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Same Building, Two Hazard Levels, Opposite Verdicts
Under the DD-2 earthquake, 25 of 26 walls are inadequate. Under DD-3, all 26 are in Limited Damage. Same building, same walls, same method. This is not a contradiction — but understanding why it is not is understanding why the two levels are asked separately.
1. Four hazard levels
The code defines four separate earthquake ground motion levels. Two are used here:
| Level | Return period | SDS | Meaning |
|---|---|---|---|
| computing… | |||
DD-2 is the "design earthquake" — the main check for most buildings happens here. DD-3 is a much more frequent, smaller earthquake: it asks how the structure behaves in an ordinary shaking.
2. When is scaling valid?
The DD-3 demands were not obtained from a fresh analysis but by scaling the DD-2 results:
Had the period fallen outside the plateau — in a long-period flexible structure, say — this simple ratio would be invalid and a separate analysis would be required at each level.
3. The unexpected part: σd changes too
So far the story looks simple: the demand falls to 0.307 times, and so does the DCR. But the data shows otherwise.
When the demand is smaller, so is the overturning. So at DD-3 the axial load is reduced less, σd comes out higher — and the capacity rises with it.
This holds in all 26 of the 26 walls. A systematic effect, not a coincidence.
| Storey | Pier | σd (DD-2) | σd (DD-3) | Increase |
|---|---|---|---|---|
| computing… | ||||
4. The two levels compared
| Storey | Pier | DCR (DD-2) | DCR (DD-3) | Ratio |
|---|---|---|---|---|
| computing… | ||||
5. The verdict reverses
| Level | Max DCR | Inadequate | Collapse zone | Significant damage | Limited damage |
|---|---|---|---|---|---|
| computing… | |||||
6. The finding does not rest on the contested choice
In TAR-02 we saw that the choice of fvk0 is contested and that three different values can be defended. A natural question follows: does this finding rest on that choice?
| fvk0 | DD-2 inadequate | DD-3 inadequate | Does the verdict reverse? |
|---|---|---|---|
| computing… | |||
Testing whether a finding is sensitive to an uncertain input is as important as reporting the finding itself.
7. Why is this not a contradiction?
The two levels ask two different questions:
- DD-3: "Will this structure come through undamaged in an ordinary earthquake it will see several times in its life?" Answer: yes.
- DD-2: "Will this structure survive a rare but credible large earthquake?" Answer: no.
A structure doing well in small earthquakes and badly in large ones is expected behaviour. That is precisely why performance-based assessment exists: instead of a single "safe / unsafe" label, it states what happens at which level.
8. Which level is binding?
Table 3.4 of the code sets out which performance level is targeted at which hazard level. The distinction there matters:
- Normal performance target (design classes 1, 2, 3, 3a, 4, 4a): no requirement at DD-3; the target is Controlled Damage at DD-2 only.
- Enhanced performance target (design classes 1a, 2a): Limited Damage at DD-3 is binding as well.
For this structure there is a further layer: being registered, it is outside the scope and none of these are formally binding. The targets are used here as an engineering criterion, not a compliance criterion.
9. What to do in practice
- Write every performance result together with its hazard level. A DCR or damage zone stated without the level is incomplete.
- Demonstrate that the scaling is valid. Is the period in the plateau of both spectra? If not, a separate analysis is needed.
- With the simultaneous V-P method, remember σd changes with the level. Carrying a capacity from one level to another is an error.
- Test your finding's sensitivity to uncertain inputs. If the result depends on a contested choice, say so; if it does not, say that too.
- Do not present a good result at a low level as reassurance. That is the sentence most open to misreading.
- For an out-of-scope structure, use the targets as criteria, not as declarations.
10. Test yourself
- What is the difference between DD-2 and DD-3? Which is more frequent?
- Under what condition is demand scaling valid? What must be done if it is not met?
- Why does σd come out higher at DD-3? How does that affect the capacity?
- Why is the fall in DCR larger than the fall in demand?
- Why does it matter to show the finding is independent of the fvk0 choice?
- Why is "all walls are in the Limited Damage zone" misleading on its own?
- What is the difference between the normal and enhanced performance targets as regards DD-3?
References
- TBDY 2018 (Turkish Building Earthquake Code) — Clauses 1.1.8, 1.1.9, 2.2 (hazard levels), 3.5.1 and Table 3.4 (performance targets), 11.2.10 and Eq.(11.1), 15.1.5. Read from the full official text.
- Case data: finite element model of a registered primary school building and its pier assessment at two hazard levels; published with the permission of the data owner.
All figures in this article are produced by engine/masonry.js and separately pinned in tests/masonry.test.js (49/49). The data set stores raw demands and σd values; the capacity is computed with a chosen fvk0, so the contested choice from TAR-02 stays open here too. The source report's DD-3 verdict (26/26 Limited Damage) is reproduced by a separate test case.
A performance result without its hazard level is incomplete.