When are Compression Ties Req'd?

[JAE]

The ACI code specifies spacing and arrangement of lateral ties for compression members in Chapter 7. It specifies that all bars, or every other bar when clear spacing <= 6&quot; be enclosed by lateral ties.

I seem to remember somewhere that ACI defined what a &quot;compression&quot; member was....in other words, you have flexural beams with 0 axial forces and you have building columns with lots of axial forces. Where is the line dividing the two? Isn't there somewhere in ACI something that reads: Compression members are those members with axial loads that create stress in the concrete > some number or some percent?

 

[Taro]

Compression members have axial stress greater than 0.1f'c. This is the dividing line that is consistently used in sections 10.3.5, 21.3.1.1, etc.

 

[rapt]

Compression reinforcement also needs to be tied in flexural members where no axial compression exists, only flexural compression. If the member requires compression reinforcement for ductility then it must be tied.

 

[ishvaaag]

I entirely agree with rapt and yet note how frequent is to have reinforced plates in slabs and walls with compressed rebar that does not enjoy the luxuries of any confinement against buckling out of the plane, and hence spalling. This no doubt adds to the fragility of such structural subassemblies, since the ability to stand the peak stress and any further cycles is much diminished.

 

[ishvaaag]

And I further comment myself ... we are always thinking of extraordinary loadings (well, limit ones, such is our job when talking about life safety). Yet for service levels, and with nonstructural attachments, movement is almost unnoticeable (as is the intent for buildings). Hence, and in the same way weak bracing forces are able to forestall the lateral buckling of a member, the adhesion of the rebar to the concrete, in this cover, itself causes such lateral restraint that the outwards buckling of unconfined compressed rebar becomes unlikely ... even if the steel gets to yield stresses upon slow creep transfer of compression stresses to the rebar steel!

So much true is the case that I myself heard over 20 years ago the tale of a mason that indicated me he had participated in the 40's in the construction of a 10 story or more tall reinforced concrete structure building, and the columns had NO stirrups. The building is still in use and perfectly standing, but is heavily infilled with attached brick masonry.

Where I live the earthquake acceleration stands per code under 0.04g at ground level, so weak earthquake area fortunately, but we have experienced two or so 5 Richter scale events in these years -enough to stress our things beyond any expected wind loads- without any consequences whatsoever, nor for this named building.

 

[JAE]

ishvaaag - haven't seen you around here much lately? Where have you been? Good to hear from you again.

 

[ishvaaag]

I also missed the site and many of you ... just did things, and not all as fun and useful as coming here.

Thanks for your greetings, and best wishes for all.