Slide 16 of 67
- Why don’t material strengths combine?
- Material strengths cannot combine because different sheathing materials reach their ultimate strength at different wall displacements.
- Shear walls braced with cement plaster and gypsum wallboard reach their ultimate strength at about ½-inch of movement of the top of the wall. After this point, gypsum wallboard buckles off the studs, frequently leaving its fasteners in place. Portland cement plaster has similar behavior.
- Structural wood panels reach their ultimate strength when the top of the wall moves about 1½ inches.
- This means that plywood will share seismic loads with Portland cement plaster or gypsum wallboard when the top of the wall moves less than one-half inch.
- When earthquake forces cause more than ½-inch displacements, the wood structural panel sheathing will have to resist the entire load.
- The sheathing material that stays on the walls the longest will have to resist the total seismic load. Normally, this sheathing material will be wood structural panel.
- The figure on the left shows brick, rock, plaster, drywall, and plywood materials (back to front)
- The figure on the right shows the relative maximum displacement of each before it will fail. As shown, plywood is able to tolerate the greatest amount of displacement before failing.
- Plywood is the best for resisting earthquake forces
- We have briefly discussed the strength of shear wall material; before discussing the stiffness of shear wall material, it is important to note that….