Here in Memphis, our aging unreinforced brick buildings (URMB) stand as monuments to our cultural heritage. But their safety in seismic events remains a critical concern, even as many of these buildings are reaching the end of their intended service life and facing demolition or unintentional collapse due to instability. Meanwhile, developers are searching for value and adapting these URMBs to new uses and higher occupancies, in a tough market. Fortunately, there's a method to retrofit these structures affordably and without compromising their historical integrity, all while satisfying building Code requirements.
This blog discusses the four main components of our approach to seismic strengthening for URMBs with wood-framed floors and roofs. By applying them, architects, engineers, and developers in the Memphis area can achieve seismic stability that meets IEBC Code requirements while preserving their unique charm.
Four Essential Components of Seismic Strengthening.
These buildings' structural and aesthetic hearts are proudly displayed for all to see. The heavy exterior brick walls provide support for the roof and floor framing, protection from the elements, and decoration for aesthetic pride. But they also tend to lean, settle, crack, and lose capacity over time. They must be anchored to the floors or risk collapsing into the street. Stabilizing them relies on four interconnected elements that work like links in a chain to keep the walls standing plumb by delivering their lateral forces to the foundations.
Shear Walls and Footings are the primary lateral force-resisting elements that deliver forces from each floor of the building to the ground.
Floor and Roof Diaphragms (i.e. sheathing), collect and distribute seismic loads horizontally to the shear walls.
URM wall anchors secure brick walls to the diaphragms, preventing separation.
Critical Connections for a Continuous Load Path that link up the above elements.
CMU Shear Walls: A Smart, Cost-Effective Solution for Stability.
Common, cheap, and simple: Grouted and reinforced CMU shear walls (Concrete Masonry Units) are the most economical vertical component for stabilizing aging URM structures.
CMU shear walls offer great flexibility - they can serve adequately even with large penetrations. Their length (in plan) can be as little as 10 to 12 feet and placed in locations that minimize their impact on floorplans (although they must be stacked vertically to maintain structural continuity). This compact, flexible design allows CMU shear walls to support the building without significantly imposing on the architectural layout.
For the engineers reading, we're developing an approach to make the shearwalls more ductile, thereby reducing seismic forces and improving compatibility with the existing structure, all while reducing costs further.
Floor and Roof Diaphragms: Collect and Distribute.
Floor and roof diaphragms (also known as Sheathing or Sub-Floor) are crucial in stabilizing URM buildings by gathering the seismic forces from the heavy brick walls and distributing them to the shear walls.
Our retrofit approach utilizes new horizontal wood elements—typically OSB sheathing and conventional lumber—with many hundreds of essential nails and screws and clips, like stitches, to reinforce the diaphragms. The use of wood in this setup offers the added benefit of flexibility, accommodating the often variable dimensions common in older buildings and allowing for easy on-site adjustments without the need for costly steel fabrication.
URM Wall Anchors: Grab and Go.
Seismic strengthening of URM buildings would be incomplete without URM wall anchors, which secure the brick walls to the floors and roof. You've seen the cast-iron stars on many buildings downtown. Each one is connected to a steel rod that penetrates the wall and is anchored to the ends of the wood joists. They are essential in reducing the risk of exterior wall collapse during an earthquake.
Simple square steel plates can be used instead of the "stars" to save even more money. These through-wall anchors are a common and economical solution, ensuring that URM walls remain securely connected to the floors and roof.
Connections: Creating a Continuous Load Path from Roof to Foundation.
Ensuring strong connections between all elements—shear walls, footings, floor and roof diaphragms, and URM anchors—is essential for creating a continuous load path. This is the backbone of structural stability during seismic events, allowing seismic forces to travel seamlessly from the heavy walls down to the foundation.
Prefab steel connectors with specified fasteners and anchors offer practical, highly adaptable solutions for making these connections.
Conclusion: Code Compliance on a Budget
For those tasked with renovating historic URM buildings, CMU shear walls combined with wood framing offer an economical and adaptable path to IEBC compliance. Both materials are easily modifiable on-site, avoiding costly and time-consuming shop fabrication.
By incorporating these components, seismic strengthening costs can be limited to just 3% to 5% of the total construction budget, a significant reduction compared to the 10% to 20% typically seen in similar projects nationwide. By replacing steel members with CMU and wood, this system saves money, helping make these projects more feasible.
This approach allows us to preserve the historical character of these buildings, fulfill present-day safety standards, and ensure their longevity. By focusing on this practical approach and using materials consistent with the historical fabric, we protect both the project budget and the building's architectural integrity, making this method an ideal choice for developers committed to preservation.
Stay tuned for Part 2 where I discuss the role Construction phase support plays in ensuring satisfactory results in the finished project.
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