Three contributing sources of shearwall deflection in a light-frame wood shearwall include:
- Shrinkage of wood members due to moisture content finding equilibrium with the local environment
- Localized crushing of wood members under compressive loading
- Construction gaps in the framed members
These three sources of deflection in the shearwall system need analysis by the building Designer because a top of shearwall lateral deflection calculation is dependent on the vertical movement of the restraint system. The code limits shearwall drift when resisting seismic loads. Excessive drift in wind events can also damage elements in the building envelope and allow internal pressure to increase resulting in poor building systems performance. Recently, ICC-ES developed an acceptance criterion, AC316, for shrinkage compensating devices. This criterion requires that the Designer submit calculations to the local building official showing that the continuous rod tie-down system story (incremental) deflection does not exceed 0.20 inches under Allowable Stress Design (ASD) level loads for shearwall overturning restraint. This includes rod elongation and the total deflection of the shrinkage compensator.
Shrinkage compensators were developed to help the continuous rod tie-down system take up gaps that form in between the plate washers, the sill plates or bridge blocks and the nuts responsible for shearwall overturning restraint.
There are three types of shrinkage compensators recognized by ICC-ES in AC316. The current product line offered by Simpson includes two of these types which are the ATUD/TUD and RTUD.
Expanding screw style that utilizes a torsional spring to force an inner cylinder to expand and is loaded in compression. This is also referred to as compression-controlled shrinkage compensation device (CCSCD). This style has very low device deflection due to having no incremental movement required for engagement
Ratcheting style is a nut composed of internal threaded wedges. This allows the rod to pass through in one direction as the structure moves downward. This is also referred to as tension-controlled shrinkage compensating device (TCSCD). This style has higher device deflection typically proportional to the thread pitch of the rod it is attached to.
Acceptance Criteria 316 refers to two device deflection values that must be determined from witnessed testing and submitted to ICC-ES for approval. These tested values must be published in the manufacturer's evaluation report. These values must be used by the Designer in the story drift calculations.
- ΔR is the average travel and seating increment. This is the average deflection from a seated position to the reseated position. This value also includes the device's structure attachment component deflection:
- ΔA is the allowable load deflection.
|Shrinkage Compensator Style||ICC-ES Designation||Device Deflection Examples|
|ΔR (in.)||ΔA (in.)||Δtotal (in.)|
|Expanding Screw (ATUD/TUD)||CCSCD||0.004||0.022||0.026|