Load Bearing Analysis Approaches

The Equivalent Lateral Force (ELF) procedure and the Response Spectrum Analysis (RSA) remain two essential yet distinct approaches to seismic design. ELF is a static equivalent method that transforms seismic effects into lateral forces distributed along the height of a building. It provides engineers with a fundamental estimate of seismic shear and overturning demands and is invaluable for developing a clear understanding of how a lateral load resisting system carries earthquake forces. Because of this transparency, ELF continues to be used for preliminary design, code compliance in regular low- to mid-rise structures, and as a benchmark to validate more complex analysis results. RSA, in contrast, is a dynamic analysis method that considers multiple vibration modes and their combination. This allows it to capture higher mode effects, irregular mass and stiffness distributions, torsional response, and setbacks or podium conditions that ELF cannot realistically address. For tall, complex, or irregular buildings, RSA gives a much more accurate representation of seismic behavior. ASCE 7-16 and the latest ASCE 7-22 both explicitly recognize RSA as an alternative when ELF assumptions are not valid. The recent amendments in ASCE 7-22 have refined ELF provisions further. The code now provides clearer requirements for irregular structures, revised criteria for the two-stage analysis, and improved guidance for distributing forces in buildings with setbacks and podiums. Importantly, ASCE 7-22 emphasizes that ELF should not be artificially modified by scaling base shear at setback levels. Instead, irregularities should be explicitly identified, and engineers are encouraged to use RSA or nonlinear procedures when ELF cannot distribute the seismic demands properly. In practice, both approaches have significant roles. ELF establishes the baseline and provides the conceptual foundation for understanding seismic load paths. RSA delivers the refined reality, capturing the actual seismic response of irregular or tall structures. Together, they ensure that engineers respect both the fundamental principles and the true dynamic behavior of structures under earthquake excitation.

  When structural engineers model any building or tower using numerical analysis software, especially in cases involving earthquakes and wind, they typically apply a fixed base assumption, meaning that the building is considered to have a fixed relationship with its foundations. However, the question arises: Is this modeling approach accurate? And what are the different ways to represent the relationship between a building and its foundations?   there are two commonly known approaches that structural engineers use in modeling: 1. The first and most common method: It assumes that the relationship between the building and its foundations is fixed. This assumption is often used to avoid discussions with the geotechnical engineer, who usually provides low soil stiffness values to avoid the need for a soil-structure interaction model as conservative approach, which is often not mentioned in the contract between the owner or contractor and the engineer. When these low values are used, the results can deviate from the typical figures the structural engineer is accustomed to based on their experience in similar cases.   Is this approach correct? This method may be appropriate if the ground under the foundations is very strong (such as rock) and the foundations themselves are rigid. In this case, assuming that the foundations do not move or rotate is close to reality, and the results based on this assumption can be reasonably accurate.   2. The second and more accurate method: When the ground beneath the building is expected to settle or move, the first method is not accurate. In this scenario, a more precise model should be used by representing the soil as having stiffness. Some might complain that the forces and displacements (drifting) increase unreasonably. However, this is not an issue with the method itself but with how it is applied. How is it applied? First, it is essential to ensure that the stiffness values provided by the geotechnical engineer are correct. Second, a soil-structure interaction model (SOIL-STRUCTURE INTERACTION MODEL) should be developed to accurately calculate the stiffness, and if possible, perform load tests and calibrate the model accordingly. Additionally, if there is a basement, the passive resistance of the soil surrounding the basement walls should be taken into account. This resistance, along with the friction between the foundations and the soil, helps reduce horizontal movement and increases the overall stiffness. Therefore, using a soil-structure interaction model is necessary because it provides more realistic results when applied correctly compared to assuming a fully fixed base. In conclusion: If the soil is rocky, the first method might be suitable and provide results close to reality. However, the more accurate approach is to use a model that accounts for soil-structure interaction, which gives more realistic values when calculating movements and forces.

[Cracks in Structural Walls Under Localized Load Concentration] In the images, a concentrated load (force F) is applied vertically onto a structural wall. If calculated solely based on classical beam bending theory, there’s a risk of cracks appearing in the wall, as shown in the figure. So why does this phenomenon occur, and are there solutions to prevent it? Answer: When the load is applied too close to the wall’s edge, a localized failure zone of about 30° tends to form, starting from the load application point to the edge of the wall. The redistribution of stresses within the wall also impacts the positions of tension and compression zones. Using finite element software with an appropriate model (2nd Image) can clearly illustrate the stress distribution along the wall. It shows that stress concentrations occur between the localized load application point (the edge of the structural wall) and the lower edge of the footing or support. This causes a difference in horizontal and vertical tension forces in these areas. Apart from ensuring proper stress redistribution vertically and horizontally through adequate reinforcement, local bearing stress in the wall should also be strengthened to prevent excessive deformation or cracking under the applied loads. Understanding this phenomenon, we should rely on a detailed model (3rd Image) to determine reinforcement placement and verify the adequacy of the structural design. Simultaneously, stress concentration calculations in reinforced regions will help ensure stress distribution is compatible with the structure's overall capacity. These insights not only optimize reinforcement placement but also ensure durability, load-carrying capacity, and better local bearing stress performance for the wall connections. Hope this helps you in refining your structural analysis, ensuring safety and reliability in your designs! 🌱

How Do Structures Transfer Their Base Shear to Soil, and Why Is It Crucial? Understanding how lateral loads move through a structure and into the soil is a basic but often overlooked part of structural engineering. This knowledge is essential for checking an important assumption in our structural analysis: the fixed base model. This approach simplifies structural analysis by assuming that there is no movement at the soil level, which makes calculations easier. However, this can lead to significant discrepancies between analytical predictions and the actual behaviour of structures. This assumption is no longer the most efficient approach and may not be safe either. Mechanisms of Lateral Load Transfer to Soil: Many engineers are familiar with vertical foundation movements related to uplift forces and soil bearing capacity. However, the lateral movements of the foundation and their effects on structures are less frequently discussed. Here is a brief description of the mechanisms through which foundations transfer lateral loads to the soil: • Friction: This is the resistance that occurs as the foundation moves relative to the soil. • Passive Resistance: Lateral forces push the foundation against the soil through elements like ground beams and engage the soil to provide resistance (via minor axis bending of beams). • Piles: These function by pushing against the soil, utilizing a mechanism similar to passive resistance described above. Slab on Grade as a Transfer Floor: In scenarios where these mechanisms under lateral resisting elements are inadequate, how well the foundation system is connected becomes vital. This is particularly true if there are missing tie beams or insufficient reinforcement in the slab on grade. Recognizing the slab on grade as a crucial “transfer floor” is essential for addressing these issues. Here are strategies to enhance foundation design and performance: • Reinforcement: A diaphragm analysis of the slab on grade is crucial. It should include reinforcement details similar to those in suspended floors, often determined through methods like grillage analysis (refer to Section 5 - Appendix C5D of the NZ seismic assessment guidelines). • Tie Beams: These are essential for providing both passive resistance and functioning as diaphragm ties, facilitating load transfer across the foundation. • Ductile Reinforcement: Using ductile reinforcement in the slab is essential to maintain tensile capacity and manage large strains. • Connections: Strong connections between the slab on grade, lateral resisting elements, and footings are crucial for effective load transfer. By designing the foundation floor to function effectively as a diaphragm, we significantly enhance the building's efficiency and resiliency to withstand lateral forces. Keep an eye out for a future post, where I will discuss soil-structure interaction modelling and lateral assessment of piles. #structuralengineering #earthquakeengineering #seismicdesign #resilience