Changes for page Seismic analysis

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 ... ... @@ -258,8 +258,42 @@ 258 258 259 259 == Calculation methods selection == 260 260 261 +(% style="text-align: justify;" %) 262 +National codes always provides, which Seismic calculation method to be performed for different structure, where and when it should be performed and what other effects to be considered (torsional effect, P-Δ effect). 263 + 264 +(% style="text-align: justify;" %) 265 +As an example in Norwegian code NS3491-12, seismic calculation is not necessary if the acceleration from the design spectrum is (% class="mark" %)S,,d,,(T,,1,,) ≤ 0,5 m/s^^2^^(%%) where T,,1,, is the base vibration period. In EC8 3.2.1 some criteria can be found. 266 + 267 + 261 261 == Other setting possibilities == 262 262 270 +(% style="font-size:18px" %)Horizontal direction setting for seismic calculation 271 + 272 +Generally codes speak about the seismic calculation in X-Y directions. However results in these directions give the maximum effect if the mass and elastic properties of the structure ensure that the calculated mode shapes lay in X-Z or Y-Z plane. Nevertheless it is not always achieved in practice. To achieve the unfavorable direction, where the results from a ground motion are maximum, the user can Set the Alpha angle or may get the program suggestion by using Auto but- ton. 273 + 274 +The most unfavorable direction can be found when any of the mx', my' is zero and the other is maximum in a row. Using Auto button, program gives the most unfavorable directions, but there are certain restrictions: this directions can be ensured only for one mode shape. The program selects the row where the effective mass is the maximum. 275 + 276 +As an example, on the left hand side figure you can see a badly adjusted x'-y' direction. Appling Auto button, program arranges the direction for the 73,8% effective mass and correct it to 98,3%. 277 + 278 +(% style="margin-top:.15pt; margin:0cm 0cm 0.0001pt" %) 279 +[[image:1536241385931-685.png||height="68" width="345"]] 280 + 281 + 282 +(% style="text-align: justify;" %) 283 +Of course this also can be reached if the user rotates the whole geometry with a specified angle. 284 + 285 + 286 +(% style="font-size:18px" %)Effective mass setting 287 + 288 +FEM-Design always takes into account the entire mass of the structure in the calculation of base shear force which was mentioned in Lateral force method. It was also mentioned, EC8 defines the total mass without the basement, this is called Reduced mass in this manual. The effective masses are generally compared to the Reduced mass, but this is not valid for the massive basement with elastic foundation. 289 + 290 +If the above mentioned situation is the case, it might happen that the sum of the effective masses of a column is larger than 100%. The user may compare the modal effective masses to the total mass or reduced mass by pushing the Eff. mass button. 291 + 292 +(% style="text-align: justify;" %) 293 +In FEM-Design Reduced mass means the difference between the total mass of the structure and the basement mass. The basement mass is the sum of all masses which lay on the foundation level which can be set in the Others tab of seismic load. 294 + 295 +It is uninteresting from the calculation point of view that effective masses are compared to the total or the reduced mass because these values are given in percentage and only gives information about which mode shape is the fundamental or which shapes are dominant in a given direction. 296 + 263 263 == Combination rule, rotation and second order effects == 264 264 265 265 (% style="text-align: justify;" %)