Seismic Retrofit of Unreinforced Masonry Buildings - Civil and ...

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: rgoel@calpoly.edu 

R.K. Goel URM Buildings Slide No. 1

Acknowledgement 

•Sam Vigil, Cal Poly, SLO 

•United State Geological Survey (USGS) 

•California Integrated Seismic Network 

(CISN) 

•The Tribune (San Luis Obispo, CA) 


Outline 

•San Simeon earthquake 

– Ground motions 

– Performance of URM buildings 

•Retrofit of URM buildings 

– Objectives 

– Methods 

– Cost 


Prior Earthquake in Vicinity of 

San Simeon 

•M 5 to M6 (?) earthquake in 1853 

•M 5.7 earthquake in 1906 

•M L 6.2 earthquake in 1952 

•M w 6.5 earthquake in 2003 

•Is there a pattern of significant event 

about every 50 year? 


Recorded Accelerations 

December 22, 2003 San Simeon Earthquake 

Station Name 

Station Network Dist. Horiz Apk (g) 

No./ID 

(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 


Comparison with Design Code 

Information: CISN 


Performance of URM 

Buildings 


Vulnerability of URM Buildings 

• 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 

• Unreinforced masonry buildings have been 

known to be seismically vulnerable 

– State and local codes require retrofit but the 

deadline was 2008 to 2018 


• Two-story URM 

building built in 

1892 

• The building 

collapsed during 

the San Simeon 

earthquake killing 

two people 

Acorn Building 

Photo: Paso Robles Chamber of 

Commerce 


Collapse of Acorn Building 

Photo: Rakesh Goel 


Acorn Building 

• Roof not tied properly to 

the walls 

• East-west motion 

imposed large 

deformations on the 

outer wall in second 

story leading to its 

collapse 

• 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 


Marlow Interior Building 

• Two-story URM 

building located on 

corner of 12 th and 

Park Street 

• Large open windows 

on street sides of 

the building 

• Solid walls with few 

openings on other 

two sides 



Marlow Interior Building 

• Significant 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 Building 

• One-story URM building 

• Retrofitted by tying roof 

diaphragm to the walls 

• Only minor to moderate 

structural damage 

– No cracks in the masonry 

walls 

– Few bricks from the 

decorative parapet 

separated 



Bistro Laurent Building: 

Retrofit Details 




Ali’s Persian Rug Building 

• Three-story URM 

building built in 1918 

• 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 


Rustic Ranch Gallery 

• One-story URM 

building sandwiched 

between two other 

buildings 

• No structural 

damage apparent 

except for broken 

window glass 



Atascadero City Hall 

• Reinforced concrete 

space frame with 

URM façade 

• Built in 1918 by 

town founder E. G. 

Lewis 

• Partially retrofitted 




Photo: Lew Rosenberg 



• Brick façade peeled 

off the rotunda 

• Moderate cracking 

in interior partition 

walls 

• City Hall closed after 

the San Simeon 

earthquake 




•Retrofit prevented parapet collapse 



San Luis Obispo (SLO) 

• More than 125 URM buildings in the city 

• About 25 have been seismically upgraded 

• 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 


Summary 

• URM 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 


Summary 

• 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 

• 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 


Retrofit of URM Buildings 

•Retrofit Objectives 

•Retrofit terminology 

•Retrofit Methods 

•Cost 


Retrofit Objectives: FEMA-356 

• Building 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 Objectives 

• 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 

• Higher cost for enhanced performance and 

lower cost for limited performance 


Terminology 

•Rehabilitation 

•Repair 

•Strengthening 

•Upgrading 

•Retrofit 

– Generic term used for strengthening, 

upgrading, repair, or rehabilitation 


Strengthening 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 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 Building 

• 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 

• Rehabilitation if damaged 

– Repair only brings the building back to the original 

strength 

– Building was damaged because original strength 

was not sufficient 


• Re-pointing 

Retrofit Methods 

– Improving the grout condition 

– May not be sufficient for seismic retrofit 

• Epoxy Injection 

– Fill minor cracks with epoxy to restore composite 

action 

• Anchoring & Tying 

– Tie the floor/roof to the wall 

– Anchor unsupported masonry walls 



•Overlays 

– 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 



• 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 

• External reinforcement 

– Attach reinforcement (steel plates or angles) to the 

surface of the URM wall 

– Improves in-plane and out-of-plane flexural 

capacity 


• 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 

• Base isolation and energy dissipation devices 

– Used for retrofit of historical buildings 

– Expensive 


Cost of Seismic Retrofit 

• Higher the rehabilitation objectives, higher the 

cost 

– Aim for basic safety objectives 

– Limited objectives may be acceptable if cost is 

prohibitive 

• 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

Seismic Retrofit of Unreinforced Masonry Buildings - Civil and ...

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