Portal Frame Method Rules of Thumb

[StrEng007]

Are there any basic rules that apply to the applicability of the portal frame method?

For instance:
1. Column height to beam length ratio should be approximately 1.0?
2. Column to beam inertia should be equivalent and must be of the same elastic modulus?
3. Results from a portal frame method are expected to be 20% higher than FEA or moment distribution method?

 

[gte447f]

No. 3 - Not necessarily. It depends.

Not helpful, I know, but No. 3 is just way too much of a generalization to be useful.

If I was going to use the portal method by hand, I would just use the results as is. I wouldn't factor them up or down to try to match the "real" results.

 

[STpipe]

I don't know if #1 is applicable. In my Kassimali structural analysis book, the example they provide has a height to length ratio of 0.5. The only stipulation to the method is that it should be limited to "relatively low building frames", although not clarifying what's the threshold.

For #2, I think the relative stiffness of the beams and columns should be such that the deflected shape of the frame matches the assumption of the portal frame method (beams in double curvature, column deflection that looks like fixed-guided rather than cantilevered). I would also think that you want the height of each level to be relatively the same to ensure the hinge is as close to the column mid-point as possible.