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Rigid Frame Analysis w/ Portal Frame Method


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#1 Hoven

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Posted 21 February 2012 - 06:30 PM

This might be a dumb question, but its better to know the answer than to get the problem wrong on the test.
My question is when analyzing a rigid frame using the portal frame method (as shown in the SERM) the base reactions are different that if you looked at the frame as a whole and calculated the overturning moment and found the base reactions by a simple force couple. I know the base reactions are different when using the portal frame method because you assume a point of inflection at the midheight of the column and you calculate the foces about that point.

Am I wrong, or are these numbers supposed to be the same?

Thanks.

#2 McEngr

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Posted 21 February 2012 - 09:59 PM

If you have the MacGregor book, it says on page 251, "the portal method is most suitable for buildings having low elevation and uniform framing. The reason for this has to do with the structure's action under load. In this regard, consider the frame as acting like a cantilevered beam that is fixed to the ground. Recall from mechanics of materials that shear resistance becomes more important in the design of short beams, whereas bending is more important if the beam is long."

#3 McEngr

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Posted 21 February 2012 - 10:00 PM

Sorry. I get James MacGregor and RC Hibbeler mixed up. I meant Hibbeler in my comment above.

#4 dakota_79

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Posted 22 February 2012 - 04:37 AM

Hoven - your instinct is correct, which is why there's also the cantilever method, which is more suitable for taller multistory frames where it's more important to be accurate in approximating the column axials (and thus the vertical reaction couples). For a squat frame, as McEngr noted, the portal method is more accurate for the governing frame force estimations.




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