Clause 6.4.5 provides the design acceleration response spectrum Sa/g as a function of the natural period T of the structure for three types of soil: Type I (rock/hard soil), Type II (medium soil), and Type III (soft soil). The spectral values are used in conjunction with Z, I, and R to compute the design horizontal seismic coefficient Ah. The spectra correspond to 5% damping.
Key Requirements
•Sa/g shall be determined from the response spectrum curves in Fig. 2 based on soil type and natural period T
•Soil type classification: Type I (rock or hard soil, SPT N > 30), Type II (medium soil, 10 < N ≤ 30), Type III (soft soil, N < 10)
•The spectrum is valid for 5% damping; for other damping values, multiply by factors from Table 4
•For periods beyond 4 seconds, site-specific spectrum should be obtained
•The spectrum shape applies to both equivalent static and response spectrum methods
Reference Tables
Sa/g for Type I Soil (Rock/Hard Soil) — Fig. 2
Period Range
Sa/g Expression
0 ≤ T ≤ 0.10 s
1 + 15T
0.10 < T ≤ 0.40 s
2.50
0.40 < T ≤ 4.00 s
1.00 / T
T > 4.00 s
Site-specific study required
Type I: Rock or hard soil sites with SPT N > 30.
Sa/g for Type II Soil (Medium Soil) — Fig. 2
Period Range
Sa/g Expression
0 ≤ T ≤ 0.10 s
1 + 15T
0.10 < T ≤ 0.55 s
2.50
0.55 < T ≤ 4.00 s
1.36 / T
T > 4.00 s
Site-specific study required
Type II: Medium stiff soil sites with 10 < SPT N ≤ 30.
Sa/g for Type III Soil (Soft Soil) — Fig. 2
Period Range
Sa/g Expression
0 ≤ T ≤ 0.10 s
1 + 15T
0.10 < T ≤ 0.67 s
2.50
0.67 < T ≤ 4.00 s
1.67 / T
T > 4.00 s
Site-specific study required
Type III: Soft soil sites with SPT N < 10.
Table 4 — Multiplying Factors for Sa/g for Other Damping Values (Clause 6.4.2)
Damping (%)
Factor
0
3.2
2
1.4
5
1
7
0.9
10
0.8
15
0.7
20
0.6
25
0.55
30
0.5
5% damping is the default for RC and steel structures. Use these multipliers only when damping differs from 5%.
Formulas
Sa/g = 1 + 15T (for T ≤ 0.10 s, all soil types)
Rising portion of response spectrum from PGA to plateau
Sa/g = Spectral acceleration coefficient (dimensionless)T = Natural period of the structure in seconds
Sa/g = 2.50 (plateau region)
Constant spectral acceleration plateau region — period range depends on soil type
Sa/g = Spectral acceleration coefficient = 2.50 for all soil types in the plateau
Practical Notes
✓The plateau value of Sa/g = 2.5 is the same for all soil types; only the transition period differs
✓Soft soil (Type III) has a longer plateau period (up to 0.67 s), amplifying forces for flexible structures
✓SPT N-value is the most common basis for soil classification — use corrected N₆₀ values
✓For T < 0.1 s (very stiff structures), Sa/g < 2.5 — this benefits rigid low-rise buildings
Common Mistakes
⚠Using the wrong soil type — Type II (medium) is commonly assumed without proper geotechnical investigation
⚠Not recognizing that soft soil (Type III) extends the plateau and increases forces for buildings with T > 0.40 s
⚠Applying damping correction when 5% damping is already appropriate for the structure type
⚠Using spectra beyond T = 4 s without a site-specific study