Examples of Dangerous Wooden Floor Plans (3)

Part 1: Common examples with a north-facing entrance
Part 2: Two-story building with a wall-less built-in garage
Part 3: Buildings with stairs on the short side
Part 4: Built-in garage exterior wall line is misaligned with the second floor

Part 3: Buildings with stairs on the short side

This is a floor plan common in the latter half of the Showa era. Often, the frontage is even narrower. It is a pattern where the north side is used for stairs because it is difficult to use as a living space.

Buildings with relatively narrow frontage and stairs on the short side, such as along the entrance, are actually dangerous. In this example, there is a north-facing entrance with stairs to the west of it. At first glance, there are three load-bearing exterior walls lined up on the north side, so it looks like there are plenty of walls.

However, once you understand that the principle of a load-bearing wall is to receive seismic force transmitted to the floor of the upper level, you realize it is a scary floor plan. The three walls on the north side face the stairs (atrium) and cannot transmit the force. Furthermore, the east side has windows and the entrance, and there are no other walls. Since there is an area above the entrance, it is not that it cannot be supported by load-bearing walls at all, but it is clear that the structure is weaker than it looks. Also, generally, the wall facing the stairs (the south wall) is rarely made into a load-bearing wall, so the only wall that can be used effectively ends up being the one between the hall and the living room.

If it were a strong floor without joists like in modern new construction, the above might hold true, but in older wooden houses, the floor is not strong because the joists are often just laid down, so seismic force may not be transmitted as well as expected.

Wall quantity calculation is a very convenient and simple tool, but it assumes a general building. It does not take into account complex shapes or situations like this one. These are matters that the designer should consider at the stage of creating the floor plan, not through calculation. In the case of new construction, such a plan might be designed safely depending on the structural design, but it would be difficult for a used house.

The same applies to an atrium instead of stairs. An atrium can also be designed safely if a proper structural plan is made. You must not neglect the study and build dangerous buildings, and even as a layperson or client, you should not allow such plans in the design of your own home where you will live for decades. A user-friendly plan is of course important, but I recommend a structurally safe plan. That is a problem that precedes obtaining a seismic resistance grade.