Base plate type
Base plate type
(OP)
Hi ,
I HAVE A BASE PLATE SUPPORTING HE300A COLUMN. THE ANCHOR BOLTS ARE PLACED OUTSIDE THE FLANGE PLEASE REFER THE ATTACHMENT .
THIS BASE PLATE HAS TO BE DESIGNED AS FIXED OR PINNED ?
I HAVE A BASE PLATE SUPPORTING HE300A COLUMN. THE ANCHOR BOLTS ARE PLACED OUTSIDE THE FLANGE PLEASE REFER THE ATTACHMENT .
THIS BASE PLATE HAS TO BE DESIGNED AS FIXED OR PINNED ?






RE: Base plate type
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Base plate type
RE: Base plate type
RE: Base plate type
DaveAtkins
RE: Base plate type
When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.
-R. Buckminster Fuller
RE: Base plate type
RE: Base plate type
If you end up designing it as a fixed connection, consider the assumption of a small amount of rotation due to the fact that a small amount of rotation will most likely occur.
RE: Base plate type
RE: Base plate type
RE: Base plate type
In order to resist a fixed base moment, the column anchors would need to be sufficiently large to minimize rotation of the base, the base plate would need to be sufficiently thick and/or stiffened to minimize rotation of the base and the foundation has to be sufficiently large and rotationally stiff to minimize rotation of the base. If these three requirements are met than you may consider designing the column base as fixed.
RE: Base plate type
RE: Base plate type
Normally, fixed based connection design proceeds as follows:
1) You determine a moment demand based on your overall structure analysis.
2) You design the fixed connection to deal with the moment from step #1.
3) You detail the connection in a way that will promote connection stiffness, even though you rarely actually evaluate connection stiffness.
I know, it's a bit sketchy.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Base plate type
1) the rigidity of the base plate (it's thickness), and
2) the rigidity of the connection (fastener preload, fastener washer/bearing pad).
as posted above, surely the moment reacted by the individual fasteners is small (tiny?) compared to the moment reacted by the fasteners (as pins on their PCD)?
another day in paradise, or is paradise one day closer ?
RE: Base plate type
I agree with PE2012. I design/use pinned base plates 95% of the time, and only design for moment/fixity if it is some kind of cantilevered, unattached column or frame. And if I must carry a moment, I make the base plate wide enough to give the anchor bolts some moment arm to resist the imposed moment - OR direct embed the post/column in the ground or concrete to get some rigidity to carry a moment.
RE: Base plate type
RE: Base plate type
RE: Base plate type
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Base plate type
why do you want to have fixed bolts (rather than pinned) ?
another day in paradise, or is paradise one day closer ?
RE: Base plate type
RE: Base plate type
RE: Base plate type
Some stats:
- Base plate is 1 1/4" x 14" x 20" (will reduce thickness now)
- Four 1 1/4" diameter anchor bolts (intend to re-size)
- Mf if assumed 100% fixity = 96 kip*ft from typical frame analysis
- W18x76 Column
- View magnification = 150 in attached images.
- Anchor rods assumed to be 100% fixed.
- Modeled compression block to be 3" wide. This is wider than it should, but I found the base plate deformation did not appear correct with a narrower zone. I have not determined a good method to model a compression only support in this software package.
- Modeled anchor head with a 0.15" gap between nut and plate. This was done to ensure there was no bonding between the nut and the base plate that would cause a compression force on the anchor rod.
From this I found the maximum tensile force on the two anchor rods totaled 23,500lb. This equated to a base moment equal to 30kip*ft or approximately 27% fixity. With a thinner base plate and tighter anchor bolt spacing that value will reduce to the point where my initial pinned assumption would be very reasonable.
RE: Base plate type
analyze as pinned,
give yourself a healthy margin;
if moments develop, well, first there's the margin that allows you to accept some additional loading (over and above the worst code loading, with minimum material property);
and second, the worst i see happening is some local plasticity that'll help relieve the moment.
i think there's more to designing the preload than worrying about moments.
another day in paradise, or is paradise one day closer ?
RE: Base plate type
The equations for rotation are already worked out in the PCI Manual based upon:
1) Elongation of the anchor bolts
2) Bending of the base plate
3) Rotation of the foundation
I don't have the reference anymore (since I retired) otherwise I would scan it and post it. You can easily write a spreadsheet if you want to look quickly at the fixity of various arrangements.
RE: Base plate type
RE: Base plate type
RE: Base plate type
RE: Base plate type
RE: Base plate type
i found it on the site, search for "MNL-120"
another day in paradise, or is paradise one day closer ?