Seismic Design Forces Research Articles

福岡県西方沖地震における港湾施設の被災について

On March 20th, 2005 an earthquake with a magnitude of 7.0 struck the port of Hakata in the northern Kyushu. A design seismic force of the earthquake motion is estimated 0.2 while the present design seismic force is 0.05 for a many of facilities at the port. The damage has been found throughout the port.This paper presents a comprehensive review of damage of Hakata port. The facility damage is analyzed in terms of structural design, construction age and level of availability of the facility. As the result, the facility displacement has a close relationship with the structural design. An occurrence of apron holes is resulted in the structural design and the construction age. This study also shows that the damaged facilities over 30cm displacement and 10% of apron hole are out of use.

建築基準法での限界耐力計算法における表層地盤の取り扱い

Buildings in Japan must be designed and constructed according to Building Standard Law, the Enforcement Order and the Notification by Ministry of Construction and the Ministry of Land, Infrastructure and Transport notification. The buildings in our country are suffered the extensive damages cause by earthquake, typhoon, and fire, etc. Then, earthquake is adopted as one of forces/loads that should be endured by the buildings. Through these valuable experiences, we learned that the ground condition strongly influences earthquake ground motion and earthquake damage. By conventional Allowable stress design method, seismic design force level is defined taking into account the surface geological condition at construction site. In addition, by the Response and Limit Strength Calculation design method newly adopted for performance-based design concept by the revised Building Standard Law in June, 1998 the seismic force is defined at engineering bedrock as an acceleration response spectrum corresponded to the seismic activity in a region concerned with the site amplification effect by subsurface layers and soil-structure interaction effect. In this paper, I would like to introduce the consideration of subsurface layers to evaluate site amplification effect under the Building Standard Law and relating regulation especially with relation to the Response and Limit Strength Calculation design method.

Reply to the discussion by R.D. Watts on "Overview of seismic provisions of the proposed 2005 edition of the National Building Code of Canada"

The discussion by R. Watts raises several questions that are concerned with the changes in seismic force levels and the increased complexity in the proposed 2005 seismic provisions. Watts uses examples of large changes in the seismic hazard parameters to question the validity of the design forces themselves. The reasons for such large changes are due primarily to better knowledge of the geographical distribution of seismic hazard and improved methodology for estimating that hazard, as described by Adams et al. (1999). While the nature of earthquakes and their effects as well as the limited amount of historical data result in considerable uncertainty, the median values of seismic hazard parameters, which are used in the proposed 2005 seismic provisions for the determination of design forces, are robust values that are expected to remain stable over some period of time. Of course, the experience of future earthquakes (locations, magnitudes, and resulting ground motions) will have an effect on estimates of seismic hazard that will be calculated in years to come. Given the potentially catastrophic effects of large earthquake ground motions, the design of buildings must take these rare events into account, which is precisely the approach taken in treating earthquake loads in the National Building Code of Canada. Watts also raises several questions arising from the increased complexity of the proposed 2005 seismic provisions. The question of the justification of increased building design and construction costs are certainly relevant to the development of seismic design provisions in a building code. The cost associated with increased design effort is, in the author’s view, largely one of educating designers to enable them to use the code easily and effectively as a design tool. The increase in the total cost of a building, even in the cases in which the seismic design forces have increased significantly, is likely to be relatively small. Both costs are certainly justified in the interests of improved life safety to the occupants of buildings; significant increases in forces arise primarily because the improved knowledge of seismic hazard has demonstrated that previous seismic hazard estimates were too low. Watts’questions and comments on code enforcement and site supervision are not issues that can or should be addressed in a seismic design code, nor are they germaine to the content of the paper.

A Comparative Study of U.S.-Japan Seismic Design of Highway Bridges: I. Design Methods

This paper summarizes the results of a comparative study on seismic design of highway bridges jointly undertaken by the U.S. Federal Highway Administration and Japan's Public Works Research Institute. The seismic design specifications for highway bridges of the two countries are reviewed and compared with respect to their design philosophies and procedures. Some major design parameters including design seismic forces, response modification factors and minimum support lengths are addressed in detail. The differences between the two specifications are illustrated via a design example of a reinforced concrete column for simple, two-span bridges common in both countries. Three different scale models of the column are designed in accordance with the seismic design specifications of the United States and Japan, and tested on a shake table for their comparative seismic performance. The results of the shake table tests are discussed separately in a companion paper.

Estimates of the yield curvature for design of reinforced concrete columns

A recent advance in displacement-based seismic design procedures relies on an estimated yield displacement as the basis for determining seismic design forces in order to satisfy seismic performance objectives. Simple expressions for the yield curvature of circular and rectangular reinforced concrete column cross sections are presented here, based on moment–curvature analyses. The estimated yield curvatures correspond to the flexural strengths determined using Eurocode 2, and thus are useful for the practice of earthquake-resistant structural engineering. Unlike earlier proposals, the expressions presented here account explicitly for the axial load ratio. These expressions are presented in terms of the depth of tension steel, which is shown to be preferable to the overall section depth, used in a previous recommendation. An example illustrates the preliminary design of a bridge pier using Yield Point Spectra and the recommended expressions for estimating the yield curvature.

Seismic design implications of revisions to the National Building Code of Canada

This paper begins with a brief introduction to Canadian seismicity and the history of seismic code development in Canada; a summary of major changes planned for the 2005 edition of the National Building Code of Canada follows. Areas of major change include seismic hazard, site effects, irregularities, force reduction factors and methods of analysis (dynamic analysis now being preferred). The implications of the proposed changes are presented in terms of impact on seismic design force for several structural systems located in regions of high, moderate and low seismicity; implications for seismic level of protection and the seismic design process are also discussed. The paper concludes with a discussion of ongoing seismic code development issues.

Ground motion amplification factors for the proposed 2005 edition of the National Building Code of Canada

Foundation factors are used in seismic codes to capture the amplification effects of local soil conditions on ground motions and, hence, on seismic design forces. Recent developments in categorizing site conditions for seismic codes and assigning intensity- and frequency-dependent amplification factors to the various site classes are presented to provide a basis for understanding the new foundation factors proposed for the 2005 edition of the National Building Code of Canada.Key words: design spectra, site characterization, amplification factors.

STRENGTH REDUCTION FACTORS FOR DUCTILE STRUCTURES WITH PASSIVE ENERGY DISSIPATING DEVICES

In recent years, current seismic codes started contemplating the design of structures with passive energy dissipating devices. One important issue for the rational seismic design of these devices and the structure that contains them is the formulation of numerical methods to estimate their design seismic forces. From the study of the dynamic response of single-degree-of-freedom systems subjected to accelerograms recorded in Mexico during the last two decades, expressions to estimate the strength reduction factor that should be used to reduce the elastic design strength spectra for 5 percent damping, to establish the design seismic forces for structures having different combinations of plastic and viscous energy dissipating capacities, are formulated.

Dynamic analysis of buildings for earthquake-resistant design

The proposed 2005 edition of the National Building Code of Canada specifies dynamic analysis as the preferred method for computing seismic design forces and deflections, while maintaining the equivalent static force method for areas of low seismicity and for buildings with certain height limitations. Dynamic analysis procedures are categorized as either linear (elastic) dynamic analysis, consisting of the elastic modal response spectrum method or the numerical integration linear time history method, or nonlinear (inelastic) response history analysis. While both linear and nonlinear analyses require careful analytical modelling, the latter requires additional considerations for proper simulation of hysteretic response and necessitates a special study that involves detailed review of design and supporting analyses by an independent team of engineers. The paper provides an overview of dynamic analysis procedures for use in seismic design, with discussions on mathematical modelling of structures, structural elements, and hysteretic response. A discussion of the determination of structural period to be used in association with the equivalent static force method is presented.Key words: dynamic analysis, earthquake engineering, elastic analysis, fundamental period, hysteretic modelling, inelastic analysis, National Building Code of Canada, seismic design, structural analysis, structural design.

Determination of seismic design forces by equivalent static load method

In the proposed 2005 edition of the National Building Code of Canada (NBCC), the seismic hazard will be represented by uniform hazard spectra corresponding to a 2% probability of being exceeded in 50 years. The seismic design base shear for use in an equivalent static load method of design will be obtained from the uniform hazard spectrum for the site corresponding to the first mode period of the building. Because this procedure ignores the effect of higher modes, the base shear so derived must be suitably adjusted. A procedure for deriving the base shear adjustment factors for different types of structural systems is described and the adjustment factor values proposed for the 2005 NBCC are presented. The adjusted base shear will be distributed across the height of the building in accordance with the provisions in the current version of the code. Since the code-specified distribution is primarily based on the first mode vibration shape, it leads to an overestimation of the overturning moments, which should therefore be suitably adjusted. Adjustment factors that must be applied to the overturning moments at the base and across the height are derived for different structural shapes, and the empirical values for use in the 2005 NBCC are presented.Key words: uniform hazard spectrum, seismic design base shear, equivalent static load procedure, higher mode effects, base shear adjustment factors, distribution of base shear, overturning moment adjustment factors.

崩壊メカニズムが異なる建物の構造特性係数の極値について

Observation of the damage induced by earthquakes indicates that buildings suffered severer damage in certain specific parts and collapsed in one story. This damage concentration is not preferable from the view point of seismic safety of buildings. Therefore, the weak-beam-strong-column concept is introduced to seismic design in order to avoid single story collapse. This design concept is due to the design basis relying on the ductility of the structure in case of severe earthquake ground motions. Therefore many seismic codes in the world adopt a structural characteristic factor that is to reduce seismic design forces considering an energy absorbing capacity of a structure. This paper discusses the extreme of the structural characteristic factor for different types of collapse mechanism.

A COMPARATIVE STUDY OF PERFORMANCE OF VARIOUS ISOLATION SYSTEMS FOR LIQUID STORAGE TANKS

A comparative study of performance of various isolation systems for liquid storage tanks is investigated under real earthquake ground motions. The various base isolation systems considered are the laminated rubber bearings (with and without lead core) and sliding isolation systems (with and without restoring force). The isolated liquid storage tank is idealized with three-degrees-of-freedom associated with convective, impulsive and rigid mass under uni-directional earthquake excitation. Since the force-deformation behaviour of the isolation systems is non-linear, as a result, the equations of motion are solved numerically by step-by-step method. In order to measure the effectiveness of the isolation systems, the seismic response of the isolated liquid storage tanks is compared with the corresponding response of non-isolated tanks. Further, the effectiveness of the isolation is also explored for wide range of practical liquid storage tanks considering the influence of tank aspect ratio. It is observed that the isolation systems are quite effective in attenuating the earthquake acceleration transmitted to the tank, which reduces the design seismic forces significantly. Further, it is also found that the sliding type isolation systems are more effective in controlling the response of liquid storage tanks in comparison to the elastomeric bearings. Among the various sliding systems, the resilient-friction base isolator is found to be most effective for seismic isolation of the tanks.

Earthquake response of tall reinforced concrete chimneys

The results from an experimental program have been used to develop a non-linear dynamic analysis procedure for evaluating the inelastic response of tall reinforced concrete chimney structures. The procedure is used to study the inelastic response of ten chimneys, ranging in height from 115 m to 301 m subject to earthquake excitation. Based on the study, a series of code design recommendations have been prepared and incorporated into the 2001 CICIND code to encourage reliance on the development of ductility in reinforced concrete chimneys and to prevent the formation of brittle failure modes. The basis for the selection of a structural response factor of R=2 which halves the seismic design forces is presented. The design recommendations result in both improved performance and cost savings of up to 20% compared with designs undertaken with the 1998 ACI307 and 1998 CICIND codes.

Energy-based Seismic Design of Structures using Yield Mechanism and Target Drift

The use of energy as means to derive seismic design forces is discussed in this paper. The energy balance concept used in deriving seismic design forces for single-degree-of-freedom systems is exte...

Effect of near-fault earthquake on bridges: lessons learned from Chi-Chi earthquake

The objective of this paper is to describe the lessons learned and actions that have been taken related to the seismic design of bridge structures after the Chi-Chi, Taiwan earthquake. Much variable near-fault ground motion data was collected from the rupture of Chelungpu fault during the Chi-Chi earthquake, allowing the seismic response of bridge structures subjected to these near-fault ground motions to be carefully examined. To study the near-fault ground motion effect on bridge seismic design codes, a two-level seismic design of bridge structures was developed and implemented. This design code reflects the near-fault factors in the seismic design forces. Finally, a risk assessment methodology, based on bridge vulnerability, is also developed to assist in decisions for reducing seismic risk due to failure of bridges.

Approximate Evaluation of Deflection Amplification Factor

In seismic design provisions, the maximum story drift occurring in major earthquakes is estimated by amplifying the drifts computed from an elastic analysis at the prescribed design seismic force level with a deflection amplification factor. A survey of several seismic design provisions indicates that the deflection amplification factor, in most of the provisions, is independent of significant factors such as ductility ratio, fundamental period, and number of stories. In this paper, a statistical evaluation of deflection amplification factor is presented in order to indicate the parameters that have an effect on this factor. Elastoplastic multidegree of freedom (MDOF) models with various dynamic characteristics are first considered. Consequently, nonlinear and corresponding linear time history analyses for 21 severe earthquake ground motions are performed in order to obtain structural response data, which are then used to derive an empirical formula for the deflection amplification factor. Finally, an app...

FACTORES DE REDUCCIÓN DE FUERZAS SÍSMICAS PARA EL DISEÑO DE ESTRUCTURAS CON SISTEMAS PASIVOS DE DISIPACIÓN DE ENERGÍA

Los reglamentos actuales de diseño sísmico no contemplan explícitamente el diseño de estructuras con sistemas de disipación pasiva de energía. Uno de los aspectos más importantes para el desarrollo de una metodología para el diseño de este tipo de estructuras es el planteamiento de métodos para estimar sus fuerzas sísmicas de diseño. Con base en el estudio de la respuesta de sistemas de un grado de libertad ante la acción de acelerogramas registrados en México durante eventos sísmicos recientes, se proponen expresiones para estimar el factor de reducción que debe utilizarse para reducir el espectro de diseño elástico de resistencia para establecer fuerzas sísmicas de diseño para sistemas con diferente capacidad de deformación plástica y de disipación de energía viscosa. Se ofrecen expresiones para suelo firme y suelo blando, en donde el valor del factor de reducción se estima en función del periodo de la estructura, de su demanda máxima de ductilidad y de su coeficiente equivalente de amortiguamiento.

Strength reduction factors for structures that simultaneously dissipate plastic and viscous energy

In recent years. current seismic codes started contemplating the design of structures with passive energy dissipating devices. One important issue for the rational seismic design of these devices and the structure that contains them is the formulation of methods to estimate their design seismic forces. From the study of the dynamic response of single-degree-of-freedom systems subjected to accelerograrns recorded in Mexico during the last two decades, expressions to estimate the strength reduction factor that should be used to reduce the elastic design strength spectra for 5 percent damping, to establish the design seismic forces for structures having different combinations of plastic and viscous energy dissipating mechanisms, are formulated.

Seismic design force for single-span slab-girder skewed bridges

This paper investigates the validity of seismic design force recommended by AASHTO for single-span bridges. The prescribed force is equal to the products of soil factor , acceleration and tributary weight of the structure. A three dimensional finite element analysis of straight and skewed bridges with skew angles varying from 0 to 60 degrees is used for this study. In the longitudinal direction, the bridges are assumed to be supported either by elastomeric bearings or a pinned support. In the transverse direction, the stiffness of end cross-frames is considered in the analysis. AASHTO ‘s recommended seismic design force for single-span bridges is compared with the El Centro time history and response spectrum analysis. It is concluded that AASHTO’s recommended design force for single-span straight and skewed bridges could be unsafe in certain cases. An increase in the design force to a level equal to response spectrum value is recommended for such cases.

Seismic Behavior of 3D Steel Moment Frame with Biaxial Columns

Three-dimensional nonlinear dynamic time-history analyses of a three-story steel moment-resisting frame designed according to the Uniform Building Code for Los Angeles seismicity were carried out. Beams in both directions had moment connections to the hollow rectangular columns. Code drift limits, rather than code considerations for bidirectional horizontal shaking, governed the member sizes. It was found that design level shaking caused the structure to exceed story yield drifts in both directions simultaneously and significant column yielding occurred above the base. Shaking in the direction orthogonal to the main shaking direction increased drifts in the main shaking direction by 46 and 64% for the design level and near-fault records, respectively, indicating that 2D analyses would not estimate the 3D response well. Also, maximum horizontal seismic components of axial force in the corner columns and side columns were 4.87 and 5.30 times the code estimations, respectively. It was shown that by increasing the column strength above the present code levels, that drifts during near-fault shaking were significantly decreased. A methodology to encourage strong-column weak-beam behavior, and to more realistically estimate the column axial forces during design level shaking is described. Building structures in seismic zones in the United States have traditionally been designed to resist lateral force with seismic frames. Other frames, referred to as gravity frames, are designed to resist gravity forces only. This design ap- proach, with separate seismic and gravity frames, requires only a few expensive moment connections, the load path is easy to follow, and 2D frame analyses may be used to model 3D frame behavior. However, as a result of the 1994 Northridge earth- quake, fracture occurred at many welded beam-column con- nections in these moment-resisting steel seismic frames (Yang and Popov 1995). The occurrence of fracture was influenced by a large number of factors, including the use of large mem- ber sizes and large welds. The lack of redundancy in these frames (Roeder 1997) and the possibility of yield in gravity columns (Mattheis 1998) are also of concern. An alternative design method in which structures are de- signed as a 3D seismic frame with beams connected to the columns, with moment connections in both directions, has some advantages. It increases the redundancy, no columns are designed to carry gravity force only, and beam and weld sizes are smaller. While many more seismic connections are re- quired, all members contribute to the frame lateral stiffness and strength allowing drift limits to be met with less material volume. However, the possibility of undesirable behavior due to significant column bidirectional loading effects should be considered. A brief summary of considerations for column bi- directional loading effects in 3D steel moment-resisting frames due to bidirectional ground shaking is given herein. The Uniform Building Code (UBC) (ICBO 1997) states that bidirectional orthogonal shaking effects must be considered if a column of a structure forms part of two or more intersecting lateral-force-resisting systems and that ''the requirement that orthogonal effects be considered may be satisfied by designing such elements for 100% of the prescribed design seismic forces in one direction plus 30% of the prescribed design seis- mic forces in the perpendicular direction. The combination re- quiring the greater component strength shall be used for de- sign. Alternatively, the effects of the two orthogonal directions may be combined on a Square Root of the Sum of the Squares (SRSS) basis.'' (ICBO 1997). The SRSS procedure (Clough and Penzien 1993) is based on the assumption that superpo- sition of the actions due to loading in each direction indepen- dently may be combined to obtain the total response. Phase incoherency of response, which may result from ground mo- tions in each direction not being in-phase or from significantly different response periods in each direction, is also assumed. The 30% rule (ICBO 1997) is based on the following con- siderations (Mattheis 1998; Tagawa 2000) following the rea- soning of Clough and Penzien (1993). If the ratio of the re- sponse due to y-direction shaking divided by that due to x-direction shaking of equal magnitude is B, and if the shaking magnitude in the y-direction is a of that in the orthogonal x-direction, then the response due to y-direction shaking, R y, is related to that due to x-direction shaking, Rx ,a s