Earthquakes in San Francisco, California

- Structural Engineering Solutions -

Why Do Earthquakes Occur?

Earth’s Lithosphere (crust) made up of numerous plates

Plates can move on earths asthenosphere: Solid, but in geological

timescales is relatively viscous

Plates move slowly - creep

Plates can become “Locked” to each other - earthquakes

Why is San Francisco at Risk?

San Andreas fault

Major fault line between North

America and Pacific plates: 800 miles long, can extend to

10miles deep

Right lateral strike slip

Geologists estimate the total

displacement 350 miles

Surveying shows drift of

2inches/year

Geology of San Francisco

The geology of San Francisco amplifies the problems caused by earthquakes

Much is alluvium and sand deposits that lack rigidity

The Great 1906 Earthquake

5:12 AM 18th April 1906

Death toll up to 3000

Earthquake strength estimated at 7.8 on Richter scale

Max observed displacement 20feet

Main shock 45-60s

Economic centre of the west coast

Cost of damage $450m ($650b in 2006)

Up to 300,000 out of 410,000 left homeless

Most damaged caused by fires

Pictures from 1906 Earthquake

Engineering Solutions

Earthquakes produce two types of vibrations: Horizontal movement Vertical movement

For buildings and their contents horizontal movement is the most dangerous

There are two basic methods to protect against earthquakes: Isolate the building from the grounds movement Make the building strong enough to withstand it

Engineering Solutions

Base Isolation: Allows for horizontal movement of the ground under the

building Isolation need a significant level of mechanical damping

Rubber Dampening Rolling Dampening

Engineering Solutions

New Modern Techniques:

PTED Frames

No welding between beams and columns

Beams and columns can twist and move without structural damage

Still in the testing stages

‘Energy dissipating’ bars – Reduce the cyclic

movement within the frame

Post tensioned bars keep the frame together under

movement

Engineering Solutions

New Modern Techniques:

Tuned Mass Damper

Large weight positioned at the top of a building

Tapia 101 - Oil dampeners connected to the steel ball stop movement of the building due to the inertia of the mass

Method currently used

Also helps with wind dampening

Tapia 101 building, Taiwan. Tuned Mass Damper on 88th floor.

One Rincon Hill

Situated in San Francisco

South of the North Bay

Reduce sway: This is the first residential building in America to make use of a liquid tuned mass damper

A water system is used, the energy from the swaying building is dissipated through filters/screens.

One Rincon Hill

Make the building strong: The building has large steel outriggers that transfer load into the serpentine rock

It is the highest US Skyscraper to have ‘buckling resistance braces’ and the first ever to have these as outriggers

The outriggers transfer the load from the central core down into the base foundations

At 48ft they are braced back onto the concrete cored for extra strength