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

[JJProczka]

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)

 

[Ron]

JJP...wrong forum. Ask the question in the Structural Engineering Other Technical Topics forum or the ACI Code Issues forum.

Red flag your post (just click on the "Red Flag this post" at the bottom of your posting) in this forum and forum management will remove it. It is against forum rules to post the same question in multiple forums.