Stiffened Column Base Plate Needing Modification

[MasterB]

Given: A steel 14" WF column within a 1960's 5 story steel framed structure welded to a 2" thick, rather narrow (14") and very long (60") base plate. The existing drawings indicate a triangular gusset plate extending out from the column flanges to keep the base plate from bending. This column occurs adjacent to a elevator pit hence the reason there was no pilaster and this work around to achieve the required bearing area.

Problem: Conveniently our architect located a corridor perpendicular to the gusset plate, which is now exposed, and has asked me to look into removing it, or shortening it up. From my frame analysis, the column base appears to be pinned. I ran a quick base plate design and found that we really do need these gussets.

All the design procedures I have seen design to a cantilevered condition. I was thinking to keep that design thought and extend to design the plate like a beam with two cantilevers. Main span is between the AB's. Then to solve the problem add additional AB's to reduce the positive moment. Negative moment will be on the same order if the bolts are placed in the right place. But then am I localizing the bearing pressure to a smaller area and run the risk of crushing the concrete? The gussets are in place to help distribute the load uniformly across the BP.

What are your toughts? Any other ideas or concerns? I welcome it. Thank you in advance.

 

[DaveAtkins]

The first thing I would try is to determine what size base plate is needed, assuming the bearing pressure under the base plate is at its limit (this may be < 60" in length). Then I would recheck the base plate thickness assuming this length.

DaveAtkins

 

[jike]

Since the codes have changed quite a bit since the 60's, you might, as Dave suggests, see what area is actually needed under the new codes taking into account the increase in bearing pressure that is allowed.

 

[MasterB]

In following your suggestions it appears that the base plate length can be reduced significantly but still requires stiffening as the plate alone is not thick enough to handle the bending stress. I did have to make one large assumption. It so happens that the side requiring the modification has enough concrete on either side to properly confine the concrete below the plate, thus 0.7 f'c was chosen. Whereas at the opposite end, adjacent to the elevator shaft, the concrete is not confined to this extent.

From this analysis the stiffening plate can be cut back as it is not required past the difference in the plate length. Correct? And to cut this plate back even further I would have to stiffen or tie down the base plate in some other fashion.

It may be in our best interest to have a testing lab come out and take a core sample to see if the compressive strength after all this time has increased to 4000 psi and past the design value. If it did, the plate can be cut back behind the wall and problem solved. However, before I can justify authorizing the lab to get on site I think it would be proper to have a fix for the plate.

Any ideas?
Thanks again.

 

[spats]

You cannot design the base plate for 0.7f'c one side, and something less on the other side. There is only one bearing pressure under the required bearing length if the column is assumed centered on the base plate, or you have to assume that the load is eccentric on the required base plate, and you wind up with a trapazoidal-shaped bearing pressure (as long as the axial load is within the middle one-third of the required base plate).

Did you re-check the plate thickness for the required bearing length? You can probably cut the stiffener back further than the edge of the required bearing length to the point where 2" thickness works.

 

[MasterB]

I resolved for the baseplate area and the actual bearing pressure using the entire baseplate. Solved for the distance beyond .95D (m) and got my plate length.

I plan to cut back the plate and put a 1/2" header plate to keep the plate rigid on the edge. Close enough to hide in the wall.

Thanks for all you help.