Key Considerations for Designing an Anchor Tiedown System for Shearwall Overturning Restraint
A. Wood Shrinkage
2018 International Building Code® (IBC) Section 2304.3.3 requires that designers evaluate the impact of wood shrinkage on the building structure when bearing walls support more than two floors and a roof. It is important to consider the effects of wood shrinkage when designing any continuous rod tiedown system. As wood loses moisture, it shrinks, but the continuous steel rod does not, which potentially forms gaps in the system.
ICC-ES AC316 limits rod elongation and shrinkage compensating device deflection to 0.20" at each level or between restraints unless shearwall drift is determined to be within code limits. Rod diameter and take-up device choice are obviously important.
Simpson Strong-Tie take-up devices (TUDs), aluminum TUDs (ATUDs) and ratcheting TUDs (RTUDs) have been designed to minimize deflection (ΔA + ΔR) in the device and therefore reduce the contribution of device displacement to the 0.20" deflection limit, which allows for smaller rod diameters.
- Delta R is the device average travel and seating increment.
- Delta A is the deflection limit of the device at allowable load.
See the Simpson Strong-Tie Wood Shrinkage Calculator for more information regarding wood shrinkage.
B. Rod Elongation
A continuous rod tiedown run will deflect under load. The amount of stretch depends on the magnitude of load, length of rod, net tensile area of steel and modulus of elasticity.
In a continuous rod tiedown system designed to restrain shearwall overturning, the rod length is defined since it is tied to the story heights and floor depths. The modulus of steel is also a constant (29,000 ksi for steel) and steel strength does not affect elongation. The only variables then per run are the load and rod net tensile area, which will be controlled by:
- Quantity, location and length of shearwalls provided to support the structure.
- Choice of rod diameter, which will be used in determining the rod net tensile area, Ae.
Note: It is important to use the net tensile area, Ae, for determining rod elongation. Gross rod area, Ag, will be used for the strength calculation.
See the Simpson Strong-Tie Rod Elongation Calculator for more information.
C. Restrain Each Floor
A skipped floor system restrains two or more floors with a single restraint point to provide overturning resistance. A continuous rod tiedown system with all floors tied-off provides overturning restraint at every floor.
D. Bearing Plates
Bearing plates are key components in transferring loads from the posts and top plates to the rods in an Anchor Tiedown System for shearwall overturning restraint. Bearing plates must be designed to spread the loads across the sole/sill plates to minimize the effects of wood crushing. Bearing plate bending must also be checked to ensure proper steel plate thickness. These plates transfer the incremental bearing loads via compression of the sole/sill plates and bending of the bearing plates to a tension force in the rod.
E. Anchorage by Designer
For Anchorage by Designer information, visit the ATS Shallow Podium Anchor page.