Wood Doesn't Mean Weak!
I’m a little mystified as to why someone would have a problem with a domestic washer of any type installed in a properly built wood structure – the washer doesn’t weigh a lot and vibrations shouldn’t be a problem. Remember that many American homes are constructed over a raised foundation with a crawl space underneath and a conventional wood structure for the first floor consisting of wood sheathing over floor joists. Baths and laundry rooms generally don’t need any special reinforcing. Likewise for the second floor, although the structural engineer might double up some joists if a big bathtub is placed over a large room such as a living room where the joist span is long.
Wood framed buildings can be exceptionally strong. Wood is by nature elastic and thus will bend before it breaks, while masonry is by nature brittle and will fracture if overloaded. This is why masonry buildings require a lot of steel reinforcing to be strong; without the steel a masonry structure is super weak and will crumble if presented with a lateral load such as that in an earthquake. There are some old non-reinforced buildings here in California that date from pre-WWII days, and most of these have had to be “earthquaked”. This entails installing small steel reinforcing plates throughout the building to tie the floor and roof structures to the walls to prevent a total collapse in a quake. It won’t save the building, but is intended only to allow it to collapse slowly enough to prevent occupants from being trapped or killed inside as happened in Haiti.
Quite a lot of reinforced concrete block is used California for industrial buildings, but it’s not good for most residential buildingsbecause all the block cavities are filled with grout and steel so there is no insulation value whatsoever. This can be made to work in the desert if the thermal mass of the masonry is used to mitigate daily temperature swings, but you still end out with a structure that’s not amendable to later modification and is very heavy, which can increase foundation costs. Running electrical and plumbing lines and air conditioning ducts in a masonry building is also difficult.
Properly engineered wood buildings are as strong as the engineer decides to make them. This strength comes from the combination of the wood framing and the floor and roof diaphragm panels and shear panels on the walls. These panels are not of particle board, but made of engineered plywood or OSB (oriented strand board) which is specified by the engineer. The engineer also specifies the nails and nailing pattern to accommodate the design load. As an architect, I’ve designed enough buildings that I have a good feel for what an engineer will ask for in the way of shear panels, so I’ll make sure I have some wall areas that are suitable in length and location. Once the structural engineering package is done the engineer will have marked the length, material, nailing pattern, and design load for each shear panel. If there are large clear spans in the building with insufficient wall area for shear panels he may have to use steel moment frames. These are steel assemblies, usually made of wide flange steel beams welded together, which resist lateral loads in a manner like shear panels.
It should be noted that here in California all the structure, including foundation, shear panels and diaphragms, and welds on moment frames must be inspected by both the design engineer and local building officials before they are covered up with finish materials.
The photo below was taken in a house I designed. Because there wasn’t enough shear panel area on the first floor at this locstion you can see a large grey steel moment frame with a horizontal beam and vertical post. That steel post goes into the foundation, which in this case sits on reinforced concrete piles that go deep into the ground, as specified in the soils and geology report. On the second floor you can see that the upper wall is sheathed with OSB; this is because that particular wall is a shear wall. If a wall isn’t a shear wall then there will be no OSB, just drywall. In an earthquake the roof will shake, but this load will be resisted by the shear panels on the second floor, which will then transfer the load to the moment frame on the first floor, which will then transfer the load to the foundation and thus to the soil and rock below. The shaking is therefore mitigated and the house won’t fall down - the same thing would happpen with high winds.
