John is a principal with Pillar Consulting Group, Inc. in Corvallis(1). He and his colleague Joe McCormick, P.E. delivered a comprehensive primer for architects who are involved with the design of renovations to unreinforced masonry (URM) buildings.
John began his presentation with a sobering account of the damage and death toll wrought by two recent tremblors. The December 2004 event in North Sumatra registered as a magnitude 9.1 earthquake. It resulted in 227,898 fatalities, many as a result of the devastating tsunami it would trigger along the landmasses bordering the Indian Ocean. The January 1994 Northridge, California earthquake killed sixty people and injured 7,000 more. While “only” a magnitude 6.7 quake, it caused $20 billion in property damage, making it one of the costliest natural disasters in U.S. history.
The North Sumatra earthquake (the second largest ever measured) was caused by subduction. Subduction occurs when the boundary of one tectonic plate slides beneath another, suddenly releasing the strain associated with plate convergence. The Northridge occurrence was another type of earthquake, the product of a crustal fault.
Here in Oregon, we are at risk of experiencing earthquakes of both types. Fault-induced quakes occur more frequently; their magnitude seldom exceeds 6.5. Nevertheless, they can cause significant damage (such as during the magnitude 6.0 earthquake that struck Klamath Falls in 1993). The Cascadia subduction zone – the boundary where the Juan de Fuca plate is sliding beneath the North American plate – can generate a huge quake, as much as magnitude 9.0. The last known subduction event occurred in 1700.(2) They’re estimated to occur every 300 to 500 years, so we’re coming due again.
The bottom line is that there may be greater potential for seismic activity in our State than is often recognized by Oregonians. To minimize the risk to life and property, it behooves us to understand the effect of earthquakes and the lateral forces they generate upon the vulnerable, older building stock in our communities, particularly those constructed of unreinforced masonry.
John described two standards, ASCE-31, “Seismic Evaluation of Existing Buildings”, and ASCE-41, “Seismic Rehabilitation of Existing Buildings”, which have been constantly updated and are available to structural engineers to help address the problems posed by URM buildings. ASCE 31 provides a series of evaluation tools to evaluate the seismic adequacy of existing buildings. ASCE 41 defines three performance levels that serve as the basis of a rehabilitation objective: immediate occupancy, life safety, and collapse prevention.
Typical URM failures and their respective “fixes” include:
- Falling parapets: Remove, brace, or core and reinforce vulnerable parapets to prevent them from falling
- Walls failing out of plane: Add connections at the floors and roof; reduce the wall span; and/or add collapse supports at beams and columns.
- Walls failing in plane (because of low shear capacity): Re-point mortar joints; apply sheets of fiber-reinforced plastic panels; and/or add a new lateral force resistance system.
- Weak or soft stories: Add concrete shearwalls; add steel braced frames; and/or add steel moment frames.
Weak story damage in San Francisco as a result of the 1989 Loma Prieta earthquake (photo courtesy of John Evans)
Our venue for the August chapter meeting – the Water Street Market in Corvallis – was a convenient example of a building that previously lacked adequate reinforcing and has been extensively upgraded to improve its earthquake performance level. Faced with a mandate to address seismic safety in a building over 90 years old, the owner turned to Pillar Consulting Group for assistance. The firm developed an investigation, testing and rehabilitation scheme to upgrade the building without displacing the tenants. Extensive field investigation and testing was required. Construction of the rehab measures occurred while the building remained fully occupied and functional. Exposed to view, the newly installed connectors, braces, and ties add visual interest to the original architecture. These new additions demonstrate that, approached creatively, URM improvements can be integrated successfully and attractively.
(1) Pillar Consulting Group, Inc. is a professional engineering and design consulting firm, specializing in civil engineering, structural engineering, and industrial plant planning and design.
(2) From Wikipedia: "Evidence supporting the occurrence of the 1700 earthquake suggests that it took place at about 9:00 PM on January 26, 1700. Although there were no written records in the region at the time, the earthquake's precise time is nevertheless known from Japanese records of a tsunami that has not been tied to any other Pacific Rim earthquake. The most important clue linking the tsunami in Japan and the earthquake in the Pacific Northwest comes from studies of tree rings which show that red cedar trees killed by lowering of coastal forests into the tidal zone by the earthquake have outermost growth rings that formed in 1699, the last growing season before the tsunami. Oral traditions describing a large quake also exist among the region's inhabitants, although these do not specify the date."