Considerations for Seismic Retrofitting

Mike Tong and Mike Mahoney FEMA, NEHRP

Structural Vulnerability –Warding off the 3 UGLIES Earthquake, Flood and Fire2015 International Hazard Mitigation Practitioner’s Symposium

Building a Safer World to Thrive in the New Normal

Earthquake Hazard and Building Vulnerability

• 406 counties in 27 States and Territories have high seismic hazard (with areas in SDC D or above).

• 75 million people and 24 million housing units are exposed to high seismic hazard.

• Annualized Earthquake Loss (AEL) is estimated to be $5.3 billion (FEMA 366 /2008).

• Northridge EQ 1994 is the 2nd costliest natural disaster in the US ($76 billion total cost, 449,000 homes and 9000 commercial buildings damaged or destroyed, and 57 fatalities).

Considerations for Seismic Retrofitting

• Local Seismic Hazard • Building Codes • Building Stock Inventory• Prioritization by Seismically Hazardous

Construction Types or Building Weaknesses

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San FranciscoLos AngelesSeattleSalt Lake CitySacramentoMemphisCharlestonSt. LouisNew York CityChicago

Hazard Curves for WUS vs. E/CUS Cities

Western US EQ Frequency~ 1 in 10 yr.

Central US EQ Frequency~ 1 in 100 yr.

MCE

FEMA/USGS – ‘07 Seismic Design Procedures Group

WUS & CUS with similar largest EQ

Risk and Timeframe Considerations for Seismic Retrofitting Policies

• Seismic hazard levels vary largely from west to east coast; therefore, seismic design requirements are also different in different communities.

• Hazard curves show that a damaging western US earthquake can be expected roughly once every 10 years, while a damaging eastern or central US earthquake can be expected roughly once every 100 years or more.

• The central and eastern US should have more time between damaging earthquakes to allow States and locals to address risk from existing buildings.

• This additional time could be taken into account in planning a State and/or local existing buildings seismic retrofitting policy.

• For all high seismic hazard areas, impact of potentially very large earthquake events should not be underestimated.

• This would require retrofitting to focus on essential and seismically hazardous buildings.

Building Codes for Seismic Risk Mitigation• New constructions in compliance to a current model building code (IBC or IRC) are

expected to perform much better in earthquakes than those existing pre-1970 buildings.

• Code compliant construction provides adequate structural protection against collapse, and is considered life safe.– Code compliant means built to a building code equivalent to the NEHRP Provisions, or the

ASCE/SEI 31-03 benchmark codes. – However, code does not always provide adequate protection against non-structural damage, but

that is a different presentation.

• Existing buildings built prior to the ASCE/SEI 31 benchmark codes may not provide adequate life safety protection.

• ASCE/SEI 31-03 and the current ASCE/SEI 41-13 allow existing buildings to be retrofitted to meet 75% of seismic resisting capacity for new buildings.

Building Code Adoption and Enforcement• One requirement for existing buildings seismic

retrofitting policy would be the adoption and effective enforcement of a suitable building code for new buildings and triggered code upgrades.

• Existing buildings retrofitting should follow IEBC and ASCE 41-13.

• Evaluation of local building codes and code departments is performed by ISO Building Code Effectiveness Grading Schedule (BCEGS).

• A minimum requirement for BCEGS score level could be established to recognize an effective building code program as a prerequisite to a seismic retrofitting policy.

Evaluation and Inventory of Existing Buildings• Inventory local hazardous buildings is

a first step for developing a suitable seismic retrofitting policy.

• FEMA P-154 and P-154 ROVER provide convenient tools for seismic screening and inventory of buildings.

• FEMA NETAP provides training support to state and local communities.

• Some communities target critical facilities for seismic evaluation (e.g. schools, state or local government buildings).

Prioritization for Seismic Retrofitting• Critical buildings such as hospitals, schools, EOCs and hazmat

facilities should be given high priority in seismic retrofitting policy. • The more hazardous the construction, the greater the risk and the

greater the need for a seismic retrofitting program.• Starting with most hazardous, the types of construction and

weaknesses to be considered and prioritized for retrofitting include:– Unreinforced masonry (URM)– Non-ductile reinforced concrete (pre-San Fernando)– Tilt-up buildings (rigid wall & flexible diaphragm)– Steel frame (pre-Northridge)– Soft story multi-unit wood frame– Wood frame residential (cripple walls and/or masonry chimney)– Nonstructural retrofitting

Unreinforced Masonry• Unreinforced masonry (or URM), is the most

seismically hazardous building type. This type of construction is considered collapse hazard in earthquake.

• Commonly seen failure mechanism for URM is out of plane failure of the masonry walls, resulting in loss of the floor support structure, collapsing the floors.

• URM construction is no longer permitted in high seismic risk areas in the US.• FEMA P-774 is a guide on establishing a URM

seismic retrofitting program.

Estimated Number of URM Buildings by Census Tract in the Continental US

by Northeast States Emergency Consortium (NESEC) • Number of URM

Buildings in the Northeastern US is estimated to be 1,637,517 units based on HAZUS.

• Total number of URM Buildings in the nation is estimated to be 17,117,254 units.

Retrofitted Unreinforced Masonry Buildings

• CA law requires localities to establish a seismic retrofit program for URM buildings.

• Napa URM retrofit ordinance– Passed in 2006, mandatory within 3 years– Objective: “to reduce the risk of death or injury”

• Chapter 15.110, Napa Municipal Code

Non-Ductile Concrete Buildings

• Non-ductile reinforced concrete buildings are concrete frame or wall buildings that were built prior to 1975.

• Non-ductile concrete building collapse was first learned in the 1971 San Fernando earthquake.

• Two non-ductile frame buildings were responsible for most of the fatalities in the Christchurch earthquake.

Non-Ductile Concrete Retrofitting• Non-ductile concrete frame

buildings were a collapse hazard.– Right: UC Berkley student dorm.– Below: Tohoku Univ. engr. bldg.

• Seismic retrofit was a new steel braced frame connected into the existing concrete structure.

Light Frame Residential Structures• Light frame residential structures

perform generally well in earthquakes. This is due to their light weight and redundant walls.

• However, there are weaknesses that can cause significant damage.

• These weaknesses may be due to irregularities (split levels), unreinforced masonry components (chimneys), or inadequate foundations (cripple walls).

• These weaknesses can be retrofitted.

Chimneys

Cripple Wall Foundation Homes

• California State Code has seismic retrofit criteria in Appendix A3.• Criteria only applies to cripple walls to 4 feet. Napa has many

higher, partly due to floodplain requirements. • There is no retrofitting criteria for walls taller than 4 feet.

South Napa Recovery Advisory for Chimneys

• Repair of Earthquake-Damaged Masonry Fireplace Chimneys.

• South Napa Earthquake Recovery Advisory – FEMA DR-4193-RA1.

• Recovery Advisory recommends replacing masonry chimney with a light weight metal flue chimney or abandoning the unit.

• Previously recommended bracing to roof is not practical.

South Napa Recovery Advisory for Cripple Walls

Figure 2: Cripple wall with plywood strengthening that was undamaged in the South Napa earthquake. Photo credit: ZFA Structural Engineers.

• Earthquake Strengthening of Cripple Walls in Wood-Frame Dwellings.

• South Napa Earthquake Recovery Advisory – FEMA DR-4193-RA2.

• Recovery Advisory includes a FEMA Plan Set, which is a set of design drawings that leads a contractor through a strengthening of a cripple wall which can then be submitted to local building department.

Nonstructural Retrofitting • Nonstructural damage accounts for most earthquake damage

and can result in loss of use of a building.– Piping failures closed ½ hospitals in the 1994 Northridge earthquake.

• Nonstructural components include:– Architectural building components.– Mechanical, electrical and plumbing components.– Furniture, fixtures and equipment.

• Types of nonstructural risk include:Life safety Property loss Functional loss

Nonstructural Mitigation Guide• Nonstructural Design Guide (FEMA E-74)

– Web-based and CD design guide.– Provides design guidance for over 70 different

nonstructural components.– For each component, guide provides examples of

damage and plans or photos of the recommended mitigation technique.

– Includes technical specifications, risk rating forms and sample inventory checklists.

– Short web-based and longer NETAP-based technical training materials also available

– Recently updated to capture Chile, Christchurch and Japan earthquake data.

http://www.fema.gov/plan/prevent/earthquake/fema74/index.shtm

Conclusions• Existing buildings are a risk that need to be addressed by mitigation policy

makers and professionals.

• Seismic retrofitting policies need to be based on the local seismic hazard and risk.

• Adopting and enforcing national building codes for new buildings and code

triggered upgrades should be required as pre-requisite of seismic retrofitting policies.

• Seismic retrofitting policies should encourage screening and inventory seismically hazardous buildings.

• Prioritization of seismic retrofitting should target critical facilities , hazardous

buildings in the community and non-structural components.

Questions?