Casting a steel post in a concrete fooring with strong uplift.

[Parrapit]

Hey Guys

I need to design or check a connection of a steel post 100 X 100 X 2MM square tube that is cast in the concrete footing.
The post at the bottom has 2 steel re-bar drilled through the post and horizontally imbedded into the concrete. There is a big uplift force of around 30 KN.

So the part that needs attention would be min thick of concrete needed not to counter weight, the up lift force, have done that all ready, but for the punching shear or pull out force. As well as the actual bonding between the bars, post and concrete.

Normally you would have a base plate so that’s easy but how do you determine the (pull out) strength of a horizontal rebar holding the post.

Also probably have to check the shear on the post so the rebar does not tear out the post.


Concrete strenght of 25Mpa Rebar around Y12.

Any suggestions

 

[msquared48]

Check the shear capacity of the rebar. Being comfined in the concrete, that will probably control.

If you do not have enough cpacity with the rebar, consider welding a base plate to the end of the steel columnn that extends beyond the column sides to pick up the concrete and develop a shear cone.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

30 kN is not all that much. Your detail is probably fine, as long as the footing is big enough and the base is deep enough to engage the footing.

 

[Parrapit]

Thanks for the Input

I decided on the following. I will check the shear on the re-bar, then the shear of the rebar and post connection. Then just allow min anchorage for the re-bar in the concrete and bobs your uncle.

I have another question when you design a base without steel then let say the base is 1m X 1m then the thickness of the base should be from the wall or in my case a post 45 degree down to get the thickness of the base. Therefore the base thickness will be 500MM. But now my post is imbedded do I take the 45 degree from the bottom part of the post. So my post is imbedded say 150mm and my base is 1m then is my base thickness 150 + 500.

Also this is now for punching of a downwards force. Does this count for the uplift force as well to determine the thickness?

The way I see it won’t cause the force is reversed and therefore the load does not press against the ground downwards but merely relay on the self weight to keep the base in the ground so it does not work the same as for a downward force.

Any Suggestions

 

[hokie66]

I don't think 150 embedment is enough. And a 1000 x 1000 x 650 footing is only about 15 kN, clearly not enough to resist the 30 kN uplift which you reported, unless there is something else contributing.

 

[Parrapit]

Yes I know its not enough just trying to give a hypothetical example let say it was enough or my load was only 3 KN.

Want to know if you agree with my thinking. There is also other things contribution soil on top of the base as well as the self weight of the roof pressing down on the base.

I calculated a 1.5m x 1.5m a 300mm base but now want to make sure of the thickness.