House performance: 01 Structure

I want to think about recent high-spec houses.

First, what spec are we evaluating? Under Japan's Housing Performance Indication Standard, there are these performance categories:

01 Structure: seismic class, wind class

02 Fire safety: detectors/alarms, fire endurance time

03 Reducing deterioration: rot/termite prevention, ventilation measures

04 Maintenance considerations: inspection access for plumbing, buried piping

05 Thermal environment: insulation performance, primary energy consumption

06 Air quality: VOC emissions, ventilation measures

07 Light/visual: number of windows

08 Acoustic: sound insulation of windows etc.

09 Considerations for elderly: degree of barrier-free design

10 Crime prevention: intrusion-resistance of openings

When performance evaluation is performed, you can evaluate up to all 10. In general practice, however, only four are commonly evaluated: structure, deterioration, maintenance, and thermal.

This time I'll take up "01 Structure." It's the easiest to understand, and the lowest acceptable level is set by the Building Standards Act. What level does that act require?

Under the Building Standards Act, three things are checked: earthquakes, wind, and snow. How much structural strength do you need against seismic force?

Put simply, the act's minimum (Seismic Class 1) is: if you bodily lifted the house off its foundation and laid it on its side (with the foundation supported), it should just barely not break. Earthquake intensity is described in gal (cm/s²); roughly speaking, this means withstanding about 1000 gal. A 400-gal seismic wave can have an effect of around 1000 gal on a building. 400 gal corresponds to Japanese seismic intensity 6-plus. The important part is "just barely not break" — in other words, walls cracking is acceptable. The level at which nothing breaks at all is about 200 gal.

How do you then ensure seismic performance? Under performance evaluation, you roughly: place load-bearing walls (shear walls) in balance, ensure sufficient strength in horizontal diaphragms (floor and roof surfaces), and check the joints at beam ends.

This "roughly" is important. Buildings are divided into "heavy" and "light," and seismic force is estimated by floor area. Tiles, partitions, etc. can change the seismic load greatly, but the calculation lumps them together — that's why it's called "wall-quantity calculation" (the loose method).

The non-loose calculation is what's called "allowable stress design." It computes the weight per component for walls, roof, and floor; estimates a more reliable seismic force; and checks whether strength is sufficient.

Current law permits either method. Comparing the two, the wall-quantity result can be off by a factor of 2 — yet both methods are legal.

Now seismic classes. Setting the law's minimum strength as 1, a house with 1.25× strength is Class 2 and 1.5× is Class 3. If requested, we can design for strength above Class 3.

Finally, wind class. In wooden houses, especially those with sheet metal roofs and exterior walls, the house can be quite light, so wind force can exceed seismic force. In three-story wooden houses, wind force is usually greater. This is a wooden-construction-specific feature; in reinforced concrete and steel houses, seismic force is almost always greater.

Wind force is checked by, roughly: multiplying the side-elevation area by a pressure coefficient to get lateral wind force, then computing similarly to seismic. If the building just meets wind strength, it's Wind Class 1; 1.2× is Class 2.

Seismic strength gets attention, but for plan-narrow buildings or tall buildings, wind class deserves attention.