US vs Japanese Building Codes

Japan has sort of famously strict building codes (to the extent that any building codes can be ‘famous’, anyway). During the 2011 earthquake, Japanese buildings suffered relatively little damage, a fact credited to diligent enforcement of very strict building codes.

But how strict are these codes, anyway? How would a typical Japanese building compare to a typical US one?

Let’s look at a sort of “typical” building that might be built, and see how it’ll differ under US and Japanese seismic design. For this example we’ll use a 4 story steel building, with composite deck flooring and concentric braced frames to resist lateral forces. This could be an apartment complex, or a hotel, or a classroom, or something else — it’s an extremely common method of construction. Since earthquake forces are a function of a buildings’ weight, we’ll assume it weighs 100,000 pounds (100 kips).

Our building might look something like this before the architects get a hold of a caption

In both codes, the seismic design procedure is relatively similar. First, calculate the amount of force an earthquake is likely to inflict on a building. Scale this value using various factors based on construction type, building height, type of lateral system, etc. Design the lateral system to withstand this much force. Finally, perform checks to make sure the building doesn’t sway too much, and that, should it fail, it fails slowly and safely, allowing people time to get out.

For the US codes I’ll be using ASCE 7–10 and AISC 341–10. For the Japanese code I’ll be using the design method from the 1981 Building Standard Law Enforcement Order.

(If you don’t feel like following along with these calculations, go ahead and skip to the TLDR at the end).

We’ll start with the Japanese code. Here’s the Japanese formula for lateral seismic forces:

To get the seismic force Q, we multiply the building weight W by a shear coefficient Ci, which is a function of the following factors:

  • Z is the seismic zone factor, which comes from the map below. (I believe the numbers represent the ground acceleration, as a fraction of gravity, that would be experienced in a 500-year earthquake. Most countries use similar criteria, but the Japanese code is somewhat unusual and I couldn’t find good information on this). The lowest number on the map is 0.7, or 70% of g, which is an extremely high seismic force. We’ll use a value of 1.0.
  • R is the vibration characteristics factor. We’ll conservatively set this to 1.
  • A is a factor for evaluating the force at different levels. For this example we’ll just consider the force at the ground (called the ‘base shear’), so we’ll use 1. (Japanese and US codes use similar methods for evaluating force as a function of building height).
  • C is the standard shear coefficient. For a building less than 31 meters tall and without any major irregularities, we can use a shear coefficient of 0.2.
Map for finding seismic zone factor.

The Japanese code also has some provisions for steel buildings specifically, the most relevant being one that requires multiplying the calculated seismic force by up to 1.5. (The other provisions are about member size/strength and building shape, and seem to have similar analogues in the US code).

Factor for increasing seismic forces in steel buildings.

This gives us a base shear of 100 kips x 1.0 x 1.0 x 0.2 x 1.5 = 30 kips. Our lateral-resisting system has to be designed to withstand 30,000 pounds of force.

Now let’s consider the same building built in the US. Here’s the equation for lateral seismic forces (from ASCE 7–10):

  • Sds is roughly the equivalent of the Z factor in the Japanese code, the ground acceleration felt under a 500 year earthquake. We’ll use the same value as 1 (100% of g), the sort of acceleration which could be experienced along the west coast, or in places like Charleston or Missouri.
  • This isn’t in the above formula, but an acceleration this high will put our building into seismic design category ‘D’. This is a category for buildings which are likely to experience extremely high seismic forces. It will, among other things, force us to multiply our seismic forces by an overstrength factor of 2.0. (Courtesy of AISC 341–10).
  • R here is the response modification factor, which lets us reduce the design forces for systems which are highly ductile. Because we’re in seismic design category D and taller than 35 feet, our frames will have to be “Special Concentric Braced Frames”, which have an R value of 6. (The Japanese code also uses a ductility factor, but it’s values are generally lower, and it’s only used in a design procedure we’re able to skip).
  • I is importance factor, which is higher for buildings which meet certain importance criteria. We’ll assume our building is unimportant and use a value of 1.

This gives us a base shear for the US codes of 100 kips x 2.0 x 1.0 / (6 / 1) = 33.33 kips.

This is in fact slightly more than the force required under the Japanese code. We have being in seismic design category D and being taller than 35 feet to thank for that — those facts combined mean some extremely burdensome design requirements.

Besides lateral force, the other major component of seismic design codes are drift limits, which dictate how much the building can shake back and forth. Limiting drift prevents damage to drywall, windows, face brick, and other non-structural portions of the building that might be damaged during an earthquake even if the structure itself remains standing.

For this sort of building, the Japanese code limits story drift to 1/200th of the story height. For a story height of 12 feet, this is about 3/4 of an inch.

The US code limits deflection for this category of building to 2% of the story height. However, it also requires multiplying the calculated deflection by a “deflection factor”, which in this case is 5. This results in an allowable drift of 5/8 of an inch. Once again, the US code is slightly more strict than the Japanese code.

Formula for calculating deflection in ASCE 7–10.

So, to sum up: The US building code requires designing for a slightly higher lateral force, and slightly lower story drift. The Japanese code is less strict than the US one, though the requirements for each are very close.

A few caveats to this surprising conclusion:

  • We’d get slightly different results with different types of construction, and different building heights. Buildings taller than 31 meters in Japan require an additional, stricter design procedure. And buildings taller than 60 meters require special approval from the government, so it’s hard to tell how restrictive the design for them has to be (though US buildings of this height also have very restrictive provisions). But in both countries the majority of construction is going to be short buildings similar to our example.
  • I am not an expert on either the Japanese building code, or designing US buildings in high seismic areas. It’s very common for building codes to have a single, obscure line that completely changes the design requirements. It would have been very easy for me to miss something in the code that substantially alters the above calculations.
  • Building codes in the US vary from state to state. The calculations above were done with the latest versions of the code, and states that still use older versions would have less strict requirements.

However, for a building of moderate height in a seismically active area, the most recent US code provisions seem to be just as strict, if not stricter, than the Japanese ones.

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