Why is my structural engineer so concerned about adding a strap?
Straps are structural hardware that are used to transfer tension forces (loads). Straps can be used for different types of structural systems. For example, straps can be used to provide continuity between hollow core slabs on framed walls; they can be used to connect steel deck floor and roof assemblies; and straps can be used on concrete-steel composite decks. In light wood-framed or mass timber structures, straps are most commonly used to transfer diaphragm forces (drag straps) and shear wall forces (vertical straps). Straps can also used to connect balconies and canopies to the main building structure to restrain them from pulling away from the structure. In mass timber buildings, straps are usually made of light gauge cold formed steel or hot-rolled steel plates and are commonly fastened with nails or screws . Straps are discrete and can often look insignificant in spite of the important role they play – so why are they needed?
Diaphragm Straps & Drag Struts
When does a floor or roof diaphragm need a strap? When there are openings in diaphragms (for example, at mechanical shafts, stairs, elevators, and skylights), drag struts can be placed around the floor (or roof) opening to transfer lateral forces around the opening to transfer forces in the plane of the floor or roof. Other changes in geometry, such as jogs in floor framing around set-in balconies, may also require drag struts to transfer stress concentrations. In cases where shear walls or braces are offset in plan, but continuous from foundation to roof, straps may be required to transfer diaphragm forces into the walls. These are often called drag struts since they create a path that “drags” the force.
Shear Wall Straps
Wood stud shear walls that are not heavily loaded with gravity loads often experience high tension uplift forces at the ends of the walls from overturning effects. Straps can provide a path to transfer the tension from one floor to another. Note that in buildings susceptible to high levels of shrinkage, long tension rods with shrinkage compensators are often used instead of strap since straps will develop slack once shrinkage occurs. In mass timber buildings, straps may be required for CLT shear walls also to resist uplift forces. In some cases, flat straps can be replaced with hold downs, depending on the design.
Balcony/Canopy Restraints
When balconies or canopies are hung off buildings, they likely need to be secured back to the building to prevent lateral movement of the balcony (or canopy) away from the building. Adding a strap may be one way to tie the balcony or canopy to the building.
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