Daniel Barton, Renovation Design Group
Last time we introduced you to Bob and Char Nelson’s remodeled 1927 Tudor-style home in the Harvard-Yale area of Salt Lake City. The remodel stayed within the floor plan of the historic home and was initially focused on energy efficiency and updating the exterior facade.
The exterior of the home was in rocky shape: The stucco was cracked and peeling, and the half timbers had fallen off sections of the home. The home was an unreinforced masonry structure, making it vulnerable to earthquake damage. This is a real concern in Utah where experts say a serious earthquake is not a matter of if, but when. Even relatively small levels of ground shaking can cause significant damage in an unreinforced masonry structure, while moderate to large earthquakes can make them completely or partially collapse.
This is not an uncommon scenario in older homes, especially brick homes built before 1970. It is estimated that 185,000 structures are built of unreinforced masonry in the Salt Lake region. For a typical residence, we don’t think in terms of an "earthquake proof" house, but focus on creating a structure that won’t entirely collapse and which will allow occupants to escape with minimal damage or injury. The house may indeed be a total loss, but the people inside won’t.
Because the Nelsons were already committed to remodeling — and to remodeling right — they knew they would do whatever it would take to make their home as structurally sound as possible. A structural engineer joined the team and worked with the architect and contractor to reinforce the foundation. Basically, the walls were not even tied to the foundation, so this correction was made.
Another major potential damage zone was their interior chimney. The roof beam was actually tied to the unreinforced masonry chimney. These types of chimneys are notorious for collapsing in an earthquake and, in this case, would have taken down the whole house with it. They rebuilt the interior chimney and redesigned the structure to properly support the roof with 6-by-6 posts. Now the house is solid from the roof beam to the foundation.
The intent of such a structural upgrade is to strengthen each part of the house to resist lateral (sideways or twisting) forces. This is done by providing connections to transfer these forces from one element to another, moving them down to the ground where they can dissipate. Horizontal forces on chimneys must be transferred to bracing and the roof structure. The forces on the roof must be transferred to the walls, and the walls also receive the forces from ceilings and floors. Forces in the walls must be transferred to the foundation and, finally, back into the ground. Similar force transfers must happen between almost every building part. If any part is weak or poorly connected, it may fail and other members or connections must pick up the extra load.
Since the walls take forces from several other building parts, certain walls need to be strengthened. These are known as "shear walls," since they are meant to absorb a greater share of the lateral or shear forces. Each building ideally needs two shear walls that run perpendicular to each other. Strengthening these walls involves removing the gypsum board or plaster and installing plywood panels over the studs with a required pattern of nailing. The sheetrock or plaster is replaced, so in the end these walls do not look any different from any other wall. A structural engineer can analyze your home to determine which walls will function best as shear walls.
Remember, earthquake forces exploit any weak or damaged "link." A thorough investigation and understanding of the structure of a house is required to identify, and then strengthen, these weak links.
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