How do you align girders, beams and columns?

[mexicali8]

How do you arrange columns, girder and beam? Do you align the beams with the plane of maximum moment of inertia for the columns, or do you align the girders with the plane of maximum moment of inertia for the columns?
Many -if not all- AISC documents show plan views with the beams aligned with the column's maximum moment of inertia plane, when girders transmit more shear or moment to columns. What is the logic behind that?

Rafael Guerra

 

[slickdeals]

Typically to keep girder sizes small, you want to span them in the shorter direction. Research has shown that an economical bay has a length to width ratio of 1.25 to 1.5. I don't think for a gravity system the orientation matters a lot. However, you want to make sure that the perimeter columns have bigger moment of inertia in the direction in which the girders frame in (due to unbalanced moment). Interior girders typically don't have a lot of unbalanced moment.

In your above picture, since there is almost no difference in the span of the girder vs. the beam, it would make sense for the girder to frame into the column strong axis.

HTH

 

[msquared48]

Irrespective of gravity loads, the orientation may too depend on the requirements of any moment frames in the system.

Mike McCann
MMC Engineering

 

[mexicali8]

Thank you both for answering. Yes, what you say is what I do.
But yesterday I downloaded the Floor Framing utility from the AISC Solutions Center, and the way it works is only with the beams aligned with the columns' plane of maximum moment (or I haven't find the way to make it the other way), and that, combined with many other drawings I have seen put me to think about it.

Well, now I know I can do the job for another 5 years (LOL).

Rafael Guerra.

 

[hokie66]

Each orientation has its advantages.

Girder to column web connections are more compact (less eccentricity), can use less bolts, but may be more difficult to erect, and the girder span is slightly longer.

Girder to column flange connection simpler to erect, but eccentricity of load on column is greater, albeit on column strong axis.

Each job requires its own assessment. The edge conditions and method of lateral load resistance usually determine the column orientation.

 

[trainguy]

I've always been reminded by more experienced engineers that minimizing column load eccentricity is key, so that no checks of axial plus bending will be required when selecting the column.

Framing a girder to a column flange will apply eccentricity, esp. for outer columns.

I agree with hokie66 about the determination of column orientation.

tg

 

[lkjh345]

Agree with hokie66 and trainguy.

If at all possible, try to frame the girder into the column web to minimize the eccentricity of the larger load (in comparison to the single beam framing into the flange).

 

[mexicali8]

Correction about my saying on the Floor Framing utility: it works only for gravity loads, so there's no point on whether it works one way or the other.

Rafael Guerra.

 

[rmurph17]

Do a check on the moments acting about both the major and minor axis.

If using a I beam remember that it will be significantly weaker in the minor axis, therefore try using H sections.

You will have to check for lateral torsional buckling also.

Generally orientate the column so that the major axis is taking the highest loads or in the direction of the largest moment.

the formula to use is M = (e + b/2) x F

where M = Moment
e = eccentricity
b = breadth of column
F = Force (summation of shear force in a particular plane)



R. Murphy
MICE MIStructE MASCE