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Storey Drift Limit: A Measure for Protecting Infill Walls
A building can stay standing in an earthquake while its walls collapse, its windows break and its doors jam. The TBDY storey drift limit exists to protect these non-structural elements, not the structure itself.
1. What drift measures
Storey drift is the difference in horizontal displacement between the top and bottom slabs of a storey. Divided by the storey height, it shows how much the storey "racks". The structural system can usually tolerate this racking; the masonry infill inside the frame cannot. It cracks, breaks up and may collapse. A wall that blocks an escape route or falls on people is a life-safety problem even if the structure survives.
2. From reduced drift to real drift
Seismic analysis uses forces reduced by Ra (TBDY-01). The drift Δ obtained with these forces is smaller than the real drift. To approach the real value, Δ is scaled back up by the reduction ratio:
This is a direct application of the "equal displacement" principle described in TBDY-01: the displacement of a yielding structure is about equal to that of an elastic structure with the same period.
3. λ: why another hazard level?
The drift check is made not for the design earthquake (DD-2) but for a more frequent one: DD-3. The reasoning: damage to infill walls is acceptable in the design earthquake, but their collapse in a smaller earthquake that is likely to occur during the building's life is not. λ is the ratio of the spectral accelerations of the two levels:
λ is computed from the hazard map for the site; it is not a fixed number.
4. The limit: 0.008κ or 0.016κ
The check is (Clause 4.9.1):
λ · δi,max / hi ≤ 0.016 κ (infill flexibly connected, Eq. 4.34b)
Flexibly connected walls need not follow the frame's movement, so twice the drift is accepted. κ depends on the material: 1.0 for reinforced concrete and 0.5 for steel buildings.
5. Worked example
A 3 m storey, drift under reduced loads Δ = 4 mm, R = 8, I = 1, and λ = 0.5 as an example value (in a real project it comes from the site hazard map). With the site's engine:
| Case | δ = (R/I)·Δ (mm) | λ·δ/h | Limit | Result |
|---|---|---|---|---|
| RC, infill attached to frame | 32 | 0.0053 | 0.008 | passes |
| RC, infill flexibly connected | 32 | 0.0053 | 0.016 | passes |
| Steel, infill attached to frame | 32 | 0.0053 | 0.004 | fails |
The same drift is within the limit for an RC frame and beyond it for a steel frame. κ = 0.5 halves the limit for steel buildings. Steel frames are usually more flexible and drift more for the same strength, so a tighter limit is needed to protect the infill.
6. Two points to watch
- The check is made for every storey. The largest ratio is often not at the bottom but somewhere in the middle of the building; checking a single storey is not enough.
- Exceeding the limit does not mean the building will collapse. The drift limit is a damage-limitation check, not a strength check. When it is exceeded the usual remedy is more stiffness: adding walls or enlarging sections.
References
- TBDY 2018 — Clause 2.2 (earthquake ground motion levels), Clauses 4.9.1 and 4.9.1.1, Equations (4.34a) and (4.34b), Clause 4.7.3.2, Equation (4.19). Official Gazette, 18.03.2018, No. 30364 (repeated).
- Table values computed with the drift() function of engine/tbdy-assessment.js (κ = 1.0 RC, 0.5 steel). λ = 0.5 is an example value.
The numbers in this article were produced by the site's engines; code values are given with their clause numbers.