetric systems. Hence, the existing code provisions for asymmetric buildings can be considered adequate for the same constructed by RC, only if the strength deterioration in RC structural elements can be minimized through adequate reinforcement detailing.

Effect of strength deterioration on inelastic seismic torsional behaviour of asymmetric RC buildings

The paper presents a limited study on the inelastic torsional behaviour of reinforced concrete (RC) asymmetric buildings using idealized one-storey models. The strength deterioration characteristics of reinforced concrete members is included in the hysteresis rules of the load-resisting elements in this study. It is observed that these characteristics may largely magnify the displacement and ductility demand in structural elements due to successive localized unsymmetric yielding and progressive strength deterioration; resulting in continuous shifting of the centre of strength and thereby increasing strength eccentricity. This magnification effect is generally found to be increasing with the rate of strength deterioration. This effect is not recognized in numerous studies on inelastic behaviour of asymmetric buildings as these studies considered a bilinear hysteresis behaviour devoid of strength deterioration characteristics for lateral load-resisting elements; and as a consequence comparatively lower displacement and ductility demand in load-resisting elements were observed. While using the results of existing literature on inelastic seismic torsional behaviour of asymmetric buildings to predict the behaviour of RC asymmetric buildings, this limitation should seriously be considered.