2. How Diagonal Wood Braces (Sukikai) Resist Sideways Forces in Earthquakes
The diagonal wood is called "Sukikai". It is a member that connects columns and horizontal members like the base or girts diagonally. When installed, the frame becomes better able to resist horizontal forces from earthquakes and wind [1]. If you push a square frame from the side, it squashes into a parallelogram. If you add one diagonal piece, the frame becomes harder to tilt. Thinking of Sukikai in this way makes its function easier to understand. However, this is just an analogy for clarity. In actual walls, the method of connecting the wood to the wood also affects the strength.
The Building Standards Act Enforcement Order Article 45 sets standards for Sukikai, including requirements to fasten it to columns and bases with metal fittings [1]. Simply putting in diagonal wood does not automatically make the wall strong. There are also rules for the "thickness" of the diagonal wood. Depending on the thickness, points are assigned to the wall.
3. How the Wall Multiplier Gives a Wall's Strength a Number
The agreement to express wall strength with a number is called the wall multiplier. For a wall with a multiplier of 1, the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) uses a resistance of 1.96 kN/m (horizontal force) when calculating wall quantity [2]. Let us think about 1.96 kN/m in everyday units. If we take the acceleration due to gravity as about 9.8 m/s², then 1.96 × 1000 ÷ 9.8 = 200. This is roughly a force equivalent to about 200 kg of weight. The image is that this is a guideline for the resistance used in wall quantity calculations.
The multiplier for Sukikai changes depending on the thickness of the wood. In the tables from the MLIT documents, for Sukikai with a width of 9 cm or more, a thickness of 1.5 cm or more is a multiplier of 1, 3 cm or more is 1.5, 4.5 cm or more is 2, and 9 cm square or more is 3 [1]. A 9 cm square piece is 6 times thicker than a 1.5 cm piece. Yet the multiplier is only 3 times higher. Even if the thickness doubles or triples, the multiplier does not increase at the same rate. If Sukikai is installed in an "X" shape (cross bracing), the multiplier becomes even higher [1].
4. How Hollow Wall Spaces and Outer Layers Help Stop Heat from Escaping
Insulation and soundproofing materials can be packed into the hollow space of an Ookabe wall [3]. However, the parts for heat in a wall are not just insulation. Documents from an agency related to the MLIT show that the basic method for outer walls is a ventilated structure. From the outside, layers overlap: exterior material, ventilation layer, breathable waterproof sheet, insulation, and interior finish. The breathable waterproof sheet prevents rainwater from entering the wall while letting moisture inside the wall escape to the outside [7]. Direct application without a ventilation layer is not recommended because there is no place for moisture to escape [7]. The manufacturer also explains that gypsum board has properties that make it hard to conduct heat [6]. The wall is a structure where many parts slowly delay the movement of heat.
The MLIT documents state that because buildings have become heavier due to increased insulation and energy-saving equipment, it is necessary to consider revising standards for required wall quantities to ensure structural safety [1]. However, this document mainly targets non-residential wooden buildings, so it cannot be said that it applies directly to general houses without qualification. After heat, we look at fire. There is a story about the board inside the wall working during a fire.
The MLIT documents note that energy-saving measures (installing insulation and energy-saving equipment) have made buildings heavier. Therefore, it is considered necessary to review standards for required wall quantities to maintain structural safety [1].
5. How the Crystal Water in Gypsum Board Turns to Steam and Helps Against Fire
According to the manufacturer, gypsum board contains about 21% crystal water. For a 100 g board, this calculates to 21 g of crystal water. Crystal water is usually stable, but it decomposes into steam when heated by fire. The manufacturer explains that until all the water turns to steam and escapes, the temperature of the gypsum does not rise above a certain point [5].
The manufacturer compares this to applying a burner to ice. Ice stays at 0°C or below until it has all melted. Similarly, the board’s temperature stays low until the crystal water uses up the heat by turning into steam. There is also an explanation comparing crystal water to firefighters, and the wall releasing steam to a wall with a built-in sprinkler [5]. It is important to note that the description is not "does not burn," but "suppresses the rise in temperature." The manufacturer’s explanation describes the function of suppressing the rise in heat [5]. This board also has features such as being hard to conduct heat, blocking sound, and resisting changes in size. It is also said to be easy to handle, cutting with a stationery cutter [6]. The function against fire was an invisible mechanism: crystal water inside the board. Let us look back at how the parts sharing the roles of earthquake, heat, and fire were divided inside the wall.
6. How to Look for Visible and Hidden Columns in Your Own Home
If you live in a wooden house, try observing the walls in each room. Look for rooms where columns are visible and rooms where they are hidden. If columns are visible, it is a Shinkabe wall; if hidden, it is an Ookabe wall [4]. However, even if it looks like a column, it might be a decorative fake column [4]. Also, in housing that is not wooden, such as apartments, the wall structure is different. If you do not know, ask an adult. Do not make holes in the wall or drive nails. Just look.
If you want to go one step further, you can compare the table of Sukikai multipliers in the MLIT documents [1] with the document that shows 1.96 kN/m for a wall multiplier of 1 [2] with an adult. It is also good to calculate yourself what multiple of thickness corresponds to the multiplier number. On the Yoshino Gypsum web page [5], the analogy of crystal water and ice is explained. Knowing the parts inside the wall changes how you see your home.