Rebar in a Sonotube

What the bar in a tube actually has to do

A sonotube pier is a short concrete column that is almost never loaded like a column. It sits on a footing, carries a post or a beam, and the load arrives as compression plus whatever the deck does to it sideways: wind, a guardrail, a slope, a truck backing into it. Concrete is fine at the compression. The bar exists for the other three things.

Three jobs, and each one produces a different number:

• Tie the pier to the footing. This is the one that sets bar length, because the bar has to develop into the footing concrete, not just sit in it. ACI 318-19 25.4.9.2.
• Hold the cage in shape. A tied cage needs verticals to hold a tie, and a tie needs verticals to hold a shape. Four is the floor, not a design.
• Take tension from uplift and bending. This is where bar size and count come from, and it is the only one of the three that needs an engineer or a drawing.

Everything on the rest of this page is the first two. The third one you cannot get from a table, and any page that pretends otherwise is selling you a number.

01 Size the cage, not the count

The cage diameter is not the tube diameter. It is the tube minus two covers minus one bar diameter:

D_cage = D_tube − 2 × cover − db

For a 12 in tube, #4 bar and 1.5 in of cover: 12 − 3 − 0.5 = 8.5 in. That is the circle the bars sit on, and its circumference is 26.70 in. At the ACI 318-19 25.2.1 minimum clear spacing of max(1 in, db) — 1 in for #4 — seventeen bars fit. So when somebody asks how many bars go in a 12 in tube, the geometry says up to seventeen and the answer they get is four, which tells you the number came from somewhere other than the tube.

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Where the four-bar habit comes from: a rectangular or circular tie needs at least four corners, and a cage with two bars cannot be tied into a shape that resists anything. Above 12 in the habit becomes six or eight, again for shape and for spreading the bar around the perimeter rather than for strength. None of those three numbers is a code requirement.

02 Cut the bar to the footing, not to the tube

This is where the internet answer and the code answer part company. The forum answer for a 12 in × 4 ft tube is 45 in, derived as 48 in of tube minus 3 in of cover. It is neat, it is repeatable, and it is wrong, because it treats the bottom of the tube as the end of the member. The member continues into the footing, and the bar has to develop there.

bar length = tube height − top cover + development length

ldc = 0.0005 × fy × db   (ACI 318-19 25.4.9.2, fy ≤ 60 ksi)

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*The starred cells are the ones that fail. A 12 in footing with 3 in of bottom cover leaves 9 in of concrete for the bar to grab. #4 needs 15 in. There is no cut length that fixes this: the bar has to end in a standard hook, or the footing has to get deeper. If your plan shows a 12 in footing and four #4 bars, one of those two things is missing from the drawing.

The other number in the formula is top cover. The bar should stop 1.5 to 2 in below the top of the pier so the post base, the anchor bolt or the bracket bears on concrete rather than on a bar end. Two inches is the usual detail; if your hardware is a stand-off base plate, read its height instead and use that.

So the four cases that matter, all with a 2 in top cover:

• 3 ft tube on a 22 in footing, #4: 36 − 2 + 15 = 49 in
• 4 ft tube on a 22 in footing, #4: 48 − 2 + 15 = 61 in
• 4 ft tube on an 18 in footing, #5: 48 − 2 + 15 = 61 in — the footing is the limit, not the bar
• 5 ft tube on a 22 in footing, #4: 60 − 2 + 15 = 73 in

If you are checking a whole foundation, the same development arithmetic runs the other way on the development length page, and the footing calculator handles the mat.

03 Two covers live in one pier

ACI 318-19 Table 20.5.1.3.1 gives two different numbers that both apply inside a single sonotube pier, and this is why two people can both be quoting the code and disagree:

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The tube is a form that stays in the ground, so the wall face is formed and takes the 1.5 in line for #5 and smaller. The bottom of the footing is cast straight against soil and takes 3 in, for every bar size. A pier therefore has a 1.5 in cover on its sides and a 3 in cover under its steel, and if somebody applies 3 in all round they lose an inch of cage diameter on each side for no reason.

Conservative practice sometimes applies 3 in to the below-grade part of the tube wall as well, on the argument that the cardboard is in contact with soil. That is defensible and it costs you cage diameter: on a 12 in tube, #4 bars go from an 8.5 in circle to a 5.5 in circle. Both are legal; the 3 in version just packs four bars into 3.82 in of clear gap instead of 6.18 in.

What is not defensible is the third habit: setting the cage on the soil. Three inches of clear below the steel, on a dobie or a precast base, or the bottom of the cage is at the corrosion line and the pier spalls from inside in ten years.

04 Tie at 8 in, not at 16

For a tied cage, ACI 318-19 25.7.2.1 caps tie spacing at the least of three things:

s ≤ min( 16 × db_vertical, 48 × db_tie, least dimension of member )

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Notice what drives it: for #4 verticals the answer is always 8 in, no matter how big the tube, because 16 bar diameters is the smallest of the three. The tube only starts to matter above #6.

Now compare that to the detail that circulates on forums for a 4 ft pier: three ties in the top 16 in, one in the middle, one at the bottom. Three ties inside 16 in is roughly 8 in on centre — correct at the top. Then it jumps to a 16 in gap, which is twice the limit, and the bar is free to bow exactly where the pier is most likely to crack. A 4 ft pier at 8 in on centre takes six ties, not five, and they are evenly spaced.

The tie is also a purchase, not a detail. Its diameter is the cage diameter from section 01 — tube minus two covers minus the vertical bar — so a 12 in tube with #4 bars takes a ring bent to 8.5 in, not to 12 in. One turn around that circle is 26.70 in of #3; add the lap you close the ring with (6 in here) and the cut is 32.70 in. Six ties over a 4 ft cage is 196.2 in, 16.35 ft, 6.15 lb, and it comes off one 20 ft stick with 43.8 in left over.

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Read the last column as a buying rule: 8 in through 14 in tubes take all six rings out of one 20 ft stick of #3, and 16 in and up need a second stick. A 14 in tube is the tight one — 6.1 in left, so a seventh ring will not fit. Ordering rings at the tube diameter instead of the cage diameter is the same mistake as cutting the bar to the tube: the steel sits inside the cover, not at the form face.

05 Pour volume, bags and the cut list

Volume is the easy half: ft³ = 0.7854 × D² × H ÷ 144 with D in inches and H in feet. A 12 in tube holds 0.785 ft³ per foot, so 3.14 ft³ over 4 ft, which is 0.116 yd³.

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Steel, for twelve piers of 12 in × 4 ft with four #4 bars each at 61 in:

• One bar: 61 in = 5.083 ft × 0.668 lb/ft = 3.40 lb. A pier is 13.6 lb. Twelve piers are 163.2 lb net.
• Pieces: 12 × 4 = 48 bars.
• In 20 ft sticks: 240 ÷ 61 = 3 pieces per stick, 57 in left over. 16 sticks, 320 ft, 213.8 lb — a 31% allowance.
• In 24 ft sticks: 288 ÷ 61 = 4 pieces per stick, 44 in left over. 12 sticks, 288 ft, 192.4 lb — an 18% allowance.

The 24 ft order is the better buy for this length and almost nobody runs it, because 61 in does not divide 240 in and the offcut is invisible on a receipt. Weight per bar for every size is on the weight calculator, and the size designations are on the size chart.

06 Four ways to get it wrong

1. Cutting 45 in because the tube is 48 in

The bar ends at the bottom of the tube and the pier is held to the footing by nothing but friction and a bit of aggregate interlock. This is the single most repeated wrong answer for this question, and it is wrong by exactly the development length.

2. Pounding the bar into the soil to hold it upright

Convenient, and it puts a chloride path through the full height of the pier. Rust occupies several times the volume of the steel it consumes, so the failure is a vertical split, not a rust stain. Set the cage on dobies and let it develop in concrete.

3. Using 3 in of cover all round on a small tube

On an 8 in tube that leaves a 1.50 in cage and four bars at 0.68 in clear — below the 1 in minimum, and physically impossible to place. The 1.5 in line in Table 20.5.1.3.1 exists for exactly this case.

4. Tying the top and letting the middle go

Three ties in the top 16 in and then a 16 in gap is a bar with a free length where the pier is in bending. Six ties at 8 in on centre cost about a dollar and remove the question.

If you are setting chairs under a slab rather than a cage in a tube, the spacing rules are a different problem and they are on the chair spacing page.

Watch: 61 in, in one screen

Common questions

How long should rebar be in a 4 ft sonotube?

61 in for #4, not 45. The 45 in figure is the tube height minus 3 in of cover, which leaves the bar ending at the bottom of the tube with nothing embedded in the footing. The bar has to develop into the footing: ldc = 0.0005 × fy × db under ACI 318-19 25.4.9.2, which is 15 in for #4 Grade 60, 18.75 in for #5 and 22.5 in for #6. So 48 in tube minus 2 in of top cover plus 15 in of development is 61 in.

How many pieces of rebar go in a sonotube?

Four is the convention for 8 in to 12 in tubes, six for 14 in to 18 in and eight for 20 in to 24 in, and the number is not in the code. Geometry never limits it: a 12 in tube at 1.5 in cover has a 26.70 in cage circumference, which fits seventeen #4 bars at the 1 in minimum clear spacing of ACI 318-19 25.2.1. What sets four is that a tied cage needs at least four verticals to hold a tie shape, and then whatever uplift and lateral load the pier has to take.

What size rebar for a 12 inch sonotube?

Four #4 bars covers most deck piers; four #5 is the step up for taller piers or heavier uplift. Going to #5 is not a heavier bar in the same hole: the cage diameter drops from 8.5 in to 8.375 in, the tie spacing opens from 8 in to 10 in, and the bar length grows from 61 in to 64.75 in because the development length grows with diameter.

How deep does the rebar go into the footing?

As deep as it takes to develop the bar, and no deeper than the footing allows. A 22 in footing with 3 in of bottom cover leaves 19 in available, which covers the 15 in that #4 needs. A 12 in footing leaves only 9 in, which is short of 15 in, so the bar needs a standard hook or the footing has to be deeper. This is the number nobody quotes and it is the one that fails inspection.

Do you need rebar in a sonotube?

For a pier that carries uplift, lateral load or a guardrail, yes: the bar ties the pier to the footing and takes the tension concrete cannot. For a plain deck pier on flat ground with no uplift, a cage is optional and some engineered sets call for none. The prescriptive residential deck provisions do not demand a cage in every pier, so the drawing decides. What is not optional is that a bar, once you put it in, has to be placed with proper cover and proper development, or it does less than nothing.

How far apart do the ties go in a sonotube?

For a tied cage, ACI 318-19 25.7.2.1 caps tie spacing at the least of sixteen vertical bar diameters, forty-eight tie diameters and the least dimension of the member. With #4 verticals and #3 ties in a 12 in tube that is min(8, 18, 12) = 8 in. The common forum habit of three ties in the top 16 in, one in the middle and one at the bottom leaves 16 in gaps, twice the limit.

Should you pound rebar into the ground under a sonotube?

No. A bar driven into soil gives chlorides a path straight up into the pier, and rust occupies several times the volume of the steel it replaces, so the pier spalls from the inside. Set the cage on dobies or a precast base with 3 in of clearance below it, and let the bar develop into the footing concrete instead of into dirt.

How much concrete does a 12 inch sonotube hold?

0.785 cubic feet per foot of height, so a 4 ft tube is 3.14 cubic feet, or 0.116 cubic yards. At the 0.6 cubic feet yield of an 80 lb bag that is 5.2 bags, so buy 6. A 10 in tube holds 2.18 cubic feet over 4 ft and a 16 in tube holds 5.59, which is 4 and 10 bags respectively.

What size rings do I need for a sonotube cage?

The ring diameter is the cage diameter, not the tube: tube minus two covers minus the vertical bar. A 12 in tube with #4 verticals at 1.5 in of cover takes 8.5 in rings, so one turn is 26.70 in of #3 and the cut is 32.70 in once a 6 in lap is added. Six rings for a 4 ft cage is 196.2 in, 16.35 ft and 6.15 lb — one 20 ft stick with 43.8 in left over. Rings are bent to the cage for the same reason the bar is cut to the footing: the steel sits inside the cover, not at the form face.

Bar weights and areas per ASTM A615, Grade 60. Cover, clear spacing and tie spacing per ACI 318-19 Tables 20.5.1.3.1 and 25.2.1 and Section 25.7.2.1. Concrete yield assumes 0.6 ft³ per 80 lb bag. Development length per ACI 318-19 25.4.9.2 for Grade 60 in normal-weight concrete.