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+---- Thread: ETABS (/thread-167.html)
RE: ETABS - essenza - 08-22-2009
(08-22-2009, 10:37 AM)fashion Wrote: essenza, thanks for your answer, the main reason for my question is to know the fund. period of the structure so as to manually calculate the seismic forces (i.e. base shear) so as to hand check the results from etabs. I know i can aproximate the period with the formulas, but eigenmode analysis is kind of precise.
whoa...stop at that, eigen analysis in finite element is approximate..the finite element itself is approximate. Most of the shape function for finite elements is polynomial which only satisfy deformation of the element under static load. In vibration problem, the exact shape function is usually trancendent, that's why if you model a structure using finite element for vibration problem (eigen analysis etc), all structural analysis program requires meshing to capture a "correct" mode shape (even after meshing it is still approximate:P). You may wonder so why we never mesh or divide frame member (beam, column etc) in highrise building structure, well don't get it wrong you actually mesh it with the floor, imagine your structure as a big cantilever tower. People who study fluid, aerodynamics and buckling especially for aerospace industry that i know only use finite element as comparison. They use exact vibration program, one of them called VICONOPT and being used by NASA to analyze exact vibration problem. You may now understand why finite element researchers compete to find the finite element "holy grail" either creating adaptive meshing or increase the polynomial degree in the shape function or number of dof in the element because they need to cope with vibration problem.
The only problem with exact vibration is the goddamn mathematic while with finite element is of course computer speed and storage because the solver for large number of equation is already available. I'm not an expert in either of these two method but i know there are war between this exact and approximate method in journal/paper.
RE: ETABS - fashion - 08-23-2009
essenza, I think what your r saying applies more to surface f.e. rather than to linear ones. While, for a simple frame structure, you (or the software) form the stifness matrix, it is easy to and straightforward to compute the eigenvalues of the matrix by simple algebra, and find the eigenmodes, for a structure modeled with shear walls (i.e. surface f.e.) I agree that eigenmode analysis is aproximate. I will try to find some papers about the issue
RE: ETABS - essenza - 08-23-2009
(08-23-2009, 11:11 AM)fashion Wrote: essenza, I think what your r saying applies more to surface f.e. rather than to linear ones. While, for a simple frame structure, you (or the software) form the stifness matrix, it is easy to and straightforward to compute the eigenvalues of the matrix by simple algebra, and find the eigenmodes, for a structure modeled with shear walls (i.e. surface f.e.) I agree that eigenmode analysis is aproximate. I will try to find some papers about the issue
there where you got it wrong. Stiffness matrix in finite element (frame, surface,solid) that we know is static stiffness matrix. Try to find out about dynamic stiffness matrix, you will not find that 12EI/L3, EA/L, 6EI/L2 etc :D, because now it is frequency dependent or in mathematical form it is trancendental.
RE: ETABS - fashion - 08-24-2009
yes of course is static and of course trying to model earthquake as a static load or a dynamic load under static analysis is a bit aproximate, but what else can you practically do?
RE: ETABS - essenza - 08-24-2009
(08-24-2009, 01:14 PM)fashion Wrote: yes of course is static and of course trying to model earthquake as a static load or a dynamic load under static analysis is a bit aproximate, but what else can you practically do?
Oh no i think you don;t understand what i mean, dynamic load under static analysis I just get confuse:(
RE: ETABS - engr1900 - 08-24-2009
Hy
what is differece between shell and membrane in Etabs, how are the wall load, 2way slab and beam loads tranfered in etabs, how to chek manually
bye
RE: ETABS - fashion - 08-24-2009
(08-24-2009, 01:19 PM)essenza Wrote:(08-24-2009, 01:14 PM)fashion Wrote: yes of course is static and of course trying to model earthquake as a static load or a dynamic load under static analysis is a bit aproximate, but what else can you practically do?
Oh no i think you don;t understand what i mean, dynamic load under static analysis I just get confuse:(
you mean that the formation of the stifness matrix refers to static only analysis?
RE: ETABS - techno - 08-27-2009
can anyone explain more about stiffness modifier?
which structural elements do we need to make changes? what value is recommended?
RE: ETABS - aslam - 08-27-2009
(1) If we assign some stories as "similar stories" then reinforcement requirement for beams in such stories will be displayed same eventhough it is different?
(2) I want to flush columns with beam face using "cardinal point". At a paticular node(point) beam width is oriented centrally. When i flush columns using "cardinal point", column is flushed with node and not with beam face. To flush beam and column i need to offset beam by half beam width. it is too labourous. Is there any simple way?
Manual input of offset by a value will be exact but when column size is reducing towards top it is difficult to give offset to all columns.
(3) I have 300 x 1200 mm column with two beams falling at different locations on columns. Actually column is defined at a point. So only one beam meets column at a point, other beam which is actually falling on this column will fall on beam suppored on this column. How rigidity can be achieved between these two points on column where beams are falling. people use master-slave option in STTAD. Is there any option in etabs?
(4) Many seismic codes states that any mass in between two floor leves (say brick masonry, column mass) shall be equally lumped to two connected floors (levels). To follow this codal provision, how brick masonry mass shall be modeled?
By default, how etabs is lumping masses between two levels to defined levels?
RE: ETABS - essenza - 08-29-2009
(08-24-2009, 04:23 PM)fashion Wrote:(08-24-2009, 01:19 PM)essenza Wrote:(08-24-2009, 01:14 PM)fashion Wrote: yes of course is static and of course trying to model earthquake as a static load or a dynamic load under static analysis is a bit aproximate, but what else can you practically do?
Oh no i think you don;t understand what i mean, dynamic load under static analysis I just get confuse:(
you mean that the formation of the stifness matrix refers to static only analysis?
no there are two different approach for vibration analysis which have different stiffness matrix. Just be careful if you are using the one used in ETABS, SAp and all those structural analysis software unless you understand the problem at hand.