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ACI 318-08 Appendix A Prismatic Vs Bottle-Shaped Struts

ACI 318-08 Appendix A Prismatic Vs Bottle-Shaped Struts

(OP)
I am wondering how other people interpret the concrete code above when it comes to classifying a strut as either prismatic or bottle shaped. Here is why the question arises:

Fig RA.1.3 shows a dark lined prismatic strut and a dashed lined bottle-shaped strut in an application that can clearly allow bottle-shaped effects to develop.

Page 385 Strut Commentary states struts are usually idealized as prismatic. Just below this the Bottle-shaped struts commentary says Bottle-shaped struts occur when the concrete CAN spread laterally.

Fig RA.1.8 seems to indicate that the width for a Bottle-shaped strut should be the same width that would come from a prismatic strut model.

The reason the distinction matters is because in equation A-3 bottle-shaped struts are much weaker than prismatic struts by either 40% or 25% if you include reinforcement that you do not need in a prismatic strut. But, back to the second paragraph of the Bottle-shaped strut commentary where it states you can treat bottle-shaped struts as prismatic if you put reinforcement in that should only allow 75% capacity now allows 100%.

I am currently of the opinion that if a Bottle-shaped strut CAN develop then the factor has to be either 0.6 or 0.75 depending on reinforcement.

This is reinforced by a paper I found called: Computer-Based Tools for Design by Strut-and-Tie Method: Advances and Challenges by Tjen N. Tjhin and Daniel A. Kuchma on page 592 where it discusses Capacity of Struts w.r.t. its figure 11.

I would love to hear some thoughts on this topic,

John-Jozef Proczka (Structural EI)

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