Anchor Tiedown System for Shearwall Overturning Restraint
A continuous load path is essential to a building’s structural performance. Directing the diaphragm loads from roofs, floors and walls to the foundation in a prescribed continuous path is a widely accepted method to prevent shearwall overturning. The installation of continuous rod systems has grown in popularity with the increase in mid-rise wood (3- to 6-story) construction. Specifying a Strong-Rod Anchor Tiedown System (ATS) for shearwall overturning restraint from Simpson Strong-Tie offers several advantages for Specifiers and installers alike:
- An ATS restraint provides the high load capacities required for mid-rise wood construction
- System components provide low deflection to help limit shearwall drift
- Steel tension elements of the structural lateral force resisting system can be designed for the Specifier by Simpson Strong-Tie Engineering Services
- Wood compression components of the shearwall system can be designed for the Specifier by Simpson Strong-Tie Engineering Services
- Simpson Strong-Tie Engineering Services can perform checks to ensure that your plans have the optimally designed system
- Our knowledge of rod system performance through years of testing ensures that all system design considerations have been met
Beyond the tension and compression aspects of a continuous rod tiedown system, wood shrinkage must also be addressed. In these types of structures, shrinkage and settlement can cause a gap to develop between the steel nut and bearing plate on the wood sole or top plate (see photo below), as the shrinkage increases cumulatively up the building and is the greatest at the uppermost floor. This can cause the system not to perform as designed and can add to system deflection. As a result, take-up devices must be used with most wood structures greater than two stories tall as is noted in the IBC 2015 Section 2304.3.3 at each level to mitigate any gap creation and therefore ensure optimum system performance.
What is the Load Path?
Traditional Shearwall Load Path
A traditional shearwall relies either on holdowns or straps attached to posts to transfer the net shearwall overturning forces to the foundation.
Lateral forces are transferred from the floor/roof to the plywood sheathing. The following steps describe the traditional load path:
- Step 1. Nails are typically used to transfer loads from the sheathing to the wall framing.
- Step 2. The outermost framing boundary elements transfer the tensile forces, resulting from the net overturning, to the holdown that is attached to the post at the boundary.
- Step 3. The holdown system then transfers the load in tension to an anchor that is embedded into a concrete foundation
Continuous Rod Tiedown System Load Path
A continuous rod tiedown system utilizes a combination of threaded rods with bearing plates and take-up devices at each level to transfer the forces to the foundation. The following steps describe the continuous rod tiedown system load path:
- The end posts deliver the sheathing load to the top plates and bearing plate.
- Bearing plate transfers the load through a nut into the rod system.
- Rod system transfers the load from the plate through tension in the rods to the foundation.
Strong-Rod System Components to Achieve This Load Path
- Aluminum take-up devices (ATUD) allow for multiple rod diameters.
- Ratcheting take-up devices (RTUD) fit 1/2", 5/8" and 3/4" diameter rods.
- Optimized bearing plates accommodate the new ATUD and RTUD sizes.
- New options for compression post configurations that standardize anchor layout and reduce non-structural lumber in the upper stories.
- Shallow podium anchors provide test-proven solutions for anchoring high loads to relatively shallow podium slabs at interior and edge conditions in conformity with ACI 318, Anchor Provisions.
