Seismic Detailing Class Lookup
Intensity, system and height in; detailing class and height check out.
Quick answer
With the default inputs: Design intensity 7 · Structural system Moment frame · Building height 30 etc.Frame detailing class Class 2
Edit the inputs below to recalculate.Inputs
Result
Frame detailing class
Class 2
| Intensity used | 7 |
| Frame | Class 2 |
| Max height at intensity 7 | 50m |
| Building height | 30m |
| Height check | Within limit |
Based on GB 50011-2010 (2016 edition) Tables 6.1.1 and 6.1.2. The height limit is looked up at the original intensity; the one-degree upgrade for class B affects detailing measures only, not the height limit. Partial frame-supported shear walls, frame-core tube, tube-in-tube and slab-column systems are not covered here.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Lookup | Class = Table 6.1.2 (intensity × system × height band) |
| Height bands | Frames at 24 m; frame-wall at 24 / 60 m; walls at 24 / 80 m |
| Class B | Detailing measures taken one degree higher; no upgrade at intensity 9 |
| Height limit | Table 6.1.1, at the original intensity, not upgraded |
| Over-limit | Above the maximum height requires an out-of-code review |
Intensity 7, moment frame, 30 m, class C: above 24 m so the frame is detailing class 2; the 7-degree limit for frames is 50 m, so 30 m is within it.
How to work it out
- Establish the seismic design intensity from the national ground-motion zoning map for the site. Note this is the design intensity, not the basic intensity — they differ in some regions.
- Identify the structural system: frame, frame-shear wall, or shear wall. Each has different height thresholds.
- Apply the height bands: frames break at 24 m; frame-shear wall at 24 m and 60 m; shear wall at 24 m and 80 m. The band determines the grade.
- Read off the grade (class 1 highest, class 4 lowest). The pattern is that higher intensity and greater height raise the grade, and frames sit one grade above shear walls under the same conditions.
- Handle the importance category: class B (key defence) takes seismic measures one intensity higher, except at intensity 9 where no further increase applies. Height limits stay at the original intensity.
- Check the maximum permitted height. Exceeding the limits requires a formal out-of-code review — raising the grade does not substitute for it.
Where it goes wrong
- Raising every parameter by one intensity for class B buildings. The code raises the seismic measures only; height limits and the seismic action calculation stay at the original intensity. Raising everything causes false out-of-code assessments.
- Measuring building height including parapets and plant screens. The code measures from external ground to the main roof, excluding local projections; overstating height misclassifies the grade.
- Treating large-podium multi-tower or basements structures as a single tower for height. These have specific provisions, and a simple total height gives the wrong answer.
- Setting the grade and moving on. The grade fixes link spacing in confinement zones, axial load ratio limits and anchorage lengths — one grade wrong means redesigning the whole reinforcement package.
- Classifying short-limb shear walls as ordinary shear walls. Short-limb walls have their own provisions requiring a higher grade.
Material and cost
Each grade step adds 5-10% to reinforcement while formwork and labour stay flat. On a 100,000 m² residential project a single grade difference runs into millions, so intensity and site class must be fixed at concept stage — later changes are extremely expensive.
On site
- Seismic grade and seismic importance category are different things: the former is a detailing class (1 to 4), the latter a building importance class (A to D). Do not conflate them.
- Frames gain a grade above 24 m, so the difference between a 23.9 m and a 24.1 m scheme is real money — worth watching that threshold at concept stage.
- Shear walls break at 24 m and 80 m, more generous than frames, which is one reason high-rise housing favours shear wall systems.
- Out-of-code review takes two to three months, so determine at concept stage whether the project triggers it, or the whole programme slips.
Basis
- GB 50011-2010 (2016 edition) Code for seismic design of buildings, maximum heights and seismic grades
- GB 50011-2010 (2016 edition) Code for seismic design of buildings, seismic action and verification
- GB 50223-2008 Standard for classification of seismic protection of building constructions
FAQ
Why one degree higher for class B - intensity or detailing?
Detailing, not seismic action. Class B buildings (hospitals, schools, large retail - those that must stay in service or hold crowds after a quake) are analysed at the local intensity but detailed one degree higher, which drives the detailing class, confinement and minimum reinforcement. No upgrade applies at intensity 9.
Does a lower class number mean stricter detailing?
Yes - class 1 is the strictest, class 4 the loosest. A higher class means the member is expected to deform and dissipate more energy, so ductility demands rise: longer confinement zones, tighter hoop spacing, higher minimum reinforcement and stricter axial load ratios.
What if my building exceeds the maximum height?
It becomes an out-of-code high-rise and needs a special seismic review by a provincial expert panel before construction drawings can be approved. In practice you either change the system (a frame at 50 m becomes a frame-wall at 120 m) or go to performance-based design. Adding reinforcement does not solve it.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Seismic Detailing Class Lookup. 2026-10-25. https://jianzhujisuan.com/en/seismic-grade
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.