Strut and Tie Models for Concrete Pilecaps

[ZeroStress]

Hello

Usually the strut in the STM is in compression but I have seen many cases where the struts come out in tension. For example when a pilecap has a large bending moment and it experiences lift off on one side, this would result in some of the struts being in tension.

Eurocode gives guidance on how to design compressive struts but no guidance on what to do with tensile struts.

Can anyone help me with this please. What do you normally do with a tensile strut? How do you design the reinforcement in the tensile strut region?

Thanks.

 

[hokie66]

By definition, struts are in compression. If it is in tension, it is a tie.

 

[TLHS]

If you've built a truss and are assuming that some areas are struts, you need to remove them from your model if analysis indicates that they go into tension. If you need that tension for stability then you need a tie in that location for that load case, and you will need to put bars in.

It is not unusual for some areas to be struts in one load case or load direction and ties in another.

 

[KootK]

How do you design the reinforcement in the tensile strut region?

- Depends on your local code but, in my area of practice, bars will be sized to 0.75 x Fy x As for the tensile demand.

- Keep an eye on anchorage of your tension ties at nodes. That's often more critical that the quantity of bar required.

- In proportioning your ties, give some thought to how much strain is implied by your design. Strut and tie doesn't have much to say about serviceability but, obviously, it still matters.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.