Steel portal frame: unequal columns.

[tony1851]

Hi guys,
Client needs a 6-bay extension to an existing shed, with the roof line continued, but with a narrower internal width; frames will be at approx 5.5m centres. This presents two problems;
1) one side of the frame will be higher than the other, and I can't find any design info. on this situation;
2) client wants the extended roof at the side to form a canopy over a loading bay. Is it best to run the rafter continuous over the column, or could the canopy be a separate element, though presumably with a hefty moment-connection at the column?
Snow load is not great, and no seismic issues, but I'm concerned about wind loading in particular, and how to approach this structure in general.
Section diagram with approx. dimensions attached.
Any advice appreciated.

 

[BAretired]

1) What kind of design info are you expecting to find? These frames are usually analyzed by computer.

2) It could be done either way. My choice would be to run the roof beam continuous over the column.

BA

 

[KootK]

Other than the fact that your solution may be asymmetric, like your columns, I don't see that the unequal column lengths bring any new challenges into play. I like everything pretty much the way that you're showing it. At the cantilever condition, you could throw a bolted splice plate on top to moment connect the two top flanges. And, of course, stiffeners where your need them. AISC has design guides for both bolted moment connections and tapered members. Both of those documents are geared towards this kind of structure. If you're not already familiar with them, you may find them useful.

Keep a close eye on LTB bracing requirements, particularly if your going to be designing plastically.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[dhengr]

Tony1851:
I agree with BA, run the beams over the columns. That canti. moment connection to the column is a real dog to do well and properly. Let that moment run right through the beam. Maybe put a kicker from the middle of the canti. span back and down to the column. Assemble them on the ground and tip them up.

 

[paddingtongreen]

I agree with the others, continue the beam, particularly if you have a wind uplift greater than your dead load. That canopy picks up full pressure underneath and suction on top and might put the bottom flange in tension. It could be awkward designing the haunches to handle that situation.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[tony1851]

Many thanks for the responses fellas.
TBH I don't often do portals as they're not in my usual line. I end up doing them by hand, and have perhaps been over-concerned about different column heights on this one.
The suggestions to continue the rafter over the column is of course sound - (can't think why I would have suggested a separate cantilever!).
And I will watch out for LTB, and wind uplift.
Thanks again.