The Escondido Village Midrise Buildings are a series of five similar buildings on the Stanford University Campus in Palo Alto, Califoria which are used as married student apartment housing. The five buildings are Abrams, Barnes, Hoskins, Hulme, and McFarl...

Prepared By: Sarah Bettinger
Occupancy: Residential
Year Built: 1961
Height:
Number of stories: 8
Stories below ground: 1
Size: 56000 gsf
Original Code: 1961
Modification: none
Year Modified: N/A
Code of Modification: N/A
Lateral Load System: Shear Wallrete
Other Load System: "Exterior and interior r/c shear walls controlled by flexure" (EQE International, 1994)
Vertical Load System: One-way slab and beams with columns
Other Vertical Load System: "12" one-way concrete core slab (7" diameter hollow cores spaced 9" apart) carry floors to walls and columns". (EQE Internationa
Foundation : Spread Footings
Other Foundation : "Cont. strip footings support walls; isolated spread footings support columns"
Country: United States
State: California
City: Palo Alto
Street: various; near El Camino Real and Stanford Avenue
Latitude: 37.4258
Longitude: -122.1563


 

Escondido Village

Earthquake Information

 

 

Earthquake Date 32798
Moment Magnitude 6.9
Epicentral Distance 51
Local Intensity VII MMI
Site Description The Escondido Village site is underlain by approximately 200 feet of alluvial soils over Franciscan formation bedrock. The alluvial soils are generally quite dense interbedded layers of clayey sand, sandy clays, sand, and gravel. (EQE International, 1994)
PGA Lateral 0.21 (g)
PGA Vertical 0.09 (g)
SaT
Ground motion recording stations The closest station to the buildings with confirmed similar soil conditions (deep alluvium) is CGS-CSMIP Station 58264, "Palo Alto 2-Story Office Building".
Distance to station 5
Station Latitude 37.4531
Station Longitude -122.113
Ground Motion Summary The October 17, 1898 Loma Prieta earthquake occured along the San Andreas fault near the summit of Loma Prieta Mountain at a depth of approximately 18 kilometers. Geodetic and seismic network data suggest right-lateral strike-slip and reverse movement on a northwest-striking plane dipping 70 degrees to the southwest. The rupture zone had an area of roughly 300 square kilometers. The earthquake and subsequent aftershocks filled a spatial gap in observed seismicity over the previous 20 years.

 

Damage Information

 

 

Performance summary

The buildings experienced moderate cracking of the vertical concrete walls and spalling of concrete floor slabs at window openings in these walls.

Damage state description

All buildings experienced "moderate but widespread cracking of the cast-in-place concrete walls, including both shear cracking in classic diagonal x-patterns, flexural cracking consisting of cracks which were approximately horizontal near the bases of the walls, and horizontal cracking along construction joints present at floor levels. The walls around the stair towers experienced the most heavy damage. Most damage to the walls was repaired shortly after the earthquake with the injection of epoxy grout... "Several walls in the basement were observed to have vertical cracks ranging in size to 1/8 inch width, in a variety of patterns. Some cracks were observed to be nearly vertical and run floor to ceiling of the basement. Others are the typical pattern of diagonal cracks extending from the corners of door openings." (EQE International, 1994)

Summary of causes of damage

1. "The walls at the ends of the building are interrupted by a series of long windows with openings... which are not provided with specially detailed spandrel beams. A number of these short sections of slab which were forced to behave as coupling beams experienced extreme damage, with chunks of concrete spalling off and falling to the ground below. 2. "These [basement] cracks may be from any of several causes including the Loma Prieta Earthquake or minor building settlements that have occurred over the years." (EQE International, 1994)

Observed Design and Construction Characteristics

 

Construction Quality

MaterialsNotesContribution to Damage
Concrete Low strength concrete in hollow floor slab due to improper curing procedures.
Reinforcing steel

ExecutionNotesContribution to Damage
Conveyance/placement of concrete
Rebar Coupling beams are generally poorly reinforced and have no stirrups and inadequate lap splices in wall boundary elements
Field variance with design documents
OtherNotesContribution to Damage
Other Factors Construction Quality Low strength concrete in hollow floor slab due to improper curing procedures.

Configuration

Plan IrregularitiesNotesContribution to Damage
Torsion
Perimeter boundary Re-entrant corners at several column locations, without special chord bars; distribution of shear walls makes slab reinforcing adequate to handle corner stresses.
Diaphragm
Out-of-plane offsets in lateral resisting system
Non-orthogonal systems

Vertical IrregularitiesNotesContribution to Damage
Soft story
Weak story "The story strength at each story is at least 80% of the stories above, however, there are local discontinuities in some of the vertical elements of the lateral force resisting system" (EQE International, 1994)
Geometric variablility of lateral resisting system
In-plane discontinuity of lateral resisting system "Vertical discontinuity in major shear resisting systems" (EQE International, 1994)
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 "Dowels provided between the floor slabs and walls are not adequately embedded to fully develop their yield strength. Consequently, the connection of diaphragms to walls cannot develop the diaphragm strength. In addition, most walls do not extend the full length of the diaphragm, and collector reinforcing has not been provided to drag diaphragm loads into the walls. "(EQE International, 1994)
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 "moderate but widespread cracking of the cast-in-place concrete walls, including... shear cracking in classic diagonal x-patterns..." (EQE International, 1994)
Sliding Shear "...horizontal cracking along construction joints present at floor levels." (EQE International, 1994)
Flexure/shear "...flexural cracking consisting of cracks which were approximately horizontal near the bases of the walls..." (EQE International, 1994)

FlexureNotesContribution to Damage
Compression zone buckling capacity
Discontinuity of wall
Boundary reinforcing fracture/buckling "Inadequate boundary reinforcing in shear walls" (EQE International, 1994)
Boundary Reinforcing at openings "Inadequate boundary reinforcing in shear walls" (EQE International, 1994)

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 No infills described in report

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 "Inadequate overturning resistance of foundation" (EQE International, 1994)

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

Life safety

Hazard Level

500 yr.

Retrofit or Repair Code

Unknown

Other Retrofit or Repair Code

Lateral Analysis

Unknown

Other Lateral Analysis

Design Strategy

Evaluation and retrofit design were performed by EQE International, San Francisco. They identified the following deficiencies: shear critical columns at the first floor and basement; inadequate boundary steel lap in some walls; and potential punching shear in exterior columns at high displacement. The retrofit performance objectives were life safety in 10%/50yr and collapse prevention in 10%/100 year. The retrofit measures were to jacket the shear critical columns, to retrofit the boundary steel laps, and to add steel collars to the tops of columns.

Retrofit Summary

Jacketing of shear critical columns was done by fiber-wrapping susceptible columns. Boundary steel laps had additional steel welded to them. Steel collars were designed to be held in place by compression at the tops of columns to support the slab in case of shear failure.

References

 

EQE International, July 1994. "Seismic Evaluation of Escondido Village Midrise Buildings". Stanford University. Prepared for Stanford Facilities Project Management.


http://eqs.eeri.org/resource/1/easpef/v6/iS1/p25_s1?isAuthorized=no
Borcherdt, R. D. and Donovan, N. C., 1990. Ground Motion,Earthquake Spectra,6, Supplement, 2580.


http://db.concretecoalition.org/static/data/3-additional-ground-motion/USA004_Ground_Motion_1.jpg
United States Geological Survey (USGS), 2009.CISN Rapid Instrumental Intensity Map for Loma Prieta Earthquake,http://earthquake.usgs.gov/earthquakes/shakemap/nc/shake/LomaPrieta/ (20 July 2012).


http://db.concretecoalition.org/static/data/6-references/USA004_Reference_4.pdf
Plafker, G., and J. Galloway, 1989. Lessons learned from the Loma Prieta, California earthquake of October 17, 1989. USGS Circular 1045, 48 pp.


http://eqs.eeri.org/resource/1/easpef/v6/iS1/p7_s1?isAuthorized=no
Borcherdt, R. D. and Donovan, N. C., 1990. Geosciences,Earthquake Spectra,6, Supplement, 724.


Comartin, Craig, 2012. Personal photo collection.