Download Seismic Retrofit of Unreinforced Masonry Buildings

Seismic Retrofit of Unreinforced
Masonry Buildings
Rakesh K. Goel, PhD, PE
Professor
Civil & Environmental Engineering Department
Cal Poly, San Luis Obispo, CA 93407
Phone: (805) 756-2052
Email: [email protected]
R.K. Goel
URM Buildings
Slide No. 1
Acknowledgement
zSam Vigil, Cal Poly, SLO
zUnited State Geological Survey (USGS)
zCalifornia Integrated Seismic Network
(CISN)
zThe Tribune (San Luis Obispo, CA)
R.K. Goel
URM Buildings
Slide No. 2
Outline
zSan Simeon earthquake
– Ground motions
– Performance of URM buildings
zRetrofit of URM buildings
– Objectives
– Methods
– Cost
R.K. Goel
URM Buildings
Slide No. 3
Prior Earthquake in Vicinity of
San Simeon
zM 5 to M6 (?) earthquake in 1853
zM 5.7 earthquake in 1906
zML6.2 earthquake in 1952
zMw 6.5 earthquake in 2003
zIs there a pattern of significant event
about every 50 year?
R.K. Goel
URM Buildings
Slide No. 4
Recorded Accelerations
December 22, 2003 San Simeon Earthquake
Station Name
Station Network
No./ID
Cambria – Hwy 1 Bridge
San Antonio Dam
Templeton – 1-story Hospital
Parkfield – Vineyard Canyon
Los Osos – Point Buchon
San Luis Obispo - Rec Ctr
37737
36258
36695
36441
36427
01083
CGS
CGS
CGS
CGS
CGS
USGS
Dist.
(km)
13
22
38
49
52
62
Horiz Apk (g)
Ground Struct.
.179
-.12
.22
.483
1.28
.09
-.09
-.165
--
Information: CISN
R.K. Goel
URM Buildings
Slide No. 5
Comparison with Design Code
Information: CISN
R.K. Goel
URM Buildings
Slide No. 6
Performance of URM
Buildings
R.K. Goel
URM Buildings
Slide No. 7
Vulnerability of URM Buildings
z Unreinforced masonry buildings suffered the
most damage
– Many such buildings are more than 100 years old
– Not designed for seismic loads
– Lack detailing to transfer seismic forces from
structure to the foundation
z Unreinforced masonry buildings have been
known to be seismically vulnerable
– State and local codes require retrofit but the
deadline was 2008 to 2018
R.K. Goel
URM Buildings
Slide No. 8
Acorn Building
z Two-story URM
building built in
1892
z The building
collapsed during
the San Simeon
earthquake killing
two people
Photo: Paso Robles Chamber of
Commerce
R.K. Goel
URM Buildings
Slide No. 9
Collapse of Acorn Building
Photo: Rakesh Goel
R.K. Goel
URM Buildings
Slide No. 10
Acorn Building
z Roof not tied properly to
the walls
z East-west motion
imposed large
deformations on the
outer wall in second
story leading to its
collapse
z The roof slides to the
side, bends over the
first floor wall, and
crushes cars and two
victims
Sketch: The Tribune (1/24/03), SLO, CA
R.K. Goel
URM Buildings
Slide No. 11
Marlow Interior Building
z Two-story URM
building located on
corner of 12th and
Park Street
z Large open windows
on street sides of
the building
z Solid walls with few
openings on other
two sides
R.K. Goel
URM Buildings
Photo: Rakesh Goel
Slide No. 12
Marlow Interior Building
z Significant asymmetry in
building plan
– Torsional (or twisting)
motions in addition to
swaying motions during the
earthquake
z Significant cracks in walls
facing the street sides
– Large demands on these
walls due to torsion (or
twisting)
R.K. Goel
Photo: Sam Vigil
URM Buildings
Slide No. 13
Bistro Laurent Building
z One-story URM building
z Retrofitted by tying roof
diaphragm to the walls
z Only minor to moderate
structural damage
– No cracks in the masonry
walls
– Few bricks from the
decorative parapet
separated
R.K. Goel
URM Buildings
Photo: Rakesh Goel
Slide No. 14
Bistro Laurent Building:
Retrofit Details
Photo: Rakesh Goel
Photo: Rakesh Goel
R.K. Goel
URM Buildings
Slide No. 15
Ali’s Persian Rug Building
z Three-story URM
building built in 1918
z Damage to masonry
façade and parapets
– No ties between façade
and the main wall
– Bricks peeled due to outof-plane motion and
stresses imposed on the
joints due to drift
Photo: Sam Vigil
R.K. Goel
URM Buildings
Slide No. 16
Rustic Ranch Gallery
z One-story URM
building sandwiched
between two other
buildings
z No structural
damage apparent
except for broken
window glass
R.K. Goel
URM Buildings
Photo: Rakesh Goel
Slide No. 17
Atascadero City Hall
z Reinforced concrete
space frame with
URM façade
z Built in 1918 by
town founder E. G.
Lewis
z Partially retrofitted
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URM Buildings
Photo: Rakesh Goel
Slide No. 18
Atascadero City Hall
Photo: Lew Rosenberg
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URM Buildings
Slide No. 19
Atascadero City Hall
z Brick façade peeled
off the rotunda
z Moderate cracking
in interior partition
walls
z City Hall closed after
the San Simeon
earthquake
R.K. Goel
URM Buildings
Photo: Rakesh Goel
Slide No. 20
Atascadero City Hall
zRetrofit prevented parapet collapse
Photo: Rakesh Goel
R.K. Goel
URM Buildings
Slide No. 21
San Luis Obispo (SLO)
z More than 125 URM buildings in the city
z About 25 have been seismically upgraded
z No significant structural damage reported
– Accelerations were about 17%g in SLO compared
to perhaps more than 50%g in Paso Robles
– Performance of URM buildings in SLO would not
be significantly different than in Paso Robles if the
shaking in SLO had been stronger
R.K. Goel
URM Buildings
Slide No. 22
Summary
z URM buildings are highly susceptible to
seismic loading
– Potential for collapse
– Significant hazard from falling bricks from façade
z Even basic seismic retrofit – tying floor/roof
diaphragm to walls – minimized the collapse
potential for URM building
– San Simeon earthquake tested several retrofitted
buildings in Paso Robles
– Most retrofitted buildings survived without major
damage
R.K. Goel
URM Buildings
Slide No. 23
Summary
z Buildings on street corners performed poorly
– Asymmetric in building plan due to large open
windows on street sides
– Torsional motions impose larger demands on
lateral load resisting elements during earthquake
z Buildings at the end of the block performed
poorly compared to similar buildings in midblock
– End buildings gets kicked out by neighboring
building
– Need special attention during retrofit
R.K. Goel
URM Buildings
Slide No. 24
Retrofit of URM Buildings
zRetrofit Objectives
zRetrofit terminology
zRetrofit Methods
zCost
R.K. Goel
URM Buildings
Slide No. 25
Retrofit Objectives: FEMA-356
z Building Performance Levels
–
–
–
–
Collapse prevention
Life safety
Immediate occupancy
Operational
z Earthquake Hazard Level: Probability of
exceedance in 50 years (Return Period in years)
–
–
–
–
50% (72): Frequent
20% (225): Occasional
10% ( 475): Rare
2% (2,475): Extremely rare
R.K. Goel
URM Buildings
Slide No. 26
Retrofit Objectives
z Select a combination of acceptable level of
building performance and earthquake hazard
– Basic Safety Objectives: Life safety during 10%
and collapse prevention during 2% event
– Enhanced Objectives: Basic safety + Better
performance during lower level event
– Limited objectives: Basic safety – Poorer
performance during lower level event
z Higher cost for enhanced performance and
lower cost for limited performance
R.K. Goel
URM Buildings
Slide No. 27
Terminology
zRehabilitation
zRepair
zStrengthening
zUpgrading
zRetrofit
– Generic term used for strengthening,
upgrading, repair, or rehabilitation
R.K. Goel
URM Buildings
Slide No. 28
Strengthening or Upgrading
z Technical measures to enhance the
performance of an undamaged structure
– Strengthening: Performance of the structure did
not satisfy the existing requirements at the time of
design/construction
– Upgrading: Structure does not meet the new
requirements introduced after the building
construction such as new code provisions
R.K. Goel
URM Buildings
Slide No. 29
Repair or Rehabilitation
zRemedial work to fix the damaged
structure
– Repair: Restoring, but not increasing, the
original performance of the structure
– Rehabilitation: Restoration of original
geometry and performance but also
increasing the strength (or resistance)
R.K. Goel
URM Buildings
Slide No. 30
Terms for URM Building
z Upgrading if undamaged
– Strengthening brings the building only up to the
requirements at the time of building’s original
construction
– Strengthening would not satisfy the more stringent
new codes
z Rehabilitation if damaged
– Repair only brings the building back to the original
strength
– Building was damaged because original strength
was not sufficient
R.K. Goel
URM Buildings
Slide No. 31
Retrofit Methods
z Re-pointing
– Improving the grout condition
– May not be sufficient for seismic retrofit
z Epoxy Injection
– Fill minor cracks with epoxy to restore composite
action
z Anchoring & Tying
– Tie the floor/roof to the wall
– Anchor unsupported masonry walls
R.K. Goel
URM Buildings
Slide No. 32
Retrofit Methods
zOverlays
– High-strength cement mortar ½ inch to 1
inch thick, reinforced with thin steel wire
mesh
– Fiber (Glass or Carbon) Reinforced
Polymers (FRP) layers
zBracing
– Steel sections, reinforced masonry,
concrete buttress, or FRP strips
R.K. Goel
URM Buildings
Slide No. 33
Retrofit Methods
z Internal reinforcement
– Steel bars inserted in holes drilled in plane of the
URM walls
– Improves in-plane and out-of-plane flexural
capacity and connection between walls/roof
z External reinforcement
– Attach reinforcement (steel plates or angles) to the
surface of the URM wall
– Improves in-plane and out-of-plane flexural
capacity
R.K. Goel
URM Buildings
Slide No. 34
Retrofit Methods
z Post-tensioning
– Used for URM walls that develop tension due to
in-plane or out-of-plane bending
– Insert pre-stressing steel to create compression in
the wall
z Base isolation and energy dissipation devices
– Used for retrofit of historical buildings
– Expensive
R.K. Goel
URM Buildings
Slide No. 35
Cost of Seismic Retrofit
z Higher the rehabilitation objectives, higher the
cost
– Aim for basic safety objectives
– Limited objectives may be acceptable if cost is
prohibitive
z Cost can range from $10 to $100 per square
foot
– Cost can be minimized if seismic retrofit work
combined with other upgrades
– Typical cost in SLO has been about $20 per
square foot
R.K. Goel
URM Buildings
Slide No. 36