Portal Frame - Uplift Force 2

[Eng-Tareq]

Hello All,

I am having a problem with designing portal frames, I am realizing that I am getting excessively big uplift forces, so I made a simplified model in Risa to show what I mean to you guys.

As it can be seen the uplift force is nearly 250 kN while running some load combinations with an arbitrary 20kN/m lateral load on the first column. Is there a different arrangement of braces usually adapted for these portal frames, or is this sort of uplift normal? I just do not think it is because I know it would be really difficult to design a baseplate and anchor that has the capacity for such tension/pull-out. In other cases, I got uplift forces even double or triple that based on the loading. I am not sure if I am doing something wrong or anyone has an idea. Thanks!

 

[GeorgeTheCivilEngineer]

You've drawn a braced bay, not a portal frame so I'm not sure what you are driving at.

 

[Eng-Tareq]

Yea sorry about that, the problem remains the excessive uplift forces in this frame. Thanks!

 

[BAretired]

Seems pretty straightforward. Could quibble about including the low roof in 'h', but this is the way I would do it for anchorage purposes. See below:

If uplift turns out to be 250 kN, that should not be a big deal. Just anchor as required.

 

[BAretired]

In my neck of the woods, there would be grade beams which could be doweled into grade slabs; they would provide a little more dead load to reduce the uplift. Pile foundations, if used would have some dead load and uplift resistance to offer as well.

Another option is to transfer one chevron brace to the other bay. No big deal!!!