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

Re: base plate software

[Subject Prev][Subject Next][Thread Prev][Thread Next]

We do a lot of it here. I haven't said much in the thread as most of the
comments are quite valid. However, for what it's worth here's what we do.

I favor heavy base plates because they are the most ductile and easily
controled of the structural elements routinely available. I would far
rather have my column putting moment into a base plate than a girder or
other framing element. If the column has a reasonable axial load, the plate
develops its moments via contact stresses with the foundation or anchors,
not through high stresses in welds and shears in the members due to a sharp
moment gradient. Also, properly detailed, I can almost guarantee the
strength of the plate and adjust its thickness easily to avoid a weak
column condition.

There has been some discussion regarding thicknesses and economics. My
experience is that plate is cheap, and holes are not a problem. Flame cut
holes are permitted for foundation anchors, and many shops now drill them
on numerically controlled machines anyway.

Lamellar tearing has seldom been a problem for us. The fact that the far
side of the joint has no restraint no doubt helps a lot. I have yet to hear
any data or anecdotes where laminations have ever prevented a flat plate
from developing its full plastic bending capacity.

I avoid stiffeners. Not only are they costly to fit, and reduce ductility
as discussed above, but they are quite hard to analyze (at least for me).
Finite element modeling on a regular basis for steel which costs 30 cents a
pound is a quick way to go broke in a small office. If manual (mainly yield
line) methods can't cut it, we thicken the plate. Better to put the
client's money into the structure than our pocket trying to save some

As for using bolts to drag the flange forces out bodily into the
foundations via stiffeners, we also do this if the axial loads are low.
Otherwise, you're relying on the welds to take the forces out in tension on
a thick plate. We already know how bad an idea that is. In that case, we
use stiffeners fillet welded to the flanges engaging the foundation 
achors. The fillets are now working parallel to the anchors, and we simply
increase their length as required to ensure proper ductility (usually the
full flange tension). The only problem with this approach is field fit up.
The anchor bolts had better be where they're supposed to be, or its a real
mess. I find that they're seldom located well enough to use this detail
without extensive field rework on some of the connections. A real pain
that, so we avoid it.

Peter Higgins, SE