Cantilever to Columm and induced moments (Diagram)

[Kurtisreeve]

I have the following two questions. I am hoping someone can confirm a few dilemmas;

I have a 2.6m steel beam cantilever attached at the center of a 15m column.
The column is restrained by tie beams every 4M in one direction , but unrestrained in the direction of the cantilever. ---|-| (cantilever attached to flange of column by end plate)

Question 1.

The connection consists of a haunch beneath the cantilever, and stiffeners within the column. So the connection will be checked for combined axial, bending and shear.
I sized the cantilever for deflection max L/180... what design checks should be followed on the beam itself?

Question 2.

This question is about the column.. column is fixed at the base and pinned at the top. I have bracing against global lateral, however I had to fix the base due to the moment present due to these cantilevers at each column.

The attachment does not include the wind load. Does it make sense to have both bracing for general stability, and fixed base for stability of columns?
usually I am used to design with either one or the other, but the general bracing (vertical and roof) are not at each column but at edges of structure.

Question 3

Also , does the cantilever reduce Leff of the column, since that is what the model is indicating.

THANK-YOU.
Much appreciated

Kurtis Reeve

 

[BAretired]

1. I sized the cantilever for deflection max L/180... what design checks should be followed on the beam itself?
Did you consider rotation of the column at the cantilever connection?

2. This question is about the column.. column is fixed at the base and pinned at the top. I have bracing against global lateral, however I had to fix the base due to the moment present due to these cantilevers at each column. Not true. You could have left them pinned top and bottom.

The attachment does not include the wind load. Does it make sense to have both bracing for general stability, and fixed base for stability of columns? If you specify fixed bases for the columns, they are fixed for all loading.
usually I am used to design with either one or the other, but the general bracing (vertical and roof) are not at each column but at edges of structure. Have you considered one cantilever unloaded?

3. Also, does the cantilever reduce Leff of the column, since that is what the model is indicating. No...the cantilever does not brace the column at point of attachment.

BA

 

[Kurtisreeve]

Thankyou, that was really helpful.

Not sure I understood the ans for 2, i.e. 'If you specify fixed bases for the columns, they are fixed for all loading'

by leaving the column pinned top and bottom, the wind girders would take the moment induced by the cantilever at each column then?
P.S, I provided vertical bracing, continuous with roof wind girder every 35 M... (building 70mlong), so the columns in between will be around 20m far from the bracing.

Thanks, again

 

[BAretired]

If columns are fixed at the base, the structure will behave differently than a structure with pinned bases for wind load or for unbalanced cantilever load because one degree of freedom has been removed from the base of each column.

If the cantilevers are equally loaded, the horizontal reactions at the top of each column are equal and opposite in direction. There is no net lateral force to the structure as a whole. The column reactions compress the bottom chord of the truss but have negligible effect on the bracing system of the structure as a whole.

If the cantilevers are not equally loaded, whether the columns are pinned or fixed at the base, there is a net horizontal force applied to the structure as a whole which must be added to the wind force and resisted by your bracing system.

In the absence of specific plans and details of your building, I can't comment on the spacing of wind girders. That would be a matter of engineering judgment.

BA

 

[Kurtisreeve]

Thank you, I understand completely.
The system is completely symmetrical, thus I will opt for a pin based column and design the bracing against wind only.


Regards,

Kurtis

 

[BAretired]

If one cantilever can be loaded while the other is unloaded, a horizontal force will be generated at the top of the columns. This will add to the wind force and must be considered in designing the bracing.

BA