The St. John's Hospital and Health Center is a healthcare complex that includes three functionally linked but structurally separate wings (North, South, and Main). The South Wing is a 7-story concrete structure with a full basement and a mechanical pentho...

Prepared By: Quinn Peck
Occupancy: HospitalHealthcare
Year Built: 1966
Height: 175 ft
Number of stories: 7
Stories below ground: 1
Size: 170000 gsf
Original Code: 1961 UBC
Modification: none
Year Modified: N/A
Code of Modification: N/A
Lateral Load System: Shear Wallrete
Other Load System:
Vertical Load System: One-way slab and beams with columns
Other Vertical Load System:
Foundation : Spread Footings
Other Foundation :
Country: United States
State: California
City: Santa Monica
Street: 1328 22nd Street, Santa Monica, CA, 90404
Latitude: 34.02
Longitude: -118.48


 

St. John's Hospital

Earthquake Information

 

 

Earthquake Date 1/17/1994
Moment Magnitude 6.7
Epicentral Distance 23
Local Intensity VII MMI
Site Description
PGA Lateral None (g)
PGA Vertical None (g)
SaT
Ground motion recording stations CSMIP Station No. 24538
Distance to station 1.8
Station Latitude 34.0112
Station Longitude -118.492
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 St. John's Hospital and Health Center was not instrumented to record ground motion, a recording station 1.8 km away at the Santa Monica City Hall Grounds recorded a PGA of 0.93g.

 

Damage Information

 

 

Performance summary

The South Wing was extensively damaged in the Northridge earthquake and receieved a "red tag". While the North Wing was demolished following the earthquake, officials decided that the South Wing would be restored to its full functionality using temporary retrofit methods to meet the immediate needs of the community.

Damage state description

The South Wing experienced severe structural damage, primarily to the main tower rising above the second floor. Characteristic X-pattern shear cracking was widespread along the north and south walls at the third, fourth, and fifth floors. Nonstructural damage was also significant, including the collapse of a cooling tower on the roof and damage in stairwells, interior hollow-clay partitions and ceilings.

Summary of causes of damage

1. Inadequate seismic detailing of the perforated, load bearing concrete walls led to widespread shear cracking in the north and south walls primarily above the second floor where the structure was set back significantly. 2. Asymmetrical location of concrete walls around elevator shafts and stair wells may have also contributed.

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 "The design base shear for the South Wing was approximately 0.09 W, which is approximately 1/3 of the design base shear used for the design of an equivalent structure in accordance with the current version of Title 24 of the California Building Code (CBC)." (SSC, 1994)

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

Other

Other Retrofit or Repair Code

1961 UBC

Lateral Analysis

Unknown

Other Lateral Analysis

Design Strategy

New steel braced elements were installed around the perimeter of the tower portion of the South Wing from the second level to the fifth level. These new braces were attached to the existing concrete walls with grouted bolts. Overturning forces were carried by installing supplementary steel columns in various locations in the first and second floors. Care was taken to ensure that the dynamic characteristics of the original building were preserved by matching the stiffness of the supplementary steel braced frame to that of the original concrete shear walls.

Retrofit Summary

A retrofit to current (1994 UBC) code was considered, but ultimately it was decided that the community was in immediate need of the facilities provided in the South Wing, so officials decided to fully restore the structure's functionality for a temporary basis of five years. While OSHPD policies mandated that hospitals be brought up to current code strength levels, facilities were allowed to temporarily re-open for a limited amount of time once returned to a pre-event condition.

References

 

http://www.seismic.ca.gov/pub.html
Seismic Safety Commission (SSC), 1994. Saint John's Hospital and Health Center, South WIng.Northridge Building Case Studies Project. Asher, J. W., Hart, G. C., Srinivasan, M., Dygean, M., and Ekwueme, C., KPFF Consulting Engineers for the California Seismic Safety Commission. Second Draft, September 26.


http://eqs.eeri.org/resource/1/easpef/v12/iS1/p49_s1?isAuthorized=no
Osteraas, John, and Peter Somers, 1996. Reinforced Concrete Structures.Earthquake Spectra,11, Supplement C, Vol. 2, 5253.


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://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).


http://db.concretecoalition.org/static/data/6-references/USA003_Reference_6.pdf
Pickett, M., 1997. Hospitals with emphasis on lifelines.Northridge Earthquake: Lifeline Performance and Post-Earthquake Response,National Institute of Standards and Technology,A. J. Schiff (ed.).