5.2 Lateral Load resisting system design – Part 1

Summary

This lecture introduces the concept of seismic forces and how they travel from their point of origin to the ground through horizontal and vertical systems. It covers the behavior of floors and structural elements during earthquake motion, focusing on design considerations like floor shape and void placement. Additionally, the lecture explains the three types of vertical systems—shear walls, braced frames, and moment frames—and their role in resisting seismic forces. The foundation’s role in handling gravity, shear, and moment forces is also discussed, setting the stage for the next part of the lecture on design principles.

Takeaways

• 😀 Seismic forces originate at a specific location and travel through horizontal and vertical systems to the underground.
• 😀 The first part of the lecture covers the basics of lateral load-resisting system design, while the second part focuses on the seven basic design principles.
• 😀 Forces during earthquake motion result from inertia of masses, and their direction is opposite to the direction of motion.
• 😀 Seismic forces are transferred to the foundation and ground through the horizontal system, which includes the floors, and then through the vertical system.
• 😀 The horizontal system in the design acts like a beam, with tension at the top and compression at the bottom.
• 😀 In the design of floors, it's crucial to avoid slender shapes to prevent large deflections, which could lead to damage.
• 😀 Void locations in the floor should be carefully chosen, avoiding areas with tension, compression, or high shear forces.
• 😀 Modern building designs focus on making floors stiff in their planes to avoid out-of-plane loading of the vertical system, which can cause damage.
• 😀 There are three basic vertical systems: shear walls, braced frames, and moment frames. Each offers different architectural possibilities.
• 😀 The vertical systems (shear walls, braced frames, and moment frames) must extend from the top floor to the foundation.
• 😀 The foundation of a building must resist gravity, shear forces, and moments by using footings or piles, depending on the design.

Q & A

What is the main focus of the lecture in the provided transcript?

The main focus of the lecture is on how seismic forces travel from their origin to the underground through horizontal and vertical systems, and the design considerations for lateral load resisting systems.

What are the two parts of this lecture series?

The first part of the lecture discusses how seismic forces travel through horizontal and vertical systems, while the second part will focus on the seven basic design principles for lateral load resisting systems.

What causes forces during an earthquake, according to the lecture?

Forces during an earthquake result from the inertia of masses, with the direction of the forces being opposite to the direction of motion.

How do seismic forces transfer through a building's systems?

Seismic forces are transferred through the floors (horizontal system), then to the vertical system (such as shear walls, braced frames, or moment frames), and ultimately to the foundation and ground.

What is the role of the horizontal system in seismic design?

The horizontal system consists of the floors, which are loaded horizontally. The floors are supported by the vertical system and act like beams, experiencing tension on the top and compression on the bottom.

What are the two important considerations in designing a floor for seismic loads?

The two important considerations are: 1) the shape of the floor plate, which should not be slender to avoid large deflections, and 2) the location of voids, which should not be in areas of tension, compression, or near vertical supports.

What are the three basic vertical systems used to resist seismic forces?

The three basic vertical systems are shear walls, braced frames, and moment frames. These systems must extend from the top floor to the foundation.

What architectural limitations do the three vertical systems impose?

Shear walls offer limited penetration possibilities, braced frames allow larger triangular penetrations, and moment frames allow larger rectangular penetrations but require larger beams and columns with stiff, strong joints.

What forces act on the vertical system due to seismic motion?

The vertical system experiences three types of forces: gravity force, shear force, and moment. The moment causes both tension and compression forces.

How does the foundation resist seismic forces?

The foundation resists seismic forces by using friction at the bottom to resist shear forces, and distributed vertical soil pressure to resist gravity force and moment. If piled, shear forces are resisted by the piles, and axial forces are transferred to the ground via skin friction and pile tip forces.