Sherman Oaks Towers is a 12-story building rectangular in plan with typical dimensions of approximately 180 feet by 121 feet at the foundation through the second floor and 156 feet by 76 feet at the third floor and above. The building has a steel-framed m...

Prepared By: Quinn Peck
Occupancy: Unknown
Year Built: 1965
Height: 161 ft
Number of stories: 12
Stories below ground:
Size: 160000 gsf
Original Code: 1964 Los Angeles City Code
Modification: Unknown
Year Modified:
Code of Modification:
Lateral Load System: Moment Frame and Shear Wall Combination
Other Load System:
Vertical Load System: Flat Slab with Columns
Other Vertical Load System:
Foundation : Piles or Piers
Other Foundation :
Country: United States
State: California
City: Sherman Oaks
Street:
Latitude: 34.152
Longitude: -118.4536


 

Sherman Oaks

Earthquake Information

 

 

Earthquake Date 1/17/1994
Moment Magnitude 6.7
Epicentral Distance 11.2
Local Intensity VIII MMI
Site Description "Soil-boring logs provided in the structural drawings indicate the site soil to consist of interbedded layers of silty sands and silty clays, to depths of at least 70 feet below the ground surface. Depth to bedrock is not known. The site water table was measured to be between 25 and 30 feet below ground level." (SSC, 1996)
PGA Lateral 0.46 (g)
PGA Vertical None (g)
SaT
Ground motion recording stations CSMIP Station No. 322
Distance to station 1.4
Station Latitude 34.154
Station Longitude -118.465
Ground Motion Summary The earthquake occurred along the Pico thrust fault, a previously undiscovered Northridge blind thrust fault, and produced some of the strongest ground motions ever recorded in North America. The earthquake started at the down-dip, southeastern corner of the Pico fault plane and ruptured up northwest approximately 15 km, with no evidence of slip above 7 km below the earth's surface. The hypocenter is believed to lie at a depth of about 19 km km at a location of 34.213, -118.537. An overall maximum horizontal ground acceleration of 1.93g was recorded at Tarzana, about 11.2 km from the epicenter. While the Sherman Oaks Tower was not instrumented to record gorund motion, a 13-story commercial building 1.4 km away recorded a peak ground acceleration of 0.46g.

 

Damage Information

 

 

Performance summary

The building experienced a moderate level of structural damage, primarily flexural cracking in the perimeter shear walls on the east and west faces. The building was "yellow tagged" and subsequently restored to occupancy in April 1994.

Damage state description

Structural damage was primarily concentrated in the exterior shear walls on the east and west faces of the building. Flexural failure of the south boundary chord at the fifth story of the west shear wall and fracturing of slab edge beams connected to the north side of the west shear wall were two of the most severe structural failures. Additionally, minor shear cracks were present in the coupling beams in the elevator core. Nonstructural damage included minor cracking in interior wall partitions and significant cracking in the exterior stucco of the mechanical room.

Summary of causes of damage

1. The damage to the boundary elements of the exterior shear walls was likely caused by insufficient transverse reinforcing. While these walls were detailed with additional tension reinforcing, they were not detailed with the closely-spaced transverse reinforcing that is now required.

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

Improved Performance

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

Although the building was yellow-tagged following the Northridge earthquake, the structural damage was actually relatively minor and readily accessible for efficient repair. The cracks throughout the shear walls and the slab edge beams were repaired with epoxy injections and the fractured reinforcing in the boundary elements of the damaged shear walls were replaced with an exterior steel strap. A thin exterior wall finish was applied to the repaired elements to hide the blemishes from the epoxy injections and the new steel straps.

Retrofit Summary

The structure was restored to occupancy in April 1994, just three months after the earthquake hit. Despite the fact that the structure experienced shaking that was not design-level in terms of strength or duration, the relatively acceptable performance of the Sherman Oaks Tower reinforced the belief that stiff, cast-in-place shear wall structures with regular geometries tend to perform well in strong earthquakes, even if they are not designed to current design provisions.

References

 

http://www.seismic.ca.gov/pub.html
Seismic Safety Commission (SSC), 1994. Sherman Oaks Towers. Northridge Building Case Studies Project. Ronald O. Hamburger, EQE International, Inc. for the California Seismic Safety Commission. Second Draft, September 26.


http://db.concretecoalition.org/static/data/6-References/USA001_Reference_2.pdf
Earthquake Engineering Field Investigation Team (EEFIT), 1994. The Northridge, California Earthquake of 17 January 1994: A Field Report by EEFIT. EEFIT, Institute of Structural Engineers.


http://db.concretecoalition.org/static/data/3-Additional Ground Motion/USA001_Ground_Motion_1.jpeg
United States Geological Survey (USGS), 2009.CISN ShakeMap for Northridge Earthquake,http://earthquake.usgs.gov/earthquakes/shakemap/sc/shake/Northridge/ (9 July 2012).


http://eqs.eeri.org/resource/1/easpef/v12/iS1/p49_s1
Osteraas, John, and Peter Somers, 1996. Reinforced Concrete Structures.Earthquake Spectra,11, Supplement C, Vol. 2, 5961.


http://eqs.eeri.org/resource/1/easpef/v11/iS2/p13_s1
Huang, M.J., and A. F. Shakal, 1995. Recorded ground and structure motions.Earthquake Spectra,11, Supplement C, Vol. 1,1396.


http://db.concretecoalition.org/static/data/3-Additional Ground Motion/USA003_Ground_Motion_2.jpeg
United States Geological Survey (USGS), 2009.CISN Peak Accel. (in %g) for Northridge Earthquake,http://earthquake.usgs.gov/earthquakes/shakemap/sc/shake/Northridge/#Peak_Ground_Acceleration (9 July 2012).