Base plate fixity 2

[BAGW]

Hi,

I have a perimeter column with four anchors outside the flanges. I am assuming this a pinned condition.

Trying to achieve pin at the base by keeping the base plate thickness small, so that the base plate yields before the anchor fails in any of the failure mode (steel and concrete). With this design approach, I am considering the base plate to be pinned and still relying on anchors to transfer shear.

Any thoughts on the above design approach.

Thanks

 

[human909]

Seems pretty straight forward. Most analysis techniques and codes would assume a pinned baseplate if the bolts are within the flanges and the plate is thin. Your design approach is quite justifyable.

On the other hand if you REALLY need a pinned connection at your baseplate and the consequences of not achieving a flexible connection are serious then you might need to dig deeper.

Look at THIS discussion for further details:

https://www.eng-tips.com/viewthread.cfm?qid=499617

 

[HTURKAK]

For nominally pinned base , you may choose the anchor bolts within the flanges , moreover with two anchors rather than four anchors..

I will suggest you to look to the following doc.





If you put garbage in a computer nothing comes out but garbage. But this garbage, having passed through a very expensive machine, is somehow ennobled and none dare criticize it. ( ANONYMOUS )

 

[DaveAtkins]

For nominally pinned base , you may choose the anchor bolts within the flanges , moreover with two anchors rather than four anchors.

Two anchor column base plates are only permitted in very specific situations, so I recommend four anchors.

I wouldn't worry about proving this is a pinned base. Assume it is pinned, and design everything else accordingly.

DaveAtkins

 

[BAGW]

Contractor does not want to install the anchors within the flanges.

Just to clarify its 2 anchors outside of each flanges (total 4 anchors). Thats why going down the path of yielding baseplate (thin baseplate) to behave like a pin support and transfer shear only.

 

[dik]

I generally consider them pinned. Some jurisdictions require 4 anchor rods, for safety reasons. Unless I'm designing the connection for developing a moment, it's pinned.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[canwesteng]

I would consider it pinned as long as there is a proper (stiffer) load path and not think twice of it. With a thin base plate, the difference in anchors inside or outside the flange should be negligible anyway.

 

[human909]

I think everybody's responses here are 'correct' and pretty normal assumptions. Canwesteng's comment about here being a proper (stiff) load path is an important and necessary requirement and in most cases you have this.

Despite my earlier comments about bolts inside of flanges, sometimes there just isn't any room. I design tall steel towers and the columns can be relatively slender but can have significant shear and uplift the bolts end up outside but the design is nominally pinned.

In the are rare exceptions where you don't have a proper stiff load path (either through poor design, or failure) your nominally pinned base plate will end up taking moment. This might actually be a good thing in the first two case. I can think of a few cases where not having a proper pin could be detrimental but they are fairly unique flexible structures.

 

[Tomfh]

Steel to steel “pin” connections yield a bit as the pinned condition is approached, but anchors pulling out can be somewhat unsightly. So yes it makes sense to soften a “pinned” baseplate (eg by going to thin plate) if it might otherwise rip out the outer row of anchors.

 

[structSU10]

PCI has a method to determine a rotational spring stiffness accounting for base plate, anchor, and foundation stiffness. I have applied it in my structural model in the past to find a reasonable base moment to design against when I think its critical. In many cases the moment is small and manageable.

 

[Peedge]

The distinction between pinned and fixed column bases can be difficult to define in practical details unless there is an in-depth analysis of the interaction between the foundation and supporting ground complex. Nevertheless, the use of 4-holding down bolts are common details which are frequently deemed to be pinned in the analysis, but could also be classed as moment resisting. In the case of boundary columns, the base detail must be capable of resisting moment, although such bases are generally modelled as pinned for the frame analysis.

Base plate design should also consider the following:

* Effect of minimum eccentricity
* OSHA requirement - minimum of 4 anchor rods in column-base plate connections (except for post-type columns weighing less than 300 lb.)
* Flexible base plate will increase tension on anchors due to prying force

See also AISC Steel Design Guide 1 for reference.

 

[KootK]

Two anchor column base plates are only permitted in very specific situations, so I recommend four anchors.

I wouldn't worry about proving this is a pinned base. Assume it is pinned, and design everything else accordingly.

This is usually my approach as well. I feel that structural engineering is primarily about the moderately intelligent, rough proportioning of things. There isn't a lot of precise "knowing" involved. Viewed in this context, I see little profit in trying to game things out with a base plate that's thin or an anchor bolt pattern that's less safe to erect.

For situations where there is genuine concern, I've seen a few approaches:

1) The PCI thing.

2) The Eurocode component method.

3) Assume the base takes 50% of the moment that it would if it were rigidly fixes.

4) For appropriate seismic situations, design for the plastic yield moment of the column with over strength.

5) FEM... always FEM.

For serious columns where there is often a shim stack involved, I don't have much faith in the predictive capability of any of these methods.

 

[Tmoose]

mmmm, load path.

https://media.tenor.com/G9vpM5i72KUAAAAC/fobidden-homer-simpson.gif

On rotating equipment "Thin" base plates are a problem.
And anchors positioned very far at all from gussets or column/beam flanges turn 1" thick base plates into thin base plates.

Attached is a medley of images from a project 15 years ago. A German fan company that started making fans in 1873 sold us a few nicely made Primary Air fans.
Regrettably there were some design issues right from the beginning plus some unfortunate complicating installation issues.

Eventually it was shown One of the main "problems" was the anchor bolts securing the fabricated pedestals to the massive concrete pedestals were 6 inches or more from the structural gussets. The resulting "Load path" was a big old floppy Z instead of the expected direct line that is required for a stiff structure.
The 1" thick base plates that were part of the handsome welded steel bearing pedestals were incapable of doing a good job with the anchor bolts so far away.

A structural building perimeter column or even a light pole most assuredly ain't a rotating machine support structure, but ..........