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Research and Development of a Uniform Seismic Analytical Method

The purpose of this research and development is to investigate the background of the theories adopted by the various jurisdictions, using actual projects to demonstrate the issues and prepare the comparative analysis for similar structure types using the various design codes in order to bring out the differences between the various requirements of jurisdictions across Canada.

Having completed the detailed comparative research, the understanding gained will provide for greater harmonisation in the seismic criteria and the manner in which it is incorporated into the design process. The proposals for more harmonised seismic criteria will thus create a more uniform analytical methodology for seismic impacts.

A summary of the research and development, and the proposals for a unified methodology that are going to be generated,will be useful as a complete current professional reference of the state of the art in the construction industry with respect to seismic design.

A review of the proposed updates that will be required to the codes of practice will be carried out from the standpoint of the practising engineer to facilitate and enhance the common understanding and treatment of this important geotechnical phenomenon that has serious structural consequences particularly given the recent and relatively severe earthquakes in Quebec and Ontario. The intention is to produce a rational unified methodology for seismic impact in structures by a summary of proposed changes to the existing codes.

The aim in the more current seismic design of structures and bridges is to design a structure that will achieve a specified deformed state under a specified design level earthquake rather than by ensuring the design forces are less than the specified material strength limit. By allowing some deformation that is within an acceptable amount at the design earthquake, the peak forces may be significantly reduced. The deformation allowed is usually limited to what is termed as repairable damage.

The current methods of analysis involving more and more powerful softwares have a degree of complexity and detail that is quite incompatible with the coarseness and the number of assumptions made regarding the structural characteristics. The problem is not about the models developed, but more about the assumptions made so far.

This current approach consists of the following:

– To develop further the existing analytical methods in the building code and the bridge code to improve their accuracy with consistent logic.

– To combine the strength approach with a displacement assessment which has not been fully addressed.

– To develop a more practical determination of the displacements that define the specified deformed state of a structure following an earthquake in various regions. The current procedure is complicated, theoretical and difficult to apply.

– To adopt a method to for deriving a more accurate structural member stiffness for determining deformation and possibly combining it with a known relatively accurate method for determining the seismic response of a structure, by use of inelastic time history analysis or pushover analysis to determine the peak deformation demand.

– To investigate discrepancies arising from the paucity of knowledge regarding the ground motion. Geotechnical research and ground movement monitoring are required to validate the seismic criteria being used in the structural analysis

The main technological advancement would be to ensure that the importance of deformation rather than strength in assessing seismic performance is better appreciated and understood, and to develop an improved and unified method that combines the strength approach with a displacement assessment.

The design for seismic impact has been a difficult aspect of the design process for projects throughout Canada due mainly to the many uncertainties associated with earthquakes, the different design methods set out in the different standards and codes of practice, and the various alternative requirements that have been specified from time to time.

In order to initiate some standardization in the design process, the Canadian Highway Bridge Design Code has been adopted for all bridge design across Canada and the National Building Code of Canada for all building designs across Canada. In these two design codes, the design for seismic impact can be done by using the different seismic values and factors given in the codes to address the level of seismic activity in the different regions of Canada. In this way, the design engineers have in essence, one procedure for the seismic design for bridges, and another procedure for structures that do not fall into the bridge category.

However, there are major differences in the level of seismic activity used in the two codes. The bridge design code considers a one in 475 year return period earthquake, which is equivalent to a 10% probability of occurrence in 50 years. On the other hand, the national building code classifies the design level of earthquake to be a one in 2475 year return period, which is equivalent to a 2% probability of occurrence in 50 years.

The two quite different levels of seismicity can create difficulties for the design engineers as they move from project to project. Research work is directed at finding a harmonised method that integrates the clear logic and solid theoretical sections of both design levels into one methodology that can be used for all types of structures throughout Canada.

In addition, it has become apparent that the values for the seismic parameters for Ontario, as given in the current National Building Code of Canada were less than those for British Columbia. Recently however, with the relatively severe earthquake in Quebec and Ontario, it has caused some engineers to believe that the factors used for seismic design may need to be increased.

The geotechnical parameters used in the seismic impact analysis for the structure such as ground accelerations, have been developed for inclusion in the design codes based on relatively short period of recent historical data, as compared to geological time. Furthermore, the recording process has not been active for many years. Given that the appropriate probability distribution for seismic activity is currently under discussion and that some recent earthquakes appear to have generated impacts more severe than anticipated, it is quite possible that in the design for future earthquakes, a significant increase to the design accelerations adopted for the project will be made with concomitant erosion of load factors and an increase in the assessed risks involved.

An attempt has been made to add the displacement assessment into the design tasks but the lack of guidance in the codes make this process difficult as the challenges are numerous, including key assumptions in structural analysis such as the real structural member stiffness during earthquakes, the expected deformed state under various types of ground motion, and the maximum displacement assessment for the various types of earthquakes.

There are also other technological uncertainties inherent with this design method. In addition to the different assumptions that have to be used for analysing the stiffness of the structural members and the distribution of the required resistive strength between the different lateral force-resisting elements of the structure, the manner of determining the seismic response or seismic demand is uncertain.

Different assumptions with respect to seismic impact are made in the different design codes particularly with reference to the determination of the appropriate stiffness for reinforced concrete and masonry members. Consequently, in determining member stiffness, there is considerable variation in the calculated fundamental period of the structures resulting in variation in the design seismic forces. The different approaches to the method of analysis for the seismic impact on a structure and the structure’s response also creates considerable uncertainty in the values generated in the seismic demand evaluation.

In traditional strength calculations, the moment demands from elastic analysis are checked against the moment capacity. Once the demand exceeds the elastic range of the material, the strength method becomes very conservative. To improve on the seismic analysis and design, a displacement approach was used in a number of ways, but not limited to:

– defining the specified deformed state of a structure following an earthquake in various regions,
– deriving a more accurate structural member of stiffness,
– calculating the yielding moment demands at a certain level of deformation.

In general terms, the impact of seismic activity on the different types of structures and their foundation is addressed during the design stage through determining the forces that will arise within any particular structural component during the period of ground excitation. The overall structural geometry is determined based on considerations of the other loads either applied or inherent, together with assumptions that have to be made for the individual member stiffness.

The fundamental period is then calculated for the particular structure and a force reduction factor is determined corresponding to the ductility capacity assessed for the overall structural system based on the specific materials used for each member selected as specified by the design code. Having determined the force reduction factor, it is then applied to the base shear force in order to arrive at the design shear force. The design shear force is then distributed to the different members and parts of the structure providing the lateral force resistance, in proportion to their individual elastic stiffnesses.

The moment capacities at the locations of potential inelastic action, ie plastic hinges are then determined using the magnitude of the displacements estimated as possibly occurring under the seismic impact. A further check is made to ensure that the estimated displacements are within the limiting values specified in the referenced research being used for the design.

If the displacement values are not exceeded, then the strength of members that are not subjected to any plastic hinging can be obtained. A check is made for the strength in shear and the moment capacity of the members and sections where plastic hinging must not occur do not exceed the maximum values possible. These values correspond to the forces that could be generated when the maximum possible resistance of the potential plastic hinges is included in a back analysis of the structure and its members.

Major differences in ground motion parameters as given in the codes was noted and a research was done to better understand and obtain an explanation for the reasons behind the differences. The building code appears to target a higher level of design earthquake to prevent severe loss if such a rare event does happen. However, it is very difficult for a practising engineer to judge the impact of the higher level earthquake and to evaluate in the design a proper level of protection for civilians. A recent magnitude 5.0 earthquake in Quebec raised questions to the adequacy of the design ground motion in traditionally less active zones.

It may become apparent that a higher value for the design ground motion may be necessary with further findings submitted to the various authorities having jurisdiction for review and subsequently endorsement. Without the appropriate endorsement from the governing authorities, it would understandably be difficult to recommend the appropriate revisions to the codes of practice.

Currently, there are still many technological unknowns contributing to a design process that is difficult. Factors such as the real seismic demands, the level of protection required, the ductility capacity of structures, the characterization for the design of the structure member stiffness, the distribution of required strength between the structural elements, and the detailing of sections for seismic responses,all contribute to a complex procedure in the design process.

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