Column load eccentricities 2

[phuduhudu]

Using the British standard, columns in simple braced construction need to be designed for eccentricities of load from attached beams of 100mm from the face or the web. What eccentricities do others use? In my particular case I have 2no. 800mm deep plate girders framing into the top of a corner column with heavy loads obviously and the moments are giving me a much larger column size than the others. One idea I had was to rather put the plate girder on a cap on the column and connect the other girder into the side of this one at the end but its a bit of an awkward detail. I could also extend the end plate for the girder down the column to give myself the additional flanges of the plated column as far down as I need to go for the moment. Any other good ideas for this situation?

 

[BAretired]

In a shear tab beam connection, a hinge is usually assumed to be at the bolt centerline, so 100 mm from the web or flange of column seems reasonable.

If you are using end plates on the beam, the eccentricity may be less, but the connection is semi rigid, so you will develop continuity moments from beam to column. With 800mm depth, depending on the girder span and load, rotation may be negligible.

Try calculating the end rotation of the girder under simple span loading, apply that rotation to the column and see how the eccentricity compares with the British standard. Then use your judgment as to whether or not you want to use a lesser eccentricity.

Personally, I think I would use the code value as a minimum.

BA

 

[asixth]

I would assume the column reaction is bearing at the face of the column minimum. I am not designing to British standards but my design standard does require this minimum eccentricity to be used.

 

[Lion06]

AISC allows you to neglect eccentricities (in the column design) for shear tab connections.

 

[phuduhudu]

For column design obviously the minimum eccentricity is at the bearing face. I am surprised AISC neglects these eccentricities for shear tabs where the load will come on the centreline of the bolt holes in the shear tab which will be away from the column face.

 

[Lion06]

I am also surprised. I've tried to rationalize it in my own head, but I'm not making any headway!

 

[enginerding]

The reason AISC considers no eccentricity is that they assume the plate, not the bolts, acts as the hinge mechanism. The moment capacity of the bolt group must be higher than that of the plate (see pg 10-103 in AISC 13th ed.)

You have to ensure that the bolt group has a capacity large enough to carry the full shear force applied at the weld line of the single plate. This is why the table values for the shear tab connections are less than simply summing the shear capacities of the bolts.

 

[Lion06]

Others have suggested that the bolts act as the hinge, which is why they don't endorse, welded-welded shear tabs.