Minimum Seismic Base Shear

This calculation determines the minimum seismic base shear forces for buildings

Input:

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Floors:

Output:

    Definitions
    • Ta = Approximate Period (s)
    • Ss = Mapped MCEr spectral response parameter at T=0.2
    • S1 = Mapped MCEr spectral response parameter at T=1.0
    • SDS = Design spectral acceleration at short periods
    • Sd1 = Design spectral acceleration at T=1.0
    • TL = Long-period transition period
    • R = Response modification coefficient
    • hx = Height of story above base
    • ex = Eccentricity for calculating story torsion, Tx
    • Cvx = Vertical distribution factor (portion of V assigned to story)
    • Fx = Lateral seismic force for story as Cvx*V
    • Tx = Seismic torsion calculated as Fx*ex
    • Vx = Story Shear (sum of Fx above story)
    • Mx = Story Moment (sum of Fx*hx above story)
    Calculate Design Spectra
    Sds=1.12g Sd1=0.40g
    • g
    • g
    Calculate Period, T
    T = 0.36 s
    • s
    • s
    Calculate the Seismic Base Shear, V
    Cs=0.223 V=668.7
    • kip
    Notes and Explanation

    • Steps to Calculate Minimum Base Shear in ASCE 7

      Step 1: Determine the Seismic Weight (W)

      Calculate the total seismic weight of the structure, which includes dead loads, permanent loads, and other applicable loads specified in ASCE 7.

      Seismic weight typically includes the weights of floors, walls, roof, partitions, and portions of live loads.

      Step 2: Determine the Seismic Design Category

      Based on the building's location (seismic zone), occupancy category, and structural system, determine the Seismic Design Category (SDC). This is derived using the site's seismic hazard data: the Short-Period Spectral Response (Ss) and the 1-Second Spectral Response (S1).

      Step 3: Determine the Response Modification Coefficient (R)

      Based on the chosen structural system (e.g., steel frame, concrete shear wall, etc.), find the appropriate R value from ASCE 7 Table 12.2-1.

      This factor accounts for the system's ability to absorb energy through inelastic behavior.

      Step 4: Obtain the Seismic Coefficients (Cs)

      For short periods:

      Cs = (Sds) / (R / Ie)

      • Sds = Design Spectral Response Acceleration at Short Periods (typically provided by local building codes or seismic maps).
      • Ie = Importance factor based on the building's use and occupancy category (see ASCE 7 Table 1.5-2).

      For long periods:

      Cs = (Sd1) / (T × (R / Ie))

      • Sd1 = Design Spectral Response Acceleration at 1 second.
      • T = Fundamental period of the structure (calculated or based on code provisions).

      Ensure the calculated value of Cs does not exceed or fall below code-specific limits.

      Step 5: Calculate the Base Shear (V)

      Using the seismic coefficient Cs from Step 4, calculate the base shear:

      V = Cs × W

      Step 6: Distribution of Base Shear

      Distribute the base shear vertically among the structure’s levels using the distribution factor formula in ASCE 7. Typically, the shear is distributed in proportion to the story weights and heights.

      Step 7: Review Additional Considerations

      Account for irregularities, torsional effects, and other factors if applicable.

      Depending on the system and site conditions, other adjustments or checks may be needed, such as for near-fault effects.

    User Notes: