How does ideCAD perform modal response spectrum analysis, according to ASCE 7-16?
Individual directional results are automatically combined by CQC. And orthogonal loading effects are combined with SRSS.
90% of mass participation ratio is automatically controlled and reported by the program.
Modal response spectrum analysis specified in 12.9.1 is done automatically.
In accordance with 12.9.1.1, the sufficient number of vibration modes to be taken into account is calculated automatically as 90%.
With the modal response analysis, seismic effects are combined automatically by CQC.
Symbols
mi = total mass of the ith story
miθ = mass moment of inertia of the ith story
mixn (X) = (X) for the earthquake direction, the i'th story modal effective mass of the nth natural vibration mode of the building in the x-axis direction
miyn (X) = (X) for the earthquake direction i'th story modal effective mass
miθn (X) = (X) of the building's nth natural vibration around the z-axis for the earthquake direction i'th storey modal effective mass moment of inertia
mj (S) = Finite Element Analysis node j to effect individual masses
mtxn (X) = (X) earthquake direction for building the x-axis direction of the nth vibration mode of base shear modal effective mass
mtyne (Y) = (Y) earthquakes base shear in the building along the y axis for the direction of modal effective mass
rmax (X) = (X) earthquake direction for any behavior variables (displacements and relative story drift, strain component) corresponding to the coupled typically to maximum modal behavior of size
rn (X) = Typical unit modal behavior magnitude corresponding to any action magnitude (displacement, relative floor displacement, internal force component) for the earthquake direction in the nth natural vibration mode (X),
rn,max (X) = nth natural vibration mode ( X) Typical largest modal behavior magnitude corresponding to any action magnitude (displacement, relative floor displacement, internal force component) for the earthquake direction
SaR (Tn ) = reduced design spectral acceleration for the nth vibration mode
Tn = nth mode natural vibration period
βmn = ratio of mth and nth natural vibration periods
Φi (X) n = nth natural vibration mode shape amplitude at i'th story (X) earthquake direction
Φixn =nth natural vibration mode shape amplitude ati'th story in x-axis direction
Φiyn = y-axis at i'th story nth natural vibration mode shape amplitude in the direction
θiθn = nth natural vibration mode shape amplitude as rotation around the z-axis at the ith story
Γx (X) = (X) for the earthquake direction, modal contribution of the nth vibration mode multiplier
ξn = modal damping ratio of the nth vibration mode
ωn = Natural vibration angular frequency of the nth vibration mode
ρmn = Cross-correlation coefficient of the mth and nth natural vibration modes in the Complete Quadratic Combination Rule
Modal Response Analysis Method
In the modal response spectrum analysis method, the structure is decomposed into a number of single degree-of-freedom systems, each having its own mode shape and natural period of vibration. The number of modes available is equal to the number of mass degrees of freedom of the structure, so the number of modes can be reduced by eliminating mass degrees of freedom.
For a given direction of loading, the displacement in each mode is determined from the corresponding spectral acceleration, modal participation, and mode shape. Because the sign (positive or negative) and the time of occurrence of the maximum acceleration are lost in creating a response spectrum, there is no way to recombine modal responses exactly. However, statistical combination of modal responses produces reasonably accurate estimates of displacements and component forces. The loss of signs for computed quantities leads to problems in interpreting force results where seismic effects are combined with gravity effects, produce forces that are not in equilibrium, and make it impossible to plot deflected shapes of the structure.
Modal analysis provides the entire response history for a given ground motion record. For design
purposes, its application requires a design ground motion record that is representative of the seismic
hazard at the site. For design purposes, we usually use the maximum value of a response parameter and
not the entire response history. Since every mode can be treated as an independent SDOF system, the
maximum response values of a mode can be easily obtained from the corresponding response
spectrum. If Sd(Tn, x), Sv(Tn, x), and Sa(Tn, x) denote the spectral displacement, velocity and acceleration,
respectively, the maximum modal displacements are obtained from a response spectrum as
The maximum displacement and the equivalent lateral force of the jth storey
It is used in the horizontal elastic design spectrum in the direction of a given earthquake and the maximum values of the response magnitudes in each vibration mode are calculated with the modal analysis method. The largest non-synchronous modal behavior magnitudes calculated for enough vibration modes are then combined statistically to obtain approximate values of the largest behavior magnitudes.
For each vibration mode considered, the largest modal behavior magnitudes namely displacements, relative floor displacements, internal forces and stresses are found. Located in the largest size modal behavior of Complete Quadratic Combination. It is combined using the (CQC) rule. In this analysis, it does not give information about when the said behavior magnitude occurred and its correlation with other loadings.
The Square Root of the Sum of Squares (SRSS) Rule
The most common rule for modal combination is the Square Root of Sum of Squares (SRSS) rule.
According to this rule, the peak response of every mode is squared and then the squares are summed.
The estimation of the maximum response quantity of interest is the square of the sum.
The major limitation is that in order to produce satisfying estimates, the modes should be
well separated, i.e., the eigenfrequencies should not have close values. If this condition is not met,
the CQC method should be used instead. A criterion to determine if two modes are well separated is
βnm = wm/wn =Tn /Tm ζn and ζm the damping ratio of modes n and m.
The Complete Quadratic Combination (CQC) Rule
where ϵnm is a correlation coefficient that takes values in the 0,1 range and is equal to 1 when n=m.
βnm the correlation term is calculated as
If the same modal damping is used for modes n and m (ζn = ζm = ζ), the equation reduces to
