5.3 Lateral Load resisting system design – Part 2

Summary

This lecture explores the seven basic design principles for creating earthquake-resistant buildings. It emphasizes the importance of using ductile materials, ensuring all building elements are connected, minimizing mass and eccentricities, avoiding vertical irregularities, designing for energy absorption, and ensuring multiple lateral load paths. The goal is to ensure buildings perform well during seismic events, absorbing energy, minimizing damage, and maintaining stability even when parts of the structure are compromised. The lecture offers insights into practical strategies for engineers to consider when designing for seismic safety.

Takeaways

• 😀 Use ductile materials for earthquake resistance, as they absorb energy and allow for nonlinear behavior, reducing the risk of brittle failure.
• 😀 Ensure all elements of the building are positively connected to act as one system, preventing isolated failures during an earthquake.
• 😀 Minimize mass in the building to reduce the forces experienced during seismic events, particularly by choosing lightweight materials where possible.
• 😀 Align the centers of mass and resistance to avoid eccentricities that lead to torsional shaking, which can cause significant damage.
• 😀 Avoid irregular building shapes and re-entrant corners, as these can create eccentricity and increase torsional motion during earthquakes.
• 😀 Minimize discontinuities in the vertical system (such as non-continuous members or transfer floors) to prevent localized damage and ensure smooth load transfer.
• 😀 Design buildings with energy absorption capabilities through hysteresis damping of ductile elements to prevent structural failure while maintaining load-bearing capacity.
• 😀 Use 'plastic' hinges or fuses in structural elements to localize energy absorption without affecting the overall load-bearing capacity of the building.
• 😀 Design for redundancy by ensuring multiple lateral load paths in the building, so that failure of one element does not lead to the collapse of the entire structure.
• 😀 Prioritize materials with high strength-to-mass ratios, such as steel and timber, instead of concrete and masonry, to reduce seismic forces on the building.

Q & A

What are the seven basic design principles for earthquake-resistant buildings?

The seven principles are: 1) Use ductile materials, 2) Connect all building elements, 3) Minimize mass and mass/strength ratio, 4) Minimize eccentricities and plan irregularities, 5) Minimize discontinuities and vertical irregularities, 6) Ensure energy absorption by designing for ductile failure mechanisms, and 7) Design multiple lateral load paths.

Why are ductile materials important in earthquake-resistant design?

Ductile materials allow buildings to behave in a nonlinear way and absorb energy during an earthquake, preventing brittle failure. This leads to smaller structural components and more economical designs.

What is the role of building connections in seismic design?

Building connections should ensure that all elements act as a single system. Connections must be designed stronger than the elements they join to avoid damage or failure at connection points.

How does minimizing mass help in earthquake resistance?

Minimizing mass reduces the seismic forces acting on the building. Lighter materials like steel and timber are preferred over concrete and masonry to reduce these forces.

What is the significance of aligning the center of mass and the center of resistance in a building?

Aligning the centers helps prevent torsional motion during an earthquake, which can lead to more significant damage. Disalignment of these centers can cause harmful torsional shaking.

Why should eccentricities in building shapes be avoided?

Eccentricities lead to torsional motion, which can cause severe damage during an earthquake. This can be avoided by designing symmetrical floor plans and balanced support systems.

What are the potential risks associated with discontinuities in building mass and stiffness?

Discontinuities can cause localized damage and failure. This includes issues like discontinuous vertical systems, varying story heights, and irregular vertical shapes.

How is energy absorbed during an earthquake in a ductile building?

Energy absorption occurs through hysteresis damping in ductile structural elements, where 'plastic' hinges or fuses absorb energy without compromising the building's vertical load-bearing capacity.

What is the purpose of using 'plastic' hinges in building design?

'Plastic' hinges are designed at specific locations where energy absorption is needed during seismic events. They allow structures to bend and dissipate energy while maintaining their overall load-bearing function.

What does designing multiple lateral load paths ensure in an earthquake-resistant building?

Designing multiple lateral load paths ensures that if one element fails during an earthquake, the building can still distribute forces safely to the foundation, preventing total collapse.