Return to index: [Subject] [Thread] [Date] [Author]

RE: base plates subject to uplift

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Title: RE: base plates subject to uplift

>I am curious as to why AISC feels that the "usual application" of
>a column base plate is to transfer only compression. <what about> shear forces <etc.>....
>It seems that AISC should address these situations as
>well as the "simple compression" case.

I didn't mean to imply that other conditions don't exist. I just meant that the majority of base plates used in buildings transfer compression loads only. There are certainly many cases that transfer some combination of axial force, shear and moment. AISC doesn't ignore them. They are treated in AISC Design Guides No.1 (Column Base Plates), No. 7 (Insdustrial Buildings: Roofs to Column Anchorage) and No. 10 (Erection Bracing of Low-Rise Structural Steel Frames).

>an oversized hole creates problems for load transfer
>under many conditions.

I think Rick Drake gave a good explanation and summary in this regard. One thing I'll add is that there is also a method of design for shear resistance covered in those AISC Design Guides called "shear-friction". Essentially, this method recognizes that any lateral movement of the base plate would induce a tension in the anchor rods, which would in turn act to further restrain the base.

In the end, I think the hole sizes used in base plates are necessary for constructability (see Harold Sprague's post on this subject). At the same time, the design must account for the size of the holes and their effect on the design. For axial compression only, I don't think the hole sizes make much difference. For heavy shear loads, you are driven to shear keys pretty quickly. For uplift and moments, plate washers to cover the base-plate holes become very important.

Charlie