Stiffness Irregularity Research Articles

The Effect of Irregularity of Lateral Stiffness in Estimating the Separation Gap of Adjacent Frames

Structural pounding can lead to local or total damage to the stories at the collision level or to the overall collapse of the building. On the other hand, lateral stiffness irregularity is common in the form of soft or very soft stories, which is due to the alternation in the type of function of the first story of the building. This paper estimates the demand for the normalized separation gap (NSG) at adjacent buildings highest collision level that were a combination of regular and irregular frames. For this purpose, the steel moment resisting frames (MRF), compounds with a total of 700 adjacent states and their NSG, is calculated by the dynamic time history analysis. In addition, irregularity increment in lateral stiffness for the first story could lead to an increase in the NSG of 84% of the adjacent combinations. In this study, a new relationship is proposed to estimate the demand for the NSG with the consideration of the effects of irregularity of lateral stiffness in the lowest story.

Response of Multi-Storeyed Buildings Having Vertical Irregularities using ETABS

The objective of this research is to analyze the regular and vertical irregular buildings for understanding the response of vertically irregular buildings by using advanced software like ETABS. The analysis is carried out to understand the behaviour of the buildings by performing Non-Linear Dynamic analysis (Nonlinear Time-History Analysis) in software. The response of buildings is analyzed for three different irregularities, they are i) mass irregularity ii) stiffness irregularity iii) setback irregularity. The response of the vertically irregular buildings with regular building is done by considering the Base shear, Displacement and Story Drift of the buildings. The buildings which are irregular in plan will undergo to torsion effects easily because their centre of mass does not coincide with the centre of gravity, since the torsion will be developed in the building. When comes to the buildings which are irregular in elevation and located in seismic zones, the understanding of behaviour of such vertically irregular buildings will be difficult. It is a challenging task to the structural engineers to study the behaviour of vertically irregular buildings. This paper will provide the response of vertical irregular buildings so the engineers can design the buildings accordingly.

Seismic and Wind Analysis of Regular and Irregular RC Structures with Tuned Mass Damper

Latest trend in the development high rise structure demanding taller and lighter structures, which are progressively adaptable with very low damping ratio. As the structures developing vertically, they are ending up all the more affecting by powerful excitation forces, for example, wind and seismic forces. For the more safety of structure and inhabitant's solace, the vibrations of the tall structures become a major issue for both structural designers. So as to control the vibration, various methodologies are proposed out of the few systems accessible for vibration control. Out of numerous methods, TMD has been observed to be increasingly powerful in controlling the dynamic forces caused due to seismic and wind excitations. In this paper, the adequacy of TMD in controlling the dynamic reaction of structures and the impact of different ground movement parameters on the seismic viability of TMD is researched. Essentially, a TMD is a vibratory subsystem appended to a bigger scale host structure so as to lessen the dynamic reactions. The frequency of damper will tuned to essential structure's frequency, so when frequency is high, the damper will results to resonate out of phase along with structural movement. The objective of this work is to study the impact of TMD on the dynamic forces brought about by seismic tremor and wind excitations in standard just as unpredictable in tall RC building structures. For that three 22 story RC building structures are considered with a similar arrangement out of which one ordinary regular structure and the other two are irregular RC structures are demonstrated in Etabs. In irregular RC structures, Stiffness irregularity and torsional irregularity are considered. For assessing seismic and wind reactions of structures, time history analysis, and static analysis used, with and without the tuned mass damper in ETABS. The outcomes acquired from the investigation of three 22 story RC structures with and without tuned mass damper are compared.

Strengthening of reinforced concrete buildings with soft story irregularity

Abstract Turkey is one of the most earthquake-prone countries in the world. The studies on the damaged buildings with reinforced concrete structural systems, especially done after the 1999 Gölcük earthquake and other earthquakes in the past, showed that the negative effects of the structural irregularities in the buildings come to light through the impact of earthquake load. They also revealed that the regularity of structural systems is one of the main principles in the design of earthquake resistant reinforced concrete structural systems. In this study, two-story single-span 1/3 scaled reinforced concrete test specimens having soft story irregularity (interstory stiffness irregularity) and insufficient earthquake resistance were tested by applying different strengthening methods. The purpose of strengthening applications is to eliminate the adverse effects of the soft story irregularity. Reversed-cyclic lateral load was applied on the test specimens to simulate the earthquake effect. While the upper stories in all the test specimens had infill walls, the lower stories had been strengthened using different types of steel bracings. The first frame was strengthened with K-type shaped (<>), the second one with straight V-type (V) and the third with turned sideways V-type (>) steel diagonal elements. As a reference to these three strengthened frames, the following two frames, which were taken from another study, were used; a frame with both stories having infill walls and a frame with infill walls on the upper story, which is empty on the lower story. As a result of the study, load and displacement histories, hysteresis and envelope curves and energy dissipation graphs of these three strengthened reinforced concrete frames were obtained. Furthermore, all the experimental results, obtained graphs and the data from the reference frames were compared and interpreted. The results show that the frame strengthened with K-type steel bracings is the best solution to reduce the effect of soft story irregularity.

Analysis on interstory drift of high-rise residential quasi-megastructure

Based on the superior lateral resistant performance of mega-X layout pattern, the concept of quasi-megastructure was put forward to introduce quasi-mega mechanism in residential buildings, using normal members only, avoiding large architecture space occupation of mega members. Unlike the megastructures that are dominated by their primary structures, the quasi-megastructures mainly realize a synergistic mechanism between the frame structure and quasi-mega braces. In order to investigate its deformation mechanism and reduce the lateral stiffness irregularity, a simplified method for evaluating interstory drift of high-rise quasi-megastructure was proposed, based on three fundamental assumptions. Thus, the interstory drift formula for any specific layout scheme of quasi-megastructure can be deduced. The formula was validated by different layout schemes. By this method, the interstory drift proportion of quasi-megastructure was investigated, showing that the interstory drift of a target story mainly consists of drift due to rigid body rotation, pure shearing drift, and shear-lag added drift. And the vertical stiffness irregularity of the original scheme was due to great changes of shear-lag added drift in different neighboring stories. Thereafter, stiffened schemes were proposed, which effectively eliminate the interstory stiffness irregularity by complementing a quasi-mega cantilever wall of full height. Compared with mega frame structure, the original scheme has a similar overall deformation mechanism, while the properly stiffened quasi-megastructure scheme behaves quite close to mega braced frame structure, both in deformation mechanism and in structural efficiency.

Analysis of Irregular Structures under Earthquake Loads

Behavior of a multi-storey building during strong earthquake motion depends on structural configuration. Irregular configuration either in plan or in elevation is recognized as one of the major causes of failure during earthquakes. Thus irregular structures, especially the ones located in seismic zones are a matter of concern. Structures generally possess combination of irregularities and consideration of a single irregularity may not result in accurate prediction of seismic response. The choice of type, degree and location of irregularities in the design of structures is important as it helps in improving the utility as well as aesthetics of structures. Hence, the present study addresses the seismic response of reinforced concrete structures possessing various combinations of irregularities. A nine-storeyed regular frame is modified by incorporating irregularities in various forms in both plan and elevation to form 34 configurations with single irregularity and 20 cases with combinations of irregularities. Along with the regular configuration, 54 irregular configurations are analyzed and compared. All the frames are subjected to seismic loads and the response of the structures is computed numerically. It is observed that irregularity considerably affects the seismic response. Out of various types of single irregularities analyzed, stiffness irregularity is found to have maximum influence on the response. Among the cases having combinations of irregularities, the configuration with mass, stiffness and vertical geometric irregularities has shown maximum response. The results of this study would aid in designing of irregular structures judiciously without compromising their performance.

Feasibility Studies on Adaptive Interface for Square Infilled Frame

Abstract High rise buildings are becoming more common nowadays to face the increasing demand on the land area for industrial and agricultural needs apart from urbanisation. Such buildings require special systems to carry lateral loads resulting from wind, earthquake etc. One of the more common lateral load resisting system is an infilled frame. Infilled frame is a structural system developed through the composite action between the bounding frame and the infilling wall under in plane lateral loads. Such an interaction has proved beyond doubt that an increase in the lateral stiffness of the bare frame. However there are certain limitations reported in the literature due to the composite action like increased base shear, short column effect, vertical stiffness irregularity, reduced ductility etc.It is worth considering development of a provision to make the frame to behave as an infilled frame during working load levels and converting its behaviour similar to that of ductile bare frame towards its failure stages. The main objective of the present investigations is to seek the evolution of feasibility of an infilled frame system whose stiffness can be altered during its service without altering physically any of the component materials. This is sought to be achieved in the present investigation by providing an air medium (pneumatic medium) at interface and altering the air pressure as required. Under main conclusions include the possible to alter lateral stiffness and cracking and failure load levels of infilled frame through use of pneumatic medium interface.

Study on The Effect of Soft Story on Infill RC Frames Under Seismic Effect

Construction practice of reinforced concrete (RC) frames infilled with unreinforced masonry is quite common now-days in urban cities in Nepal and elsewhere. Previous study shows the lateral load transfer mechanism is different than that of bare frames in infill buildings. Because of the unavoidable circumstances like elimination of central columns, elimination of infill wall in basement for parking purpose and reducing the size of frame members etc. may cause the particular story to be soft.In this study the infill RC frames with stiffness irregularity has been analysed with linear time history method using Gorkha-2015 earthquake as ground motion using structural analysis and design software (ETABS 2000 V.16). In total 8-numbers of 6-story RC infilled frames were analysed introducing the soft story in each story level respectively from basement to top. Regular frame was designed as per IS 1893:2002 load combination considering torsional effect. After analyse of bare frame, regular frame and irregular frames the global and story level seismic demand parameters were studied comparatively. Base/Story shear, Story displacement, inter-story drift and fundamental time period were the parameters compared taking regular frame as reference case.Results showed that, there is significant effect of location of irregularity on the seismic demand. The global and story level seismic demand is higher when the irregularity is introduced in bottom part of the buildings and further it showed that the lateral strength of RC frames get highly enhanced due to introductions of infill in analytical models.Kathmandu University Journal of Science, Engineering and TechnologyVol. 13, No. 2, 2017, page:79-91

Locating optimum torsion axis in asymmetric buildings subjected to seismic excitation

Seismic torsional effects in buildings are attributed to the distance between the centres of mass of the floor slabs and the centres of rigidity of the building. Various procedures have been developed to locate these points, but their scattering over the height of the structure makes their use very difficult in implementing the code torsional provisions. As an alternative, the concept of the optimum torsion axis has been introduced: a vertical axis through which application of lateral in-plane forces would cause minimum torsional response. A convenient method is presented for the determination of this axis by means of the discrete-element approach (stiffness method) and its accuracy is demonstrated in common building structures with mass and stiffness irregularities.

Extension of Direct Displacement-Based Design to Include Higher-Mode Effects in Planar Reinforced Concrete Frame Buildings

Now that problems with force-based seismic design have been clearly identified, design is inclined toward displacement-based methods. One such widely used method is Direct-Displacement-Based Design (DDBD). Yet, one of the shortcomings of DDBD is considering higher-mode amplification of story shear, moments, and displacements using equations obtained from limited parametric studies of regular planar frames. In this paper, a different approach to account for higher-mode effects is proposed. This approach determines the lateral secant stiffness of the building frames that fulfill the allowable inter-story drift without exceeding the desired story displacements. Using the stiffness, an elastic response spectrum analysis is carried out to determine elastic higher-mode force effects. These force effects are then combined with DDBD-obtained first-mode force effects using the appropriate modal superposition method so that design can be performed. The proposed design procedure is verified using Nonlinear Time History Analysis (NTHA) of twelve planar frames in four categories accounting for mass and stiffness irregularity along the height. In general, the NTHA response outputs compared well with the allowable limits of the performance objective. Thus, it fulfills the aim of minimizing the use of NTHA for planar frame buildings, thereby saving computational resources and effort.

Fundamental periods of reinforced concrete building frames resting on sloping ground

Significant research efforts were undertaken to evaluate seismic performance of vertically irregular buildings on flat ground. However, there is scarcity of study on seismic performance of buildings on hill slopes. The present study attempts to investigate seismic behaviour of reinforced concrete irregular stepback building frames with different configurations on sloping ground. Based on extensive regression study of free vibration results of four hundred seventeen frames with varying ground slope, number of story and span number, a modification is proposed to the code based empirical fundamental time period estimation formula. The modification to the fundamental time period estimation formula is a simplified function of ground slope and a newly introduced equivalent height parameter to reflect the effect of stiffness and mass irregularity. The derived empirical formula is successfully validated with various combinations of slope and framing configurations of buildings. The correlation between the predicted and the actual time period obtained from the free vibration analysis results are in good agreement. The various statistical parameters e.g., the root mean square error, coefficient of determination, standard average error generally used for validation of such regression equations also ensure the prediction capability of the proposed empirical relation with reasonable accuracy.

Soft story retrofit of low-rise braced buildings by equivalent moment-resisting frames

Soft-story buildings have bottom stories much less rigid than the top stories and are susceptible to earthquake damage. Therefore, the seismic design specifications need strict design considerations in such cases. In this paper, a four-story building was investigated as a case study and the effects of X-braces elimination in its lower stories studied. In addition, the possibility of replacement of the X-braces in soft-stories with equivalent moment resisting frame inspected in two different phases. In first phase, the stiffness of X-braces and equivalent moment-resisting frames evaluated using classic equations. In final phase, diagonals removed from the lowest story to develop a soft-story and replaced with moment resisting frames. Then, the seismic stiffness variation of moment-resisting frame evaluated using nonlinear static and dynamic analyses. The results show that substitution of braced frames with an equivalent moment-resisting frame of the same stiffness increases story drift and reduces energy absorption capacity. However, it is enough to consider the needs of building codes, even using equivalent moment resisting frame instead of X-Braces, to avoid soft-story stiffness irregularity in seismic design of buildings. Besides, soft-story development in the second story may be more critical under strong ground excitations, because of interaction of adjacent stories.

Identifying stiffness irregularity in buildings using fundamental lateral mode shape

Soft or extreme soft storeys in multi-storied buildings cause localized damage (and even collapse) during strong earthquake shaking. The presence of such soft or extremely soft storey is identified through provisions of vertical stiffness irregularity in seismic design codes. Identification of the irregularity in a building requires estimation of lateral translational stiffness of each storey. Estimation of lateral translational stiffness can be an arduous task. A simple procedure is presented to estimate storey stiffness using only properties of fundamental lateral translational mode of oscillation (namely natural period and associated mode shape), which are readily available to designers at the end of analysis stage. In addition, simplified analytical expressions are provided towards identifying stiffness irregularity. Results of linear elastic time-history analyses indicate that the proposed procedure captures the irregularity in storey stiffness in both low- and mid-rise buildings.

Comparison of Seismic Responses for Reinforced Concrete Buildings with Mass and Stiffness Irregularities Using Pushover and Nonlinear Time History Analysis

Pushover analysis or also known as nonlinear static procedures (NSP) have been recognized in recent years for practical evaluation of seismic demands and for structural design by estimating a structural building capacities and deformation demands. By comparing these demands and capacities at the performance level interest, the seismic performance of a building can be evaluated. However, the accuracy of NSP for assessment irregular building is not yet a fully satisfactory solution, since irregularities of a building influence the dynamic responses of the building. The objective of the study presented herein is to understand the nonlinear behaviour of six story RC building with mass irregularities at different floors and stiffness irregularity at first story (soft story) using NSP. For the purpose of comparison on the performance level obtained with NSP, nonlinear time history analysis (THA) were also performed under ground motion excitation with compatible to response spectra design. Finally, formation plastic hinges and their progressive development from elastic level to collapse prevention are presented and discussed.

Emission of Train-Induced Ground Vibration - Prediction of Axle-Load Spectra and its Experimental Verification

Train passages induce forces on the track, train-induced vibrations propagate through the soil and excite neighbouring buildings. The emission, which is the first part of the prediction of vibrations near railway lines, is presented by focusing on the dynamic axle loads. The calculation of the axle loads is based on the vehicle-track-soil interaction. This interaction calculus utilises the dynamic stiffness of the vehicle (the inertia of the wheelset) and the dynamic stiffness of the track-soil system. Based on various time consuming finite-element boundary-element calculations, an approximate track-soil model has been established. The vehicle-track-soil analysis yields several transfer functions between the various geometric or stiffness irregularities and the axle loads of the train. Geometric irregularities of the vehicle (the wheels) and the track (rail surface and track alignment) are the simplest components. Geometric irregularities of the subsoil (trackbed irregularities) have to be transferred to effective irregularities at rail level. The bending stiffness of the track is filtering out the short-wavelength contribution. Stiffness irregularities occur due to random variations in the ballast or the subsoil, which must also be transferred to effective track irregularities, and due to the discrete rail support on sleepers. All necessary transfer functions for the prediction of axle-load spectra are presented as general formula and as specific graphs for differing vehicle and track parameters. The prediction method is applied to a ballast track and a slab track and compared with corresponding axle-box measurements. Moreover, ground vibration measurements at numerous sites are exploited for the axle-load spectra and the validation of the prediction method. All theoretical and experimental results confirm that the dynamic axle-load spectra have an approximate value of 1 kN per third of octave and increase with train speed, track stiffness and around the vehicle-track resonance.

Performance evaluation of setback buildings with open ground storey on plain and sloping ground under earthquake loadings and mitigation of failure

Setback structures are highly vulnerable during earthquakes due to its vertical geometrical and mass irregularity, but the vulnerability becomes higher if the structures also have stiffness irregularity in elevation. The risk factor of such structure may increase, if the structure rests on sloping ground. In this paper, an attempt has been made to evaluate the seismic performance of setback structures resting on plain ground as well as in the slope of a hill, with soft storey configuration. The analysis has been performed in three individual methods, equivalent static force method, response spectrum method and time history method and extreme responses have been recorded for open ground storeyed setback building. To mitigate this soft storey effect and the extreme responses, three individual mitigation techniques have been adopted and the best solution among these three techniques is presented.

Using water hammer to enhance the detection of stiffness changes on an out-of-round pipe with distributed optical-fibre sensing

Summary Over the last few decades, distributed optical fibre sensor (DOFS) has been introduced to monitor the structural health of water pipelines. Most of the previous studies show that DOFS is very effective as a static measurement and monitoring platform. However, there is still a lack of research being done using DOFS to monitor the dynamic response of the pipeline. This paper will first demonstrate the dynamic capability of optical frequency domain reflectometry-based DOFS on a pipe. To be specific, the primary monitoring work is conducted on an out-of-round plastic pipe subjected to water hammer. It is important to monitor the dynamic response of the pipe as it is well known that water hammer can occur in any pressurised pipeline system due to changes in the operating conditions. The ability to detect local stiffness irregularity on the noncircular pipe subjected to water hammer is also demonstrated. The result shows that the presence of the local stiffness change is accentuated when the pipe is subjected to water hammer. The dynamic capability of DOFS facilitates the application of water hammer as a stimulus and hence shows the potential to enhance pipeline health monitoring.

Seismic Analysis of Vertically Irregular Buildings

The greatest challenge for any structural engineer in today's scenario is to design seismic-resistant structures. A regular building, i.e. having mass and stiffness uniformly distributed through its height behaves normally. The presence of vertical irregular frame subject to devastating earthquakes is a matter of concern. Points of sudden change in stiffness, mass and strength in buildings are known as weak points. For the design of safe irregular buildings it is necessary to study the effect of irregularity on the response of buildings to lateral loads. Here we study the proportional distribution of lateral forces evolved through seismic action in each storey level due to changes in mass and stiffness of frame on vertically irregular structures. The effect of mass and stiffness irregularity of G + 10-storeyed vertical geometric irregular building is studied using finite element method-based software. Two methods of analysis, namely linear static and linear dynamic analysis are used to evaluate response of the structure in the form of storey shear, storey displacement and storey drift. Responses are plotted and compared, and conclusions have been made from the results.

Seismic response of subsystems in irregular buildings

The component-mode synthesis method is applied to investigate the seismic response of secondary subsystems multi-connected to primary structures with irregularities. The proposed approach is more accurate than the cascade approximation, which is often used in the design practice, as the primary–secondary dynamic interaction is considered through the modes of vibration of the two components. The results of parametric analyses on a representative case study reveal similar trends in the displacements of the two components for mass irregularities in elevation, while stiffness irregularities in plan can result in significant torsional motion in both components, with the effects in terms of absolute accelerations being in general larger than those associated with the lateral drifts. This suggests that dynamic analyses with the component-mode synthesis method are particularly indicated for the seismic assessment of acceleration-sensitive secondary subsystems.

Evaluation of Effective Location of Pendulum Tunned Mass Dampers for Building with Mass Irregularity

The building may have the stiffness irregularity due to some unavoidable circumstances. While doing the seismic retrofitting of such buildings the location of dampers plays a vital role. The stiffness irregularity may be categorized as vertical or horizontal. The stiffness irregularity may rise due to unsymmetrical location of shear wall or similar stiff element, such irregularity is called as irregularity in plan. The dampers can be located towards the stiffened side of building or may be located away from the stiffened side. In this paper a G+20 building frame having stiffness irregularity in plan is selected and it seismic resistance is evaluated for three different location of PTMD. The mass ratio of was kept equal to 0.03. The Nonlinear modal time history analysis was performed using finite element software SAP. The performance parameters such as displacement, storey drift and maximum forces are compared and presented. The pendulum damper is modeled using nonlinear link element. The results indicates that along the axis where the building is symmetrical in plan the location away from the stiff element proves to be efficient location where as in other direction the damper located at centroid of building is more efficient.