Slide 6 of 45
IMPORTANCE OF PROPER INSTALLATION
- It is critical to install hold-downs correctly.
- Here is an example to illustrate this:
- A tall and narrow shear wall is 8 feet tall and 2 feet 3 inches wide (height to width ratio of 3 ½ to 1).
- Apply an earthquake induced horizontal load at the top of the wall - the wall will begin to overturn.
- For every one inch that the bottom, left corner of the wall goes up, the upper, right corner will move laterally a distance of 3 ½ inches.
- Cyclic testing of wood shear walls
- Lateral movement of as little as 2 inches can cause loss of vertical support and cause building collapse.
- In a building with a height to width ratio of 3 ½ to 1, this means if a hold-down of a shear wall slips as little as 5/8 inch up, the building could suffer significant damage and possible collapse!
- Narrower walls fall sooner.
- Consider also the effects of plywood, bending of shear wall etc.
- Even a properly installed hold-down usually allows some upward movement because of:
- Localized crushing of the wood fiber around the fasteners
- Crushing of the sill or sole plate under the end stud.
- Stretching of the metal hold-down hardware.
- Random nut spin or thread stripping during the earthquake.
- Bending of the fastener (nail, screw or bolt)
- Wood shrinkage
- Additional rotation allowed from one-sided connection
- If errors are made in the installation, the upward movement may be significant