Concrete Beam Question 2

[XR250]

Suppose I have a 24" wide x 12" thick strip footing with a point load from say, a shearwall chord, that can be either up or down. If I design top and bottom rebar to resist the demand, do I need to add ties to confine the reinforcing? The rebar is not necessary to increase the compression face capacity of the beam - it is only there to take care of the stress reversal. Shear reinforcing in not required based on the demand and footing size.

Thanks in advance!

 

[r13]

If concrete shear capacity is adequate, no tie is required, but you should provide straight transverse bars to hold the longitudinal bars firmly in place, a good practice for construction concerns.

 

[BAretired]

I agree with retired13. Us retired folks have to stick together.

BA

 

[r13]

 

[XR250]

Thank you retired contingent!

 

[BAretired]

You are entirely welcome! We don't have much else to do these days.

BA

 

[XR250]

LOL.

So is there a point at which you need ties to confine the reinforcement? Won't the buckling of the rebar eventually overcome the concrete's ability to confine it w/o ties?

 

[r13]

You won't get that much of compressive stress required to "buckle" the rebars in a strip foundation. Also, it is placed in a somehow confined environment, especially top and bottom.

 

[dold]

Off the top of my head (for a special wall or wall pier) i think boundary elements and confinement reinf is required when max compressive stress exceed 0.2f'c.

While were on this bit, and since we've got the retired contingent hanging around...In a SMF column, tie spacing can get down to 2" o.c. pretty quick. A lot of that is for strength and preventing long bar buckling, but is there any consideration of trying to "hold the rubble" in the column cage to provide *some* degree of capacity? I'm out in left field, by the way...

 

[KootK]

A lot of that is for strength and preventing long bar buckling, but is there any consideration of trying to "hold the rubble" in the column cage to provide *some* degree of capacity?

I actually believe that those very tight spacing have more to do with concrete confinement than longitudinal bar buckling. You shouldn't have to get anywhere close to a 2" spacing to take your longitudinal column bars well into the inelastic range.

 

[BAretired]

@dold,

As long as you're expecting help from the retired contingent, you should realize that we don't know what the heck you're talking about when you say SMF column. I have never in my life used a column with ties spaced at 2". Must have a lot of shear, I'm guessing. Not required to prevent bar buckling.

BA

 

[r13]

dold,

Retired can be "retired dumb" Like I don't understand the connection between "SMF" and tie, which is used for reinforced concrete structures but "steel moment frame". In a concrete column, if tie spacing done to that small, spiral reinforcement may be more beneficial than ties.

but is there any consideration of trying to "hold the rubble" in the column cage to provide *some* degree of capacity?

Isn't "hold the rubble" to provide "capacity" has already answered by yourself in saying "...A lot of that is for strength and preventing long bar buckling..."? Confinement increases the compressive capacity by tightly hold the compression steel in place to ensure the concrete core is well protected from busting, thus provide the desirable strength. This been the reason why column reinforced with spiral cage allowed a higher compressive force than a tied column.

Did I mentioned shear? Look up to ACI definition for "stirrups", you will find it refers to ties...

 

[KootK]

So is there a point at which you need ties to confine the reinforcement? Won't the buckling of the rebar eventually overcome the concrete's ability to confine it w/o ties?

That's an interesting question that I've struggled with myself.

1) In situations such as high seismic elements expected to experience serious inelastic strains, clearly you can buckle unrestrained compression bars. We've all seen pictures of shear wall zone rebar all gimped up after earthquakes.

2) There are gobs, and gobs, of common, elastic-ish design situations where we inadvertently have unrestrained compression bars in the high curvature zones of flexural members and nothing bad ever seems to happen. Almost every two way slab has untied bottom bars running near the columns and you never hear of those buckling out the bottom. Ditto for grade beams at piles. So, on an anecdotal basis, there would certainly seem to be little to worry about for your application.

3) As for the "why" of it, I think that it simply takes more strain than concrete's 0.003 US limit to induce bar buckling in situations with common densities of rebar and amounts of cover. If your flexural design limited your compression side strain to 0.003 at ultimate, you probably never even see 0.002 there.

4) Columns are different animal from purely flexural members in that creep tends to transfer some of concrete compression into the steel over time. So there's a very real possibility that your column rebar will indeed see strains in excess of 0.003.

5) This kinda raises the question of whether or not beams designed to utilize compression reinforcement need that reinforcement to be tied column style? Per code, I believe that this is necessary. In terms of real world performance, I very much doubt that's the case unless we're talking plastic hinge zones etc.

6) If your design were my design, I probably would toss a gaggle of stirrups in near the anchor. That, to restrain the beam rebar locally and to improve the anchorage condition, both in terms confinement and robustly dragging the imposed load back to the far side of the beam.

 

[KootK]

SMF = Special Moment Frame. US designation for a system designed for maxim ductility levels.

 

[r13]

KtootK

I've thought it later, thanks.

 

[KootK]

For the curious, this is a brief but thorough document outlining what an SMF is and what is required of it: Link

 

[EZBuilding]

Following up on KootK's #6 if it were my design

Would probably ignore the effects of compression steel in the design and I do see the benefits of having some additional stirrups near the anchors. (3) #3 stirrups @ 6" o.c. each side of anchors and 24" o.c. balance. The field guys will need some type of vertical steel to tie off the top bars to, might as well include the those in your design for good measure.

Following up on KootK's #2

It would be very atypical to have compression steel be anchorage through similar provisions to columns. Double stirrups are already pretty rare, and even those may not provide sufficient confinement per the column tie provisions.

 

[XR250]

Thanks for the info!

 

[dold]

Sorry, SMF, special walls and wall piers - all concrete. Should've clarified. As for "strength" I was talking about shear strength. I was surprised by the amount of steel that can be needed to develop the required shear strength based on maximum probable moment in a column/wall pier (ACI 318-14 chapter 18). I was getting jammed up with this on some really tall tilt wall piers - the long out-of-plane span required a ton of vertical steel, which in turn caused the maximum probable moment of the wall pier to be unmanageable (i.e. <2" tie spacing). Luckily you can limit max probable moment in the pier by applied shear times overstrength. And depending on dimensional limits, some wall piers must be designed/detailed as a (concrete) special moment frame column, which require, from the get-go, maximum tie spacing of 6".

As for "tie" vs. "stirrup" - i've always referred to them as "ties" (or "hoops") when used in a column or wall application (resisting horizontal forces); "stirrup" when talking about beam shear as the word "stirrup" is something you stand on when riding a horse, thereby resisting vertical forces. Terminology in ACI follows that loosely (sans the horse part..)

XR250, apologies for the derailment. And please excuse my rambling, incoherent prose...

 

[r13]

dold,

Again, I'm confused on "...be needed to develop the required shear strength based on maximum probable moment...", and feel the need to connect the dots - shear strength and moment. Do you mind to open a new thread, with simple sketch depict the situation, as your question could be quite involved, and can be benefited from more inputs from a wider pool.