Base plate type

[allimuthug]

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 ?

 

[KootK]

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.

 

[allimuthug]

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

 

[allimuthug]

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

 

[DaveAtkins]

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

DaveAtkins

 

[manstrom]

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

 

[allimuthug]

 

[WiSEiwish]

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.

 

[allimuthug]

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 .

 

[allimuthug]

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.

 

[jike]

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.

 

[jike]

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

 

[KootK]

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.

 

[rb1957]

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 ?

 

[Jc67roch]

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.

 

[WiSEiwish]

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.

 

[allimuthug]

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.

 

[KootK]

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.

 

[rb1957]

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 ?

 

[allimuthug]

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