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Building Height Class (BYS): Same Height, Different Class
A 20-metre building can be BYS 5 in Istanbul and BYS 6 in a low-hazard region. BYS measures not height alone but height combined with seismic hazard — and it decides which analysis method may be used.
1. Two classes, one decision
Before design begins, TBDY 2018 describes a building with two classes: the Seismic Design Class (DTS) represents the severity of shaking and the importance of the building; the Building Height Class (BYS) represents its height. Together they decide which structural systems and which analysis methods may be used. Getting either class wrong can send the whole design down the wrong path.
2. Seismic design class (DTS)
DTS depends on the short-period design spectral acceleration SDS and the building use class (BKS) (Table 3.2). Examples computed with the site's engine:
| SDS | DTS (BKS 2–3) | DTS (BKS 1) |
|---|---|---|
| 0.20 | 4 | 4a |
| 0.40 | 3 | 3a |
| 0.60 | 2 | 2a |
| 1.08 | 1 | 1a |
For BKS 1 buildings (hospitals, schools and the like) an "a" is appended; these buildings are designed with stricter rules under the same hazard.
3. Building height class (BYS)
BYS is set from HN, the building height used in seismic calculations, and runs from 1 to 8. BYS 1 is the tallest class: these are "tall buildings" governed by the separate rules of Chapter 13 (Clause 3.3.2.2). BYS 8 is the lowest.
The limits themselves depend on DTS (Table 3.3). For DTS 1 and 2, for instance, they are 70, 56, 42, 28, 17.5, 10.5 and 7 metres.
4. Why the same height gives a different class
The table shows the BYS of four buildings of different heights under three seismic design classes (computed with the site's engine):
| HN (m) | DTS 1 · 1a · 2 · 2a | DTS 3 · 3a | DTS 4 · 4a |
|---|---|---|---|
| 9 | BYS 7 | BYS 8 | BYS 8 |
| 20 | BYS 5 | BYS 6 | BYS 6 |
| 45 | BYS 3 | BYS 4 | BYS 4 |
| 75 | BYS 1 | BYS 2 | BYS 3 |
A 20 m building is BYS 5 under DTS 1 but BYS 6 under DTS 3 and 4. A 75 m building is a tall building (BYS 1) under DTS 1 but BYS 3 under DTS 4. The logic: as the hazard decreases, simpler rules suffice for a building of the same height. BYS is therefore not a pure measure of height but height "weighted" by seismic hazard.
5. What BYS determines
The most important consequence of BYS is the choice of analysis method. Under TBDY Table 4.4, the equivalent lateral force method may be used, in the "all other buildings" row, only from certain BYS values onward: BYS ≥ 5 for DTS 1–2a and BYS ≥ 6 for DTS 3–4a. Taller buildings (smaller BYS numbers) require modal analysis. The site's equivalent lateral force example shows this check step by step.
BYS also limits the height up to which some structural systems may be used. It should therefore be determined at the start of design, before the structural system is chosen.
References
- TBDY 2018 — Table 3.2 (seismic design class), Table 3.3 and Clause 3.3.2.2 (building height class, tall buildings), Table 4.4 (analysis methods). Official Gazette, 18.03.2018, No. 30364 (repeated).
- All classes were computed with the designClassTBDY() and heightClassTBDY() functions of the site engine engine/seismic-tr.js.
The numbers in this article were produced by the site's engines; code values are given with their clause numbers.