Strut-Tie

[Juz110]

Hi,

I have a 2mx2m pile cap that is 1 meter deep on 4 piles with 1.8 meter span and 1.4m span in another direction. A 300x300mm column with 2500kN is applied on top with a eccentricity of 300mm in 1.8m span. Refer below.

I have designed the pile cap with strut-and-tie method, check the strut capacity and provide 10N28 Bars at the bottom in both direction with cogs as tie. But I didnt provide any shear reinforcement. Now my question is, if strut-and-tie is used, do we need to check beam shear as well? (Because if we do, there is no chance the concrete shear capacity is adequate with given 2500kN axial load). I use Australian Standards but open to anyone's opinions.

Much appreciated.

 

[Tomfh]

You don't specifically check beam shear. You check the struts and ties. You often end up with strut bursting reinforcement, which is effectively shear reinforcement.

 

[Juz110]

Thank you. When you said strut bursting reinforcement how do you provide them in pile cap? What I did was just top and bottom reo with cog at end (and top and btm reo cog doesn’t lap together)

 

[Tomfh]

Look in the code. It shows required bursting reo that needs to cross the struts. We tend to use regular ties between top and bottom reo.

You may or may not need it. You have to assess the bursting forces, the strutting angles, etc, which is somewhat tedious.

 

[Juz110]

I did check the strut capacity without bursting reo though. And here comes my another question. What width should I choose to calculate the strut capacity? This one is a bit odd. For some reason screw pile is used instead of concrete pile. So, let’s say there are 300x300 cap plates on top of the screw piles. What width should I adopt to check strut capacity?
Thank you.

 

[Juz110]

Also, since the depth is 1m while the max distance between support to column is 1.1, meaning the angle is less than 45 degree. Is strut tie method still applicable here without considering beam shear?

 

[Juz110]

I just went through the code but have some doubt.
So I have four struts here. With 2500kN axial forces I have roughly 850kN per strut. So I can work out the burst force to be around 450kN.

But I am wondering when comparing to the capacity, what width b should I adopt? Can I take half of the pile cap width, which is 1000mm?

Also, for the strut strength check, what dc should I use considering the cap plate is 300mm wide?

Much appreciated if anyone can help.