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P-Delta Effects per ASCE 7-16 with ideCAD

How ideCAD defines p-delta effects according to ASCE 7-16?

  • The stability coefficient θ and θmax are determined by eq. 12.8-16 and 12.8-17 automatically.

  • Where the stability coefficient θ is greater than 0.10, but less than or equal to θmax, ıt’s multiplied displacements and member forces by 1.0 / (1 − θ) automatically.

  • For both directions, P-Delta Effect is automatically calculated with Equation 12.8-16 at each floor.

  • If the stability coefficient (θ) determined for all floors conforms to the limit value calculated by Equation 12.8-17, the stability coefficient are not taken into account in the calculation of the internal forces based on the design.

  • If the condition given in Equation 12.8-17 is not fulfilled, all internal forces are automatically increased according to Section 12.8.7.


Px = Total vertical design load at and above level x [kip (kN)]
Δ = Design story drift as defined in Section 12.8.6 occurring simultaneously with Vx [in. (mm)]
Ie = Importance Factor
Vx = Seismic shear force acting between levels x and x − 1 [kip (kN)]
hsx = story height below level x [in. (mm)]
Cd = Deflection amplification factor in Table 12.2-1.
β = The ratio of shear demand to shear capacity for the story between levels x and x − 1.

P-delta effects are determined with ASCE Section 12.8.7. Moreover, the base shear used to determine the story shears and the story drifts is determined in ASCE Section 12.8.6.

P-delta effects on story shears, internal design forces, and the story drift are considered with stability coefficient (θ) as determined by the following equation:

P-delta effects are not required to be considered where the stability coefficient (θ) is equal to or less than 0.10

The stability coefficient (θ) does not exceed θmax, determined with the following equation:

If the stability coefficient (θ) is greater than 0.10 but less than or equal to θmax, the incremental factor related to P-delta effects is determined by rational analysis. Alternatively, displacements and member forces can be multiplied by 1.0 / (1 − θ). Where θ is greater than θmax, the structure should be redesigned.
Where the P-delta effect is included in an automated analysis, Eq. (12.8-17) is still satisfied; however, the value of θ computed from Eq. (12.8-16) using the results of the P-delta analysis is permitted to be divided by (1 + θ) before checking Eq. (12.8-17).

The load-bearing system calculation, made as a standard under the influence of various loads, is essentially made over an unchanged (not deformed) system. However, considering the deformed shape, additional behavior magnitudes (internal forces and displacements, etc.) can be calculated in the structural system. The effect of the structural system's deformed shape can be considered in two ways. In the former, the deformed shape is taken into account in equilibrium conditions. This approach called the "Large Displacement Theory," can only be applied in flexible systems with large deformation. In practice, the approach used especially for building bearing systems is the "Second Order Theory," which opposes the deformed shape to be considered only in equilibrium equations.
It is necessary to calculate the "P-Delta effects" that occur as a result of horizontal loads occurring in the vertical elements of the bearing system under the effect of earthquake ground motion in general and should be taken into account in the design because these effects cause both "stiffness loss" and "strength loss" especially due to nonlinear behavior in the conveyor system. If both effects take too large values, it may ultimately lead to loss of stability (buckling) in the conveyor system and eventually collapse.
However, it is not easy to obtain P-Delta effects in seismic calculation within the scope of design Based on Strength, and it is contented with approximate methods. In the approximate methods considered in ASCE 7-16 and which are very similar, the rigid diaphragm is accepted, and torsional effects are neglected. 

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