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A Balcony Slab Is a Cantilever: Thickness and Top Bars to NSCP 2015

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On many two-storey houses, the balcony is formed and poured together with the second floor. It looks like a small extension of the floor slab. It is short, about 1.5 m (maikli lang), so a 100 mm slab with a few bars seems like enough.

But a balcony is not a small floor slab. It is a cantilever, and a cantilever follows different rules. Here is why, with a real check to NSCP 2015.

Why a cantilever is different

An inside slab rests on beams on both sides. A balcony is held on one side only, at the beam along the wall. That changes three things:

  • The moment is bigger. For the same load and length, the moment of a cantilever is four times the mid-span moment of a simply supported slab (wℓ²/2 against wℓ²/8). It is largest at the root (ugat), at the face of the beam.
  • The tension is at the top. The balcony bends down like a diving board, so the top surface stretches. The main (working) bars must be at the top. The bottom bars are still needed for shrinkage and temperature, and for the upward earthquake force covered below, so they are anchored into the support too.
  • The tip sags more, and keeps sagging. A cantilever deflects much more than a slab of the same length supported on both sides, and the sag grows over the years as the concrete creeps.
A balcony slab is a cantilever: section, load and bending moment Section of a balcony slab with a 1.50 m clear projection from the face of the beam at the wall. The slab shown is case C, 150 mm thick, drawn to scale; the beam and wall are schematic. The top bars run back over the beam into the inside slab. A uniform factored load wu of 10.76 kN per metre acts on a 1 m strip. Below, the bending moment diagram is zero at the tip and grows to Mu minus = wu ln squared over 2 = 12.11 kN m per metre at the root, at the face of the beam (10.49 for the 100 mm slab of cases A and B). The tension is at the top. A balcony slab is a cantilever Case C drawn: slab thickness and projection to scale · beam and wall schematic wall wu = 10.76 kN/m on a 1 m strip (case C) Top bars run back over the beam into the inside slab inside floor slab beam h = 150 mm ℓn = 1.50 m clear projection from the beam face Mu− = 12.11 kN·m/m at the root (face of the beam) 10.49 for the 100 mm slab Bending moment, Mu− = wu ℓn² / 2 tension at the top of the slab zero at the tip Cases A and B use h = 100 mm. Case C uses h = 150 mm = ℓn/10.
Figure 1. A 1.50 m balcony slab in section, with the 150 mm slab of case C drawn to scale. The top bars run back over the beam into the inside slab, and the moment grows from zero at the tip to 12.11 kN·m/m at the root (10.49 for the 100 mm slab).

That last point is why NSCP 2015 Table 407.3.1.1 asks for a much thicker cantilever. If you do not calculate the deflection, and the slab does not carry walls or other construction that large deflections could damage, a solid one-way slab must be at least:

Support condition Minimum thickness h
Simply supportedℓ/20
One end continuousℓ/24
Both ends continuousℓ/28
Cantileverℓ/10

These are for normal-weight concrete and fy = 420 MPa. For other grades they are multiplied by (0.4 + fy/700), which is 1.0 for fy = 420. For a cantilever, ℓ is the clear projection from the face of the support. If the balcony carries a CHB parapet or wall, the table is not enough: the deflection must be calculated (407.3.2).

NSCP 2015 Table 407.3.1.1: minimum thickness of a solid one-way slab, worked out for 1.50 m Four support conditions with the minimum thickness when deflection is not calculated, for fy = 420 MPa and normal-weight concrete, each worked out for a length of 1.50 m. Simply supported, l over 20: 75 mm. One end continuous, l over 24: 62.5 mm. Both ends continuous, l over 28: 53.6 mm. Cantilever, l over 10: 150 mm, twice the simply supported value. The thickness bars are drawn to the same scale. Minimum slab thickness, NSCP 2015 Table 407.3.1.1 Worked out for ℓ = 1.50 m · fy = 420 MPa · normal-weight concrete · deflection not calculated SUPPORT h,min FOR ℓ = 1.50 m Simply supported ℓ/20 75 mm One end continuous ℓ/24 62.5 mm Both ends continuous ℓ/28 53.6 mm Cantilever ℓ/10 150 mm The cantilever needs 150 mm: twice the 75 mm of a simply supported slab of the same length.
Figure 2. NSCP 2015 Table 407.3.1.1 worked out for a length of 1.50 m: 75 mm simply supported, 62.5 mm with one end continuous, 53.6 mm with both ends continuous, and 150 mm as a cantilever.

So, unless deflection is calculated, a cantilever needs twice the minimum thickness of a simply supported slab of the same length. The sample that ships with our One-Way Slab Designer is a 2.80 m simply supported floor slab: its minimum is 2800/20 = 140 mm. A 1.50 m balcony, about half as long, needs 1500/10 = 150 mm.

The test case

I ran a 1.50 m balcony through our One-Way Slab Designer (NSCP 2015), set to Cantilever. Materials, cover, bars and finishes are the sheet's own sample (a two-storey residence); the live load is the balcony value:

  • Clear projection ℓn = 1.50 m from the beam face, per 1 m strip
  • f'c = 21 MPa, fy = 420 MPa, concrete 24 kN/m³
  • Clear cover 20 mm (the sample's "not exposed to weather" value; see the open balcony note below), 10 mm bars
  • Superimposed dead load 1.5 kPa (tiles and finishes)
  • Live load 2.9 kPa, the NSCP 2015 Table 205-1 value for exterior balconies
  • Factored gravity load wu = the larger of 1.4D and 1.2D + 1.6L (405.3.1); earthquake is discussed below

Only the thickness and the top bars change between the cases.

Case A: the thin slab

100 mm slab, top bars 10 mm @ 250 mm.

Self-weight is 24 × 0.100 = 2.40 kPa, so D = 2.40 + 1.5 = 3.90 kPa. On a 1 m strip, wu = 1.2 × 3.90 + 1.6 × 2.9 = 9.32 kN/m (1.4 × 3.90 = 5.46 is smaller). The moment at the root is

Mu− = wu ℓn² / 2 = 9.32 × 1.50² / 2 = 10.485, shown by the sheet as 10.49 kN·m/m

Check (NSCP 2015) Demand or limit Provided Result
Flexure, top barsMu− = 10.49 kN·m/mφMn = 8.47 kN·m/mFAIL, ratio 1.24
Minimum thickness (Table 407.3.1.1)h,min = 150 mmh = 100 mmCHECK DEFL.
Shear, no stirrups (422.5.5.1)Vu = 13.98 kN/mφVc = 43.82 kN/mPASS (0.32)

Overall: REVISE SLAB.

Case A in the One-Way Slab Designer, NSCP 2015 edition: the sheet's Design Check Summary. Flexure, top is marked FAIL at a ratio of 1.24, Minimum thickness is marked CHECK DEFL. (150 mm needed, 100 mm provided), and the overall result is REVISE SLAB.
Figure 3. Case A in the One-Way Slab Designer, NSCP 2015 edition: the sheet's Design Check Summary. Flexure, top is marked FAIL at a ratio of 1.24, Minimum thickness is marked CHECK DEFL. (150 mm needed, 100 mm provided), and the overall result is REVISE SLAB.

By hand: As = 1000 × 78.54 / 250 = 314.2 mm² per metre, at d = 100 − 20 − 10/2 = 75 mm. Then a = 314.2 × 420 / (0.85 × 21 × 1000) = 7.39 mm, and

φMn = 0.90 × 314.2 × 420 × (75 − 7.39/2) / 10⁶ = 8.47 kN·m/m

The section is tension-controlled (εt = 0.0229), so φ = 0.90. The factored moment is 1.24 times the design strength φMn, so this section does not meet the code.

Case B: more bars, same 100 mm

The usual reaction is to add steel: same 100 mm slab, top bars 10 mm @ 150 mm.

Check Demand or limit Provided Result
Flexure, top barsMu− = 10.49 kN·m/mφMn = 13.62 kN·m/mPASS (0.77)
Minimum thicknessh,min = 150 mmh = 100 mmCHECK DEFL.

Overall: OK · CHECK DEFLECTION.

Strength is now fine: As = 523.6 mm²/m, a = 12.32 mm, φMn = 0.90 × 523.6 × 420 × (75 − 12.32/2) / 10⁶ = 13.62 kN·m/m. But the slab is still 50 mm thinner than ℓ/10, and more steel helps the strength but only a little with stiffness. It does not satisfy ℓ/10. You must either calculate the deflection, including the long-term part (407.3.2), or make the slab thicker. Thickness is by far the more effective fix.

Case C: the fix, 150 mm

h = ℓ/10 = 1500/10 = 150 mm, top bars 10 mm @ 200 mm (the bar spacing of the sheet's sample).

The thicker slab is heavier: self-weight 24 × 0.150 = 3.60 kPa, D = 5.10 kPa, wu = 1.2 × 5.10 + 1.6 × 2.9 = 10.76 kN/m. The moment goes up by about 15%:

Mu− = 10.76 × 1.50² / 2 = 12.105, shown as 12.11 kN·m/m

But d grows by two thirds, from 75 to 150 − 20 − 5 = 125 mm:

As = 1000 × 78.54 / 200 = 392.7 mm²/m; a = 392.7 × 420 / (0.85 × 21 × 1000) = 9.24 mm
φMn = 0.90 × 392.7 × 420 × (125 − 9.24/2) / 10⁶ = 17.87 kN·m/m

Check Demand or limit Provided Result
Flexure, top barsMu− = 12.11 kN·m/mφMn = 17.87 kN·m/mPASS (0.68)
Minimum thicknessh,min = 150 mmh = 150 mmPASS (1.00)
Shear, no stirrupsVu = 16.14 kN/mφVc = 73.03 kN/mPASS (0.22)

Overall: SLAB ADEQUATE.

Case C on the same sheet: Flexure, top passes at 0.68 (12.11 against 17.87 kN·m/m), Minimum thickness passes at 1.00 (150 mm against 150 mm), and the overall result is SLAB ADEQUATE.
Figure 4. Case C on the same sheet: Flexure, top passes at 0.68 (12.11 against 17.87 kN·m/m), Minimum thickness passes at 1.00 (150 mm against 150 mm), and the overall result is SLAB ADEQUATE.

Notice that case C has fewer bars than case B (10 mm @ 200 instead of @ 150) and passes everything. Thickness did what extra steel could not.

Balcony slab root: factored moment Mu against design strength phi Mn for cases A, B and C Bar chart in kN m per metre of slab. Case A, 100 mm slab with 10 mm bars at 250: Mu 10.49 against phi Mn 8.47, FAIL, ratio 1.24, and thickness 100 below the 150 mm minimum, CHECK DEFL. Case B, 100 mm with 10 mm at 150: Mu 10.49 against phi Mn 13.62, PASS, ratio 0.77, thickness still CHECK DEFL. Case C, 150 mm with 10 mm at 200: Mu 12.11 against phi Mn 17.87, PASS, ratio 0.68, thickness 150 equals the minimum, PASS. Bending at the root of the balcony Mu− = factored moment · φMn = design strength of the top bars · kN·m per metre of slab 0 5 10 15 20 kN·m/m Case A 100 mm · 10 mm @ 250 FAIL · Mu/φMn = 1.24 h 100 < 150 · CHECK DEFL. Mu− 10.49 φMn 8.47 Case B 100 mm · 10 mm @ 150 PASS · Mu/φMn = 0.77 h 100 < 150 · CHECK DEFL. Mu− 10.49 φMn 13.62 Case C 150 mm · 10 mm @ 200 PASS · Mu/φMn = 0.68 h 150 = h,min 150 · PASS Mu− 12.11 φMn 17.87 Mu−: demand φMn: capacity thickness check
Figure 5. Moment at the root against the strength of the top bars, in kN·m/m: case A 10.49 against 8.47 (FAIL), case B 10.49 against 13.62 (passes in strength, but the thickness is still flagged), case C 12.11 against 17.87 (PASS, and the thickness passes).

The lesson

For a balcony, start with a thickness of at least ℓ/10, then size the bars. Adding steel to a thin cantilever fixes the strength on paper, but does not by itself deal with the sag. And because the tension is at the top, d is measured to the top bars: if they are stepped down during the pour, strength is lost.

Four things our test case does not cover:

  • A wall or heavy railing at the tip. The case has uniform loads only. A CHB parapet at the edge acts on the full 1.5 m lever arm. Work out its moment by hand and use the sheet's "Direct input" method. It also makes the deflection check necessary, as noted above.
  • An open balcony. If rain reaches the slab, the cover for these bars is 40 mm, not 20 mm (Table 420.6.1.3.1). Treat most balconies here as exposed, unless the engineer decides otherwise. Then d = 150 − 40 − 5 = 105 mm. My hand check, not a sheet run: φMn = 0.90 × 392.7 × 420 × (105 − 9.24/2) / 10⁶ = 14.90 kN·m/m, still above 12.11 (ratio 0.81). Run it on the sheet with the exposed setting.
  • Where the moment goes. The top bars must continue over the beam into the inside slab far enough to develop (see Development Length and Lap Splices). With no slab behind, the beam must carry the moment by twisting (torsion). The sheet does not check anchorage; show it on the plans.
  • Earthquake. NSCP 2015 Section 208 requires horizontal cantilevers in Zone 4, which covers most of the Philippines, to be designed for a net upward force. The engineer of record also checks the root for this reversed moment and the bottom-bar anchorage.

What to check on site

  1. Thickness at least ℓ/10 of the clear projection, unless the engineer has calculated the deflection. For 1.5 m, that is 150 mm.
  2. Main bars on top, as the outer layer, with the temperature bars under them.
  3. Firm supports under the top bars, so they keep their height when people walk on them. Check again just before concreting.
  4. Top bars run back over the beam into the inside slab, as long as the plans show. Do not cut them at the beam line.
  5. No construction joint at the root. Pour the balcony together with the inside slab, and check the closed stirrups of the supporting beam.
  6. Shores stay under the balcony until the engineer says they can come out. Do not stack hollow blocks or cement bags on it, during or after shoring.
  7. Later, a crack along the wall line (or hollow-sounding tiles there), a tip that visibly drops, or a railing that leans out are signs to call the engineer. Water running back toward the house is a drainage problem; fix it too, since standing water rusts the top bars faster.

This is not about any one trade. The fix is a clear balcony section on the plans and a quick check before the pour.

General information only. Follow the approved structural plans and the engineer of record.