seismic retrofit of unreinforced masonry buildingsr.k. goel urm buildings slide no. 1 seismic...

R.K. Goel URM Buildings Slide No. 1

Seismic Retrofit of Unreinforced Seismic Retrofit of Unreinforced Masonry BuildingsMasonry BuildingsRakesh K. Goel, PhD, PE

Professor Civil & Environmental Engineering Department

Cal Poly, San Luis Obispo, CA 93407Phone: (805) 756-2052

Email: [email protected]


AcknowledgementAcknowledgement

Sam Vigil, Cal Poly, SLOUnited State Geological Survey (USGS)California Integrated Seismic Network (CISN)The Tribune (San Luis Obispo, CA)


OutlineOutlineSan Simeon earthquake– Ground motions– Performance of URM buildings

Retrofit of URM buildings– Objectives– Methods– Cost


Prior Earthquake in Vicinity of Prior Earthquake in Vicinity of San SimeonSan Simeon

M 5 to M6 (?) earthquake in 1853M 5.7 earthquake in 1906ML6.2 earthquake in 1952Mw 6.5 earthquake in 2003Is there a pattern of significant event about every 50 year?


Recorded AccelerationsRecorded Accelerations

Horiz Apk (g)Station Name Station No./ID

Network Dist. (km) Ground Struct.

Cambria – Hwy 1 Bridge 37737 CGS 13 .179 - - San Antonio Dam 36258 CGS 22 .12 .22 Templeton – 1-story Hospital 36695 CGS 38 .483 1.28 Parkfield – Vineyard Canyon 36441 CGS 49 .09 - - Los Osos – Point Buchon 36427 CGS 52 .09 - - San Luis Obispo - Rec Ctr 01083 USGS 62 .165 - -

Information: CISN

December 22, 2003 San Simeon Earthquake


Comparison with Design CodeComparison with Design Code

Information: CISN


Performance of URM Performance of URM BuildingsBuildings


Vulnerability of URM BuildingsVulnerability of URM BuildingsUnreinforced 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 foundationUnreinforced masonry buildings have been known to be seismically vulnerable– State and local codes require retrofit but the

deadline was 2008 to 2018


Acorn BuildingAcorn BuildingTwo-story URM building built in 1892The building collapsed during the San Simeon earthquake killing two people

Photo: Paso Robles Chamber of Commerce


Collapse of Acorn BuildingCollapse of Acorn Building

Photo: Rakesh Goel


Acorn BuildingAcorn BuildingRoof not tied properly to the wallsEast-west motion imposed large deformations on the outer wall in second story leading to its collapseThe 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


Marlow Interior BuildingMarlow Interior BuildingTwo-story URM building located on corner of 12th and Park StreetLarge open windows on street sides of the buildingSolid walls with few openings on other two sides Photo: Rakesh Goel


Marlow Interior BuildingMarlow Interior BuildingSignificant asymmetry in building plan– Torsional (or twisting)

motions in addition to swaying motions during the earthquake

Significant cracks in walls facing the street sides– Large demands on these

walls due to torsion (or twisting)

Photo: Sam Vigil


Bistro Laurent BuildingBistro Laurent BuildingOne-story URM buildingRetrofitted by tying roof diaphragm to the wallsOnly minor to moderate structural damage– No cracks in the masonry

walls– Few bricks from the

decorative parapet separated

Photo: Rakesh Goel


Bistro Laurent Building: Bistro Laurent Building: Retrofit DetailsRetrofit Details

Photo: Rakesh Goel

Photo: Rakesh Goel


Ali’s Persian Rug BuildingAli’s Persian Rug BuildingThree-story URM building built in 1918Damage to masonry façade and parapets– No ties between façade

and the main wall– Bricks peeled due to out-

of-plane motion and stresses imposed on the joints due to drift

Photo: Sam Vigil


Rustic Ranch GalleryRustic Ranch Gallery

One-story URM building sandwiched between two other buildingsNo structural damage apparent except for broken window glass

Photo: Rakesh Goel


Atascadero City HallAtascadero City Hall

Reinforced concrete space frame with URM façadeBuilt in 1918 by town founder E. G. LewisPartially retrofitted Photo: Rakesh Goel



Photo: Lew Rosenberg



Brick façade peeled off the rotundaModerate cracking in interior partition wallsCity Hall closed after the San Simeon earthquake Photo: Rakesh Goel


Atascadero City HallAtascadero City HallRetrofit prevented parapet collapse

Photo: Rakesh Goel


San Luis Obispo (SLO)San Luis Obispo (SLO)More than 125 URM buildings in the cityAbout 25 have been seismically upgradedNo 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


SummarySummaryURM buildings are highly susceptible to seismic loading– Potential for collapse– Significant hazard from falling bricks from façade

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


SummarySummaryBuildings 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 earthquakeBuildings at the end of the block performed poorly compared to similar buildings in mid-block– End buildings gets kicked out by neighboring

building– Need special attention during retrofit


Retrofit of URM BuildingsRetrofit of URM BuildingsRetrofit ObjectivesRetrofit terminologyRetrofit MethodsCost


Retrofit Objectives: FEMARetrofit Objectives: FEMA--356356Building Performance Levels– Collapse prevention– Life safety– Immediate occupancy– Operational

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


Retrofit ObjectivesRetrofit ObjectivesSelect 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 eventHigher cost for enhanced performance and lower cost for limited performance


TerminologyTerminology

RehabilitationRepairStrengtheningUpgradingRetrofit– Generic term used for strengthening,

upgrading, repair, or rehabilitation


Strengthening or UpgradingStrengthening or Upgrading

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


Repair or RehabilitationRepair or Rehabilitation

Remedial 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)


Terms for URM BuildingTerms for URM BuildingUpgrading 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

Rehabilitation if damaged– Repair only brings the building back to the original

strength– Building was damaged because original strength

was not sufficient


Retrofit MethodsRetrofit MethodsRe-pointing– Improving the grout condition– May not be sufficient for seismic retrofit

Epoxy Injection– Fill minor cracks with epoxy to restore composite

actionAnchoring & Tying– Tie the floor/roof to the wall– Anchor unsupported masonry walls


Retrofit MethodsRetrofit MethodsOverlays– High-strength cement mortar ½ inch to 1

inch thick, reinforced with thin steel wire mesh

– Fiber (Glass or Carbon) Reinforced Polymers (FRP) layers

Bracing– Steel sections, reinforced masonry,

concrete buttress, or FRP strips


Retrofit MethodsRetrofit MethodsInternal 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/roofExternal reinforcement– Attach reinforcement (steel plates or angles) to the

surface of the URM wall– Improves in-plane and out-of-plane flexural

capacity


Retrofit MethodsRetrofit MethodsPost-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 wallBase isolation and energy dissipation devices– Used for retrofit of historical buildings– Expensive


Cost of Seismic RetrofitCost of Seismic RetrofitHigher the rehabilitation objectives, higher the cost– Aim for basic safety objectives– Limited objectives may be acceptable if cost is

prohibitiveCost 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

seismic retrofit of unreinforced masonry buildingsr.k. goel urm buildings slide no. 1 seismic...

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