Is the F force applied to the floor when the response spectrum is calculated?

[Ismail Hakki Besler]

Some of our users are asking this question. (Just because some programs do that) The calculation made with Kat F is equivalent or multimodal analysis. Structures higher than 28 m cannot be calculated with equivalent or multimodal analysis if there is torsional irregularity.[/b ] As a result of the calculation with the mode combination method, no F load is applied to the structure. There are only masses. As a result of the analysis, the response magnitudes are calculated by using the earthquake design spectrum for each mode. In the first part of the calculation, unit modal behavior sizes are found by using the period, modal participation factor, modal effective mass, and modal mass participation ratios for each mode. Then, modal displacements and modal internal forces are calculated using the horizontal elastic design spectrum defined in the Regulation. After the modal displacements and modal internal forces of each mode, in other words the unit modal behavior quantities, are found, they are combined with one of the Perfect Quadratic Combination (CQC) or the Square Root of Sum of Squares (SRSS) methods and the combined internal forces are found. ideCAD uses full quadratic compositing. This internal force is the highest internal force value that that point can take for a single point of the element, considering all modes. The calculation with the mod combination method should be done according to the rules and rules given in EK4B as described in the regulation. The calculation made by finding the floor forces acting on the floors as a result of the calculation made in different ways and by different names, and affecting them, is not a dynamic calculation made by mod combination, but a static calculation. Maybe, modal analysis can be named with words such as multimodal analysis, but it is not a true Mode integration (Response spectrum) analysis. The calculation made by the effect of the floor forces becomes the earthquake calculation with the equivalent earthquake load. Equivalent earthquake load can be calculated by considering only the 1st mode contribution, or it can be done by taking the contribution of all modes. But this account is never a Mod combining method, it is an equivalent account. Analysis by considering only the 1st mode Equivalent earthquake calculation is a 1-mode analysis. If you take into account the contribution of other modes, this calculation is the multimodal equivalent earthquake calculation. However, Response is not spectrum analysis. Earthquake regulation does not say to do it this way. It doesn't say you can. ideCAD performs the calculation with the Static Mode combining method, as international programs do. Therefore, there is no horizontal floor force (F) acting on the floor in the calculation made with the mode combination method. If there is torsional irregularity in the structure, structures higher than 28 m cannot be calculated by either single-mode equivalent or multi-mode analysis. If there is torsional irregularity in the structure, structures higher than 28 m can only be calculated using the Mode coupling method specified in 4.8.2.

 

[Ismail Hakki Besler]

Re: When the response spectrum is calculated, the F force is effective on the floor The new earthquake code has explained in detail how to do dynamic analysis with mode coupling and detailed how the calculation should be done in Annex 4B. Here he speaks of typical behavioral magnitude. These behavior dimensions are the beam, column, wall, foundation slab raft internal forces of all elements Combined cross-sectional effects of Polygon Walls Relative storey drifts Slab stresses Shear overturning moments Calculation of foundations are dimensions such as punching controls. All these quantities should be calculated separately for each vibration mode and combined as mentioned in Annex 4B. To elaborate a little, for example, in the calculation of relative floor hoteling, floor displacements should be calculated with the spectrum curve and reduction coefficients given for each mode and then combined. Or the maximum base tipping moment of the structure should be calculated for each mode as given in 4B 2.5 and combined according to 4B2.4.