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IwonaBudny 1.1 1 {{box cssClass="floatinginfobox" title="**Contents**"}}
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IwonaBudny 17.5 5 = **Introduction** =
IwonaBudny 1.1 6
IwonaBudny 2.2 7 (((
8 (% style="text-align: justify;" %)
9 Seismic calculation is a special case of forced vibration calculation, when the exciting effect is the ground acceleration which is time dependent and of course not periodical. The response of the structure to the ground acceleration will be a vibration like motion. The structure gets forces of inertia which is calculated ac- cording to the Newton’s law (F = m a) and it is proportional to the mass and acceleration. These equivalent forces of course cause internal forces, stresses and if they are larger than the limit value the structure may collapse.
10 )))
IwonaBudny 1.1 11
IwonaBudny 2.2 12 (% style="text-align: justify;" %)
13 From the above explanation we found that originally this is a dynamic phenomenon when the acceleration and so the inertia forces change in each second. The interaction of the ground and structure is complicated so in a given time the acceleration of the structure depends on several components:
14
15 * ground acceleration (the seismic magnitude and its development on time),
16 * the elasticity of the structure,
17 * the mass and mass distribution of the structure,
18 * the connection between the structure and ground, namely soil type.
19
IwonaBudny 40.4 20 (% style="text-align: justify;" %)
21 Another complicated problem is to define exact direction of the ground motion in the seismic investigation. Generally the ground movement is assumed as an arbitrary horizontal motion but the vertical motion also may cause problem to the structure. Fundamentally the calculation process can be divided into three methods.
IwonaBudny 2.2 22
IwonaBudny 2.4 23 (% id="HTimehistory" %)
24 == Time history ==
IwonaBudny 1.1 25
IwonaBudny 2.5 26 (% style="text-align: justify;" %)
27 This calculation is carried out as an ordinary forced vibration when the excitation is a time dependent acceleration function. These functions can be registered or simulated seismograms. Mathematically we always solve the differential equation system of the vibration by a suitable method (e.g. step-by-step method). From the results of the equation system (means the displacement of the structure) the internal forces can be calculated and the design can be performed.
IwonaBudny 1.1 28
IwonaBudny 2.5 29 (% style="text-align: justify;" %)
30 Theoretically the method is exact, but several circumstances strongly constrain the usage:
31
32 * statistically the number of seismograms are insufficient,
33 * the calculation is very complicated and the runtime is long.
34
35 (% style="text-align: justify;" %)
36 Because of the above mentioned difficulties, this method is not widespread and is not implemented in FEM-Design.
37
IwonaBudny 2.4 38 (% id="HModalanalysis" %)
39 == Modal analysis ==
IwonaBudny 1.1 40
IwonaBudny 2.5 41 (((
42 (% style="text-align: justify;" %)
43 As was mentioned in the above method, the vibrations arising from the seismic effect are difficult to predict. So the modal analysis assumption starts from the investigation of the most unfavorable ground motion directions and the period time.
44 )))
IwonaBudny 1.1 45
IwonaBudny 2.5 46 (% style="text-align: justify;" %)
47 Expected value of the maximal accelerations belongs to the individual periods which are prescribed in the national codes and named as acceleration response spectrum. The horizontal axis shows the frequency or vibration time of a single degree mass-spring system and the vertical axis shows the maximum corresponding acceleration. (In the Civil Engineering practice vibration period is used instead of frequency.)
48
49 (% style="text-align: justify;" %)
IwonaBudny 40.4 50 The results which belong to the different ground motion directions and structural eigenfrequencies are summarized on the basis of the probability theory, which assumes that not all the effects appear in the same time. Most frequently used summation rule is the **SRSS** (Square Root of Sum of Squares).
IwonaBudny 2.5 51
52 (% style="text-align: justify;" %)
53 Although the modal analysis is the most accepted method all over the world (as well as in EC8), it has some disadvantages. Some of them are listed as follows:
54
IwonaBudny 2.6 55 * the results which are calculated using the SRSS summation rule are not simultaneous. For example for a bending moment in a point of the structure we can’t show the simultaneous normal force in the same point, because the summation is carried out from component to component separately. Consequence of the summation rule, other calculations (second order application, stability analysis) are not interpreted,
56 * mainly from the application of the statistical method, the graphical results weakly can be followed compare to the results of statical calculation,
57 * in a lot of cases great number of vibration shapes should be calculated to reach reasonable results which require long calculation time.
IwonaBudny 2.5 58
59 (% style="text-align: justify;" %)
60 Despite of all disadvantages of this method, we can expect most trustable results if the code requirements are fulfilled.
61
IwonaBudny 2.4 62 (% id="HLateralforcemethod" %)
IwonaBudny 40.11 63 == Lateral force method ==
IwonaBudny 1.1 64
IwonaBudny 2.6 65 (% style="text-align: justify;" %)
IwonaBudny 40.11 66 The lateral force method called also Equivalent static load method, partly eliminates the disadvantages of the previous method with simplification in certain cases. The method postulates that the dis- placement response of the structure for ground motion can be described with one (or both x', y' directions) mode shape. While this means generally a simplification or approximation, this method is suitable for a part of the structure (EC8 prescribes the condition of application). In this method the mode shape of the structure is a linear deviation or it is equivalent to the calculated fundamental vibration shape. In the case of linear deviation or mode shape the period also can be calculated by approximate formula.
IwonaBudny 1.1 67
IwonaBudny 2.6 68 (% style="text-align: justify;" %)
69 The application of this method gives possibility to transform the seismic lateral forces to simple static loads and it is applicable as follows:
70
71 * these loads (seismic load cases) can be combined with other static loads,
72 * second order and stability analysis can be performed,
73 * it is also possible to use these loads for hand calculation, so the results can be checked easily.
74
75 (% style="text-align: justify;" %)
76 This method is usable in FEM-Design with two options if the code permits:
77
78 * assumption of linear deviation shape when the period also can be defined by the user (Static, linear shape),
79 * application of the calculated fundamental vibration shape as the deformed shape of the structure and its period (Static, mode shape).
80
IwonaBudny 1.1 81 == National codes ==
82
IwonaBudny 2.6 83 (((
IwonaBudny 40.4 84 (% class="box warningmessage" style="text-align: justify;" %)
85 (((
IwonaBudny 2.6 86 Remarks in application of national codes:
87
IwonaBudny 40.4 88 * before releasing the current version of FEM-Design, only the Eurocode and Norwegian national code contained special description for seismic calculation. In the other codes FEM-Design supports only the general modal analysis,
IwonaBudny 2.6 89 * most of the countries did not prepare the National Application Document (NAD) for the universal Eurocode, so the program uses the general pres-cription.
90 )))
91
92 Supported national codes and methods:
IwonaBudny 40.4 93 )))
IwonaBudny 2.6 94
IwonaBudny 2.7 95 |(% style="width:247px" %)British |(% style="width:1626px" %)Modal analysis
96 |(% style="width:247px" %)Code independent|(% style="width:1626px" %)Modal analysis
97 |(% style="width:247px" %)Danish|(% style="width:1626px" %)Modal analysis
98 |(% style="width:247px" %)Eurocode (NA: - )|(% style="width:1626px" %)EC8-2005 (No NAD, static method, modal analysis)
99 |(% style="width:247px" %)Eurocode (NA: British )|(% style="width:1626px" %)EC8-2005 (No NAD, static method, modal analysis)
100 |(% style="width:247px" %)Eurocode (NA: German )|(% style="width:1626px" %)EC8-2005 (No NAD, static method, modal analysis)
101 |(% style="width:247px" %)Eurocode (NA: Italian )|(% style="width:1626px" %)EC8-2005 (No NAD, static method, modal analysis)
102 |(% style="width:247px" %)Finnish (B4:2001) |(% style="width:1626px" %)Modal analysis
103 |(% style="width:247px" %)Finnish (By50:2005)|(% style="width:1626px" %)Modal analysis
104 |(% style="width:247px" %)German|(% style="width:1626px" %)Modal analysis
105 |(% style="width:247px" %)Hungarian|(% style="width:1626px" %)Modal analysis
IwonaBudny 2.8 106 |(% style="width:247px" %)Norwegian|(((
107 NS3491-12 (static method, modal analysis)
IwonaBudny 2.6 108 )))
IwonaBudny 2.7 109 |(% style="width:247px" %)Swedish|(% style="width:1626px" %)Modal analysis
IwonaBudny 2.6 110
111 Norwegian code differs from Eurocode in a few places, so they are reviewed together and the differences are marked separately.
112
IwonaBudny 1.1 113 ----
114
IwonaBudny 17.6 115 = **Input data** =
IwonaBudny 1.1 116
117 (% id="HSub-paragraph-2" %)
IwonaBudny 6.2 118 == Dynamic calculations and Mass definitions ==
IwonaBudny 1.1 119
IwonaBudny 6.2 120 (% style="text-align: justify;" %)
IwonaBudny 40.5 121 To calculate the seismic effect it is necessary to know the vibration shapes and corresponding periods, except the static method (lateral force method: linear force distribution). Therefore, a dynamic calculation should be done before performing seismic calculation, which gives sufficient vibration shapes of the structure. To perform the dynamic calculation, it is necessary to define mass distribution which can be defined in Load tab as concentrated mass or load case-mass conversion.
IwonaBudny 6.2 122
IwonaBudny 40.5 123 [[image:1536237300428-654.png||height="27" width="66"]] According to EC8 3.2.4(2), mass distribution should be made in the following way:
IwonaBudny 6.2 124
IwonaBudny 40.5 125 (% class="mark" %)ΣGk, j"" + ""ΣψE, iQk, i
IwonaBudny 6.2 126
127 where:
128
129 * ψ,,E, i,, is the combination coefficient for variable action i (see EC8 4.2.4), it shall be computed from the following expression:
130
131 (% class="mark" %)ψ,,E, i ,,= ϕ ψ^^2, ^^i
132
IwonaBudny 40.5 133
IwonaBudny 6.2 134 The recommended values for ϕ are listed in EC8 Table 4.2.
135
136 The above formula means that mass conversation is made from all dead load without any factor, also masses in gravity direction temporary loads with reduced value.
137
138
IwonaBudny 1.1 139 == Design spectrum ==
140
IwonaBudny 6.2 141 (% style="text-align: justify;" %)
142 [[image:1536237268175-179.png||height="23" width="26"]] The program contains EC8 and NS3491-12 predefined design spectra or the user can define its own spectra if necessary. The vertical spectrum is necessary when the vertical affect taken into account.
143
IwonaBudny 40.5 144 (% id="HEC8designspectrum" %)
145 === EC8 design spectrum ===
IwonaBudny 6.2 146
147 The code gives the horizontal and vertical spectra and although the value of variables is prescribed, they can be modified if necessary.
148
149 [[image:1536237376815-771.png||height="247" width="315"]]
150
IwonaBudny 40.5 151
IwonaBudny 17.2 152 **Horizontal spectra**
IwonaBudny 6.2 153
IwonaBudny 17.2 154 Data of horizontal design spectra:
IwonaBudny 6.2 155
IwonaBudny 10.2 156 * Type type of spectra, which there are two in the code,
157 * Ground ground type, which can be A, B, C, D and E,
IwonaBudny 6.2 158
IwonaBudny 10.2 159 (% style="text-align: justify;" %)
160 The above two data specify the values of S, TB, TC and TD, which can be found in EC8, table 3.2 and 3.3.
IwonaBudny 6.2 161
IwonaBudny 10.2 162 * ag is the design ground acceleration on type A ground (ag = γI ag R),
163 * S is the soil factor,
164 * q is the behavior factor, which depends on material and type of the structure,
165 * beta (β) is the lower bound factor for the horizontal design spectrum.
166
167 (% style="text-align: justify;" %)
168 The Sd(T) horizontal design spectrum is based on EC8 3.2.2.5 as follow:
169
170 [[image:1536237735225-141.png||height="224" width="400"]]
171
172 (% style="text-align: justify;" %)
IwonaBudny 17.2 173 **Vertical spectra**
IwonaBudny 10.2 174
175 (% style="text-align: justify;" %)
IwonaBudny 17.2 176 The built-in vertical design spectrum is derived from the horizontal spectrum using the aυg / ag multiplicator which can be found in EC8 table 3.4 and described in 3.2.2.5(5)-(7).
177
178 (% style="text-align: justify;" %)
IwonaBudny 10.2 179 [[image:1536237786462-739.png||height="179" width="297"]]
180
181
IwonaBudny 17.2 182 **Other input parameters (Others tab)**
IwonaBudny 10.2 183
184 [[image:1536237903146-157.png||height="81" width="170"]]
185
186 (% style="text-align: justify;" %)
187 In the Others tab, the user should set some parameters that effect the calculation and results.
188
IwonaBudny 40.5 189 * Ksi(ξ) is the viscous damping ratio, expressed as a percentage, gene- rally 5%. This data is used in modal analysis when the sum- mation of the effect of the same direction vibration shapes is carried out by the CQC (Complete Quadratic Combination), see later.
190 * qd is the displacement behavior factor, assumed equal to q unless otherwise specified.
191 * Foundation level when Static-linear shape is used, the program assumes that the foundation level is defined on that height. It means the pro- gram calculates the mass height from that level. In the other two calculation methods (Static-mode shape and Modal analysis) base shear force is drawn in that level and it is taken into consideration in the so called reduced mass calculation (details in Effective mass setting).
IwonaBudny 10.2 192
IwonaBudny 40.5 193 (% id="HNS3491-12designspectrum" %)
194 === NS3491-12 design spectrum ===
IwonaBudny 17.2 195
196 **Horizontal spectra**
197
IwonaBudny 17.4 198 [[image:1536238030672-563.png||height="220" width="281"]]
IwonaBudny 17.2 199
200 The built-in horizontal design spectrum is based on the following formula:
201
202 (% class="mark" %)S,,d,,(T,,i,,) = k,,Q,, k,,S,, γ,,1,, a,,g,, S,,e,,(T,,i,,) k,,f, spiss,,
203
204
205 where:
206
207 * Ksi(ξ) is the declining ratio for the structure, given in %. Usually 5%,
208 * k,,Q,, is a structure factor, dependent on the type of structure,
209 * k,,S,, is a soil factor, dependent on the type of ground,
210 * Gamma 1(γ,,1,,) is a seismic factor, dependent on the seismic class,
211 * a,,g,, is the maximum ground acceleration, dependent on location and reference period,
212 * S,,e,,(T,,i,,) is the acceleration for the period Ti in the normalized response spectra, see below,
213 * k,,f,spiss,, is a factor dependent on the reference period used.
214
IwonaBudny 40.5 215
IwonaBudny 17.2 216 **Vertical spectra**
217
218 [[image:1536238363669-269.png||height="32" width="119"]]
219
220 (((
221 (% style="text-align: justify;" %)
222 (% class="mark" %)S,,νd,,(T,,ν,i,,) = k,,ν,, γ,,1,, a,,g,, S,,e,,(T,,ν,i,,) k,,f, spiss,,
223 )))
224
225 (% style="text-align: justify;" %)
226 where
227
228 * k,,ν,, is the ratio between horizontal and vertical response spectra, mostly set to 0,7.
229
230 (% style="text-align: justify;" %)
IwonaBudny 40.5 231
IwonaBudny 17.2 232 The normalized response spectrum in Norwegian code is based on four different formulas, each covering a part of the possible periods from 0 to 4 seconds. Periods over 4 seconds has to be treated in a different way anyhow, and can therefore be based on a manually written response spectrum.
233
234 (% style="text-align: justify;" %)
235 In FEM-Design, we assume, the spectrum is constant for periods over 4 seconds and equal to the value of S,,d,,(T = 4).
236
IwonaBudny 31.2 237 [[image:1536238500410-574.png||height="154" width="378"]]
IwonaBudny 17.2 238
239 where:
240
241 * T is the vibration period,
242 * T,,B,, = 0,1sec,
243 * T,,C,, = 0,25sec
244 * T,,D,, = 1,5sec
245 * η is a factor describing how the swaying declines, calculated as: [[image:1536238569724-268.png||height="37" width="167"]]
246
247 (% class="wikigeneratedid" %)
248 **Other input parameters (Others tab)**
249
250 (% class="MsoBodyText" style="margin-top:0cm; margin-right:87.25pt; margin-bottom:.0001pt; margin-left:5.5pt; text-align:justify; margin:0cm 0cm 0.0001pt 5.3pt" %)
251 [[image:1536238618416-879.png||height="35" width="146"]]
252
253
254 In the NS3491-12 code only foundation level should be set.
255
IwonaBudny 40.5 256 (% id="HDesignspectraintheothernationalcodes" %)
257 === Design spectra in the other national codes ===
IwonaBudny 17.2 258
259 Except for the above mentioned two codes, the user has in all cases to define the spectra in table or in a graphical way. In the Others tab only the foundation level should be set.
260
261 [[image:1536238682205-293.png||height="220" width="271"]]
262
IwonaBudny 1.1 263 ----
264
IwonaBudny 17.6 265 = **Calculations parameters and calculations steps** =
IwonaBudny 1.1 266
IwonaBudny 40.2 267 (% style="text-align: justify;" %)
268 Calculation input parameters can be set in the Calculation dialog in Analysis/ Seismic analysis in the Setup as can be seen below.
IwonaBudny 40.6 269
IwonaBudny 40.2 270
271 (% style="text-align: justify;" %)
272 [[image:1536569776410-536.png||height="76" width="250"]]
273
274 (((
275 (% class="box warningmessage" %)
276 (((
277 Remarks:
278
279 * Setup for the Seismic calculation can be done at any time, but the Seismic calculation could be performed only after Eigenfrequency calculation
280 )))
281 )))
282
IwonaBudny 1.1 283 == Calculation methods selection ==
284
IwonaBudny 28.2 285 (% style="text-align: justify;" %)
286 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).
287
288 (% style="text-align: justify;" %)
289 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.
290
IwonaBudny 31.2 291 (% style="text-align: justify;" %)
292 FEM-Design provides three types of calculation methods in harmony with EC8 and NS3491-12.
IwonaBudny 28.2 293
IwonaBudny 31.2 294 (% style="text-align: justify;" %)
295 [[image:1536241835290-794.png||height="105" width="297"]]
296
297 (% style="text-align: justify;" %)
298 These three methods really cover two basic concepts:
299
300 * Lateral force method, where the base shear force can be distributed in two ways (Static linear/mode shape),
301 * Modal response spectrum analysis (Modal analysis).
302
IwonaBudny 40.2 303 (% id="H1.Lateralforcemethod" %)
IwonaBudny 40.6 304 === Lateral force method ===
IwonaBudny 31.2 305
306 In some codes called equivalent static analysis.EC8 as well NS3491-12 uses this method. The user may not use this method in other codes.
307
308 This method can be used to calculate the seismic effect in horizontal plan, x' and/or y' direction. The main point of this method is to calculate base shear force taking into account the base vibration period and design spectrum in x' or y' direction which is distributed into those nodes of the structure where there are nodal masses. The base shear force formula is taken from EC8 4.3.3.2.2(1)P:
309
310 (% class="mark" %)F,,b,, = S,,d,,(T,,1,,) m λ
311
312 (((
313 (% style="text-align: justify;" %)
314 where:
315
316 * S,,d,,(T,,1,,) is the value of design spectrum at T1 (means the acceleration of the structure),
317 * T1 is the fundamental period of vibration of the building for lateral motion in the direction considered,
318 * m is the total mass of the building, above the foundation or above the top of a rigid basement. Remark: the FEM-Design always ta- kes into account the total mass of the structure including the base- ment,
319 * λ is the correction factor, the value of which is equal to: 0,85 if
320 * T^^1^^ ≤ 2 TC and the building has more than two storeys, or λ = 1,0 otherwise.
321 )))
322
323 (% style="text-align: justify;" %)
324 From this formula it can be seen that the base shear force is nothing else than the total seismic force of inertia (from second Newton’s law) which acts between the ground and the structure.
325
326 (((
327 (% class="box warningmessage" %)
328 (((
329 Remarks for NS3491-12:
330
331 * There is no λ (λ = 1,0).
332 * According to the code, if S,,d,,(T,,1,,) ≤ 0,5 m/s2, seismic analysis can be suspended, so when the above condition is fulfilled, it is not necessary to incorporate seismic loads in the design.
333 )))
334
335 (% style="text-align: justify;" %)
336 Distribution of the base shear force can occur in two ways which is described below.
IwonaBudny 40.2 337
IwonaBudny 31.2 338 )))
339
IwonaBudny 40.8 340 (% class="wikigeneratedid" id="HLineardistributionofhorizontalseismicforces28Static2Clinearshape29" %)
341 **Linear distribution of horizontal seismic forces (Static, linear shape)**
IwonaBudny 31.2 342
343 In this method the distribution of base shear force happens according to a simplified fundamental mode shape which is approximated by horizontal displacements that increased linearly along the height (see EC8 4.3.3.2.3(3)). The seismic action effects shall be determined by applying to the x' or y' direction. The horizontal forces are:
344
345 (% style="text-align: justify;" %)
346 [[image:1536242331763-898.png||height="51" width="116"]]
347
348 where:
349
350 * F,,b,, is the seismic base shear force,
351 * F,,i,, is the horizontal force acting on the place of mi,
352 * z,,i,,, z,,j,, are the heights of the masses m,,i,,, m,,j ,,above the foundation level.
353
IwonaBudny 31.3 354 According to NS3491-12 the distribution formula is:
IwonaBudny 31.2 355
IwonaBudny 40.2 356 (% class="MsoBodyText" style="margin-top:3.5pt; margin-right:5.2pt; margin-bottom:.0001pt; margin-left:18.0pt; text-align:justify; margin:0cm 0cm 0.0001pt 5.3pt" %)
357 [[image:1536570150242-670.png||height="50" width="123"]]
IwonaBudny 31.2 358
IwonaBudny 40.2 359 where:
IwonaBudny 31.2 360
IwonaBudny 40.2 361 * k = 1 for T,,1,, ≤ 0,5 sec
362 * k = 2 for T,,1,, ≥ 2,5 sec
IwonaBudny 31.3 363
IwonaBudny 40.2 364 In the 0,5-2,5 interval the value of the k is interpolated linearly.
IwonaBudny 31.3 365
IwonaBudny 40.2 366 (% style="text-align: justify;" %)
367 As a matter of fact eigenfrequency calculation is not necessary for this method, because giving the base period time in x' and y' direction is enough for the calculation. Practically, eigenfrequency calculation is performed before setting this data, but these data can be defined using experimental formulas as well. Investigation can be done in x' or y' direction, or both together.
IwonaBudny 31.3 368
IwonaBudny 40.2 369 (% style="text-align: justify;" %)
370 The user may set the calculation direction to be performed by selecting the desi- red direction. To set the desired x'-y' direction user should give the α angle (α is the angle between the global x and x'). α = 0,0 means x'-y' directions coincide with global x-y directions. More details can be found in Horizontal direction setting for seismic calculation to set the correct seismic effect direction (α).
371
372 (% style="text-align: justify;" %)
373 [[image:1536570274859-276.png||height="81" width="142"]]
374
375 (% class="box errormessage" style="text-align: justify;" %)
376 (((
377 Some limitations of this method:
378
379 * unusable if the whole foundation is not in the same plane,
380 * unusable if the horizontal foundation is elastic
381 )))
382
IwonaBudny 40.6 383 (% class="wikigeneratedid" id="H" %)
384 [[image:1536570428267-379.png||height="205" width="330"]]
IwonaBudny 40.2 385
386 (% style="text-align: justify;" %)
387 If any of the above mentioned situations happen, the static, mode shape or modal analysis should be used.
388
389
IwonaBudny 40.8 390 (% class="wikigeneratedid" id="HDistributionofseismicforcesaccordingtofundamentalmodeshapes28Static2Cmodeshape29" %)
391 **Distribution of seismic forces according to fundamental mode shapes (Static, mode shape)**
IwonaBudny 40.2 392
393 (% style="text-align: justify;" %)
394 In this method the distribution of base shear force happens according to the base vibration shape (see EC8 4.3.3.2.3(2)P). The horizontal forces acting on the place of mi are:
395
396 (% style="text-align: justify;" %)
397 [[image:1536570497215-808.png||height="62" width="122"]]
398
399 (% style="text-align: justify;" %)
400 where:
401
402 * s,,i,,, s,,j,, are the horizontal displacements of masses
403 * m,,i,,, m,,j,, in the fundamental mode shape.
404
405 (% style="text-align: justify;" %)
406 The following table shows how to select the base vibration shape. The table contains all mode shapes (No.), the vibration time (T(s)) and effective masses of the mode shapes in x' and y' directions (mx~(%) and my~(%)). As you can see the effective masses are given in a relative form to the total or reduced mass of the structure. The reduced mass means the total mass above the foundation or above the rigid basement. The value of the effective mass is referred to how the mode shape respond to a ground motion direction, so the effective mass shows the participation weight of the mode shape.
407
408 (% style="text-align: justify;" %)
409 It is recommended to select that mode shape which gives the largest effective mass as the fundamental mode shape. The method allows to Select one mode shape in x´ or/and y´ direction(s).
410
411 (% style="text-align: justify;" %)
412 [[image:1536570577597-259.png||height="191" width="233"]]
413
414
415 (% class="box warningmessage" style="text-align: justify;" %)
416 (((
417 Remarks:
418
IwonaBudny 40.6 419 * The calculation of base shear force is performed according to the total mass of the structure and not the effective mass, as was introduced earlier in** Lateral force method**.
IwonaBudny 40.2 420 )))
421
422
IwonaBudny 40.11 423 === Modal response spectrum analysis ===
IwonaBudny 40.7 424
IwonaBudny 40.2 425 (% style="text-align: justify;" %)
IwonaBudny 40.6 426 This method can be used in all national codes.
IwonaBudny 40.2 427
428 (% style="text-align: justify;" %)
IwonaBudny 40.6 429 The essence of the method is the calculation of the structural response for different ground motions by the sufficient summation of more vibration shapes. Method gives possibility to take into account full x, y and z direction investigation. In the table below, more vibration mode shape could be selected in x', y' and z' directions if necessary. The last row of the table shows that in a given ground motion direction how large is the sum of the considered effective masses compared to the total or reduced mass of the structure.
430
431 (% style="text-align: justify;" %)
IwonaBudny 40.2 432 According to EC8 4.3.3.3.1(3) and NS3491-12 sum of the effective mass of the chosen mode shapes - at least in horizontal direction - should reach 90% of total mass. Additionally every mode shape has to be taken into account which effective mass is larger than 5%.
433
434 (% style="text-align: justify;" %)
435 [[image:1536570931469-309.png||height="183" width="222"]]
436
437 (% class="box warningmessage" %)
438 (((
439 Remarks:
440
441 * If the sum of the effective mass is much smaller than 90%, eigenfrequency calculation should be done for more shapes in order to reach 90%.
442 * In vertical direction lots of mode shapes should be ensured to reach the 90% of total mass; highly recommended to check the national code whether it is necessary to examine the vertical effect.
443 * (((
444 The mode shapes which have small effective mass may be neglected, because their effect in result is very small but the calculation time increases.
445 )))
446 )))
447
448 According to the EC8 and NS3491-12 the summation rule in the individual directions can be selected in the lower part of the seismic analysis setup dialog. In all other codes there is no possibility to choose, the SRSS rule is used for summation. According to EC8 4.3.3.3.2, the summation rule possibilities are the following:
449
450 (% class="wikigeneratedid" %)
451 [[image:1536570957782-355.png||height="54" width="141"]]
452
453 (% class="wikigeneratedid" %)
454 where:
455
456 * E,,E,, is the seismic action effect under consideration (force, displacement, etc.),
457 * E,,Ei,, is the value of this seismic action effect due to the vibration mode i,
458 * r,,ij,, is the interaction between two vibration periods taking into ac- count the declining ratio:
459
460 == [[image:1536571049986-697.png||height="63" width="395"]] ==
461
462 (% style="text-align: justify;" %)
463 The **CQC **(Complete Quadratic Combination) summation rule might be adopted when individual direction, two vibration modes are dependent to each other if they satisfy the following condition:
464
465 T,,j ,,/ T,,i ,,> 0,9 with T,,j ,,≤ T,,i,,
466
467
468 FEM-Design always applies the selected rule for the summation except if the **Automatic **is highlighted. If the **Automatic **is selected then the rule selection procedure is as follows:
469
470 * (((
471 (% style="text-align: justify;" %)
472 Always three directions (if there were more than one mode shape selected in that column) is investigated weather all mode shapes are independent from each other or not.
473 )))
474 * (((
475 (% style="text-align: justify;" %)
476 If at least one dependent situation exists in a direction, the program automatically uses the CQC rule for all mode shapes in that direction, otherwise SRSS rule is used.
477 )))
478
479
IwonaBudny 1.1 480 == Other setting possibilities ==
481
IwonaBudny 40.11 482 === Horizontal direction setting ===
IwonaBudny 28.2 483
484 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.
485
486 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.
487
488 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%.
489
490 (% style="margin-top:.15pt; margin:0cm 0cm 0.0001pt" %)
491 [[image:1536241385931-685.png||height="68" width="345"]]
492
493
494 (% style="text-align: justify;" %)
495 Of course this also can be reached if the user rotates the whole geometry with a specified angle.
496
497
IwonaBudny 40.3 498 === Effective mass setting ===
IwonaBudny 28.2 499
500 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.
501
502 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.
503
504 (% style="text-align: justify;" %)
505 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.
506
507 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.
IwonaBudny 40.6 508
IwonaBudny 28.2 509
IwonaBudny 1.1 510 == Combination rule, rotation and second order effects ==
511
IwonaBudny 27.2 512 (% style="text-align: justify;" %)
513 According to EC8 4.3.3.5, the combination rule of x', y' and maybe Z direction effects, namely the seismic calculation of final results (Seismic max.), can be selected from the following two possibilities:
514
515 [[image:1536240253862-277.png||height="109" width="120"]]
516
517 (% style="text-align: justify;" %)
518 The first rule which is called SRSS is implemented to all the other codes than EC8 and NS3491-12 and there is no possibility for rule selection.
519
IwonaBudny 40.3 520 (% id="HTorsionaleffect" %)
IwonaBudny 40.6 521 === Torsional effect ===
IwonaBudny 27.2 522
523 (% style="text-align: justify;" %)
524 According to EC8 4.3.2 the program gives possibility to take into account the accidental mass distribution of the structure by the calculation of the torsional effect. This means that from the horizontal seismic forces a Z directional torsional moment can be calculated according to EC8 4.3.3.3.3 (EC8 4.17 equation) as follows:
525
526 (((
527 (% style="text-align: justify;" %)
528 (% class="mark" %)M,,ai ,,= e,,ai,, F,,i,,
529
530 (% style="text-align: justify;" %)
531 where:
532
533 * M,,ai,, is the torsional moment applied at the mi point about the vertical axis,
534 * e,,ai,, is the accidental eccentricity of mass i in accordance with expression (EC8 4.3 formulas) for all relevant directions:
535
536 (% style="text-align: justify;" %)
537 (% class="mark" %)e,,ai,, = ± 0,05 L,,i,,
538 )))
539
540 * L,,i,, is the floor-dimension perpendicular to the direction of seismic action (Lx',i or Ly',i),
541 * F,,i,, is the horizontal force acting on the place of mi in x' or y' direction, when static method is used. In the modal analysis, this force is calculated, selecting the mode shape which gives the largest effective mass (fundamental shape). Using this mode shape this force is calculated according to static, mode shape. So, the total mass and not the effective mass of the structure is taken into account which belongs to this fundamental mode shape.
542
543 The explanation of the floor-dimension (L,,x',i ,,and L,,y',i,,) on the ith storey:
544
545 [[image:1536240529728-753.png||height="244" width="202"]] [[image:1536240543701-430.png||height="245" width="133"]]
546
547
548 (% class="box warningmessage" style="text-align: justify;" %)
549 (((
550 Remarks:
551
552 * To calculate the torsional effect, storey(s) should be defined.
553 * The accidental eccentricity of the masses which are not laid on the storey will be considered on the nearest storey’s eccentricity.
554 )))
555
556 (% style="text-align: justify;" %)
557 It was seen that the influence of uncertainties of mass position was modeled by the rotation effect. According to our experiment using the FE method, when a plate, a wall and beams are divided into several elements the accidental torsional effect is not reasonable.
558
IwonaBudny 40.3 559 (% id="HSecond-ordereffects28P-2206effects29" %)
IwonaBudny 40.6 560 === Second-order effects (P-∆ effects) ===
IwonaBudny 27.2 561
562 (% style="text-align: justify;" %)
563 Only EC8 gives a possibility to calculate the second order effect which is done according to 4.4.2.2(2). The second order effect is ignored if the following condition is fulfilled in all storeys and all horizontal directions:
564
565 (% style="text-align: justify;" %)
566 [[image:1536240719142-621.png||height="56" width="151"]]
567
568 (% style="text-align: justify;" %)
569 where:
570
571 * θ is the interstorey drift sensitivity coefficient,
572 * P,,tot,, is the total gravity load at and above the storey considered in the seismic design situation. Remark: this total gravity load is calculated back from the nodal masses.
573 * d,,r,, is the design interstorey drift, evaluated as the difference of the average lateral displacements ds (see Displacement calculation) at the top and bottom of the storey under consideration and calculated in accordance with EC8 4.3.4,
574 * V,,tot,, is the total seismic storey shear force,
575 * h is the interstorey height.
576
577 If 0,1 < θ ≤ 0,2, the second order effect is taken into account by multiplying the relevant seismic action effects (the internal and reaction forces) by a factor equal to 1/(1-θ).
578
579 [[image:1536240962810-689.png||height="218" width="186"]]
580
581 (% style="text-align: justify;" %)
582 According to EC8 4.4.2.2(4)P the θ coefficient shall not exceed 0,3. When θ >0.3, FEM-Design sends a warning message and continues the calculation using θ = 0,0.
583
584 (% style="text-align: justify;" %)
585 The 0,2-0,3 interval is missing in EC8. In this case FEM-Design sends a warning message and continues the calculation using calculated θ.
IwonaBudny 27.3 586
IwonaBudny 27.2 587
IwonaBudny 27.3 588 (% class="box warningmessage" style="text-align: justify;" %)
IwonaBudny 27.2 589 (((
IwonaBudny 27.3 590 Remarks:
591
592 * To calculate thesecond order effect, storey(s) should be defined.
IwonaBudny 27.2 593 )))
594
IwonaBudny 40.9 595 == Displacement calculation ==
IwonaBudny 40.3 596
IwonaBudny 21.2 597 (% style="text-align: justify;" %)
598 The displacement calculation is made according to EC8 4.3.4 using the following formula:
599
600 (% style="text-align: justify;" %)
601 (% class="mark" %)d,,s ,,= q,,d,, d,,e,,
602
603 (% style="text-align: justify;" %)
604 where:
605
606 * d,,s,, is the displacement of a point of the structural system induced by the design seismic action,
607 * qd is the displacement behavior factor, assumed equal to q unless otherwise specified,
608 * d,,e,, is the displacement of the same point of the structural system, as determined by a linear analysis based on the design response spec- trum.
609
610 (% style="text-align: justify;" %)
611 FEM-Design uses the above formula only to calculate the summarized and combined the so called final results displacements. The displacements obtained from the single shapes and torsional effects won’t be modified.
612
IwonaBudny 1.1 613 ----
614
IwonaBudny 17.6 615 = **The results of seismic calculation** =
IwonaBudny 2.1 616
IwonaBudny 21.2 617 (% style="text-align: justify;" %)
618 The seismic results are very similar to statical results with some more items as follow: equivalent seismic forces and base shear force. The results shown separately from mode shapes, torsional effects, sum of the directions (Sum, x'…) and the final results (Seismic max.).
IwonaBudny 2.1 619
IwonaBudny 21.2 620 (% style="text-align: justify;" %)
621 Desired results can be selected from the result dialog as it is shown below. Among the equivalent load results not only the nodal forces can be seen but also the base shear force, and in case of torsional effect the total torsional moment as well.
622
623 (% style="text-align: justify;" %)
624 [[image:1536239899693-361.png||height="181" width="272"]]
625
626 (% class="box warningmessage" style="text-align: justify;" %)
627 (((
628 Because of the summation rule, the summarized values by direction and the full combinations give only positive values, namely these results means the maximum envelope. Because of the summation rule, none of the displacement components in a node and none of the member’s internal forces are not simultaneous.
629 )))
630
IwonaBudny 2.1 631 ----
632
IwonaBudny 17.6 633 = **Summation of static and seismic effects** =
IwonaBudny 2.1 634
IwonaBudny 20.2 635 (% style="text-align: justify;" %)
636 The seismic effect’s results can be considered together with static effects in two ways:
IwonaBudny 2.1 637
IwonaBudny 20.2 638 * seismic forces applied as real static forces in load cases,
639 * the final results can be combined with a static load combination or taking into account in load groups.
640
IwonaBudny 2.1 641 == Seismic loads in static load cases ==
642
IwonaBudny 20.2 643 (((
644 (% style="text-align: justify;" %)
645 Horizontal seismic forces and torsional effects which were calculated using the static method according to EC8 or NS3491-12 additionally can be added to the static load cases. However it is recommended to have the seismic forces in separate load case(s) in order not to mix up them with the normal static loads.
646
647 (% style="text-align: justify;" %)
648 [[image:1536239563646-414.png||height="169" width="264"]]
649
650 (% style="text-align: justify;" %)
IwonaBudny 20.4 651 In the static calculations, load cases which contain seismic forces behave like the other normal static forces. Consequently they can be inserted in load combinations and load groups. If they are inserted in the load combinations then they can take part in the imperfection and stability analysis. Of course there is no possibility to convert the masses from these load cases. As it can be seen in the table above, this effect can be taken into account by positive or negative sign as well, because the seismic effect means vibration between +/- extreme values, but the results are shown only in positive direction for the sake of simplicity.
652
653 (% style="text-align: justify;" %)
IwonaBudny 40.10 654 As it is shown above all the seismic possible cases can be found in the list but only those cases are valid which were calculated in seismic calculation. The calculated static loads from seismic effect can be found among the seismic results in the **Equivalent loads**.
IwonaBudny 20.2 655 )))
656
IwonaBudny 40.10 657 == Final results of seismic effect in load combination ==
IwonaBudny 2.1 658
IwonaBudny 20.2 659 Final results of seismic effect (Seismic max.) always obtained from the total summation of all components. These results which actually means extreme +/- values, can be added to a load combination as a special load case with arbitrary factor and it can be applied in all codes.
660
661 [[image:1536239630246-104.png||height="62" width="298"]]
662
663 (((
664 (% style="text-align: justify;" %)
IwonaBudny 40.10 665 The combination of the Seismic max. and the other static loads results is calculated in a special way.
IwonaBudny 20.2 666
667 (% class="box warningmessage" style="text-align: justify;" %)
668 (((
IwonaBudny 40.10 669 Because the results of the seismic effect are always positive and individual components (e.g. N, My and Mz internal forces in bar) are not simultaneous FEM-Design takes action as follow: all components of the seismic results are added to the components of static results with the sign of static component.
IwonaBudny 20.2 670 )))
671 )))
672
IwonaBudny 40.10 673 == Final results of seismic effect in load groups ==
IwonaBudny 2.1 674
IwonaBudny 20.2 675 (% style="text-align: justify;" %)
IwonaBudny 20.3 676 Final results of seismic effect (Seismic max.) can be applied in load groups in all codes. Using EC8 and NS3491-12 program give possibility to have Seismic load type beside Permanent, Temporary and Accidental load type. In all other codes it is recommended to apply the Seismic load type in the Accidental load type.
IwonaBudny 20.2 677
678 (% style="text-align: justify;" %)
IwonaBudny 20.3 679 The final results of seismic effect take part with +/- sign automatically in the load group combination.
680
681 (% style="text-align: justify;" %)
IwonaBudny 20.2 682 [[image:1536239721199-472.png||height="59" width="249"]]
683
IwonaBudny 2.1 684 ----
685
IwonaBudny 17.6 686 = **Useful tips, which method to use?** =
IwonaBudny 2.1 687
IwonaBudny 17.7 688 (((
689 (% style="text-align: justify;" %)
IwonaBudny 17.8 690 It is hard to answer this question, even for experienced engineers. However, some basic concept can be formulated:
IwonaBudny 17.7 691
692 (% class="box successmessage" %)
693 (((
694 Before any decision, always run the Eigenfrequencies calculation. From these results you will experience how the structure behaves in aspect of dynamics.
695
696 Always check the effective masses, if you calculate the structure for seismic effect in the first occasion or make changes in the geometry or in the mass distribution.
697
698 If you see that the effective masses shows larger value than 100% com- pare them to the Total mass and not to the Reduced mass.
699
700 If the sum of the selected effective masses is less than the prescribed minimum (in EC8 this is 90%), calculate more mode shape.
701
702 If the sum of effective masses in case of large number mode shapes doesn’t approach to the prescribed value use the static, linear shape or static, mode shape if the code allow.
703
704 If the building has large importance or it has special geometry try to apply the modal analysis.
705
706 If the building is not too high, any of the static methods will give reliable results avoiding longer run time calculation in eigenfrequencies.
707
708 It is not always necessary to analyze the 3D model in all directions, sometimes one or two planar model is enough.
709 )))
710 )))
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