About Seismic Load Calculator
The seismic load calculator applies the Equivalent Lateral Force method to find the base shear and its distribution over a building height per ASCE 7-22, Eurocode 8, and AS 1170.4. It is used by structural engineers to estimate earthquake demands, build a design response spectrum, and assign a Seismic Design Category.
Enter the site hazard parameters, the structural system, the risk category, and the per-story weights and heights. The tool returns the fundamental period, the seismic response coefficient, the base shear, the per-story forces, shears, and overturning moments, and the design response spectrum, all updating in real time.
How It Works
- Estimate the fundamental period from an empirical formula (Ta = Ct * hn^x for ASCE) and apply the code period cap where relevant.
- Compute the seismic response coefficient Cs from the design spectral accelerations, the response modification factor R, and the importance factor, bounded by code upper and lower limits.
- Calculate the base shear V = Cs * W and distribute it vertically as Fx = Cvx * V using the height exponent k (1 to 2 depending on period).
- Accumulate story shears and overturning moments, build the design response spectrum, and determine the Seismic Design Category from the design spectral accelerations and risk category.
Worked Example
A building with SDS = 1.0 g, SD1 = 0.6 g, R = 8, importance factor Ie = 1.0, and design period T = 0.5 s (with T <= TL) per ASCE 7-22. Cs is the smaller of SDS/(R/Ie) = 1.0/8 = 0.125 and SD1/(T*(R/Ie)) = 0.6/(0.5*8) = 0.15, but not below 0.044*SDS*Ie = 0.044, so Cs = 0.125. For an effective seismic weight W = 10000 kN, the base shear is V = 0.125 * 10000 = 1250 kN.
Formulas
- Seismic response coefficient (ASCE 7)
Cs = SDS / (R / Ie)- Cs upper limit (T <= TL)
Cs_max = SD1 / (T * (R / Ie))- Base shear
V = Cs * W- Vertical force distribution
Fx = (wx * hx^k) / sum(wi * hi^k) * V- Approximate period (ASCE 7)
Ta = Ct * hn^x
Standards & References
- ASCE 7-22
- Eurocode 8 (EN 1998-1)
- AS 1170.4
Frequently Asked Questions
Which seismic codes does the calculator support?
It supports ASCE 7-22 (Equivalent Lateral Force, Chapter 12), Eurocode 8 (EN 1998-1) with the inelastic design spectrum and behaviour factor q, and AS 1170.4 with its hazard factor Z and probability factor kp.
How is the base shear calculated?
The base shear is V = Cs * W, where Cs is the seismic response coefficient bounded by code upper and lower limits, and W is the effective seismic weight summed over all story levels.
How are the lateral forces distributed over height?
The base shear is distributed as Fx = Cvx * V with Cvx = (wx*hx^k)/sum(wi*hi^k). The exponent k is 1 for periods up to 0.5 s, 2 for periods of 2.5 s or more, and interpolated in between.
What is the Seismic Design Category?
The SDC (A through F) is assigned from the design spectral accelerations SDS and SD1 and the risk category per ASCE 7 Tables 11.6-1 and 11.6-2, taking the more severe of the two, and drives detailing and analysis requirements.