ZanStructures
Structural
I am engaged in the analysis and design of a 13-storey concrete structure situated in a seismic zone. The design codes for this project are EC2, EC8, and EC7. Due to tight scheduling constraints, the preliminary analysis for the superstructure was conducted without any geotechnical data or reports. Consequently, I made initial assumptions regarding the soil type based on geological maps of the surrounding area. The superstructure was analyzed utilizing fixed/pinned supports, and member sizing was coordinated and agreed with the architects based on the initial analysis. As the project progressed, I received a geotechnical report from a local engineer, prompting the need for a more comprehensive analysis and design package for the entire structure. I have decided to use a mat/raft foundation, and my current concern lies in determining the best course of action.
In my consideration, there are four potential analysis and design approaches, each with varying degrees of engineering correctness, conservatism, and uncertainties. The fourth option is still under deliberation.
These options are outlined as follows:
1. Reanalyze the superstructure and foundation together and design based on these combined analysis results.
2. Independently design the superstructure (fixed/pinned supports) and subsequently extract reaction forces to design the foundation. The foundation will undergo analysis using finite elements and area springs, meaning a flexible foundation analysis.
3. Independently design the superstructure and then assume a rigid foundation, apply the resultant load, and design it manually as individual strips with soil pressure as the load and walls/columns as supports.
4. Reanalyze the superstructure and foundation together and design only the foundation using these analysis results.
Each analysis method comes with its own set of concerns:
1. The first method, while the most sophisticated, requires precise input data with minimal margin for error. I would like to emphasize my concern about the geotechnical report, which appeared to be rushed, raising doubts about the accuracy of the results. The geotechnical engineer provided a subgrade modulus (Ks) without me supplying details about the foundation type, dimensions, or loads. However, soil elastic properties (modulus of elasticity Es and Poisson’s ratio) were included, allowing me to calculate my own value for the area spring Ks. Utilizing this spring, I conducted an analysis and obtained results. The specific structure with fixed/pinned supports (first analysis), has a fundamental period that is approximately 1.4-1.5 seconds. In the context of response spectrum analysis in EC8, this period already indicates a low seismic acceleration that the structure will experience. Upon conducting a full model analysis (considering both foundation and superstructure), the fundamental period extends to 1.9-2.0 seconds, implying a smaller earthquake force applied to the structure.
2. The second method is a conservative approach focusing on identifying areas of the foundation requiring additional reinforcement.
3. The third method, based on past designs, is deemed the most conservative.
4. The fourth option raises questions, as the superstructure may be conservatively designed while the foundation is designed with smaller loads (superstructure reaction forces).
What is your opinion on this, which approach would you have chosen?
In my consideration, there are four potential analysis and design approaches, each with varying degrees of engineering correctness, conservatism, and uncertainties. The fourth option is still under deliberation.
These options are outlined as follows:
1. Reanalyze the superstructure and foundation together and design based on these combined analysis results.
2. Independently design the superstructure (fixed/pinned supports) and subsequently extract reaction forces to design the foundation. The foundation will undergo analysis using finite elements and area springs, meaning a flexible foundation analysis.
3. Independently design the superstructure and then assume a rigid foundation, apply the resultant load, and design it manually as individual strips with soil pressure as the load and walls/columns as supports.
4. Reanalyze the superstructure and foundation together and design only the foundation using these analysis results.
Each analysis method comes with its own set of concerns:
1. The first method, while the most sophisticated, requires precise input data with minimal margin for error. I would like to emphasize my concern about the geotechnical report, which appeared to be rushed, raising doubts about the accuracy of the results. The geotechnical engineer provided a subgrade modulus (Ks) without me supplying details about the foundation type, dimensions, or loads. However, soil elastic properties (modulus of elasticity Es and Poisson’s ratio) were included, allowing me to calculate my own value for the area spring Ks. Utilizing this spring, I conducted an analysis and obtained results. The specific structure with fixed/pinned supports (first analysis), has a fundamental period that is approximately 1.4-1.5 seconds. In the context of response spectrum analysis in EC8, this period already indicates a low seismic acceleration that the structure will experience. Upon conducting a full model analysis (considering both foundation and superstructure), the fundamental period extends to 1.9-2.0 seconds, implying a smaller earthquake force applied to the structure.
2. The second method is a conservative approach focusing on identifying areas of the foundation requiring additional reinforcement.
3. The third method, based on past designs, is deemed the most conservative.
4. The fourth option raises questions, as the superstructure may be conservatively designed while the foundation is designed with smaller loads (superstructure reaction forces).
What is your opinion on this, which approach would you have chosen?
