problem in analysis with zero displacements .

[achillesgr]

hi

i am new robot user and i have some problems trying to aanalyse a simple bar.here is the case:

i have a simple bar with two suports.one end is pinned (x and z ) and the other end is supported by a roller(there is a  x axis freedom and restrained in the z axis ).also a uniform load is presented.

the problem is that after calculations i do not see any x axis movement at the roller support end.even if i input an extreme load the xx displacement at the roller is 0,0
 .i see the bar deformed but no movement at the roller.

another strange situation i have if i convert bothsupports to pinned in both axes x and z. i would expect an Fx reaction in the supports as the bar bents downwards but i get 0.0 again.

what am i doing wrong?

can someone please help?

[Pawel.Pulak]

You observe such behavior because probably you have run static linear analysis for the load case with uniform perpendicular load.

It is necessary to consider 2nd and 3rd order geometrical non-linear effects to obtain "coupling" between perpendicular load of member and its longitudinal displacement or force:

Moreover to see such coupling the member should be relatively slender.

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Best regards,

[achillesgr]

hi,

i changed the settings per your suggestions but still the same . i am sending the project in atachment .

please take a look what is going wrong

thanks!!

[Rafal.Gaweda]

You have to make more calculation elements along this bar

Model attached.

[achillesgr]

Ok. that worked fine!

if i assume that this is the uper beam of a frame with two columns so instead of the supports  there are two columns, what buckling parameters should i input?

please consider the folowing :

1.there are 9 purlins on TOP of the beam at the nodes .

2.the beam to column is moment connections

3.there is one L that suports the lower flange to the midle purlin.

should i check the lateral buckling?

the X option at the buckling parameters means that the member will not be checked?

thanks!!

[Rafal.Gaweda]

should i check the lateral buckling?

What code and your experience says?

the X option at the buckling parameters means that the member will not be checked?

NO , it means bickling will not be checked.

[achillesgr]

Rafal,

 i am talking about Eurocode checking.

steel desighn is new for me and do not know where to start from.

what members should i check for buckling or lateral buckling?

i have 3x  two span portal with  5 purlins per span.

should i check the whole structure for buckling ?( purlins, beams and columns?

or only the suported ones?

for example the purlins are unsuported with 5meter length , should i check them in buckling?maybe in both buckling and lateral buckling?

can you please give me an idea what members do i need to check and how?

if a beam have a very small axial load, is this enouth to force me to check for buckling?

from what i have read till now , if a member do ot have axial load then it is not necesary a buckling check.

i am attaching a file to please take a look if you have time.it is a simple structure. please take a look at the bar types and buckling settings and the suports .

thanks again

[achillesgr]

Rafal,

this is the structure i am talking about.

the columns are 6m height and have z bukling suports at midle and the y buckling is set to AUTO. no lateral?

the uper beams have have z bukling suports at the purlin connections and the y buckling is set to AUTO. for lateral i have set a lower flange suport at midspan, right?

thanks

[Rafal.Gaweda]

My proposal in attached file.

Either bar 3 as a separate bar (Beam Upper  bar type) or as super-member 87 (bar 3+4) and bar type Beam Upper 34.

Anybody else can look?

Tony?

[achillesgr]

Rafal ,

in the case of the beam uper member why do you take the buck Y coef =1.4?

in the case of the supermember beam uper34 member why do you take the buck Y coef =0.7? the 0.7 is for fixed-pined bar , why do you assign this? there are two columns with moment connection type, shouldn t you asighn the fixed -fixed type 0.5?

the automatic buckling works only for columns?

how do i make the two bars supermember? i cannot finf any command to connect bars.

also, there are some offsets in the real model, for example the purlins are located at the upper flange of the beams. should i consider this and move somehow the purlins in robot to their real position?

i have the sme question in the case of rc beam and slab. should i model the slab upon the beam or at the centerline?

how should i treat these offsets in robot?

thanks!!!

[Rafal.Gaweda · Accepted solution]

in the case of the beam uper member why do you take the buck Y coef =1.4?

Whole rafter acts as a one member (3+4) so in case of designing member 3 or 4 separately you should take double lenght for buckling so 0.7 (for single) * 2 (to double)

in the case of the supermember beam uper34 member why do you take the buck Y coef =0.7?

the 0.7 is for fixed-pined bar , why do you assign this?

0.5 is for fully fixed connections but in fact beam to beam or to column is not super rigid so is it better to take 0.7. But the choice is yours. You sign the project.

there are two columns with moment connection type, shouldn t you asighn the fixed -fixed type 0.5?

Your choice 

the automatic buckling works only for columns?

for all members 

how do i make the two bars supermember? i cannot finf any command to connect bars.

movie: http://screencast.com/t/Xo7R3h5z3

also, there are some offsets in the real model, for example the purlins are located at the upper flange of the beams. should i consider this and move somehow the purlins in robot to their real position?

i have the sme question in the case of rc beam and slab. should i model the slab upon the beam or at the centerline?

Your choice, look here first:

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Beam-offsets/m-p/3332921/highlight/true#M276...

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Consequence-of-axis-position-offset-excentre...

[achillesgr]

Thanks Rafal,

you helped  a lot!!