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Base plate type

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 ?

RE: Base plate type

It could be either in practice. Are you planning to deliver significant moment to the column base connection?

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

(OP)
IF I ASSUME IT TO FIXED HOW MUCH MOMENT WOULD BE TRANSFERRED . HOW IT IS POSSIBLE TO BEHAVE FIXED AS WELL AS PINNED.

RE: Base plate type

(OP)
IF I CONSIDER AS FIXED BASE PLATE THE ANCHOR BOLTS WOULD BE SUBJECTED TO TENSION AND SHEAR . IF I CONSIDER PINNED THE ANCHOR BOLTS WOULD BE SUBJECTED TO SHEAR. WHAT IS THE BEST ASSUMPTION

RE: Base plate type

It is customary to assume it is pinned, unless you must transfer moment to the supporting member.

DaveAtkins

RE: Base plate type

no need to yell

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

You could design the base plate for either. That is a decision that you have to make based on the parameters of the project and what you need for design. As mentioned above, it seems to be customary to start with a pinned base. High moments can induce very large tensile forces in the anchors which can make it difficult to design.

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

(OP)
If so i am considering fixed base plate , can any body briefly expain how much rotation should i consider and also how to calculate moment in base plate based on the rotaion .

RE: Base plate type

(OP)
Is it the thickness of base plate which makes the behaviour as pinned and fixed . I.e if it is very thick the base plate is considerd as fixed . If the base plate is considerably thin then it behaveas as pinned.

RE: Base plate type

As stated before most steel columns are designed with pinned bases.

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

There is a way to calculate the rotation of those THREE effects and how much "FIXITY" they actually provide.

RE: Base plate type

If you search for "pinned or fixed" here, you'll get more information than you know what to do with on this topic. Here's a recent version of this with a detailed response from me: Link.

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

fixity of the bolts depends on ...
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

A great resource on this topic is AISC's Design Guide 1: Base Plate and Anchor Rod Design (Second Edition). Available free to members, or non-members for $60.

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

If you don't need the base plate to carry moment there is no need to make your life more difficult by designing it as a fixed connection.

RE: Base plate type

(OP)
There is a sketch of base plate in the link given above. The baseplate is supported by pile cap .I feel that the pile cap would not rotate much as compared with spread footing.Can anyone give an worked example of finding the rotaion in the base plate based on thickess of base plate.

RE: Base plate type

Base plate flexibility is really a pretty tough thing to calculate without resorting to FEM. If someone has a worked example, I'll be very interested to see it myself. I agree, in many instances, I expect that piles result in a greater degree of fixity.

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

the piles can be considered rigid, so the tension side of the moment is carried by the bolts, but the compression side is carried by contact between the base plate and the pile surface. the contact would be good along the base of the H beam, and around the preloaded bolts, and the plate flexibility would come into play as you move away from the H flanges.

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

(OP)
I don't want to have fixed bolts , rather i want to design what practically going to happen.

RE: Base plate type

What is the result of your structural analysis? Did you consider the base connections pinned or fixed? You should have base reactions.

RE: Base plate type

I believe the concern you have is the unintended moment that occurs after installation. We can assume whatever we like, but the structure will behave according to the actual field conditions. I have a project on my desk that I have the same problem, so I decided to complete a more detailed analysis to check the degree of fixity using the actual base plate and frame geometry I intend to use. The analysis is not as exact as it could be, but it was done within a reasonable time frame. I thought I would post a couple of screenshots in case you are interested.

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

my 2c ...
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

It is really not very difficult to calculate how much fixity a particular base plate/foundation can carry.

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

jike is correct. PG 4-46 of the PCI manual does outline the steps. I would post the forumla's if not for the copyright.

RE: Base plate type

(OP)
Can you specify the PCI manuval detailed name .

RE: Base plate type

(OP)
Hi can you give me the screen shot of front page of pci design hand book , I couldnt find from the link give above.

RE: Base plate type

"Hi can you give me the screen shot of front page of pci design hand book , I couldnt find from the link give above."

i found it on the site, search for "MNL-120"

another day in paradise, or is paradise one day closer ?

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