Changes for page Analysis
Last modified by Iwona Budny Bjergø on 2022/01/18 12:23
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edited by Fredrik Lagerström
on 2020/03/27 11:08
on 2020/03/27 11:08
edited by Fredrik Lagerström
on 2020/03/27 11:13
on 2020/03/27 11:13
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... ... @@ 742,10 +742,13 @@ 742 742 A national code always provides which seismic calculation method has to be performed for different structure, where and when it should be performed and what other effects to be considered (e.g. torsional effect, P∆ effect). //FEMDesign //provides three types of calculation methods (depending on the applied code): 743 743 [[image:1585303002015960.png]] 744 744 Figure: Settings of seismic analysis 745 ))) 746 746 747 * **Static, linear shape** 748 As a matter of fact, eigenfrequency calculation is not necessary for this method, because giving the base period time in //x’// and //y’// directions (//Tx’// and //Ty’//) is enough for the calculation. Practically, eigenfrequency calculation performs 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.((( 746 +* 747 + 748 +**Static, linear shape** 749 + 750 +As a matter of fact, eigenfrequency calculation is not necessary for this method, because giving the base period time in //x’// and //y’// directions (//Tx’// and //Ty’//) is enough for the calculation. Practically, eigenfrequency calculation performs 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. 751 + 749 749 (% style="width:700px" %) 750 750 (% style="width:225px" %)((( 751 751 [[image:1585303078921407.png]] ... ... @@ 752,9 +752,14 @@ 752 752 )))(% style="width:472px" %)You may set the calculation direction to be performed by selecting the desired direction. To set the desired //x’y’// direction, you should give //Alfa// (alfa is the angle between the global //X// and //x’//; see (% class="wikiinternallink" %)**Direction of the horizontal effect**(%%)). =0.0 means //x’y’// directions coincide with global //XY// directions. 753 753 754 754 (% style="width:107px" %)[[image:warning.png]](% style="width:1383px" %)This method is unusable, if the whole foundation is not in same plane or the horizontal foundation is elastic. In these cases, //Static, mode shape// or //Modal analysis// should be used. 755 ))) 756 * **Static, mode shape** 757 In this method the distribution of “base shear force” happens according to fundamental mode shapes (base vibration shapes).((( 758 + 759 + 760 + 761 +* 762 + 763 +**Static, mode shape** 764 +In this method the distribution of “base shear force” happens according to fundamental mode shapes (base vibration shapes). 765 + 758 758 (% style="width:826px" %) 759 759 (% style="width:258px" %)((( 760 760 [[image:1585303183304641.png]] ... ... @@ 766,12 +766,17 @@ 766 766 (% style="width:95px" %)[[image:light.png]](% style="width:1355px" %)It is recommended to select that mode shape which gives the largest effective mass as the fundamental mode shape. 767 767 768 768 (% style="width:107px" %)[[image:warning.png]](% style="width:1383px" %)The calculation of “base shear force” is performed according to the total mass of the structure and not to the effective mass. 769 ))) 770 * **Modal analysis** 771 \\[[image:1585303301268646.png]] 777 + 778 + 779 + 780 +* 781 + 782 +**Modal analysis** 783 +[[image:1585303301268646.png]] 772 772 \\The essence of the methodis 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// (=global //Z//) direction investigation. 773 773 \\In the table, more vibration mode shape could be selected in //x’//, //y’// and //z //directions if necessary. The last row (orange cells) of the table shows that how large is the sum of the considered effective masses compared to the total or reduced mass of the structure in a given ground motion direction. 774  ((( 786 + 787 + 775 775 (% style="width:823px" %) 776 776 (% style="width:95px" %)[[image:light.png]](% style="width:725px" %)According to //EC8//, sum of the effective mass of the choosen mode shapes (at least in horizontal direction) should reach 90% of total mass. Additionally every mode shape has to be taken into account where effective mass is larger than 5%. 777 777 ... ... @@ 782,10 +782,12 @@ 782 782 783 783 The mode shapes which have small effective mass may be neglected, because their effect in result is very small, but calculation time increases. 784 784 ))) 785 ))) 786 786 787  ===**Summation rule by directions** ===799 + 788 788 801 +* 802 + 803 +**Summation rule by directions** 789 789 According to the //EC8//, the summation rule in the individual directions can be selected. In all other codes always the //SRSS// rule is used for summation (there is no choice). Read more about //SRSS// and //CQC// summation rules in //Theory book//. If the //Automatic// is selected, the rule selection procedure is as follow: 790 790 791 791 (% style="width:843px" %) ... ... @@ 798,12 +798,11 @@ 798 798  If at least one dependent situation exists in a direction, the program automatically uses the //CQC// rule for all mode shape in that direction, otherwise //SRSS// rule is used. 799 799 ))) 800 800 801 === **Direction of the horizontal effect** === 802  816 +* **Direction of the horizontal effect** 803 803 Codes generally speak about seismic calculation in //XY// directions. These directions give the maximum effect, if the mass and elastic properties of the structure ensure that the calculated mode shapes lay in //XZ// or //YZ// plane. But it is not always achieved in practice. 818 +\\To achieve the unfavorable direction, where the results of ground motion are maximum, the program gives the possibility to set //x'////y'// direction for the seismic horizontal effect (//Alpha//). The program suggests the //Alfa// value, if you click on //Auto// button. It finds the most unfavorable direction, where any of the //mx’// and //my’// is zero and the other is maximum in the same row (same shape). But, there is a rule: the direction can be ensured only for one mode shape, so the program selects the row where the effective mass is the maximum. If manually definition is chosen, give an angle for //Alfa// and press the button //Set//. 819 + 804 804 805 To achieve the unfavorable direction, where the results of ground motion are maximum, the program gives the possibility to set //x'////y'// direction for the seismic horizontal effect (//Alpha//). The program suggests the //Alfa// value, if you click on //Auto// button. It finds the most unfavorable direction, where any of the //mx’// and //my’// is zero and the other is maximum in the same row (same shape). But, there is a rule: the direction can be ensured only for one mode shape, so the program selects the row where the effective mass is the maximum. If manually definition is chosen, give an angle for //Alfa// and press the button //Set//. 806  807 807 [[image:1585303554467548.png]]((( 808 808 On the left hand side figure you can see a badly adjusted //x’y’// direction (//Alpha = 0//). Appling //Auto// button, the program arranges the direction for the 58.5% effective mass //my’// and correct it to 78%. 809 809 ... ... @@ 811,7 +811,13 @@ 811 811 812 812 Figure: Settings of Alpha 813 813 ))) 828 +))) 814 814 830 + 831 + ((( 832 + 833 +))) 834 + 815 815 === **Effective mass** === 816 816 817 817 The modal effective masses can be compared to the total mass or reduced mass at //Eff. mass//: ... ... @@ 820,12 +820,11 @@ 820 820 [[image:1585303685867313.png]] 821 821 )))(% style="width:1362px" %)In //FEMDesign// “//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 (set at //Seismic load/ Others//). 822 822 843 +(% style="width:107px" %)[[image:warning.png]](% style="width:1383px" %)((( 844 +//EC8// defines the total mass without basement (//Reduced mass//). The effective masses are generally compared to the //Reduced mass//, but this is not valid for the massive basement with elastic foundation. If the above mentioned situation is the case, it might happen that the sum of the effective masses of a column is larger than the 100%. 823 823 824 In //FEMDesign// “//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 (set at //Seismic load/ Others//). 825  826 **[[image:file:///C:/Users/Fredrik/AppData/Local/Temp/msohtmlclip1/01/clip_image006.wmzalt="MCj04113200000%5b1%5d"]] **//EC8// defines the total mass without basement (//Reduced mass//). The effective masses are generally compared to the //Reduced mass//, but this is not valid for the massive basement with elastic foundation. If the above mentioned situation is the case, it might happen that the sum of the effective masses of a column is larger than the 100%. 827  828 828 It is uninterested in 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, that which mode shape is the fundamental or which shapes are dominant in a given direction. 847 +))) 829 829 830 830 At //Options//, more calculation properties can be set: 831 831