In any major house remodel it is important to consider the shear supports as well as the weight bearing structure. Adding a new door or window may look great, or removing a wall might open new space, but such a change may weaken the shear capacity of the house and put the structural stability of the house at risk.
Vertical Loads and Shear Loads
Uprights in a timber wall handle the vertical loads, keeping the second story above the first and holding up the roof. Remove too many uprights and the house will collapse in on itself. Similarly, the shear strength of the walls resists loads that push the building from side to side, primarily wind loads and earthquake loads. The importance of shear capacity is often missed by the layperson because removing it won’t cause the house to collapse immediately. Rather, the house will seem to be as it was before, until a windstorm or an earthquake knocks it over.
Shear is resisted by all walls to some degree, but like columns are built to carry roof load to the foundation, specific walls are designed to carry shear load safely and efficiently. The uprights of these walls are the same as any other wall in a house. The important factor for carrying the shear load is the sheathing: drywall, plywood, OSB, and stucco.
Shear Sheathing in Older Homes
Drywall and stucco shear walls are used mostly in older construction as they were phased out of practical use by the building codes of the ’90’s. Unfortunately, these types of shear walls are hard to identify without the original blueprints. Long interior walls in older homes are most likely drywall shear walls. This means removing an interior wall to merge two rooms or create a loft space may weaken your home.
Similarly, long exterior stucco walls without windows are likely candidates for stucco shear walls, which means adding a window or a door to this space is a risky proposition. There are ways to compensate for removing a shear wall, but to do so properly it is best to consult with a professional engineer.
Shear Sheathing in Newer Homes
Any wall where plywood or OSB is found under the outer layer of wall sheathing can be identified as a shear wall. So, when adding windows and doors, removing a wall, or making plumbing changes, an important goal is to not damage or remove this sheathing. If removing the sheathing is the only option to complete a project, consult a professional engineer about installing a new shear wall elsewhere in the house to compensate.
Other Possible Shear Walls
Shear loads can also be resisted by steel frames, masonry and brick walls, and concrete walls, so don’t assume just because your home isn’t timber framed that shear loads aren’t a concern. When in doubt, consult a professional engineer before any major remodeling project.
Even major house remodels can be made safely if on considers both vertical load and shear load when designing the changes.
Rating: 3 out of 5