Footing Top Steel 1

[EngTipper.]

Lets say you have a square spread footing with a column with a small moment and large axial load such that the entire surface under the footing is pushing up on the footing (i.e. e<B/6), and the gravity load in this case is only due to the superstructure (not the footing or soil self weight). Will there be any tension in the top of the footing at all?

I'm thinking maybe locally in order to distribute the moment from the top of the footing to get the expected bearing pressure, that there may locally be tension at the top?

In the case that soil/footing self weight is required to keep e<B/6 (i.e. not enough gravity load from the superstructure itself to keep e<B/6)) i'm of the opinion top steel is definitely required (or concrete tensile design).

Thanks

 

[KootK]

Lets say you have a square spread footing with a column with a small moment and large axial load such that the entire surface under the footing is pushing up on the footing (i.e. e<B/6), and the gravity load in this case is only due to the superstructure (not the footing or soil self weight). Will there be any tension in the top of the footing at all?

I don't believe so.

I'm thinking maybe locally in order to distribute the moment from the top of the footing to get the expected bearing pressure, that there may locally be tension at the top?

I see your point and, frankly, it's got me wondering myself now. My tentative thoughts:

1) I would expect the "in the kern" logic to apply to a rigid base plate much as it does the footing itself.

2) For a flexible baseplate, I think that you might get some local tension depending on your Mf/Pf ratio. That said, this would normally be dealt with in your baseplate anchorage calcs, quite possibly by utilizing the tensile strength of the surrounding concrete. Regardless, I don't believe that engineers typically provide a top steel mat for this.

In the case that soil/footing self weight is required to keep e<B/6 (i.e. not enough gravity load from the superstructure itself to keep e<B/6)) i'm of the opinion top steel is definitely required (or concrete tensile design).

I mostly agree. I certainly agree that the condition produces flexural tension in the top of the footing. That said, it's not uncommon, in my experience, for designers to use the flexural strength of plain (unreinforced) concrete to deal with that.

 

[human909]

Based on your description in your first example the top of the pad footing will be in compression all around so not tensile reinforcing required.

The "local tension" you are expecting isn't local tension it is just reduced compression. You can still moments with only compression loads. The same thing occurs in an axially loaded column with small moments. (And my favourite example is a bicycle wheel spoke whose role is to transfer axial load from the axle to the tyre/ground and it does it purely through tension.)

In the case that soil/footing self weight is required to keep e<B/6 (i.e. not enough gravity load from the superstructure itself to keep e<B/6)) i'm of the opinion top steel is definitely required (or concrete tensile design).

The analysis approach of e<B/6 is not something I've encountered before but if I understand the jist of what you are saying then yes your pad will have tension in the top layer as there is insufficient compression from the superstructure.

 

[phamENG]

and it does it purely through tension.

You just blew my mind. I had never realized that before...

(Also, you should have said "a bicycle wheel spoke whose roll is to transfer axial load...")

 

[JAE]

it is just reduced compression.

Amen!

 

[dik]

If your column load is within the kern, there is no part of the footing that is loaded in tension. The footing has to be larger than the column and therefore the load on the footing occurs within the kern of the footing... My $.03 (two cents American)

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

-Dik

 

[human909]

You just blew my mind. I had never realized that before...

(Also, you should have said "a bicycle wheel spoke whose roll is to transfer axial load...")

Glad somebody enjoyed that interested titbit. It gets ever more interesting when the internet starts arguing whether the bicycle hub "hangs" from the top or is "supported" from the bottom.

It can be show through tests and by inspection that tension largely remains the same in ALL the spokes except for the local ones around the contact patch which reduces. Too loose spokes undergo tension reversal at then fantigue will occur and the J-hook (more from movement than the reversal as such) and the spoke fails. And of course the spoke aren't perfectly radial as otherwise they can't transfer torque! Only front wheels with rim brakes can have all radial spokes.

(The London Eye is another tension structure. But it is probably more obviously so and has a less uneven load distribution.

 

[KootK]

@human909: are you a wheel builder? I feel like I'm hearing a lot of Jobst Brandt here...

 

[human909]

@human909: are you a wheel builder? I feel like I'm hearing a lot of Jobst Brandt here...

I though I was more of a Sheldon Brown acolyte. The name rings a bell but I had to look up exactly who and even Sheldon defers to Jobst when it comes to bicycle wheels. Further googling and yes I had read posts/quotes from Jobst Brandt before.

Regarding building wheels. I've only built one from scratch, but I've trued and repaired many. These days I find bicycle maintenance less 'Zen' than I used to.


(I "wasted" many years in other career directions and got my physics/engineering kicks from my hobbies including cycling and rockclimbing, the latter you really do place your life in the hands of your engineering judgment! I studied engineering as a mature aged student and now spend much more time on Eng-tips and and professional development as I try to catch up on experience.)