Weak Story Research Articles

2構面モデルを用いた一軸偏心多層RC建物の制振改修設計法

In retrofitting multistory RC buildings which have uniaxial eccentricity, it is important to control torsional displacement to assure safety of structure under seismic input. In this study, elastic response of each frame of uniaxial eccentric three dimensional model is evaluated by using a pair of equivalent independent lamped mass models. The easy damper distribution method proposed for plane frames in previous paper is applied to these pair of lamped mass, and damper distribution in each plane frame are determined. The validity of proposed method is confirmed by applying to three-dimension model of typical Turkish RC building which has uniaxial eccentricity and weak stories.

損傷配分に着目したトルコにおける多層RC建物の制振改修

When retrofitting reinforced concrete buildings in Turkey with energy-dissipation devices, damage distribution between each story become very important, because the ultimate horizontal shear strength in each story is lower than Japanese buildings, and damage concentration at specific story is highly expected. In this paper, a retrofit concept adding elastic frames together with hysteretic dampers is introduced, and the effects of these elastic elements for damage distribution were researched and discussed, also for use in structures including weak stories. The easy damper distribution method using equivalent linearization techniques including the effect of additional elastic frames is proposed, followed by its validity evaluations.

필로티형 콘크리트 전단벽 구조물의 능력스펙트럼기반 손상도 기준에 대한 연구

Abstract Despite the increasing incidence of earthquakes in recent years, many of the existing buildings don't have appropriate seismic performance due to the deterioration or structural characteristics. In particular, a piloti-type RC shear wall structure, which is one of the building types in Korea, is highly vulnerable to earthquakes due to a great lack of shear function that can resist lateral force caused by the earthquake since the first floor is mostly soft story, and it is classified as weak story. In this regard, a study on the damage state criterion for the piloti-type RC shear wall structures was carried out. The capacity spectrum was calculated through the structural analysis by selecting typical type of buildings of shear wall systems, and damage state criterion was defined based on the shape of the capacity spectrum. Key Words : Capacity spectrum, Damage state criterion, Piloti-type RC shear wall structure, Seismic performance 본 논문은 산업통상자원부 산업융합원천기술개발사업(건축/대형구조물의 안전관리를 위한 내외피용 IT기반 고정밀도 패치/임플란트시스템 기술 개발)의 연구비지원으로 수행되었음.

Comparison on Dynamic Parameters of Structures with Different Additional Steel Frame Stories Constructed on the Prototype Reinforced Concrete Frame

Based on a practical project, the dynamic parameters for a 4-story reinforced concrete frame, with additional different stories of steel frame constructed on its top, are investigated, including vibration period, frequency, and seismic response under minor shock, such as inter-story maximum horizontal displacement, inter-story maximum horizontal displacement angle, inter-story shear force and so on. It reveals that the maximum number of additional stories may be determined by the inter-story maximum horizontal displacement angle. With the additional steal frame adding on the top of the original structure, the maximum horizontal displacement angle often comes into being as the second story from the top of the structure, at the same time, a certain story in the prototype structure may evolve into a weak story which has to be reinforced. However, with the increasing of additional stories, the total shear force in the bottom story decreases, which leads to a new anti-seismic design concept.

Shaking Table Test of RC Frame-Shear Wall Structures with Partial Columns Sliding at Upper Ends

A new kind of RC frame-shear wall structures with partial columns sliding at upper ends is proposed in this paper. A shaking table test of this new kind of structure model (Model B) and a conventional frame shear-wall structure Model (Model A) were carried out, and the plan layout and the member size of the two models are nearly identical. The two models are 3-story and 2-bay by 2-bay, and the second story of either structure is weak story. The test results showed that: (1) under the same intense earthquake, the damage of Model B is slighter than that of Model A; and (2) under the same intense earthquake, the interstory drift angle, the acceleration and the shear force of weak story of Model B are reduced remarkably in comparison to Model A.

The Structural Damages and Collapse Types in Turkey, Observed after the Recent Earthquake

In the last decade, the earthquakes in Turkey caused thousands of loss of life. The most unfortunate case was happened in 1999 Marmara earthquake and more than 20000 people lost their life. Recently an earthquake of magnitude 7.2 occurred in Van city and 600 dead toll was reported. Following the earthquake, the author was visited the earthquake region. In this study field reconnaissance survey the causes of failures observed on reinforced concrete (RC) structures were summarized. The main problem was the construction quality especially the concrete production process. The steel detailing was also a big problem for the region. The failed or damaged buildings were included several non-seismic details. The observed problems of damaged or collapsed buildings were short column formations, soft or weak stories, inadequate lateral confinements reinforcement, use of plain undeformed bars, poor material and concrete quality, lap spilice problems, strong beam-weak columns and weak beam-column joints.

Damages on reinforced concrete buildings due to consecutive earthquakes in Van

Consecutive earthquakes occurred on October 23rd, 2011 in Ercis and on November 9th, 2011 in Edremit that are townships located 90km and 18km far from Van city in Turkey, respectively. A total of 28,000 buildings were damaged or collapsed in the city center and the surrounding villages after the Ercis earthquake. This number reached 35,000 after the Edremit earthquake. In the area where the earthquakes occurred, almost all the reinforced concrete buildings were affected.This study presents field observations of damages on reinforced concrete buildings due to the consecutive earthquakes that occurred in Van, Turkey. Damages appearing in the buildings may occur due to several reasons such as site effect, poor construction quality, poor concrete strength, poor detailing in beam-column joints, detailing of stronger beam than column, soft stories, weak stories, inadequate reinforcement, short lap splices, incorrect end hook angle, and short columns. Aftershocks also caused progressive damages on the buildings within 17 days after the earthquakes. According to the results of this study, most of the damaged buildings were not designed and constructed according to the Turkish earthquake code, the so-called Specification for Buildings to be built in Seismic Zones.

Performance and damages of reinforced concrete buildings during the October 23 and November 9, 2011 Van, Turkey, earthquakes

In this paper, it aimed to investigate the performance and damages of reinforced concrete buildings during October 23 (Erciş) and November 9 (Edremit), 2011 Van earthquakes in Turkey. A total of 28,000 buildings are damaged or collapsed in the city center and surrounding villages after the Erciş earthquake. This number increased to 35,000 after the Edremit earthquake. Large proportion of non-engineering reinforced concrete buildings completely collapsed or were damaged heavily. Most of the reinforced concrete buildings in the affected area are not designed and constructed in accordance with Turkish Earthquake Resistant Design Code. The cracking and failure patterns of the reinforced concrete buildings are examined. Seismic code requirements are discussed and compared with observed details. It is seen that the damages are due to several reasons such as site effect, location, and length of the fault, the poor construction quality, the poor concrete strength quality and unribbed reinforcement steel, poor detailing in beam column joints, strong beam–weak columns, soft stories, weak stories, inadequate transverse reinforcement, existence of short lap splices and incorrect end hook angle, short columns, weak walls, inadequate safe distance between buildings, unconfined gable walls, concrete slab failures, insulation materials, broken staircase slab and cracks at the corners of the windows and doors. Also, it is emerged that 26% of the buildings have no building license, 66% of the buildings' ground floor is used as a shop, 36% of the buildings have no static project, building (static) projects of 57% of the structures are insufficient, 57% of the buildings are not constructed in accordance with their static project, 74% of the static projects have no detail drawings and hoops densifications, the majority of the buildings have not got sufficient compressive strength according to the related earthquake code, aggregate dimensions are larger than maximum aggregate diameter for concrete, 60% of the building was constructed using un-ribbed reinforcement steel, and the majority of the buildings (85%) have not got any soil report. In addition to these reasons, the two earthquakes hit the reinforced concrete buildings within 17 days, causing progressive damage.

Analysis on Seismic Behavior of Structures Supported by Foundations with Different Locations

In order to know more about the seismic behavior of structures supported by foundations with different locations, using the elastic and static elastic-plastic analysis method, the research is respectively from the aspects of the distribution characteristic of elastic inter-story displacement angle, the distribution law of story shear, and the distribution characteristic of elastic-plastic displacement angle and the sequence of plastic hinges. In the end, it is got that the characteristics of force and deformation of structures supported by foundations with different locations. It is found that the position of weak story would change along with the change of story stiffness. In order to guarantee the structures supported by foundations with different locations to have good seismic behavior, it needs to take the necessary internal force adjust measures and details of seismic design.

硬化型復元力特性を有する部材により耐震補強した多層骨組の振動特性

When making repairs to buildings affected by earthquakes, restoration by just adding strength is commonly used. However, this causes a large change in the axial force on the piles because of the increase in the overturning moment. In this paper, when applying seismic retrofit by using a device with hardening type hysteresis, it was revealed that the increase in overturning moment could be controlled and the maximum ductility factor could be reduced. Furthermore, we proposed a prediction formula for the maximum ductility factor when there is a weak story in the building.

On the behaviour of concentrically braced frames subjected to seismic loading

This paper deals with the causes and the development of weak storey mechanisms in concentrically braced frames subjected to seismic action. In order to investigate this phenomenon, different braced frames were designed in accordance with Eurocode provisions. The design parameters and the obtained structures are presented in the paper. Later the results of a comprehensive nonlinear time history analysis carried out with various accelerograms are shown emphasizing the presence of weak storeys. The connection between the behaviour of the brace and the occurrence of the weak storey is investigated. It is described how the plastic deformation of the diagonals and the development of the weak storey are related. A theoretical influence of the brace deformation on the modal response of the braced frames is introduced and also proven by means of signal processing of the displacement time series of the numerical experiments. In the last chapter a possible redesign method is introduced to rule out the occurrence of weak stories.

Seismic Fragility Analysis of Reinforced Concrete Frames Based on the Service Age Variation

The seismic fragility of R/C frames was redefined as the conditional failure probability of the structure with regard to the ground motion index and structural service age in this paper. An analytical method was proposed to disclose the seismic fragility variation of reinforced concrete frames with respect to the service age. Considering the variation of nonlinear mechanical characteristics of un-carbonated concrete within the service life, the seismic fragility variation of frames was analyzed using the inter-story drift ratio in the weak storey. For the defined damage levels of frames, each seismic fragility curve variation reflects the failure probability change tendency within the service life. A R/C frame was modeled to illustrate the variation of the seismic fragility within the service life.

Pounding of San Francisco-Type Soft-Story Corner Buildings

We investigated 1920s-era four-story wood-frame corner buildings common to San Francisco, many of which were damaged in the city's Marina District during the 1989 Loma Prieta earthquake. Such buildings can have relatively weak and flexible first stories and are referred to as soft-story buildings. We calibrated two building computer models to simulate actual earthquake response. We then performed computer analysis to assess collapse performance under 22 hypothetical pounding situations of both as-built and retrofitted soft-story buildings. For this building type, we found that a typical pounding situation increased the collapse rate by 14%, and for retrofitted versions, pounding slightly decreased the collapse rate. However, there were factors that appear to significantly increase the rate, especially in combination: negligible building separations and multiple adjacent buildings having low effective damping and large mass. Based on the results, we outline an approximate way to account for pounding within the context of current design procedures. An upper bound on the pounding effect was found to be equivalent to about a 30% increase in demand.

Seismic fragilities for reinforced concrete buildings with consideration of irregularities

Poor seismic performance of non-code conforming RC buildings, mainly designed for gravity loads prior to 1970s, highlights the need for reliable vulnerability assessment and retrofitting. The vulnerability is compounded since the RC buildings are subject to different irregularities such as weak storey, soft storey, plan irregularities, and poor construction quality; and interaction of different irregularities. Fragility based seismic vulnerability of structures with consideration of soft storey (SS) and quality of construction (CQ) is demonstrated on three-, five-, and nine-storey RC frames designed prior to 1970s. Probabilistic seismic demand model (PSDM) for those gravity load designed structures is developed, using the nonlinear finite element analysis, considering the interactions between SS and CQ. The response surface method is used to develop a predictive equation for PSDM parameters as a function of SS and CQ. Result of the analysis shows the sensitivity of the model parameter to the interaction of SS and CQ. The accuracy of the predictive equations is checked for randomly selected SS values and three levels of CQ. Further, the fragility curves are developed for the three structures considering SS, CQ and of their interactions. Finally, confidence bounds on the fragilities are also presented as a measure of their accuracy for risk-informed decision-making.

Seismic evaluation and upgrading of RC buildings with weak open ground stories

The inelastic earthquake response of existing, reinforced concrete buildings with an open ground story, designed according to the old Greek codes, is investigated before and after their seismic strengthening with steel braces restricted to the open ground stories. The seismic performance evaluation is based on Part 3 of Eurocode 8 for assessment and retrofitting of buildings. Three and five-story, symmetric and non-symmetric buildings are subjected to a set of seven pairs of synthetic accelerograms, compatible with the design spectrum, and conclusions are drawn regarding the effectiveness of the strengthening solutions. Seismic behavior of the selected models confirms results of previous work regarding the insufficient capacity of the open ground stories for design level earthquakes. It is also shown that strengthening only the weak ground story, a choice having the substantial advantage of low cost and continued usage of the building during its seismic retrofitting, can remove the inherent weakness without shifting the problem to the stories above and thus making such buildings at least as strong as those without a weak first story. This partial strengthening is possible for symmetric as well as eccentric buildings, in which torsion plays a further detrimental role.

Seismic Fragility Analysis of Reinforced Concrete Frames within Service Life

An analytical method was proposed to obtain the seismic fragility curve of reinforced concrete frames within the service life. Considering the variation law of nonlinear mechanical characteristics of un-carbonated concrete within service life, the seismic fragility curve of frames was developed based on the inter-storey drift corner in the weak storey of frame structures. According to the defined frame damage states, each seismic fragility curve reflects the probability change tendency of the defined damage state in frames within service life. A numerical example was modeled to illustrate the variation characteristic of seismic fragility within the service life.

Simulation of the damping effect of a high-rise CRST frame structure

The damping effect of a Concrete-filled Rectangular Steel Tube (CRST) frame structure is studied in this paper. Viscous dampers are employed to insure the function of the building especially subjected to earthquakes, for some of the main vertical elements of the building are not continuous. The shaking table test of a 1:15 scale model was conducted under different earthquake excitations to recognize the seismic behavior of this building. And the vibration damping effect was also investigated by the shaking table test and the simulation analysis. The nonlinear time-history analysis of the shaking table test model was carried out by the finite element analysis program CANNY. The simulation model was constructed in accordance with the tested one and was analyzed under the same loading condition and the simulation effect was then validated by the tested results. Further more, the simulation analysis of the prototype structure was carried out by the same procedure. Both the simulated and tested results indicate that there are no obvious weak stories on the damping equipped structure, and the dampers can provide the probability of an irregular CRST frame structure to meet the requirements of the design code on energy dissipation and deformation limitation.

Pin‐supported walls for enhancing the seismic performance of building structures

SUMMARYAn application of a pin‐supported wall‐frame system in retrofitting an eleven‐story steel reinforced concrete frame is introduced. The retrofit aims at enhancing integrity and avoiding weak story failure in an existing moment‐resisting frame. Seismic performance of the building before and after the retrofit is assessed through nonlinear dynamic analysis. The results show that the pin‐supported walls are effective in controlling the deformation pattern of the ductile frame and hence in avoiding weak story failure. With the well‐controlled deformation pattern, carefully arranged energy dissipating devices are able to concentrate energy dissipations so that damage to the rest of the structure can be significantly reduced. Copyright © 2012 John Wiley & Sons, Ltd.

Multi-objective seismic design method for ensuring beam-hinging mechanism in steel frames

Previous research efforts have shown that the column-beam flexural strength ratios of joints in moment resisting steel frames should be higher than 1.0 or even 2.0 for a beam-hinging collapse mechanism. However, it has been pointed out that, in order to prevent a weak story mechanism in a structure, it is not practical to use a specific single value as a limit for the column-beam flexural strength ratio for all joints of a structure. Therefore, an optimal design technique is needed to determine the column-beam flexural strength ratios for joints in a structure. In this paper, a multi-objective seismic design method for ensuring beam-hinging mechanism in steel moment resisting frame structures is presented and applied to optimal seismic design of well-known steel moment frames. In addition to the constraint for ensuring beam-hinging mechanism, the relationship between the structural cost and the energy dissipation capacity of structures is provided by considering the two conflicting objective functions. In order to select the best design among the candidate designs, as a guide for structural engineers, a simple rule is presented in the form of dissipated energy density defined by the ratio of the energy dissipation capacity to the structural weight.

Applicability investigation of code-defined procedures on seismic performance assessment of typical school buildings in Taiwan

Seismic resistance of elementary and high school buildings is of great concern due to high earthquake risk in Taiwan. Existing school buildings are often built of similar configurations, and designed based on previous building codes, which are not in compliance with current codes in seismic performance objective. The use of current code procedures on seismic performance assessment of existing typical school buildings may not be applicable in predicting their structural behavior during earthquakes. This paper describes a full-size two-story, three-bay frame structure test of typical school building design in Taiwan, and examines the performance and failure mechanism of the frame structure with current building codes. Results from loading test show good agreement with the code-defined procedures in identifying soft story, weak story, and failure modes of columns, but show different failure mechanism in beam–column joint region, i.e. strong-column–weak-beam is expected by building code procedures, whereas the test specimen exhibits weak-column–strong-beam behavior. Although the joint shear check following current code procedures gives satisfactory results, i.e. is consistent with the experiment, the check on its own is not a reasonable one. Revisions of current checking procedures for failure mechanism in beam column joint and for joint shear are also proposed in this paper for existing non-ductile reinforced concrete school buildings. The revised procedures incorporate the effects of both weak story and weak-column–strong-beam behavior. After adopting the proposed procedures, the predicted structural behavior and failure modes are consistent with those observed from the full-size frame test.