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AS 1170.4:2024AS 1170.0:2002Australia

Seismic Analysis (AS 1170.4)

Australian structural engineers determining seismic base shear and lateral force distribution to AS 1170.4:2024, the current edition under NCC 2022. Covers earthquake design category, spectral parameters, and story force distribution using equivalent static analysis for regular structures.

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What it calculates

Calculate seismic base shear and lateral load distribution to AS 1170.4:2024. Determines earthquake design category, spectral parameters, and story forces using the equivalent static analysis procedure.

Code standards

  • AS 1170.4:2024
  • AS 1170.0:2002

How it calculates

The Seismic Analysis (AS 1170.4) calculator determines earthquake design forces for Australian buildings using the Equivalent Static Analysis (ESA) procedure per AS 1170.4:2024. It converts site hazard parameters into a seismic base shear and distributes that force to each building story.

Annual probability and site hazard (AS 1170.0, AS 1170.4:2024 Cl. 3)

The design seismic action is governed by the annual probability of exceedance (p_design), which depends on the structure's importance level and design working life per AS 1170.0:2002 Table 3.3. The probability factor k_p scales the hazard design factor Z:

k_p × Z - the combined site hazard parameter used for all spectral calculations

The minimum product k_p × Z is enforced per AS 1170.4:2024 Cl. 3.2 to provide a floor on seismic demand for low-hazard sites.

Earthquake design category (AS 1170.4:2024 Cl. 2.1)

EDC is determined from the combination of importance level and k_p × Z. Structures are classified as EDC I, II, or III, which governs the detailing requirements and whether ESA is permitted. The calculator reports the governing EDC and the applicable construction requirements section.

Equivalent static analysis (AS 1170.4:2024 Section 6)

The natural period T is computed from the structural height and framing type per AS 1170.4:2024 Cl. 6.2.3. The spectral shape factor C_h(T) is looked up for the sub-soil class and period. Base shear:

V = k_p × Z × C_h(T) × (mu/S_p) × W_t

Where:

  • mu - ductility structural factor (depends on structure type and framing)
  • S_p - structural performance factor (0.67 for most structures)
  • W_t - total seismic weight of the building

Vertical distribution of seismic forces (AS 1170.4:2024 Cl. 6.3)

The base shear is distributed to each story using height-proportional weighting modified by exponent k:

F_x = C_vx × V, where C_vx = w_x × h_x^k / sum(w_i × h_i^k)

The calculator tabulates the lateral force at each story level and the cumulative base shear for direct use in shear wall and diaphragm design.

Assumptions

Regular structure with no torsional irregularity is assumed. Importance level 5 (exceptional structures) is not supported. The calculator does not cover site-specific response analysis required for non-standard sub-soil classes or near-fault conditions.

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Frequently asked questions

What design standard does this calculator follow?
The calculator implements AS 1170.4:2024 - Structural design actions: Earthquake actions, alongside AS 1170.0:2002 for probability and importance level definitions. It uses the Equivalent Static Analysis procedure (ESA) from AS 1170.4:2024 Section 6, which applies to regular structures within the height and regularity limits of the standard.
What are the key inputs?
Key inputs are hazard design factor (Z) from the AS 1170.4:2024 hazard map, sub-soil class (Ae through Ee), structure importance level (1 to 4), design working life (determining annual probability of exceedance), building height and story masses, framing type, and structure type for ductility and performance factors.
What does the calculator output?
Outputs include probability factor (k_p), product k_p × Z, site natural period, spectral shape factor (Ch(T)), earthquake design category (EDC I to III), total seismic weight, base shear (V), exponent k, and the lateral force at each story level. Results are tabulated for direct use in lateral system design.
What earthquake design categories does the calculator cover?
The calculator determines EDC I, II, or III based on the combination of importance level and seismic hazard at the site. EDC I structures require only simple detailing. EDC II and III structures require specific structural detailing per AS 1170.4. Importance level 5 (exceptional structures requiring site-specific response analysis) is not supported.
When is the Equivalent Static Analysis procedure applicable?
ESA applies to regular structures within the height limit specified in AS 1170.4:2024 Cl. 5 for the applicable EDC. Irregular structures or those exceeding height limits require dynamic analysis. The calculator assumes no torsional irregularity - structures with significant plan eccentricity should be assessed separately for torsional effects.

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