unbalanced forces

[achillesgr]

hello,

please someone take a look at this model. it is a simple frame with short column that sits upon a concrete column.

i have barckets and offsets .

why i do not have the forces balanced?

i mean why the right side of beam shows My=25.48 and the column top My=39.41?

thanks. i apreciate any help

[Artur.Kosakowski]

If you used offset add axial force * offset value into the equation.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[achillesgr]

i changed the second frame to NO offset and i get these results.

the My went up tp 22.51 from 10.00 and the forces are balanced.

is it OK?

also what is the purpose of brackets? display only ?

i am asking this because i was expecting to see MY= -26.40 as support reaction because the column is only 66cm (IPE330+bracket only) and i think the whole assembly at the support worts as rigid.

my concern are the support  forces becouse i need those to calculate the concrete model with is in another file.

So what are the support reactions for the concrete medel ? MY=-26.40  My=-22.51 or the ones with the ofsett (see the rest frames)  My=-90.93?

thanks

[Artur.Kosakowski · Accepted solution]

Personally I don't like to use offsets as they influence bending moment values. A part of the bending you have for a bar without an offsets is 'hidden' in an additional axial force that you will get in the bar.  If you need to 'move' bar element from the positions of their end nodes you may consider using rigid links instead.

If you use a bracket you replace the original I section of a bar with a tapered one along the length of the bracket.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[achillesgr]

"

If you use a bracket you replace the original I section of a bar with a tapered one along the length of the bracket."

is that take into accout when the robot calculates a bar under buckling or lateral b?I mean that the max momment isnt the one at the end but something less.

OK. offsets are disaster for the main frame. what about the purlins ?do you use them in robot calculations or only at the detailing softaware?

what about multistory buildings ? do you use ofsets at the beams?it is not dificult to settup the model at the x axis of beams but when you need to change the section then  there is a problem.

ahat do you do then?

thanks!

[Artur.Kosakowski]

is that take into accout when the robot calculates a bar under buckling or lateral b?I mean that the max momment isnt the one at the end but something less.

The steel design module of Robot recognizes brackets.

OK. offsets are disaster for the main frame. what about the purlins ?do you use them in robot calculations or only at the detailing softaware?

what about multistory buildings ? do you use ofsets at the beams?it is not dificult to settup the model at the x axis of beams but when you need to change the section then  there is a problem.

ahat do you do then?

As I wrote I don't. I prefer rigid links. smileyhappy: 

[Tuctas]

"The steel design module of Robot recognizes brackets."  

This true, but there are some assumptions used in Robot in such case that are not "obvious" and the user has to be careful...(for instance, it is not obvious that the results will be more favorable by placing brackets comparing with the results obtained without using brackets and this is due to the assumption of the lateral buckling curve that is automatically used in Robot for calculating the slenderness Lam_LT in which the recommended values of table 6.4 of EC3 for welded sections are used, that sometimes aren't rational because of the specific local formulation of brackets and then the user should define manually the lateral buckling curve). 

About the offsets in Robot, my personal opinion is that they have to be replaced with another option in which rigid links will be automatically defined in proper locations, because the way that are used right now in Robot is not convenient at all and it is causes many simulation problems...

Furthermore, achillesgr you should assign concrete columns and not steel columns and then run the analysis in Robot, if the model in which you will transfer the results of Robot contains concrete columns...

[dimogrec]

About the offsets in Robot, my personal opinion is that they have to be replaced with another option in which rigid links will be automatically defined in proper locations, because the way that are used right now in Robot is not convenient at all and it is causes many simulation problems...

You mean something like rigid offsets at the ends of a member that is used in SAP for example... Yes that would be very helpful especially for RC buildings

[Tuctas]

I've just added it in the wishlist..

http://forums.autodesk.com/t5/robot-structural-analysis/offsets-in-robot/idi-p/5256537