Structures

Structures

Design

Age

Building materials

Measuring force on

structures

Acceleration

Resonance

Acceleration

• A measurement made on structures

relative to gravitational force

• 1 g = 32 ft/sec squared or 9.8

meters/second squared

• Building codes are at about 40-60

percent of that or written as .4 to .6

Acceleration

• Added strength is needed to

maintain a structure’s integrity when
subjected to lateral accelerations

Structures are built to maintain their integrity due
to gravity

• Acceleration

readings vary with

earthquakes

• What type of fault

would produce the

highest

accelerations?

Acceleration

-Horizontal
accelerations reached
1.19 and 1.02 g at the
base and 1.8 g on the
roadway.
-The Los Angeles River
sediments underlie this
bridge.
-What happened?

Freeway Collapse

Simi Valley freeway collapse due to high

accelerations.

Northridge Earthquake

• Increase with building height

Acceleration

San Jose High School, 1906
earthquake: stiff building
material and increase
acceleration with height

• Decreases with

distance from
epicenter

Acceleration

Period and Resonance

• Period is the amount of time it

takes one wavelength to pass a
point

• Seismic waves with a long

wavelength have a larger period
(2-4 seconds)

• Seismic waves with a short

wavelength have a shorter period
(1/2-20 cycles/sec)

Wavelength

Period and Resonance

• Buildings also have a period
• The period (long or short) is

determined by the number of stories

• Resonance occurs when the seismic

waves pass through the earth material

producing a particular wavelength and

this wavelength matches the buildings

period (wavelength).

• Remember: frequency is inverse of

period

• Resonance causes the motion of

the bldg to increase

• 0.1 second for a one-story building
• 1-2 seconds for a 10-20 story building

Period and Resonance

Common Building Failures

• Resonance:

when the period
of the seismic
wave matches
the period of a
structure

• 30 seconds of

shaking put the
structure into
resonance

Structural Failure associated

with the Loma Prieta

Earthquake

Bay Bridge failure

• First pier into bay

mud off Yerba Buena
Island (bedrock)

• Connection failed

due to low
frequency seismic
waves (mud) and
high frequency
seismic waves
(bedrock)

Cypress structure, Oakland

Reinforced
concrete
failure

1950 structure:
lacked seismic
design

Earth
material:
bay mud

Seismic
waves
amplified

Liquefactio
n

Marina District

Earth
material:
unengineer
ed fill

Liquefactio
n

Seismic
waves
amplified

Soft story: a floor of a multiple
story building that lacks the
structural strength or symmetry of
the other floors

Downtown Santa Cruz

Earth
material:
unconsolidate
d sediments
deposited by
the San
Lorenzo River

Seismic
waves
amplified

Liquefactio
n

Unreinforced masonry (URM)
failed

Structural Failure

Man-made structures:

– Structural design and age
– Building materials
– Fire
– Infrastructure failure: gas lines, water

lines, electrical wires or transformers, cell
phone towers

Soft Story Collapse

• Parking garage is a

soft story

• Scenes like this

were familiar near
the epicenter

• Where have you

seen this type of
structure?

• Soft story: inadequate lateral

bracing

Structural failure,

Northridge Eq

Kobe, Collapse of 5th Story

• Another example

of soft story
collapse

• 5th floor restaurant
• Open structure
• Stories above and

below have more
support

Stiff building material

Pakistan, 2005:
Mw 7.6
8:50 AM, local
time
80,000 fatalities
200,000 injuries

Wall Failure

Traditional structures failed-

unreinforced

brick

• Seismic waves travel horizontally and

vertically

• Failure occurs at the connections
• Increase in acceleration with height

Irregularly shaped buildings

• T-shaped structure
• Communication

center in Mexico City

• The city lost

international
communication after
the 1985 earthquake

Resonance

• Resonance:

when the period
of the seismic
wave matches
the period of a
structure

• 30 seconds of

shaking put the
structure into
resonance

Mexico City,
1985

Earth material

• Loosely consolidated sediments and water

saturated mud or sand amplify seismic shaking

• Liquefaction often occurs
• Failure at connections where earth material

varies

Bay Bridge

Cypress
Structure

Moss
Landing

House falls off foundation

Foundation

Sill plate

House attaches to the
foundation through the sill
plate

HOG: house over garage

Open, weakly supported garage
fails with heavier and sturdier
structure above

Cripple wall failure

• The wall between

the sill plate and
the house

• Yellow: GPS data

– Slow slip or silent

earthquakes

– Early- 2002, mid-2006

• Red/Green: seismic

stations

– Circled area,

earthquakes

Silent earthquakes: indicative of

earthquakes

• Shallow and then

becomes more steep
under Mexico City

Mexican subduction

zone

Mexico City Earthquake

• 50 x 170 kilometers of displacement

along the subduction zone

• M 8.1
• Mexico city is 400 kilometers away
• City was built on the sediments of

Lake Texcoco

Mexican subduction

zone

• Cocos tectonic plate is

subducting under the
North American Plate

• Two plates lock
• Stress builds and

energy is stored

• Stress exceeds

frictional force

• Release of energy in

terms of an earthquake

• Earthquakes are

more shallow than
other subduction
zones

Mexican subduction

zone

Mexico City

• Drained Lake Texcoco
• Clay sedimentary layers
• Low frequency surface

waves amplified

• 1-2 second frequencies
• Matched the periods of

buildings 6-16 stories

Mexico City: Common

Building Failures

• Top floors fail-resonance
• T-shaped structures
• Flexible structures

between stiff structures

Mexico City: Building

Failures

• Hammering

Soft story
collapse

Uplifted terrace with

lighthouse

Intertidal fauna
exposed

3-6 feet of uplift along the
coast

Balcony

beams and weak

internal wall caused

buckling of building.

Early

seismic construction

• Huaca Pucllana
• Lima, Peru
• 200-700 CE
• Bricks built in a

trapezoid pattern
with spacing

• Accommodates

seismic shaking

Which structural designs tend
to fail during ground shaking?

• Soft story
• Structures constructed from stiff building

materials

• One weak point initiates other failures
• Irregularly shaped structures
• Structures that move into resonance
• Earth material fails
• Hogs
• House off foundation
• Cripple wall failure

Constructing model

buildings and subjecting to

shaking

• Building must be:

– At least 30 cm high
– At least 3 stories
– No central post or uprights

• Materials are limited
• Complete construction in limited time

Document Outline