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Seismic Base Shear Calculator

Beta — preliminary loads only. This tool computes an equivalent-static base shear and its vertical distribution. Behaviour factor q (EC8) / response modification R (ASCE7) must be justified by the user against the full code — it is not auto-derived from a system-type dropdown. Seismic Design Category / lateral-force-method applicability limits are only partially checked. Has not been independently reviewed by a licensed engineer.
γI = 1
S=1.2TB=0.15sTC=0.5sTD=2s
agR = reference PGA from national zonation maps. Design ag = agR x gammaI, computed below — do not enter an already-adjusted value here unless the box above is checked (entering an adjusted value AND leaving gammaI active would double-apply the importance factor).
Ct = 0.075
q is NOT derived automatically from the system-type dropdown above (that list only supplies the period coefficient Ct). Determine q from EN 1998-1 Ch. 5-8 based on material, ductility class (DCL/DCM/DCH), regularity, and the overstrength ratio alpha_u/alpha_1. Default shown (1.5) is the DCL floor value, not a recommendation.
00.511.522.533.540.000.170.340.520.690.86T1=0.57sPeriod T (s)Se, Sd (g)EC8 Response SpectrumSe(T)Sd(T)
Lateral Force DistributionF1F2F3F4F5123.9 kN247.8 kN371.7 kN495.6 kN619.6 kN3.0m6.0m9.0m12.0m15.0mV = 1858.7 kN
Lateral-Force Method Applicability
✓
Period limit: T1 <= min(4.TC, 2.0s)
T1 = 0.572s vs limit 2.000s. OK.
?
Regular in elevation (§4.2.3.3)
Not confirmed — this tool cannot derive regularity from the inputs given. Confirm manually before relying on the lateral-force method.
?
Not torsionally sensitive (§4.2.3.2)
Not confirmed — this tool cannot derive torsional sensitivity from the inputs given. Confirm manually.
Status: Unconfirmed — engineering confirmation required
The floor-force distribution Fi = Fb.(zi.wi)/Sum(zj.wj) assumes a linear (triangular) first-mode shape (EC8 §4.3.3.2.3(1)). For buildings with irregular mass or stiffness distribution, or where higher modes matter, a proper mode-shape-based or modal response-spectrum distribution should be used instead.
Base Shear V
V
1858.7kN
% of W
37.17%
Period T1
T1
0.572s
Design Sd(T1)
Sd(T1)
0.4373g
Max Story Force
Floor 5
619.6kN
Floor Force Distribution
Floorzi (m)wi (kN)CvxFi (kN)Vi (kN)
515.010000.3333619.6619.6
412.010000.2667495.61115.2
39.010000.2000371.71486.9
26.010000.1333247.81734.7
13.010000.0667123.91858.7
Period [EC8 §4.3.3]▼
Fundamental Period
EC8 §4.3.3.2.2, eq. (4.6)Approximate fundamental period
T1 = Ct x H^(3/4)
T1 = 0.075 x 15.0^0.75
T1 = 0.572 s
Adopted fundamental period
T1 = 0.572 s
Spectrum [EC8 §3.2.2]▼
Response Spectrum
EC8 §3.2.1(3), §4.2.5Design ground acceleration = reference PGA x importance factor
ag = agR x gammaI
ag = 0.250 x 1.00
ag = 0.2500 g
EC8 §3.2.2.2, eq. (3.6)Damping correction factor eta (elastic spectrum only — NOT applied again to the design spectrum, which uses q instead)
eta = max(sqrt(10/(5+xi)), 0.55)
eta = max(sqrt(10/(5+5)), 0.55)
eta = 1.000
EC8 Table 3.2/3.3Ground type parameters
Type B: S, TB, TC, TD
S=1.2, TB=0.15s, TC=0.5s, TD=2s
Ground type B — Very dense sand, gravel, stiff clay
EC8 §3.2.2.2Elastic spectral acceleration at T1 (reference only — not used for the design base shear)
Se(T) = ag.S.eta.2.5.TC/T
Se(0.572) with ag=0.250g, S=1.2, eta=1.000
Se(T1) = 0.6560 g
EC8 §3.2.2.5, eq. (3.13)-(3.16)Design spectral acceleration — its own piecewise formula (NOT Se/q): damping is absorbed into q, not applied twice
Sd(T) = ag.S.(2.5/q).(TC/T) >= beta.ag
Sd(0.572) with ag=0.250g, S=1.2, q=1.50, beta=0.2
Sd(T1) = 0.4373 g
Base Shear [EC8 §4.3.3]▼
Base Shear Calculation
EC8 §4.3.3.2.2(1)Correction factor lambda
lambda = 0.85 if T1 <= 2.TC and stories > 2, else 1.0
T1=0.572s, 2TC=1.00s, stories=5
lambda = 0.85
EC8 §4.3.3.2.2, eq. (4.5)Seismic base shear
Fb = Sd(T1) x W x lambda
Fb = 0.4373 x 5000.0 x 0.85
Fb = 1858.7 kN (37.17% of W)
Distribution [EC8 §4.3.3]▼
Vertical Distribution
EC8 §4.3.3.2.3, eq. (4.11)Vertical force distribution — linear (first-mode) approximation. This assumes a triangular mode shape; it is not exact for irregular mass/stiffness distributions.
Fi = Fb x (zi.wi) / Sum(zj.wj)
Sum(zj.wj) = 45000.0
Distributing Fb = 1858.7 kN over 5 floors