Making Masonry Buildings Safer with Large Scale Seismic Tests

Words: Andreas Stavridis

A series of large-scale tests were recently concluded in the SEESL Laboratory at the University at Buffalo. The tests are part of a five-year project funded by the National Institute of Standards and Technology. The goal of the project is to improve the earthquake performance of unreinforced masonry buildings.

Such buildings are usually old and vulnerable to earthquakes, and they can be found in downtown areas of cities throughout the U.S., including areas of high seismic risk. They often house residences and critical infrastructure, including schools and fire departments. Hence, this research project addresses an urgent national need.

The tested specimens ranged from small individual masonry components up to buildings that were tested on a shaking table simulating earthquake motions.

Anchor Pullout Tests
The first phase of the testing program included 78 anchor pullout tests, carried out under static loading to understand the effect of factors such as the anchor diameter, location, embedment length, wall thickness, and the effect of nearby cracks. Figure 1 shows setups for the anchor pullout test. The results obtained from these tests were used to develop an analytical model that can estimate the strength of such anchors.

  
                     (a) Confined Test                                                                                                         (b) Unconfined Tests

                                                                               Figure 1 – Anchor pullout test setup


Retrofitted Wall Tests
After the anchor tests, four 10-foot-tall walls were tested. The walls were retrofitted with a hollow steel tube section that was connected to the wall using adhesive anchors. The four walls were tested until collapse to understand the effect of support conditions, strong back sizes and connection details, as well as the anchor locations. Figure 2 shows the deformed shape of the retrofitted wall under out-of-plane loading as it is being pushed. During the four wall tests, the design of the wall-to-tube connections was improved to prevent the collapse seen in wall specimens designed based on current code provisions. Furthermore, improvements were made in the design of the steel tubes as their sizes were optimized. Hence, their sizes were reduced without significant loss in deformation capacity.



Figure 2 – Deformed shape of retrofitted masonry wall under cyclic loading


First Full-Size Building Test – Specimen I
After the wall and anchor component tests were completed, a full-size single-story building was constructed and tested on top of a shake table at SEESL capable of simulating major earthquakes. The building was 12 feet tall and approximately 23 feet by 9 feet in plan. It was designed to represent a small commercial building commonly found in California and elsewhere in the U.S. The building had a parapet, openings, and was strengthened using current U.S. seismic retrofit practices. The strengthening included bracing the walls and some of the parapets with the same type of steel strong backs utilized in the aforementioned wall tests and anchoring the walls to the roof. The building was then tested until it suffered catastrophic damage from a strong earthquake. The damage to the building was concentrated at the building corners and the test was stopped when one corner fully collapsed, resulting in over 1,000 pounds of falling bricks.

This test was unique because it represented the first test of a full-size brick masonry building with typical U.S. construction and retrofitting techniques to be tested with earthquake shaking in multiple directions simultaneously. This enabled the research team to monitor how damage to real buildings would occur in detail.



Figure 3 - First full-size building after earthquake tests


Improved Full-Size Building Test – Specimen II
Based on the observations from the tests on the first building, a second building was designed to optimize the retrofit approach. The geometry and materials were kept the same as in Building I, but the design philosophy was significantly changed. All four corners of the first building sustained heavy damage. This shortcoming was addressed in the design of the strengthening of the second building. Four different approaches were considered and tested as each corner was strengthened in a different manner. Moreover, for the design of the strong backs, the stiffness and deflection limits of the current guidelines were ignored as a more rational design approach was implemented. This resulted in significantly smaller steel sections.

For the tests of the second building, the same testing sequence as the first building was followed to allow the direct comparison of the test results of the two buildings. However, the second building developed considerably less damage. Hence, the testing sequence continued as the structure was subjected to a series of strong aftershocks. The tests showed that the additional retrofit measures prevented failures at all four corners and the smaller size strong backs performed adequately. Figure 4 shows the state of the second building after the test representing the maximum considered earthquake.



Figure 4 – The research team in front of the second building at the end of the testing sequence


Conclusions
Historic masonry buildings represent an important part of the built environment in the U.S. To ensure these buildings are safe during significant earthquakes, key pieces of masonry buildings, as well as two full-size buildings, were tested at the University at Buffalo. The first rounds of testing successfully identified strengths and deficiencies in the current methods used to upgrade historic masonry buildings. The final test demonstrated that changes could be made to the design philosophy to substantially improve the seismic safety of historic masonry buildings.

The research team is currently analyzing the test data and developing computer models to investigate the performance of other building geometries with different material properties. All the data and tools will be made available to the community via doctoral dissertations, papers, and presentations.

Acknowledgements
The project is funded by the National Institute of Standards and Technology. The principal investigators at the University at Buffalo include Profs. A. Stavridis (lead), Michel Bruneau, and Kallol Sett. The doctoral students are R. Raman, G. Congdon, and R. Singh. The research team is collaborating with an advisory panel of design professionals with extensive experience in masonry structures and code development to generate new knowledge on the seismic performance of retrofitted URM buildings. The project has also received generous contributions in materials from HILTI, the Brick Industry Association, Glen-Gery, and The Belden Brick Company, and contributions in labor from the International Masonry Institute, Iroquois Job Corps, and the International Union of Bricklayers and Allied Craftworkers Local 3 New York.

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