Hi all Does anyone have an examp

[am760503]

Hi all

Does anyone have an example hand calc ( in accordance with BS5950 or similar) for a baseplate design as per sketch attached.
In particular the column/windpost has a large eccentricity to the baseplate which means the holding down bolts are not at the outside of the column.
Thanks in advance
Sam

 

[oengineer]

I do not have a calculation for the exact design you show but AISC has a design guide for Base Plate design which provides design examples.

Here is the document:

https://files.engineering.com/getfi..._Guide_1_Base_Plate_and_Anchor_Rod_Design.pdf

Hopefully it helps

 

[am760503]

Thank you oengineer, I have designed baseplates with axial load, moments and shears, but have not done one where the bolts arrangement is unsymmetrical about the major axis of the column.
Where this is the case, are there any specific additional considerations required in the design?
Any help or examples appreciated.

 

[JLNJ]

Some assumptions on how the system acts need to be made. If you think your plate will be stiff, you can assume a compression distribution under the right-hand end of the plate. Maybe you assume the NA is right at the right hand row of bolts. Then check your bearing pressure - a triangular distribution is a good place to start. If the bearing pressure is reasonable, check the plate section for flexure near the right side face of the post based on the resultant of the bearing compression stress and the distance to the face of the column. If that's OK, then look at your bolt tension and check the plate bending locally at the tube face near the bolts. If all that checks out, look at the weld stresses from the plate to the tube.

If the flexure is reversable (i.e. not just clockwise but CCW as well), the action is a little different, but the same principles apply.

 

[dik]

That 100mm at the heel should be 40mm to start with.

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

-Dik

 

[am760503]

Great stuff, thanks to all.
I will look to make my calcs available to all for future reference should anyone have a similar query.

 

[phamENG]

I've been playing around with Hilti's CBFEM base plate modeling, and I've found something interesting - the bearing pressure assumptions we typically use are wrong (assuming the FEM is right, of course). And it makes sense. Really, for the bearing pressure to be way out at the edge, the plate has to be at least as rigid as the concrete. Not going to happen. The compression side of the couple is under the compression flange or wall of the column. A thicker baseplate may help to reduce the peak bearing pressure, but it'll still be centered right there where it's applied.

So I'd look at it this way:

Tension applied at the tension wall of the column.
Reaction at the anchors adjacent the tube.
Compression reaction under the compression wall of the column.

The rest isn't doing anything for the moment - might be useful for shear, though. Just be careful how you allocate the shear loads and, if you're going to assume those take the shear and the others don't, you may want to slot the front bolts or use oversided holes with non-welded plate washers in those to be sure you don't overload an anchor.

Now, at those anchors that are in tension, you'll need to look at prying to determine either the actual anchor load or the plate thickness necessary to preclude prying as a limit state.

 

[dik]

... concrete crushes and redistributes the load... that's how a lot of things work.

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

-Dik

 

[dik]

The way I'd do it would be to determine the depth of the rectangular stress block from the moment, and apply it to the toe (based on moment shown)... determine the depth of the compression block for the axial load, and add this distance to the distance at the toe and then use the combined compresson block to determine the plate thickness... and check anchor rods required...

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

-Dik

 

[phamENG]

dik - right, if you get to a high enough load. But then, it's going to crush where the load is highest first. So that'll be under the compression flange/wall. But that concrete is confined. So even if it crushes, where does it go? Will it actually move enough to transfer the load to the edge of the plate?

For cases where you have a concentric column with a thick baseplate (moment arm from the center of the assumed rectangular stress block to the compression flange to the thickness of the plate is <3:1 or so), the traditional assumptions are close enough - they have been working for a long time, after all. But in a case like this, I think those assumptions would prove unconservative.

 

[dik]

correct... but that's hidden by the base plate... by using the load as an extreme moment on the baseplate, it will likely be thick enough... Unless there's a whole bunch of them, a more refined analysis isn't necessary.

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

-Dik

 

[Celt83]

As an example of what phamENG is talking about (the modeling process can be done in any software that has shells, rigid links, point springs, and area springs):
In the following images Shell model is on the left and Rigid plate is on the right.

Loading:

Bearing Stress:

Anchor Tensions:

Deformation:

 

[phamENG]

dik - I'm not sure I agree. By assuming the stress block is at the extreme edge, the moment arm between the resultant compression force and the tension in the anchors is longer, which allows you to design the anchors for a smaller tension load. This, then, reduces the moment on the plate induced by the cantilever bending from the tension face/flange to the anchor. If we assume the compression is centered under the flange, the arm is smaller, the tension load is higher, and the resulting base plate is thicker.

Again, this works just fine for 'traditional' baseplates. I'm not saying we've been designing them all wrong and every building is in danger of collapse. I'm just saying the typical methods are 'good enough' for typical applications and work well for hand/excel calcs. The OP does NOT have a typical situation, and I don't think it's appropriate to apply some of these oversimplified methods to the case in question. For the normal redistribution of load through the connection to occur, the OP's connection would have to undergo a considerable amount of deformation. Probably more than is acceptable.

Thanks, Celt - that's precisely what I mean.

 

[dik]

...looks good for a knife edged plate... see if that changes when you use a rectangular HSS like original problem... might be different.

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

-Dik