WF steel column retrofit question 1

[delagina]

If i retrofit an existing WF steel column by adding new plates to increase Sx and Area, at what location do i terminate the new plates at the beam-column connection?

I am confuse at the joint of beam-column connection as far as retrofitting column is concerned.

Help from the "good" engineers here is appreciated.

Thanks,

 

[Lion06]

Is there a reason that you want to add the plates to the flanges? It seems more efficient to me to add the plate across the tips of the flanges.

It doesn't appear like there would be much net moment given that you have beams framing in from sides.

With that being said, I think what you show is fine, but would require you to check capacity at mid-height of the column AND at the end where the beams frame in. The difference in the two checks is for the mid-height check use the buckling capacity per AISC for axial along with the moment and capacity at mid-height. At the end, where the beams frame in, I would check the interaction of axial and moment at that particular location, the difference being that I would use the squash load for the column, not the buckling capacity per AISC based on the kl/r and the reason is that the column isn't going to buckle at the beam location.

 

[larsacious]

I agree with StructuralEIT's approach.

Also, be sure to check the compactness of the plates per AISC 13th Edition, Table B4.1. I would treat it as case 4 with the "b" being the width of the plate if you only connect it to the existing column with welds at the flanges.

 

[delagina]

ok thanks, i think i will go back and check again whether the column actual moment at the joint is bigger than capacity. it may be failing at mid height only.

 

[JedClampett]

These are very tricky. Don't forget, unless you remove the existing load from the column there is an amount of stress locked into the existing column. Plus adding plates is a somewhat inefficient way of increasing rx or ry (but good at adding area). And the act of welding can reduce the column stiffness during installation due to the heat involved. On the other hand, plates are cheap.
What leads you to believe that the column is overstressed? Have the loading conditions changed?

 

[delagina]

@jed, yes loading conditions have changed/increased.

 

[racookpe1978]

OK.

SO you have a compressed (shortened) beam due to the initial loading conditions, and then the beam(s) got more load and were shortened even more.

But two plates welded on under load will go on "cold" (room temperature, field temperature, factory temperature, whatever) but will be welded at their extremities. The weld metal will be molten (obviously) when applied, and then it will contract more as it contracts after solidifying. This will tend to "stretch" the plate linearly (stressing the welds). Will this contraction stress be acceptable under your conditions? Physically, the movement will be little if you tack weld the reinforcing plates evenly before final welding.

Can you "unload the beams before adding the new material?

Can you accept riveted or bolted connections to the added that won't stress the beam with temperature changes?

 

[rscassar]

Is there any need for you to increase the flexural capacity of the column. It doesn't appear from the sketch you provided that the column has been detailed as part of the lateral-force resisting system.

The design approach for fixing plates to the WF column to increase the compression capacity is well documented on this website. I would possibly look at orientating the plates between the flanges as opposed to welding the plate to the flat of the WF section. This way you can have the plates running the full extent of the column if required and it will increase the ry of the section and hence increase the compression capacity further.