The West Anchorage High School was made up of a number of independent wings structurally separated by 2 inch seismic joints, including a 2-story reinforced concrete frame and shear wall classroom wing. The main portion of the school, including the primary...

Prepared By: Quinn Peck
Occupancy: Education
Year Built: 1953
Height: 26 ft
Number of stories: 2
Stories below ground: 0
Size: 67000 gsf
Original Code: 1949 UBC
Modification: Addition
Year Modified: 1955, 1956
Code of Modification: 1949 UBC
Lateral Load System: Moment Frame and Shear Wall Combination
Other Load System:
Vertical Load System: Flat Slab with Columns
Other Vertical Load System:
Foundation : Spread Footings
Other Foundation :
Country: United States
State: Alaska
City: Anchorage
Street: 1700 Hillcrest Drive, Anchorage, AK 99517
Latitude: 61.2002
Longitude: 149.9157


 

W. Anchorage HS

Earthquake Information

 

 

Earthquake Date 3/27/1964
Moment Magnitude 9.2
Epicentral Distance 120
Local Intensity MMI
Site Description
PGA Lateral None (g)
PGA Vertical None (g)
SaT
Ground motion recording stations
Distance to station None
Station Latitude None
Station Longitude None
Ground Motion Summary The Great Alaskan earthquake, as it is commonly known, occurred at 5:36 PM AST on Good Friday, March 27, 1964. The epicenter was along the northern shore of Prince William Sound, at 61.05 N, 147.05 W and a focal depth of 33 kilometers. Over 500,000 square kilometers experienced a significant vertical displacement as a result of the earthquake, ranging between 15 meters of uplift and 2.3 meters of subsidence. It is believed that the ground waves generated during the event were actually the result of several successive shocks propagating in southwesterly direction. While initial measurements indicated a magnitude of 8.4, it is now believed that the earthquake actually had a magnitude of 9.2. In the 24 hours following the main shock, there were 10 aftershocks of magnitude exceeding 6.0. While there were no seismograph stations in the area to record the main shock, several portable stations were flown in to record the aftershocks. These stations were used to pinpoint the locations of each aftershock, giving seismologists a more accurate estimation of the main shock's location. A lack of seismographs in the area also meant that the exact duration of the earthquake is unknown. However, estimates based on interviews of Anchorage residents place the strong motion duration around 1-1.5 minutes and a perceptible motion duration of about 4 minutes. A final result of the lack of ground motion recordings is that the intensity of the ground motion is unknown. Detailed studies of the damage in Anchorage and comparisons to past recordings led researchers to propose Anchorage ground motion intensities between 0.10g and 0.25g.

 

Damage Information

 

 

Performance summary

The classroom wing sustained extensive structural damage during the 1964 Great Alaskan earthquake. The other wings sustained minor damage to structural and non-structural elements, but the vast majority of damage was concentrated in the classroom wing.

Damage state description

Exterior columns at the second floor and roof levels were completely shattered and exhibited severe cracking and spalling at the first floor. Large cracks were apparent in many wall panels and several holes had opened up where concrete had completely fallen out. Many of the exterior columns had severe spalling of concrete and buckled bars leading to significant vertical displacement in the roof slab. Some second floor columns had buckled out of plane, dragging the roof slab down several feet. The junction between the classroom wing and the gymnasium also sustained extensive damage from pounding between the two structurally separate buildings, with the classroom wing's roof slab ramming the gymnasium walls inward approximately 3 to 4 inches. The second-floor slab only experienced minor cracking, but the ground-floor slab was cracked in numerous locations due to a differential settlement of 0.5 to 1 inch near some of the interior columns.

Summary of causes of damage

1. Pounding between the gymnasium and the classroom wing resulted in significant damage to both buildings. 2. The heavy concrete roof resulted in a significant seismic demand on the second story columns. 3. The classroom wing had relatively few shear walls arranged in an asymmetrical pattern in the longitudinal direction of the building, creating a torsional demand that failed the shear walls. Once the shear walls failed, the seismic forces were transferred to the non-ductile columns which failed as well. 4. Overall, the structure was not adequately designed to withstand major seismic forces. However, while the building was designed for zone 2 seismic requirements of the UBC, even if it had been detailed for zone 3 requirements (as current code required), the building would probably still have sustained significant damage. 5. The roof slab fractured where the direction of the classroom wing changed.

Observed Design and Construction Characteristics

 

Construction Quality

MaterialsNotesContribution to Damage
Concrete
Reinforcing steel

ExecutionNotesContribution to Damage
Conveyance/placement of concrete
Rebar
Field variance with design documents
OtherNotesContribution to Damage
Other Factors Construction Quality

Configuration

Plan IrregularitiesNotesContribution to Damage
Torsion
Perimeter boundary
Diaphragm
Out-of-plane offsets in lateral resisting system
Non-orthogonal systems

Vertical IrregularitiesNotesContribution to Damage
Soft story
Weak story
Geometric variablility of lateral resisting system
In-plane discontinuity of lateral resisting system
Mass distribution
Setback
Change in stiffness

OtherNotesContribution to Damage
Other Factors Configuration

Lateral Load Resisting System‐General

StrengthNotesContribution to Damage
Overall lack of strength

StiffnessNotesContribution to Damage
Extreme Flexibility

Load PathNotesContribution to Damage
Collectors/Struts
Anchorage of nonstructural elements
Out-of-plane capacity of walls
Diaphragm chords
Diaphragm openings

OtherNotesContribution to Damage
Other Factors Lateral Load Resisting System-General

Lateral Load Resisting System‐Frames

ColumnsNotesContribution to Damage
Shear strength
Flexural strength
Axial load ratio
Vertical load columns drift capacity
Interference of frame action by infill

BeamsNotesContribution to Damage
Strength relative to columns
Shear controlled behavior
Continuity of longitudinal reinforcing
Loss of vertical capacity
Interference of frame action by infill beams

JointsNotesContribution to Damage
Interior
Exterior
Corner

OtherNotesContribution to Damage
Other Factors Lateral Load Resisting System-Frames

Lateral Load Resisting System‐Shear Walls

ShearNotesContribution to Damage
Diagonal tension/compression
Sliding Shear
Flexure/shear

FlexureNotesContribution to Damage
Compression zone buckling capacity
Discontinuity of wall
Boundary reinforcing fracture/buckling
Boundary Reinforcing at openings

OtherNotesContribution to Damage
Other Factors Lateral Load Resisting System-Shear Walls

Lateral Load Resisting System‐Infills

InfillsNotesContribution to Damage
Unreinforced
Interference with frame action
Out of plane
Attachment to framing

OtherNotesContribution to Damage
Other Factors Lateral Load Resisting Systems-Infills

Lateral Load Resisting System‐Other

FoundationsNotesContribution to Damage
Liquefaction
Pounding
Surface Rupture

OtherNotesContribution to Damage
Pile/Pier tension capacity

MiscellaneousNotesContribution to Damage
Spread footing capacity
Other Factors Lateral Load Resisting Systems-Other-Foundations

OtherNotesContribution to Damage
Other Factors Lateral Load Resisting Systems-Other-Misc

Repair and Retrofit Information

 

 

Type of Retrofit or Repair

Restoration to pre-event condition

Other Retrofit or Repair

Performance Level

Unknown

Hazard Level

Unknown

Retrofit or Repair Code

Unknown

Other Retrofit or Repair Code

Lateral Analysis

Unknown

Other Lateral Analysis

Design Strategy

Because the majority of the structural damage occurred at the second floor, it was determined that the most economical method of repairing the school was to completely remove the second floor and build a new one-story classroom structure nearby to regain the lost classroom space.

Retrofit Summary

The repair was conducted in three phases. Phase one consisted of repairing the minor structural damage to the cafeteria, auditorium, gymnasium, shops and maintenance area. Phase two involved removing the entire second floor of the classroom wing and installing a new roof on top of the original second-floor slab. Phase three was the construction of the new one-story addition to the school.

References

 

Seismicity of the United States, 1568-1989 (Revised), by Carl W. Stover and Jerry L. Coffman, U.S. Geological Survey Professional Paper 1527, United States Government Printing Office, Washington: 1993.


Von Hake, Carl and William K. Cloud, 1966. United States earthquakes, 1964. Environmental Science Services Administration, U.S. Coast and Geodetic Survey.


http://books.google.com/books?id=5EArAAAAYAAJ&printsec=frontcover#v=onepage&q&f=false
National Academy of Sciences, 1973.The Great Alaska Earthquake of 1964.Committee on the Alaska Earthquake of the Division of Earth Sciences National Research Council. Washington, D.C.


http://db.concretecoalition.org/static/data/6-references/USA001_Reference_4.pdf
Comartin, Craig, Kenneth Elwood, and Heidi Faison, 2004. "Reinforced Concrete Moment Frame Building without Seismic Details." World Housing Encyclopedia.http://www.world-housing.net/WHEReports/wh100108.pdf ( 9 July 2012).


http://db.concretecoalition.org/static/data/6-references/USA001_Reference_5.pdf
Benuska, K. Lee, and Clough, Ray, 1973. Dynamic Analysis of Building Failures,The Great Alaska Earthquake of 1964, p. 283-307. National Academy of Sciences, Washington D.C.