column spirals

[ajk1]

a) Does anyone have a Table of column spirals satisfying CSA A23.3 (or presumably the equivalent clause in ACI 318), or know where there might be such a Table? The Table would account for column diameter, spiral diameter (which depends on concrete cover, which depends on fire rating as well as durability exposure classification), grade of steel, concrete strength and spiral pitch.
Or do I have to calculate such a Table myself?
Preferably the Table should be for Canadian metric bars, but even a Table satisfying American imperial bars might be helpful as a rough check on any calculation that I may have to do.

b) Are spirals that are plain bars (with no deformations) supplied in Grade 400 steel (i.e. same grade of steel as Canadian metric bars)?

 

[ajk1]

I have had no response to this posting to-date, so let me re-phrase it since I have in fact prepared an Excel spreadsheet that calculates the spiral pitch based on the new formula in CSA A23.3 Design of Concrete Structures.

My updated questions now are:

a) What is generally used for spirals: plain round bars (such as 10 mm diameter), or deformed bars? CSA A23.3 says that the minimum diameter of spirals is 10 mm. This implies to me that spirals are not generally deformed rebar or it would have said 10M.

b) If plain round bars are generally used for spirals, what grade of steel do they come in? Do they come in the same grade as deformed rebar (fy=400 MPa)?

It would be a plus if you could frame your answers, if possible, for Canada (we are in Ontario), although I would also be interested in answers from elsewhere.

 

[jayrod12]

perhaps the question should be asked of a local rebar fabricator. I know locally here there is only one supplier that can make spiral reinforcing.

 

[BridgeSmith]

Don't know what your fellow Canucks use, but we (state DOT in the USA) always use deformed bars or deformed coil (for seismic applications requiring continuous spirals)

 

[ajk1]

Thank you Hotrod10. Our work is not in a strong seismic zone. Does your reference say deformed bars in all seismic areas regardless of intensity? I now notice that the latest Cement Association of Canada (CAC) Concrete Design Handbook Table 7.7 lists only 10M bars (which of course are deformed bars) for the spirals. Hopefully they checked with suppliers before preparing this table. Someone in our office will double-check with a supplier (I am retired and my supplier contacts are also retired). I guess things have changed from 50 years ago when the CRSI Handbook I have listed only plain bars for spirals, which is the way I recall spirals in the field when I used to go on site. As far as the CSA A23.3 Standard goes, there seems nothing to say only deformed bars must be used but I will double check that.

 

[Shu Jiang PEng SE]

“Plain bars are permitted only for spiral reinforcement used as transverse reinforcement for columns, transverse reinforcement for shear and torsion, or confining reinforcement for splices.”
Yield strength less than 100ksi is normally acceptable. The tie diameter shall be determined by the diameter of the longitudinal bars.


—————————————————————
J&J Structural Consulting Inc.
Structural design, analysis, inspection, drawing review and stamping, and connection design

 

[BridgeSmith]

Under the AASHTO spec, spirals must be continuous through the plastic hinge zones of columns in Category B (Zone 2) and above, but are not required for the lowest seismic category (Cat. A, aka. Zone 1).

From what I understand it's fairly common to use coiled bar even where it's not required for ease of construction and to save steel.

 

[ajk1]

Is coiled bar deformed or plain bar?

 

[BridgeSmith]

The coiled bar we spec is deformed.

 

[Agent666]

In NZ there is no restriction on spirals being deformed or plain round bars. You'd tend to see both types even if plain round was specified. I'd say plain round is specified for the most part in smaller diameter piles (<500mm, and more likely to see deformed for larger diameter piles (500-2400mm), and/or deformed for larger diameter bars (16-20mm spirals)

 

[ajk1]

Thank you both for this information. Much appreciated.

 

[ajk1]

Another question: Is it possible to fabricate/place a tapered spiral like shown on our typical detail sheet that I am reviewing (see attached) as part of my general review and correction of our typicaldetail sheets? If so, then is it done? I do not think so, but would like your opinion.

 

[Agent666]

No, just switch to hoops at different decreasing diameters over that region and terminate the spiral above and below.

 

[ajk1]

Thanks. Much appreciated.

 

[ajk1]

But if that were done, then that part of the column would be a tied column and not qualify to be designed as a spiral column, with its greater load capacity. That would defeat the point of using a spiral to get higher strength. I think that the answer must be not to gooseneck the bars in a circular spiral column, because the spiral cannot be fabricated to follow the gooseneck. Instead, use dowels. Carry the vertical bars in the column below to 75 mm below the top of the slab, and carry the spiral to there as well.

 

[hokie66]

Just chiming in here to agree with your last post, ajk1. Using straight bars and dowels is the way to go. Taking the spiral up past the soffit may or may not be required. If you have to do that, it makes the floor reinforcement placement a bit more difficult.

 

[BridgeSmith]

Could the smaller column steel and spiral be carried down into the larger column, so that the column cages overlap? That's what we do at our column to drilled shaft interfaces when the drilled shaft is significantly larger. Of course, we don't have a slab in between...

 

[hokie66]

That's impractical for building work, HotRod. Lapping dowels are used instead.

 

[BridgeSmith]

I suspected it might be different...thanks hokie66.

 

[ajk1]

► to Hotrod10 - I suppose so, but as you noted, that is not the case that I am currently considering. In the case you describe, if you carried the column cage down into the drilled shaft, you would have to stop the drilled shaft concrete placement at the bottom of the column vertical bars and place the concrete from that location to the top of the drilled pier as a second concrete placement, likely on another day...right?

► to hokie66 - you are absolutely right. I meant to say that. Thanks for the correction.