## Column Eccentricity

## Column Eccentricity

(OP)

I would like to get some feedback from fellow structural engineers on what eccentricity is typically used for square tube steel columns (HSS's) that are concentrically loaded and are not part of a frame (resist gravity loads only). The columns would support steel beam(s) on one or both sides with equal or unequal spans. The steel beams are connected to the column with a through knife plate, so they are not connected to just one face of the column. The columns sizes would be anywhere from 4" to 10" square.

I typically use 1" eccentricity. I have heard others use 10% of the column size, (i.e. 6" col. would have 0.6" of eccentricity).

Your thoughts?

Thank you,

I typically use 1" eccentricity. I have heard others use 10% of the column size, (i.e. 6" col. would have 0.6" of eccentricity).

Your thoughts?

Thank you,

## RE: Column Eccentricity

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## RE: Column Eccentricity

I don't add an eccentricity to a load when I connect a beam to the flange of a W-section, so in the same manner, I do not an eccentricity to the load when I have a simple shear connection to the face of a tube column.

JMHO.

## RE: Column Eccentricity

Mike McCann

MMC Engineering

## RE: Column Eccentricity

## RE: Column Eccentricity

Apsix makes a valid point that the beam reaction should be taken as a minimum of 100mm (4") from the face of the column.

I assume the 1" you mention is from the column face and not the column centreline.

## RE: Column Eccentricity

## RE: Column Eccentricity

Similar applies to single span on the top.

For this reason I would never take the eccentricity any less than half the width of the column.

Also what about eccentricity perpendicular to span, assuming zero ecentricity gives no room for erection tolerance.

## RE: Column Eccentricity

## RE: Column Eccentricity

I usually ignore any eccentricity on a column if beams frame into it in two opposite directions. If a load on one side was much higher than the other side I might consider eccentricity. If I have a one sided connection I use half the column depth plus 2 inches (approximate center of bolts) for the eccentricity.

## RE: Column Eccentricity

Be careful about saying others are wrong--there are usually various ways to look at structural engineering issues!

If the bolts in a single plate connection are properly tightened, the row of bolts can take shear AND moment--and the point of inflection moves toward the column. In fact, considering the flexibility of the column wall compared to the stiffness of the row of bolts, the point of zero moment will be at the column wall. Therefore, the eccentricity is half the column depth, not the distance to the line of bolts.

DaveAtkins

## RE: Column Eccentricity

The load is applied to the plate at the location of the bolts. The plate is fixed to the column, so there's definitely a moment in the plate, and thus a moment in the column. This moment is rarely balanced by the beam on the opposite side... and it's appropriate to skip live load.

The point I made earlier was that you can design the beam connection to the plate to take moment as well, and this will reduce the moment in the plate & column. Think in terms of the plate being a beam, fixed at one end (column), and free to deflect but not rotate at the other end (bolts), with a concentrated load at the deflected end. Per standard beam diagram & formulas from the steel code book, the moment is PL/2 at each end.

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## RE: Column Eccentricity

DaveAtkins

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No I am not saying that the continuous beam will form a moment connection, the plates cantilever out to the bolts and will therefore be more flexible than direct bearing.

What I am saying is that if the beam is at a slope (from uneven load flexure) and the cap plate is horizontal then there will be only one point of contact until the column flexes to even out the bearing. But the column has to flex to achieve this and that is the reason for minimum eccentricity.

Dave, That assumption is not strictly true. The single plate shear connection relies on the ability of the plate to rotate to match the beam end rotation (or the bolts to plough through the plate). Either action will induce a moment in the column. The reason why an edge girder works is because its torsional stiffness is very low and it easily rotates.

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Never, but never question engineer's judgement

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There will be load eccentricity or induced moment in the column.

Some code(s) require that a nominal eccentricity is included in thge design.

Other code(s) allow you to ignore it, apparently with no ill effect.

Have I got it right?

## RE: Column Eccentricity

This would make an interesting research project for a university grad, if not done so already. It may be that for certain design codes and building arrangements (single vs multi-story), the inclusion of the eccentricity has little to no effect on the chosen column section.

You may ALL be right...

tg

## RE: Column Eccentricity

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