In Practical how to Achieve Purely Pinned Base in Steel Structure

[saruncivil]

Dear Creator,

We designing the Pinned steel column base plate by providing Anchor bolts near to the web of "I" section but how can we say this is pure Pinned base. It will create very small moments. how to overcome from this issue and how to achieve pure pinned.

Please all put your views to conclude this one.


Thank you
Arun S

 

[Tomfh]

If you do a google image search "pinned connection" you'll find a number of approaches.

Of course, you also need to ask yourself if you really need a true pin. A lot of the time you don't.

 

[CELinOttawa]

Giant pins are true pins, until they seize with rust.

Large rocker plates are often used where you REALLY need a pin.

That said, it is rare in the extreme that you really must have a pin. It is perfectly safe (and common practice) to assume pin and let the structure sort itself out.

 

[jrbaus]

For materials like steel, it is the ductility of the material which affords us the convenience of designing as 'pinned' even though we know some moment will exist in reality.

For argument's sake, if you had portal frames constructed of brittle glass than I would expect that these 'small' moments would indeed require a much more thorough assessment.

 

[rb1957]

if you design a small pattern of bolts, so that they can react a small moment, the real world would say that the stiffness contribution of the bolt pattern is small compared to the overall structure, leading you to analyze as pinned. then you could assume a distribution between the bolts (say 60:40) so that there is some moment taken by the base. The critical thing is not to have a structure designed for zero moment (like a single small diameter rod) that experiences some moment in the real world. For my money I`d be more concerned about flange bending and prying (real world effects of a pinned connection).

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

 

[SteelPE]

Just my $0.02

In the case you describe your column base will not act as a true pin. I seem to remember a seminar stating that the typical base plate configuration that is assumed to be pinned will actually develop about a 10% fixity. I routinely use this fixity at the base with single story moment frames. It doesn't help much, but it does help.... especially when trying to limit wind drift to H/500

As discussed above, it is really difficult to develop a true pin at the base of a column. Unless you really need it (I'm not sure why you would) I would try to avoid it for the added costs.

 

[canwesteng]

On top of the good points mentioned above, in a braced frame, the structure is much stiffer than the base plate connection so the fixity is negligible.

 

[KootK]

I feel that canwesteng's point is the crux of this.

As a result of unintended joint stiffness and joint prestress due to axial load, all base plates are pretty much fixed. Right up until they're not that is. There are two basic paths:

1) The base connection attracts moment until it fails in a ductile manner (rare). The moment in the connection then remains relatively steady at the yield + over-strength value.

2) The base connection attracts moment until it fails in a brittle manner (common). The moment in the connection then drops to zero-ish.

So the goal of design is not to create anything approximating a pin at the column bases. That's improbable given typical detailing practices. Rather, the goal is to ensure that the diaphragm providing lateral support to the tops of the columns doesn't shift very much. As long as diaphragm movement is kept small, the tops of the columns won't move very far, and the column base connections will never attract enough moment to initiate a brittle failure. Consequently, the parameter of interest is the ratio of gravity column lateral stiffness to story bracing lateral stiffness.

Consider two extreme one story building cases where the gravity columns are just a bunch of HSS6x6 pinned to the supported roof beams and conventionally "pin" fastened to the foundations:

1) The lateral system consists of stiff cross bracing at regular intervals on all four walls. In this case, as canwesteng intimated, the lateral stiffness of the story bracing will dwarf that of the gravity columns. The gravity columns will be shielded from attracting moments that would cause brittle base connection failure.

2) The lateral system consists of a handful of the HSS6x6 columns designated as flagpole cantilever moment frames and provided with kick-ass moment resisting base plate connections. In this case, the designated lateral resisting system will not shield the gravity columns from attracting moments and brittle base connection failures may well occur in the gravity columns.

The Take Away

It is counterproductive to use a tight, centrally located anchor bolt pattern in an attempt to approximate a pin connection. Baring an extreme anchor bolt / base plate arrangement designed with the intent of creating flexibility, you're going to wind up about the same amount of base connection rotational stiffness no matter what you do. By using tight, central anchor bolt patter, you just exacerbate the prying/tension loads on your anchor bolts when the inevitable base moment shows up. The goal of design for "pinned" base connections is this: moment capacity < moment demand. Reducing moment demand by stiffening the story bracing helps; reducing the moment capacity of the base connection...hurts.

It's worth noting that effective column buckling length goes another way. It takes very little flexibility in the base connection to turn a fixed based column into a column that is effectively pinned.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[TehMightyEngineer]

As seems to be the case KootK has said exactly what I was thinking.

The only addition I would make is that I try to design pinned bases with a limiting strength of flexure in the baseplate. For this reason, as KootK mentioned, bolts close to the web isn't the best idea as it's a brittle failure and you likely need those bolts for uplift and shear. If it's flexure in the baseplate then the baseplate wont be stiff enough to transfer any significant moments to the anchor bolts and then the pinned base can be assumed and both the column, welds, and anchor bolts wont ever see overload from flexure.

Maine EIT, Civil/Structural.

 

[rb1957]

"moment capacity < moment demand" ? ... moment capacity > moment demand, no?

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

 

[KootK]

Exactly right rb1957. Thanks for the fact check.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.