portal bracing

[malikasal]

can any one provide me with a design refrence/book/example if a building with a portal bracing system in it.

will the portal column help in the axial load capacity?? and what is the best way to model this system in staad??

ôIf you don't build your dream someone will hire you to help build theirs.ö

Tony A. Gaskins Jr.

 

[MotorCity]

Portal frames are moment frames and can be found in almost any book on structural analysis. They typically exhibit more drift than a braced bay, so the drift usually governs if you have finishes or cladding that are sensitive to movement. As for modeling, you can experiment with pinned or fixed bases.

 

[malikasal]

Dude you are missing the point.
I am not talikng about simple moment frames.
I.am.asking about the portal bracing wich is the practise of adding a rotated column near the main column to be used as a lateral bracing in the minor axis of the main column.



ôIf you don't build your dream someone will hire you to help build theirs.ö

Tony A. Gaskins Jr.

 

[KootK]

I agree with motorcity's respone. As shown in your sketch, it's jus a run of the mill moment frame that would not participate in axial load resistance. If the neighbouring columns will be fastened together to behave compositely, which I doubt, that's another,more complex story.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MotorCity]

Sorry dude, there was no question regarding practice of adding a rotated column next to the main column in your original post. I have never seen that done.

 

[DaveAtkins]

This is done all the time by pre-engineered metal building suppliers. As KootK stated, the portal frame takes no gravity load, but resists all lateral load. They are typically assumed to have pinned bases, so as not to transfer any moment to the foundations.

DaveAtkins

 

[JAE]

If the portal column had connectors along the length between it and the main building column, the main building column might develop more additional axial capacity in that the portal column would serve as a lateral brace against weak axis buckling in the main column. The connectors don't have to be composite.

The added capacity would only help if the main column would be helped by a smaller unbraced length L (as in a reduced kL/r value).

This would be a brace system that AISC appendix 6 covers for columns. The portal column would have to provide adequate strength and stiffness to do any good (per App 6).



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[DaveAtkins]

JAE is correct, but I don't think the pre-engineered metal building designers consider that.

DaveAtkins

 

[spieng89]

I've seen portal frames utilized a lot in pre-engineered metal building's as well. With virtually no dead load being applied by the column, the footings for portal frames can become quite large to offset the lateral and uplift forces.

 

[JAE]

DaveAtkins - yes I think you are correct - they just design the main frame - then use the portal for the wind in its own direction.

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[malikasal]

I recently started working on pre engineered buildings after 5 years of working on regular steel buildings and I am still getting used to some new concepts like this one.
The thing is I am designing a multistory building with portal bracing and because there is a huge axial load on the columns I am thinking of using a star shaped columns instead of the regular portal brace system to increase the axial capacity .

Any comments or advice guys??



ôIf you don't build your dream someone will hire you to help build theirs.ö

Tony A. Gaskins Jr.

 

[EngineeringEric]

So multi story Pre-Engineered Metal Building? or a Hybrid Building? A good W8 section can handle some hundreds of kips with an unbraced length of 15ft.

Star shapes sounds odd and therefore expensive. Cruciform shapes are an option made with two W sections. Best advice, use a different system lateral system. unless you are 6+ stories you shouldn't need this, and if you are that big you shouldn't be learning on the job without some supervision.