Portal Frame Design

[JStr]

Hi Guys,

I have a question which can be seen in link

https://unsee.cc/be5cc678/

Since the structure is resting on column on 3 sides but open on 1. Can I still assume that the main girder B (roof truss) will only take 50% of the load and the other 25% of the load will go to the columns on the top and on the cantilever end the 25% load will go on to purlin (cantilever rests above purlin on section B-B) and then eventually to the outside two columns?

If yes, what kind of support would I put at the cantilever end in the section B-B?

 

[BAretired]

Under uniform snow load, the main girder B carries 100% of gravity load while the columns along the closed (top) side carry zero.

If yes, what kind of support would I put at the cantilever end in the section B-B?

You would put no support at the cantilever end in Section B. If you did, it wouldn't be a cantilever.

BA

 

[JStr]

Hi BAretired, thanks for your response,

Would it be fair to say I have pinned support at column, a roller support at the main girder B (truss) and overhang?

 

[BAretired]

If you are talking about the analysis for gravity load, you could say that. Alternatively, you could say that you have a pinned support at Girder B and a roller support at each column.

If you are talking about analysis for wind load, neither point is pinned.

BA

 

[BAretired]

Would it be fair to say I have pinned support at column, a roller support at the main girder B (truss) and overhang?

I just re-read your question. You could say you have a roller support at Girder B but you cannot say you have a roller support at the overhang. That would make Truss A indeterminate.

BA

 

[JStr]

Hi BAretired, please see my picture below just to confirm I have understood correctly.

https://unsee.cc/be5cc678/

I understand that I could have roller on column and pinned on the main girder B for my last section aswell.

The wind load is only there for me to design the bracing as far as I know.

 

[JStr]

Also, another query my friend mentioned I could treat the cantilever part as where the wind load is acting on section B-B as a roller acting as a spring since it is resting on purlin? what does this mean

 

[BAretired]

No, your sketch is not quite right. See attached PDF. The red area goes to Girder B; the blue area goes to the three end columns (not exact because of the end bay framing, but close enough). Of course, the entire Girder B reaction goes to Column E at each end.

Also, another query my friend mentioned I could treat the cantilever part as where the wind load is acting on section B-B as a roller acting as a spring since it is resting on purlin? what does this mean

I have no idea what this means. Why don't you ask your friend?

One thing I can say is that wind analysis will be complicated by potential upward pressure acting on the underside of roof and by torsional forces on the structure as a whole.

BA

 

[JStr]

Hi BAretired, thanks for your response again and the sketch!

Just wanted to ask, do the columns near the main building not take any load? If yes, why don't we transfer some of the load to reduce load on the main girder in the initial stage?

 

[BAretired]

The only way the columns on the north side can take any load is if the snow on the new roof is unbalanced. In that case, the columns on the north side can be in tension or compression depending on which side has more snow. If the snow is uniform over the entire new roof area, the north side columns take zero load. It is a question of statics. Check it out for yourself by calculating the reactions of Truss 'A' with pin at one end, roller at midpoint and uniform load throughout.

BA