Downtown AnchorageSeismic Risk Assessment &Land Use Regulations toMitigate Seismic Risk

Loss Modeling & Risk Assessment (Tasks 5 & 6)

Donald Ballantyne, MMI EngineeringHope Seligson, MMI Engineering

Presentation to the Anchorage Geotechnical Advisory Committee

August 25, 2009

Presentation Overview

Brief project overview & status updatePrototypical buildingsRisk Assessment Approach– Shaking– Ground failure (including custom fragilities)– Combined

Results

Project Objective

Conduct a seismic risk assessment for downtown AnchorageDevelop land use regulations to mitigate seismic risk

Tasks1. Identify Earthquake Scenario2. Identify Prototypical Buildings, HAZUS, Spreadsheet 

Models3. Initial Meeting with Project Review Committee4. Introductory Public Meeting5. Modeling Losses5.1 Characterize hazard zones5.2  Develop building fragilities5.3 Model building losses5.4 Evaluate functionality of critical buildings5.5  Evaluate critical services6. Narrative‐

Land Uses, Seismic Hazards, Levels of Risk7. Meeting w/ Project Review Committee‐Modeling Results, 

Narrative8. Draft Land Use Regulations to Mitigate Seismic Hazards9. Presentation‐Modeling Results, Risk Narrative, Proposed 

Regulations10. Draft Technical Report11. Final Technical Report

Prototypical Buildings10 prototypical buildings defined by Occupancy (derived from Anchorage Downtown Comprehensive Plan) & Size (small, medium, large)Typical building sizes (square feet) taken from HAZUS or Means ModelRepresentative building construction types determined in consultation with GACEach prototypical building has been evaluated for 2 foundation types (shallow/spread footings & deep/mat foundations) and 2 – 4 structural systems (10 x 2 x <2-4> = 64 building models)

Prototypical Buildings# Use Size (SF) HAZUS Model Building

TypesEst. # Occupants (Day/Night)

Est. Bldg Value ($1,000)

1 MFR Small (12,000) W2, S2L, RM1L 7/37 2,473

2 MFR Med. (40,000) S2M, S4M, C2M 25/124 8,293

3 MFR Lrg. (145,000) S1H, S2H, C1H, C2H 90/450 35,050

4 Office Small (20,000) W2, S2L, RM1L 80/4 4,128

5 Office Med. (80,000) S2M, S4M, C2M 320/16 16,314

6 Office Lrg. (260,000) S1H, S2H, C1H, C2H 1,040/52 46,929

7 Hotel Med. (135,000) S2M, S4M, C2M 169/338 27,4528 Hotel Lrg. (450,000) S1H, S2H, C1H, C2H 563/1,125 91,761

9 Multi-use Med. (60,000) S2M, S4M, C2M 159/100 12,597

10 Parking (145,000) C1M, C2M 29/1 10,602C1 = Concrete moment frame C2 = Concrete shear wallS2 = Steel braced frameS1 = Steel moment frameW2 = Wood frame

S4 = Steel frame w/ CIP concrete shear wallsRM1 = Reinforced masonry bearing wall L = Low-rise, M = Mid-rise, H = High-rise

HAZUS Ground Failure Fragility ModelFor buildings on shallow foundations:– 2” Vertical + 12” horizontal peak ground deformation (PGD)

10% of buildings damaged; 8% in “Extensive” Damage State (DS), 2% in “Complete” DS

– 10” Vertical + 60” horizontal PGD 50% of buildings damaged; 40% “Extensive”, 10% “Complete”

Damage state probabilities (Ext., Complete) may be determined for any level of PGD. (Ground failure is not expected to produce “Slight” or “Moderate”damage)

Custom Fragility ModelsComparable damage to buildings on mat foundations is not expected to occur until the PGD is on the order of 5 times greater than that for spread footings with grade beams.For all foundation types, Mid-rise = baseline. Low-rise buildings require 1.5x the median PGD to result in the same level of damage. High-rise buildings are expected to have comparable damage when undergoing 0.67xthe median PGD.

Custom Fragility Models (cont.)For shallow foundations, wood-frame = baseline (80% “E”:20% ”C”). Fewer steel frame buildings expected to suffer Complete damage (90:10) & more concrete and masonry buildings expected to suffer Complete damage (60:40) when subject to significant PGDs.

Risk Assessment ApproachGround shaking impacts modeled using HAZUS’ Advanced Engineering Building Module (AEBM)Results include:– Casualties (injuries in 4 categories, inc. deaths)– $ losses for building, contents, comm. inventories– Other building damage-related direct economic

losses (relocation costs, lost rental income, lost business income and lost wages.)

HAZUS’ Injury Severity CategoriesSeverity 1 – “Injuries requiring basic medical aid that could be administered by paraprofessionals.”Severity 2 – “Injuries requiring a greater degree of medical care and use of medical technology such as x-rays or surgery, but not expected to progress to a life threatening status.”Severity 3 – “Injuries that pose an immediate life threatening condition if not treated adequately and expeditiously.”Severity 4 – “Instantaneously killed or mortally injured” (Deaths)

Ground Failure DamageGround failure fragilities combined with ground failure probabilities (expected displacements) by zone to estimate damage state distribution, & “HAZUS- consistent” losses

Expected displacements by Hazard Zone

Combined DamageAs in HAZUS, damage due to ground shaking is assumed independent of damage due to ground failure. To avoid double-counting, losses must be combined:Pcombined = PGroundShaking + PGroundFailure – (PGroundShaking x PGroundFailure )Where, for example:PGroundShaking = probability of being in or exceeding a given damage state as a result of ground shaking.

RESULTS

Total Direct Economic Losses due to Shaking & Ground Failure in 2500 Yr

Scenario EQ – Zone 5

Building Loss Ratio due to Shaking & Ground

Failure in 2500 Yr Scenario EQ – Zone 5

Daytime Deaths due to Shaking & Ground Failure in 2500 Yr Scenario EQ –

Zone 5

Nighttime Deaths due to Shaking & Ground Failure in 2500 Yr Scenario EQ –

Zone 5

Maximum Number of Estimated Deaths for Buildings on Shallow Foundations due to

Shaking & Ground Failure in 2500 Yr Scenario EQ

Maximum Number of Estimated Deaths for

Buildings on Mat Foundations due to

Shaking & Ground Failure in 2500 Yr Scenario EQ

Sample Summary of Performance of

Prototypical Building #9 due to Shaking & Ground

Failure in 2500 Yr Scenario EQ

Contribution of Shaking and Ground Failure to Combined Risk

Ground failure loss is a significant component of overall loss when shallow foundations are used. When mat foundations are used, ground failure losses are greatly reduced, but the building may still suffer moderate damage. MFR Example:

Foundation Range of MFR Shake Loss Ratio

Range of MFR GF Loss Ratio

Range of MFR Combined Loss Ratio

Shallow 8 – 15% 25 – 45% 31 – 48%

Deep 8 – 15% 7 – 16% 14 – 26%

Sensitivity of Results to Fragility Assumptions

Damage estimated with mat foundations 50% better (“mat 7.5x stronger than shallow”) and 50% worse (“mat 2.5x stronger than shallow”) than baseline “mat 5x stronger than shallow”)

reducing the assumed strength by 50% results in an average 42% increase in economic loss, while increasing the assumed strength by 50% reduces the average loss by 20%.

Next Steps

Develop land use regulations to mitigate seismic risk

Questions?