Strut and tie Bs question 2

[Gus14]

According to ACI, Bs for tension members or tension zones of members is 0.4. What do they mean by tension zones of members? And how would a tension member be considered a strut wouldn't it then be a tie ?

 

[Settingsun]

If your strut-tie model is in the xy plane, this is referring to tension in the z direction.

 

[Gus14]

Thank you 1503-44 and steve for replyjng, I am still wondering however what they mean tension zone member.

 

[strucbells]

1503-44, OP is asking in the context of ACI's strut and tie method, in which struts are regions of a concrete member idealized as always in compression and ties are idealized as always in tension and represent steel reinforcing. Steve has it right I think, see below:

From one of the papers below: "For struts intersecting cracks in a tensile zone, βs is reduced to 0.4. Examples include STMs used to design the longitudinal and transverse reinforcement of the tension flanges of beams, box-girders, and walls. For all other cases (e.g., in beam webs where struts are likely to be crossed by inclined cracks), the βs factor can be conservatively taken as 0.6."

There are a couple of graphics in the SEA presentation about halfway through that illustrate this concept.

Here's a couple of resources:

https://structurepoint.org/publication/pdf/pdh-Strut-And-Tie-Design-Method.pdf

https://oksea.org/wp-content/upload...sign-What-They-Didnt-Teach-You-in-Scho....pdf

 

[KootK]

I am still wondering however what they mean tension zone member.

The ACI 318-19 commentary blurb shown below has a good example of what steveh49 mentioned.

The principal to keep in mind is this:

1) Where a strut has lateral confining reinforcement or compression surrounding it, it's longitudinal compression capacity will be enhanced.

2) Where a strut has a lateral tension surrounding it, it's longitudinal compression capacity will be diminished.

The Bs values reflect this and it all ties back to the triaxial compression state stuff that you probably studied in your undergraduate concrete labs.

 

[Gus14]

Thank you KootK, and Strucbells for dumbing down the concept for me. But to make sure I understand, for the following arbitrary strut and tie model would you consider the highlighted struts 4,6,7,10,11 and 12 in tension zones because of the lateral tie surrounding it from the load application point to the support?

 

[KootK]

But to make sure I understand, for the following arbitrary strut and tie model would you consider the highlighted struts 4,6,7,10,11 and 12 in tension zones because of the lateral tie surrounding it from the load application point to the support?

1) I'm honestly not sure as it's been a while since I've tried to work with that provision and much has changed.

2) Historically, there has usually been some manner of anti-confinement penalty from struts that cross ties. Whether that's a reduction in compressive capacity or a requirement for more confining reinforcing depends on the code being used and the time period in which it has been used.

3) In my opinion, of the six highlighted struts, the only ones that have this problem are struts six and twelve as they are the only struts crossed by ties. Frankly, I consider this to be a great reason to adjust your model so as not to have any struts crossed by ties. There is no strict requirement to avoid that situation but it's certainly good practice to avoid it when it's easy to do so as is the case here I think.

4) Yeah, I feel that Bs = 0.4 probably is appropriate for struts six and seven even though their tension is in the plane of the truss rather than perpendicular to it. If others know differently, however, I'd be happy to hear about it.