6.2 BEAMS AND GIRDERS _ 6.2.1 STANDARD BAR ARRANGEMENT FOR BEAMS _ (ACI Detailing Manual-2004 FIGURE 5)

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ACI Bar Arrangement for a Continuous Beam (Two Spans L1 & L2)

This picture is one standard ACI bar arrangement for a continuous beam (two spans L1 and L2). I’ll read it for you piece by piece.

1. Geometry and symbols

  • L1 – span length between the external end support and the interior (INT. END) support.
  • L2 – span length between the interior support and the next end support.
  • D – overall depth of the beam section.
  • d – effective depth (from the top compression fibre to the centroid of the bottom tension bars).

At the bottom you see:

  • EXT. END – CENTER – INT. END – END
    These show the positions of supports and the span centre lines along the beam.

2. Longitudinal bottom bars (green bars at the bottom)

  • “EXTEND ALL BOTTOM BAR INTO SUPPORT”
    → Every bottom bar is taken into the support (column/wall) far enough to develop its stress (anchorage length). They are not cut at the face of the support.
  • At the very left support the bar bends up: “STANDARD 90° HOOK (REFER TO 1.1)”
    → ACI allows a standard 90° hook instead of a long straight development length at the end.

So: bottom bars are continuous through the span and anchored inside supports.

3. Longitudinal top bars (negative-moment bars)

3.1 Exterior span (left side)

Above L1 near the left end you see:

  • 0.25 L1 MIN.

This is for the top bar in the first span: From the face of the support the top bar must continue into the span at least 0.25 × L1.

Purpose: to resist negative moment near the support and control cracking.

3.2 Interior support (between L1 and L2)

At the middle top you see:

  • “STIRRUP SUPPORT BAR (IF NECESSARY)”
    That’s a hanger bar at the top used to hang stirrups if needed.
  • Under it: “0.3 L1 OR 0.3 L2 (GREATER)” written on both sides.

This means: Top support bars crossing the interior support must extend into each adjacent span at least the greater of:

  • 0.3 × L1
  • 0.3 × L2

So if L1 ≠ L2 you choose the longer distance on each side. Purpose: enough development, good negative-moment capacity over the interior support.

4. Stirrups (vertical green U-bars)

The zig-zag red arrow labelled “STIRRUPS” points to the shear reinforcement that runs along the full length of the beam.

4.1 Confinement zones

Under the beam, near each support, you see:

  • “(CONFINEMENT ZONE) 2D”

This means: starting from the face of each support, measure a distance = 2 × D along the beam. That region is the confinement (high-shear) zone.

Inside each confinement zone:

  • The first stirrup is placed close to the support:
    text “150 MIN.” and “150 MIN. CENTER” indicate that a stirrup must be within about 150 mm of the support centre/face (exact interpretation depends on your local practice, but idea = not far away).
  • The stirrup spacing here is very tight:
    “50 (TYP.) ≤ S₁, MAX d/2” (reading the chopped text):
    • Typical stirrup spacing near the critical region is 50 mm.
    • In any case, spacing S₁ must not exceed d/2 (ACI rule for maximum shear-stirrup spacing).

So: in 2D zones around supports you use small spacing (≈50 mm) and ensure S₁ ≤ d/2.

4.2 Central zone (mid-span)

Between the two confinement zones in span L1 you see:

  • “(CENTRAL ZONE)” under the word CENTER.

At the top, near the middle of the drawing, you see:

  • 0.125 L1 and 0.125 L2 each side of the interior support, and a 50 mm segment in the middle.

These distances divide the stirrup layout around the interior support:

  • In a short region right at the support the spacing is very small (50 mm).
  • Moving away from the support into the span, up to about 0.125 × L from the centre, spacing can increase to S₁, subject still to the ACI maximum limits (≤ d/2, ≤ some absolute max).

So, the central zone of the span (away from supports) has larger stirrup spacing S₁; the near-support confinement zones (2D) have tight spacing for shear and confinement.

5. How to use 0.25L, 0.3L, 0.125L and 2D in practice

Imagine:

  • L1 = 5.0 m, L2 = 6.0 m
  • D = 600 mm, so 2D = 1.2 m

Then:

  • Top bar at exterior support: extend at least 0.25 L1 = 1.25 m into span.
  • Top bar over interior support: extend into L1 side at least 0.3 × max(L1, L2) = 0.3 × 6 = 1.8 m, and same into L2.
  • Confinement zones: take 2D = 1.2 m from each support face; in that length use tight stirrup spacing (≈50 mm, but ≤ d/2).
  • Around the interior support, the tightest stirrups occur within 0.125 L1 (≈0.625 m) into the left span and 0.125 L2 (≈0.75 m) into the right span, with the very centre portion at 50 mm.

6. Summary in simple words

  • Green bars: main tension bars along the beam.
    • Bottom bars run continuously and go into supports.
    • Top bars extend a fixed percentage of the span length so they have enough length for anchorage in negative-moment zones.
  • Vertical green bars: stirrups for shear and confinement.
    • Very close near supports (confinement zones 2D).
    • Can be more widely spaced in the middle (central zone), but not more than ACI max spacing (≤ d/2, etc.).
  • Magenta numbers like 0.25L, 0.3L, 0.125L, 2D tell you how far to extend bars and where to change stirrup spacing.

If you want, you can combine this explanation with a calculator that uses your actual L1, L2, D, d, dแต‡ and dแต— to automatically generate bar lengths and stirrup spacing for your BBS.

7. Inputs for a calculator (concept)

You already know most of these:

  • L1 = clear span 1 (mm)
  • L2 = clear span 2 (mm)
  • D = overall depth of beam (mm)
  • d = effective depth (mm)
  • db = bar diameter of tension bar (mm) – for stirrup rules
  • dt = stirrup bar diameter (mm) – for stirrup rules

8. Key formulas from the ACI figure

8.1 Top bars

Exterior top bar in span L1

L_ext_top = 0.25 × L1 (minimum)

So, cutoff of that top bar is at x = 0.25 × L1 from the exterior support face.

Interior support top bars (negative bars over the middle support)

They must extend into each span (L1 and L2) at least the greater of 0.3L1 and 0.3L2.

L_int_neg = 0.3 × max(L1, L2)

Then:

  • Into span 1: extend L_int_neg from the face of the interior support into L1.
  • Into span 2: extend L_int_neg from the face of the interior support into L2.

8.2 Confinement zones and central zones

The figure shows confinement zones of length 2D from each support.

L_conf = 2 × D

For each span:

Span L1:
  left confinement zone:   0 → 2D
  right confinement zone:  L1 − 2D → L1
  central zone:            2D → (L1 − 2D)

Span L2:
  left confinement zone:   0 → 2D   (from interior support into L2)
  right confinement zone:  L2 − 2D → L2
  central zone:            2D → (L2 − 2D)

8.3 ACI rules for stirrup spacing

Region near supports (confinement zone) – use S1 (tight spacing):

S1_max = min( d/4 , 8 × db , 24 × dt , 300 mm )
Choose S1 ≤ S1_max.

Region away from supports (central zone) – use S2 (larger spacing):

S2_max = min( d/2 , 24 × dt , 300 mm )
Choose S2 ≤ S2_max.

8.4 Number of stirrups in each zone

For any zone of length L_zone with spacing S:

N = floor( L_zone / S ) + 1   (include stirrup at start of zone)

Example for span L1:

L1_conf_zone = 2D
L1_mid_zone  = L1 − 2×2D = L1 − 4D

N1_left_conf  = floor( L1_conf_zone / S1 ) + 1
N1_right_conf = floor( L1_conf_zone / S1 ) + 1
N1_mid        = floor( L1_mid_zone  / S2 ) + 1

9. Summary table (symbol → formula)

Item Symbol Formula (mm)
Exterior top bar extension L_ext_top 0.25 × L1
Interior negative bar extension L_int_neg 0.3 × max(L1, L2)
Confinement zone length L_conf 2 × D
Central zone length (span 1) L1_mid L1 − 4 × D
Central zone length (span 2) L2_mid L2 − 4 × D
Max stirrup spacing (zone 1) S1_max min(d/4, 8db, 24dt, 300)
Max stirrup spacing (zone 2) S2_max min(d/2, 24dt, 300)
Stirrups per zone N_zone floor(L_zone / S) + 1

You can now copy this whole block as a Blogger HTML post. Later we can attach a live calculator or sketch under this explanation using the same variables (L1, L2, D, d, db, dt, S1, S2).

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