Reinforced Concrete Columns - Detailed Explanation
Columns are the primary vertical load-carrying members of any reinforced concrete structure. Unlike beams that resist predominantly flexure, columns must simultaneously resist axial compression and bending moments arising from eccentric loads, lateral forces (wind, seismic), and frame action. The National Structural Code of the Philippines (NSCP 2015, Section 422) — which adopts the ACI 318 framework — governs the design of RC columns in the Philippines. For PRC Civil Engineer board examinees, columns are among the most frequently tested topics, appearing in both the structural design and materials/construction portions of the examination. This chapter provides a rigorous, exam-focused treatment of short tied and spiral columns under axial load and combined axial load plus bending, the steel-ratio requirements, spiral confinement, the axial–moment interaction diagram, and slenderness effects. All formulas are presented in SI units, and board-style worked examples are provided throughout.
Classification of RC Columns
Examples
A 350 mm × 350 mm column in a braced frame has an unsupported length lu = 3.5 m and effective length factor k = 0.75. The radius of gyration r = 0.30 × 350 = 105 mm. End moments M1 = 20 kN·m and M2 = 45 kN·m (single curvature). Classify the column as short or slender.
Solution
klu/r = (0.75 × 3500)/105 = 2625/105 = 25.0
Slenderness limit for braced frame: 34 − 12(M1/M2) = 34 − 12(20/45) = 34 − 5.33 = 28.67, but ≤ 40.
Since klu/r = 25.0 < 28.67, the column is SHORT — slenderness effects may be neglected.
For braced frames, the ratio M1/M2 is positive for single curvature (same sign moments). The slenderness limit increases as the moment ratio decreases (more favorable curvature). Always check this before proceeding to capacity calculations.
RC columns are classified by (1) transverse reinforcement type, (2) loading condition, and (3) slenderness. Understanding the classification is essential before applying any design formula.
By Transverse Reinforcement:
- Tied columns use individual lateral ties (hoops) to laterally restrain the longitudinal bars. They are rectangular or square in most cases, though circular tied columns are permitted. The strength-reduction factor is φ = 0.65 for compression-controlled sections.
- Spiral columns use a continuous helical spiral to confine the concrete core. The spiral provides significantly better ductility and post-peak load-carrying ability, which is rewarded by a higher φ = 0.75. Spiral columns are typically circular in cross-section.
By Loading:
- Axially loaded (pure compression): Load acts through the centroid with zero eccentricity. This is a theoretical idealization; NSCP/ACI 318 accounts for unavoidable eccentricity through the 0.80 and 0.85 cap factors.
- Eccentrically loaded (combined P and M): The factored axial load Pu acts at an eccentricity e = Mu/Pu from the centroidal axis, producing both axial force and bending moment simultaneously.
By Slenderness:
- Short columns: Slenderness effects are negligible; second-order (P-δ and P-Δ) moments are less than 5% of first-order moments. Design proceeds using nominal strength directly.
- Slender (long) columns: Lateral deflections amplify the applied moments. Moment magnification or second-order analysis is required per NSCP 2015 Section 406.2.5.
Braced vs. Unbraced Frames:
- Braced (non-sway) frames: stability provided by shear walls, bracing, or stiff cores. Slenderness limit: klu/r ≤ 34 − 12(M1/M2), max 40.
- Unbraced (sway) frames: lateral stability depends on frame action. Slenderness limit for short column: klu/r ≤ 22.
Applications
- Determining which NSCP provisions and φ values apply for a given column design.
- Deciding whether moment magnification is required in a structural analysis.
- Selecting the appropriate confinement reinforcement system for seismic zones (spirals preferred for ductility).
- Qualifying columns under NSCP 2015 Section 418 special moment frame requirements.
Common misconceptions
- Many examinees use r = h/12 (for rectangular sections, r = h/√12 ≈ 0.289h ≈ 0.30h is the approximation). For circular sections, r = D/4.
- Confusing 'braced' with 'fixed-end' — braced refers to lateral stability, not end conditions.
- Assuming all columns with ties are rectangular — circular tied columns exist and also use φ = 0.65.
- Using k = 1.0 for all columns — k depends on end conditions and frame type; for pin-pin k = 1.0, for fixed-fixed k = 0.5.
Related concepts
- Effective length factor k (NSCP Table 406.2.5)
- Radius of gyration r for rectangular and circular sections
- Braced vs. unbraced frames
- Moment magnification (slender columns)
Common exam questions
Example
'A circular column with spiral reinforcement...' → φ = 0.75, cap = 0.85.
Approach
Read whether the problem states 'tied' or 'spiral.' Assign φ = 0.65 (tied) or φ = 0.75 (spiral). Then apply the correct cap factor (0.80 tied, 0.85 spiral).
Example
Given k = 1.0, lu = 4 m, column 300 mm square: r = 0.30(300) = 90 mm; klu/r = 4000/90 = 44.4 > 22 → slender in unbraced frame.
Approach
Compute klu/r. Compare against the appropriate limit (22 for unbraced; 34−12(M1/M2) for braced). State whether short or slender.
Key points to remember
- Tied columns: φ = 0.65; spiral columns: φ = 0.75 — this difference is critical in board exams.
- The higher φ and higher cap factor (0.85 vs 0.80) for spirals reflects their superior ductility.
- Short column design ignores slenderness; verify the klu/r limit first.
- Columns are classified as compression-controlled when the net tensile strain εt ≤ 0.002 at the extreme tension steel.
- In the PRC board exam, the column type (tied vs. spiral) determines which cap factor and φ value to use — read the problem carefully.
Nominal Axial Strength and Maximum Usable Axial Load
Examples
A 400 mm × 400 mm tied column is reinforced with 8–25 mm diameter bars. f'c = 28 MPa, fy = 415 MPa. Compute the design axial capacity φPn,max.
Solution
Step 1 — Gross area:
Ag = 400 × 400 = 160,000 mm²
Step 2 — Steel area:
Ast = 8 × π/4 × (25)² = 8 × 490.87 = 3,927 mm²
Step 3 — Net concrete area:
Ag − Ast = 160,000 − 3,927 = 156,073 mm²
Step 4 — Nominal strength:
Po = 0.85(28)(156,073) + 415(3,927)
Po = 3,714,537 + 1,629,705
Po = 5,344,242 N = 5,344.2 kN
Step 5 — Design capacity:
φPn,max = 0.80(0.65)(5,344,242) = 0.52 × 5,344,242
φPn,max = 2,779,006 N ≈ 2,779 kN
Note that the combined factor 0.52 directly multiplies Po. This is the maximum factored axial load Pu that the tied column can sustain under near-concentric loading. Any Pu ≤ 2,779 kN is acceptable for pure axial.
Same section but with spiral reinforcement instead of ties. Compute φPn,max.
Solution
Po = 5,344,242 N (same as above)
φPn,max = 0.85(0.75)(5,344,242) = 0.6375 × 5,344,242
φPn,max = 3,406,954 N ≈ 3,407 kN
The spiral column carries 3,407/2,779 = 1.226, approximately 23% more load than the equivalent tied column. This premium reflects spiral confinement. In board exams, you may be asked to compare tied vs. spiral capacity — always recall the 23% approximate advantage of spirals.
The nominal pure-axial strength Po is the theoretical maximum compressive load a column can carry if absolutely no eccentricity exists. Per NSCP 2015 (adopting ACI 318-14 Eq. 22.4.2.2):
Po = 0.85f'c(Ag − Ast) + fyAst
where:
- f'c = specified 28-day compressive strength of concrete (MPa)
- Ag = gross cross-sectional area of the column (mm²)
- Ast = total area of longitudinal steel (mm²)
- (Ag − Ast) = net concrete area (critical: use net area, not gross, for the concrete term)
- fy = yield strength of longitudinal reinforcement (MPa)
The factor 0.85 on f'c accounts for the difference between in-situ cylinder strength and actual column concrete strength (size, curing, sustained loading effects).
Why there is a cap on usable axial load:
Because perfect concentricity is impossible in practice (construction tolerances, load path imperfections), NSCP limits the maximum design axial load:
For tied columns (φ = 0.65):
φPn,max = 0.80 × φ × Po = 0.80(0.65)Po = 0.52Po
For spiral columns (φ = 0.75):
φPn,max = 0.85 × φ × Po = 0.85(0.75)Po = 0.6375Po
The combined multipliers (cap × φ):
- Tied: 0.80 × 0.65 = 0.52
- Spiral: 0.85 × 0.75 = 0.6375
Memorize these two combined multipliers — they appear constantly in board problems.
Physical interpretation: The 0.80 cap for tied columns corresponds to an equivalent minimum eccentricity of about 5–10% of the column dimension. The 0.85 cap for spirals reflects their tighter confinement, allowing them to sustain load more reliably near the concentric condition.
Applications
- Checking whether a proposed column can carry the factored gravity load from tributary area calculations.
- Determining the required Ag for a column given Pu, ρg, f'c, and fy (reverse design).
- Comparing tied vs. spiral column efficiency in seismic design where ductility is paramount.
- Preliminary sizing of columns in multi-storey buildings before detailed interaction diagram analysis.
Common misconceptions
- Using Ag (gross) instead of (Ag − Ast) for the concrete term — this overcounts the steel area twice.
- Forgetting the 0.85 factor on f'c in Po — some students write Po = f'c(Ag−Ast) + fyAst.
- Applying φ = 0.90 (flexure value) to columns — columns are compression-controlled, φ = 0.65 or 0.75.
- Confusing the cap factor (0.80 or 0.85) with the φ factor — they are separate multipliers both applied to Po.
- Using the cap on Pn (not Po) — the cap and φ are both applied to Po: φPn,max = cap × φ × Po.
Related concepts
- Net concrete area vs. gross area
- Steel ratio ρg and its limits
- φ factor for compression-controlled sections
- Interaction diagram — pure axial point
Common exam questions
Example
Find the design axial load capacity of a 500 mm diameter spiral column with 6–28 mm bars, f'c = 35 MPa, fy = 415 MPa.
Approach
Identify column type → compute Ag, Ast → compute Po = 0.85f'c(Ag−Ast) + fyAst → apply correct combined multiplier (0.52 tied or 0.6375 spiral).
Example
Design a square tied column for Pu = 3,500 kN, ρg = 0.03, f'c = 28 MPa, fy = 415 MPa.
Approach
Set φPn,max = Pu. Express Ast = ρg × Ag. Substitute into Po formula. Solve for Ag as a quadratic or by iteration.
Key points to remember
- Always use NET concrete area (Ag − Ast) in the concrete term of Po, never gross area Ag alone.
- Combined multiplier: tied = 0.52, spiral = 0.6375 — these are the most frequently used numbers in column problems.
- The 0.85 factor on f'c in Po is a material-reduction factor (not φ) and is always present.
- Po is the theoretical maximum; the code-usable load is always reduced by the cap and φ.
- For the steel term, use full fy — steel in compression does not use a 0.85 reduction.
- fy is capped at 550 MPa for non-prestressed longitudinal reinforcement in NSCP 2015.
Longitudinal and Transverse Reinforcement Requirements
Examples
Verify the tie spacing for a 400 mm × 400 mm tied column with 8–25 mm longitudinal bars and 10 mm ties.
Solution
Criterion 1: 16 × db = 16 × 25 = 400 mm
Criterion 2: 48 × dtie = 48 × 10 = 480 mm
Criterion 3: Least column dimension = 400 mm
Minimum controls: s ≤ 400 mm
Use s = 400 mm maximum spacing.
In this case, criteria 1 and 3 both give 400 mm, which governs. The tie size (10 mm) is appropriate for 25 mm bars (≤ 32 mm threshold).
Find the minimum spiral ratio for a 400 mm diameter circular column with a spiral core diameter Dch = 310 mm (assuming 45 mm cover to spiral). f'c = 28 MPa, fyt = 415 MPa.
Solution
Ag = π/4 × (400)² = 125,664 mm²
Ach = π/4 × (310)² = 75,477 mm²
ρs,min = 0.45 × (Ag/Ach − 1) × (f'c/fyt)
ρs,min = 0.45 × (125,664/75,477 − 1) × (28/415)
ρs,min = 0.45 × (1.6648 − 1) × 0.06747
ρs,min = 0.45 × 0.6648 × 0.06747
ρs,min = 0.45 × 0.04486
ρs,min = 0.02019 → ρs,min = 0.0202 (2.02%)
The minimum spiral ratio of 2.02% means the volume of spiral per unit volume of core must equal or exceed 2.02%. This is then used to find the required pitch: s = 4asp/(Dch × ρs). For a 10 mm spiral wire (asp = 78.54 mm²): s = 4(78.54)/(310 × 0.02019) = 314.16/6.259 = 50.2 mm. Use s = 50 mm (round down).
NSCP 2015 (Section 410) prescribes both minimum and maximum reinforcement to ensure column ductility, constructability, and adequate strength.
Longitudinal Reinforcement (NSCP 2015 Section 410.6):
Steel ratio: ρg = Ast/Ag
Limits:
- Minimum: ρg ≥ 0.01 (1%) — to prevent sudden brittle failure from shrinkage and creep
- Maximum: ρg ≤ 0.08 (8%) — to prevent bar congestion and ensure concrete placement
- Practical limit: ρg ≤ 0.04 (4%) where bars are lap-spliced, to avoid excessive congestion at splice zones
Minimum number of bars:
- Tied columns (rectangular): minimum 4 bars
- Spiral or circular columns: minimum 6 bars
- Triangular ties: minimum 3 bars
Tie Reinforcement (NSCP 2015 Section 410.7.6):
Tie size: 10 mm (for 32 mm bars and smaller); 12 mm (for 36 mm bars and larger or bundled bars)
Tie spacing (smallest of three criteria):
- 16 × db (longitudinal bar diameter)
- 48 × dtie (tie diameter)
- Least column dimension
Spiral Reinforcement (NSCP 2015 Section 410.7.6.4.2):
The minimum volumetric spiral ratio ρs ensures the spiral provides confinement equal to or greater than the strength lost when the shell concrete spalls:
ρs ≥ 0.45 × (Ag/Ach − 1) × (f'c/fyt)
where:
- Ag = gross column area
- Ach = core area measured to the outside of the spiral (Ach = π/4 × Dch²)
- fyt = specified yield strength of spiral wire, capped at 700 MPa in this equation
- ρs (volumetric) = volume of spiral per unit length of column / volume of core per unit length
Volumetric spiral ratio computation:
ρs = (4 × asp) / (Dch × s)
where asp = area of spiral wire cross-section, Dch = core diameter (to outside of spiral), s = spiral pitch (center-to-center spacing).
Spiral pitch limits:
- Maximum: 75 mm or (1/6)Dch
- Minimum: 25 mm (to allow concrete placement)
- Clear spacing between turns: 25–75 mm
Applications
- Detailing column drawings for structural construction documents.
- Checking contractor shop drawings for code compliance.
- Designing confinement reinforcement for columns in seismic zones (NSCP 2015 Section 418 for special moment frames).
- Evaluating adequacy of existing columns in building retrofit assessments.
Common misconceptions
- Using Ag instead of Ach in the spiral ratio formula — this produces an incorrect (usually lower) ratio.
- Forgetting the 700 MPa cap on fyt in the spiral formula — if fyt > 700 MPa, use 700 MPa.
- Using the 4% practical limit as the code maximum — the code maximum is 8%; 4% is a practical guideline for lap-splice zones.
- Applying tie spacing criteria to only one criterion and ignoring the others.
- Using core diameter to centerline of spiral (not to outside) for Dch — the formula uses outside-to-outside of spiral for Ach.
Related concepts
- Column confinement and ductility
- Seismic column detailing (NSCP 2015 Section 418)
- Bar spacing requirements for concrete placement
- Development length and lap-splice requirements
Common exam questions
Example
A 500 mm spiral column has 60 mm cover to the spiral. f'c = 35 MPa, fyt = 415 MPa. Find ρs,min and the required pitch for 12 mm spiral wire.
Approach
Compute Ag and Ach. Apply ρs = 0.45(Ag/Ach−1)(f'c/fyt). Check that fyt ≤ 700 MPa. Often followed by finding required pitch s.
Example
Determine the maximum allowable tie spacing for a 450 mm × 600 mm column with 8–32 mm bars and 12 mm ties.
Approach
Apply all three spacing criteria and take the minimum. Verify tie bar size matches longitudinal bar size per code.
Key points to remember
- ρg limits: 1% minimum, 8% maximum; use 4% max at lap splices in practice.
- Minimum bars: 4 (tied), 6 (spiral/circular), 3 (triangular ties).
- Tie spacing: least of 16db(long), 48db(tie), least column dimension — all three must be checked.
- Spiral ratio formula uses Ach (core to outside of spiral), not the net core area.
- fyt in spiral ratio formula is capped at 700 MPa per NSCP 2015, regardless of actual steel grade.
- A larger Ag/Ach ratio (thicker cover relative to core) requires a higher spiral ratio — more confinement needed.
Axial–Moment Interaction Diagram
Examples
A 400 mm × 400 mm tied column with 4–25 mm bars (two each at d' = 65 mm and d = 335 mm from compression face) has f'c = 28 MPa (β1 = 0.85), fy = 415 MPa, Es = 200,000 MPa. Find the balanced axial load Pb and balanced moment Mb.
Solution
Step 1 — Balanced neutral axis:
εy = 415/200,000 = 0.002075
cb = 0.003 × 335 / (0.003 + 0.002075) = 1.005/0.005075 = 198.0 mm
Step 2 — Depth of stress block:
a = β1 × cb = 0.85 × 198.0 = 168.3 mm
Step 3 — Concrete compression force:
Cc = 0.85 × f'c × a × b = 0.85 × 28 × 168.3 × 400 = 1,597,896 N = 1597.9 kN
Step 4 — Compression steel strain and force:
εs' = 0.003(cb − d')/cb = 0.003(198.0 − 65)/198.0 = 0.003 × 133/198 = 0.002015
Since εs' = 0.002015 ≈ εy = 0.002075 (close, use fy)
Cs = As'(fy − 0.85f'c) = 2(490.87)(415 − 0.85×28) = 981.74 × 391.2 = 384,037 N = 384.0 kN
(Each 25mm bar: As = π/4 × 25² = 490.87 mm²)
Step 5 — Tension steel force:
εs = 0.003(d − cb)/cb = 0.003(335 − 198)/198 = 0.002076 ≥ εy → Ts = As × fy = 981.74 × 415 = 407,422 N = 407.4 kN
Step 6 — Balanced axial load:
Pb = Cc + Cs − Ts = 1597.9 + 384.0 − 407.4 = 1574.5 kN
Step 7 — Balanced moment (about section centroid at 200 mm from top):
Mb = Cc(200 − a/2) + Cs(200 − d') + Ts(d − 200)
Mb = 1597.9(200 − 84.15) + 384.0(200 − 65) + 407.4(335 − 200)
Mb = 1597.9(115.85) + 384.0(135) + 407.4(135)
Mb = 185,108 + 51,840 + 54,999 = 291,947 kN·mm = 291.9 kN·m
φPb = 0.65 × 1574.5 = 1023.4 kN
φMb = 0.65 × 291.9 = 189.7 kN·m
The balanced point (1574.5 kN, 291.9 kN·m) is where concrete crushes simultaneously as tension steel yields. This is the point of maximum moment on the interaction diagram. Notice how compression steel is almost at yield (εs' ≈ εy), which is typical for columns with moderate cover.
Real columns almost never carry pure axial load — eccentricities and lateral loads always introduce bending moments. The interaction diagram (also called P-M diagram or column interaction curve) is the fundamental tool for checking columns under combined axial force and bending.
What the Interaction Diagram Represents:
Each point (Pn, Mn) on the curve represents the combination of axial load and moment that causes the section to reach its nominal strength simultaneously in compression (εc = 0.003 at the extreme compression fiber). Any (Pu, Mu) plotting INSIDE the φ-reduced curve is acceptable; any point plotting OUTSIDE means the section fails.
Key Points on the Interaction Diagram:
- Point A — Pure Axial (top of curve): Mn = 0, Pn = Po = 0.85f'c(Ag−Ast) + fyAst. The design value is capped at 0.80φPo (tied) or 0.85φPo (spiral).
- Point B — Zero Tension Point (concrete controls): The neutral axis lies at the level of the tension steel, so tension steel stress = 0. Below this point, some steel is in tension.
Point C — Balanced Condition (Pb, Mb): The concrete crushing strain (εcu = 0.003) is reached simultaneously with the tension steel yielding (εs = εy = fy/Es). This point produces the maximum moment the section can carry. The balanced failure is the transition between compression-controlled and tension-controlled behavior.
- Balanced neutral axis depth: cb = (0.003/(0.003 + εy)) × d = (600/(600 + fy)) × d (with fy in MPa, using Es = 200,000 MPa)
- Note: cb = 600d/(600 + fy) is valid in SI units
- Point D — Pure Flexure (bottom): Pn = 0, Mn = φMn as in beam design. The interaction curve touches the moment axis.
Compression-Controlled vs. Tension-Controlled Zones:
- Above the balanced point (P > Pb): compression-controlled; failure initiated by concrete crushing before steel yields. φ = 0.65 (tied).
- Below the balanced point (P < Pb): tension-controlled; failure initiated by steel yielding. φ transitions to 0.90 as net tensile strain increases.
- At balanced: φ = 0.65 (tied) or 0.75 (spiral) applies.
Eccentricity:
For a given axial load Pn, the eccentricity e = Mn/Pn defines the load path. As eccentricity increases from 0, the (Pn, Mn) point moves from pure axial toward pure flexure along the interaction curve.
Constructing the Interaction Diagram (approximate method for board exams):
For a symmetric column section, select several values of neutral axis depth c (from c = ∞ for pure axial to c → 0 for pure flexure), compute the strain in each steel layer from compatibility (εs = 0.003(c − d')/c for compression steel, εs = 0.003(d − c)/c for tension steel), compute steel stress (capped at fy), find C_c = 0.85f'c × a × b (a = β1 × c), find Cs and T from steel layers, then sum forces and moments about the section centroid.
Applications
- Checking column adequacy under combined gravity and lateral (wind or seismic) loading.
- Determining if a column can sustain a given (Pu, Mu) combination from ETABS/STAAD output.
- Selecting the required steel ratio ρg for a given eccentricity.
- Evaluating existing columns in building renovation projects (e.g., change of occupancy load).
Common misconceptions
- Thinking the balanced point is the most dangerous (it is actually the maximum moment, not the minimum safety).
- Forgetting to subtract 0.85f'c from the compression steel stress to avoid double-counting concrete area.
- Using gross area Ag for the concrete compression zone — use b × a (rectangular stress block area).
- Confusing eccentricity e with the moment arm — e = M/P is measured from the load application to the centroidal axis.
- Applying φ = 0.90 throughout the interaction diagram — φ transitions from 0.65 (compression) to 0.90 (tension) through a transition zone.
Related concepts
- Whitney rectangular stress block (β1)
- Strain compatibility
- Balanced failure in beams
- φ factor transition zone (NSCP 2015 Section 421.2.2)
Common exam questions
Example
Find the balanced axial load and moment for a 300 mm × 500 mm column with specified steel layout, f'c = 21 MPa, fy = 275 MPa.
Approach
Use cb = 600d/(600+fy). Compute Cc, Cs, T. Sum vertically for Pb, then sum moments about centroid for Mb.
Example
A column carries Pu = 800 kN and Mu = 250 kN·m. Is it safe given φPb = 900 kN and φMb = 300 kN·m?
Approach
Compute φPn and φMn for the given loading. Compare against the diagram or use the eccentricity method.
Key points to remember
- The balanced point gives the MAXIMUM moment — not the maximum axial load.
- Above balanced: compression-controlled (φ = 0.65 tied, 0.75 spiral); below: tension-controlled (φ → 0.90).
- cb = 600d/(600 + fy) in SI units — this formula appears frequently in board exams.
- Any factored (Pu, Mu) must plot inside the φ-reduced interaction envelope to be acceptable.
- The interaction diagram is convex — adding a small axial load to a beam increases moment capacity (up to the balanced point).
- For eccentrically loaded columns, e = Mu/Pu. If e < ebalanced, failure is compression-controlled.
Slenderness Effects and Moment Magnification
Examples
A braced-frame column: klu/r = 36, M1/M2 = +0.5 (single curvature), Pu = 1,200 kN, Pc = 5,000 kN, f'c = 28 MPa, βdns = 0.6, M2 = 180 kN·m. Compute the magnified design moment Mc.
Solution
Step 1 — Check if slender:
Limit = 34 − 12(0.5) = 34 − 6 = 28 < 36 → Column is SLENDER, magnification required.
Step 2 — Cm:
Cm = 0.6 − 0.4(M1/M2) = 0.6 − 0.4(0.5) = 0.6 − 0.2 = 0.40
Step 3 — δns:
δns = Cm/(1 − Pu/0.75Pc) = 0.40/(1 − 1200/(0.75×5000))
δns = 0.40/(1 − 1200/3750) = 0.40/(1 − 0.32) = 0.40/0.68 = 0.588
Since δns = 0.588 < 1.0, use δns = 1.0
Step 4 — Magnified moment:
Mc = δns × M2 = 1.0 × 180 = 180 kN·m
Step 5 — Minimum moment check:
M2,min = Pu(15 + 0.03h) — assume h = 400 mm:
M2,min = 1200(15 + 0.03×400) = 1200(15 + 12) = 1200 × 27 = 32,400 kN·mm = 32.4 kN·m < 180 kN·m ✓
Mc = 180 kN·m (governs over minimum)
Even though Cm computed as 0.588 (< 1.0), the code requires δns ≥ 1.0. The final design moment equals the first-order moment M2 = 180 kN·m because Pu/Pc is small (the column is far from buckling). The Cm < 1.0 result occurs because reverse curvature (or here, single curvature with M1/M2 = 0.5) reduces the critical mid-height deflection relative to a uniform moment case.
When a column is slender (klu/r exceeds the short-column limit), lateral deflections under load cause additional moments beyond those from first-order analysis. This P-δ effect (member curvature) and P-Δ effect (lateral drift in sway frames) must be accounted for in design.
Slenderness Limits (NSCP 2015 Section 406.2.5):
Braced (non-sway) frames: Column is short if klu/r ≤ 34 − 12(M1/M2), max value = 40
- M1/M2 is positive for single curvature (inflection point between ends)
- M1/M2 is negative for double curvature (reverse bending)
- Unbraced (sway) frames: Column is short if klu/r ≤ 22
Moment Magnifier Method (Braced Frames):
The magnified moment is:
Mc = δns × M2 ≥ M2,min
where:
δns = Cm / (1 − Pu/0.75Pc) ≥ 1.0
Cm = equivalent moment factor:
- For members without transverse loads: Cm = 0.6 − 0.4(M1/M2)
- For members with transverse loads: Cm = 1.0
Pc = π²EI/(klu)² = critical (Euler) buckling load
EI = stiffness accounting for cracking, creep:
- EI = (0.4EcIg)/(1 + βdns) [simplified]
- Or EI = (0.2EcIg + EsIse)/(1 + βdns) [more accurate]
βdns = ratio of maximum factored sustained (dead) load moment to total factored moment
Ec = 4700√f'c (MPa), Ig = gross moment of inertia
Minimum Eccentricity:
M2,min = Pu(15 + 0.03h) mm·N (h in mm)
If M2 < M2,min, use M2,min with Cm = 1.0.
Sway (Unbraced) Frames:
The sway moment magnifier δs is applied to the sway component of the moment. This is more complex and typically requires software (ETABS, SAP2000) for practical design but the principles appear in board exams conceptually.
Applications
- Design of columns in open-frame structures without shear walls.
- Checking tall columns in warehouse or industrial buildings.
- Retrofit assessment of existing slender columns under increased loads.
- Design of columns in transfer floors where stories have different heights.
Common misconceptions
- Using δns < 1.0 — it must always be ≥ 1.0 by code.
- Forgetting the 0.75 factor in the denominator — using Pc instead of 0.75Pc underestimates the magnifier.
- Confusing P-δ (member curvature in braced frames) with P-Δ (story drift in sway frames).
- Using klu/r = 40 as the universal limit regardless of frame type or moment ratio.
- Applying M1/M2 as positive for double curvature — it is negative for double curvature (inflection within span).
Related concepts
- Effective length factor k
- Euler critical load Pc
- Second-order analysis
- P-Δ effects in sway frames
Common exam questions
Example
k=0.8, lu=5m, 350mm square column, M1/M2=0.6, braced frame — is it slender?
Approach
Compute klu/r. Compare against appropriate limit (22 or 34−12M1/M2). State whether magnification is needed.
Example
Given Pu, Pc, M1/M2, compute the moment magnifier δns.
Approach
Apply Cm = 0.6−0.4(M1/M2). Compute Pc. Then δns = Cm/(1−Pu/0.75Pc) ≥ 1.0.
Key points to remember
- Slenderness limit for braced frames: klu/r ≤ 34−12(M1/M2) but ≤ 40. For unbraced: klu/r ≤ 22.
- δns ≥ 1.0 always — moment magnification only increases the design moment, never reduces it.
- Cm accounts for the distribution of moments along the column height (less than 1.0 for reverse curvature).
- The 0.75 factor in the denominator (0.75Pc) is a stiffness reduction for uncertainty.
- Minimum eccentricity: emin = 15 + 0.03h mm, where h is the column dimension in the bending direction.
- For sway frames, ALL columns in the story must be checked together (the magnifier is a story-level quantity).
Practice problems
PROBLEM 1 (Tied Column — Design Axial Capacity)
A 450 mm × 450 mm square tied column is reinforced with 8–28 mm diameter longitudinal bars. f'c = 35 MPa, fy = 415 MPa. (a) Compute the nominal axial strength Po. (b) Compute the design axial capacity φPn,max. (c) Verify that the steel ratio is within NSCP limits.
Solution
Given:
Ag = 450 × 450 = 202,500 mm²
Ast = 8 × (π/4)(28²) = 8 × 615.75 = 4,926 mm²
(a) Nominal axial strength:
Po = 0.85f'c(Ag − Ast) + fyAst
Po = 0.85(35)(202,500 − 4,926) + 415(4,926)
Po = 0.85(35)(197,574) + 2,044,290
Po = 5,875,327 + 2,044,290
Po = 7,919,617 N = 7,919.6 kN
(b) Design axial capacity (tied, φ = 0.65):
φPn,max = 0.80 × 0.65 × Po = 0.52 × 7,919,617
φPn,max = 4,118,201 N = 4,118.2 kN
(c) Steel ratio check:
ρg = Ast/Ag = 4,926/202,500 = 0.02434 = 2.43%
Since 1% ≤ 2.43% ≤ 8% → Within NSCP 2015 limits ✓
The combined multiplier 0.52 (= 0.80 × 0.65) is the key shortcut for tied columns. Note that f'c = 35 MPa does not change β1 in the Po formula (β1 is only needed for the interaction diagram). Always show the net concrete area (Ag − Ast) explicitly to avoid the common exam error of using Ag directly.
PROBLEM 2 (Spiral Column — Design Axial Capacity)
A 500 mm diameter circular column with spiral reinforcement has 8–28 mm longitudinal bars. f'c = 28 MPa, fy = 415 MPa. Find: (a) φPn,max. (b) The percentage increase over an equivalent tied column.
Solution
Given:
Ag = π/4 × (500)² = 196,350 mm²
Ast = 8 × (π/4)(28²) = 8 × 615.75 = 4,926 mm²
(a) Nominal strength:
Po = 0.85(28)(196,350 − 4,926) + 415(4,926)
Po = 0.85(28)(191,424) + 2,044,290
Po = 4,549,851 + 2,044,290
Po = 6,594,141 N = 6,594.1 kN
Spiral column (φ = 0.75):
φPn,max = 0.85 × 0.75 × 6,594,141 = 0.6375 × 6,594,141
φPn,max = 4,203,765 N = 4,203.8 kN
Equivalent tied column:
φPn,max(tied) = 0.52 × 6,594,141 = 3,428,953 N = 3,429.0 kN
(b) Percentage increase:
[(4,203.8 − 3,429.0)/3,429.0] × 100 = (774.8/3,429.0) × 100 = 22.6% increase
The ~23% advantage of spiral over tied columns is a classic board exam result. It arises purely from the difference in the combined multipliers: 0.6375 vs. 0.52, a ratio of 1.226 (22.6% more). This number is worth memorizing as a sanity check.
PROBLEM 3 (Minimum Spiral Ratio)
A 450 mm diameter spiral column has 50 mm cover to the outside of the spiral. f'c = 28 MPa, fyt = 415 MPa. Determine: (a) Minimum spiral ratio ρs,min. (b) Required spiral pitch if 10 mm spiral wire is used.
Solution
Given:
Column diameter: Dc = 450 mm
Cover to outside of spiral: 50 mm
Core diameter: Dch = 450 − 2(50) = 350 mm
(a) Areas:
Ag = π/4 × (450)² = 158,962 mm²
Ach = π/4 × (350)² = 96,211 mm²
ρs,min = 0.45 × (Ag/Ach − 1) × (f'c/fyt)
ρs,min = 0.45 × (158,962/96,211 − 1) × (28/415)
ρs,min = 0.45 × (1.6522 − 1) × 0.06747
ρs,min = 0.45 × 0.6522 × 0.06747
ρs,min = 0.45 × 0.04400
ρs,min = 0.0198 (1.98%)
(b) Spiral pitch (10 mm wire, asp = π/4 × 10² = 78.54 mm²):
ρs = 4asp/(Dch × s) → s = 4asp/(Dch × ρs,min)
s = 4(78.54)/(350 × 0.0198)
s = 314.16/6.930
s = 45.3 mm → use s = 45 mm
Check: s = 45 mm is within limits (25 mm ≤ s ≤ 75 mm) ✓
The spiral pitch calculation follows directly from solving ρs = 4asp/(Dch × s) for s. Always round DOWN to the nearest 5 mm to satisfy the minimum ratio. Verify that the chosen pitch is between 25 mm and 75 mm per NSCP tie/spiral spacing requirements.
PROBLEM 4 (Steel Ratio — Reverse Design)
Design a square tied column to carry a factored axial load Pu = 3,500 kN. Use f'c = 28 MPa, fy = 415 MPa, and ρg = 0.025. Find the required column dimension h.
Solution
For a tied column: φPn,max = 0.52 × Po
Po = 0.85f'c(Ag − Ast) + fyAst
With Ast = ρg × Ag = 0.025Ag:
Po = 0.85(28)(Ag − 0.025Ag) + 415(0.025Ag)
Po = 0.85(28)(0.975Ag) + 10.375Ag
Po = 23.205Ag + 10.375Ag
Po = 33.580Ag
Setting φPn,max = Pu:
0.52 × 33.580Ag = 3,500,000 N
17.462Ag = 3,500,000
Ag = 200,441 mm²
For a square column:
h = √(200,441) = 447.7 mm → use h = 450 mm
Verification:
Ag = 202,500 mm², Ast = 0.025 × 202,500 = 5,063 mm²
Po = 0.85(28)(202,500 − 5,063) + 415(5,063)
Po = 0.85(28)(197,437) + 2,101,145
Po = 4,693,200 + 2,101,145 = 6,794,345 N
φPn,max = 0.52 × 6,794,345 = 3,533,059 N = 3,533 kN > 3,500 kN ✓
In reverse design, express Ast in terms of Ag using the assumed ρg. This linearizes Po as a function of Ag, allowing direct solution. Always round the column dimension UP to the next standard size, then verify adequacy. This is a classic board exam problem type.
PROBLEM 5 (Balanced Point of Interaction Diagram)
A 300 mm × 500 mm tied column bends about the 300 mm axis. Reinforcement: 3–25 mm bars at d' = 65 mm from compression face, 3–25 mm bars at d = 435 mm. f'c = 21 MPa (β1 = 0.85), fy = 275 MPa, Es = 200,000 MPa. Find Pb and Mb.
Solution
As = As' = 3 × (π/4)(25²) = 3 × 490.87 = 1,473 mm²
b = 300 mm, h = 500 mm (bending direction), centroid at 250 mm from top
Balanced neutral axis:
εy = 275/200,000 = 0.001375
cb = [0.003/(0.003 + 0.001375)] × 435
cb = [0.003/0.004375] × 435 = 0.6857 × 435 = 298.3 mm
a = β1 × cb = 0.85 × 298.3 = 253.5 mm
Concrete compression:
Cc = 0.85 × f'c × a × b = 0.85(21)(253.5)(300) = 1,355,963 N = 1,356.0 kN
Compression steel:
εs' = 0.003(298.3 − 65)/298.3 = 0.003 × 233.3/298.3 = 0.002347 > εy → compression steel yields
Cs = As'(fy − 0.85f'c) = 1,473(275 − 0.85×21) = 1,473(275 − 17.85) = 1,473 × 257.15 = 378,782 N = 378.8 kN
Tension steel:
εs = 0.003(d − cb)/cb = 0.003(435 − 298.3)/298.3 = 0.003(136.7/298.3) = 0.001375 = εy ✓ (yields at balanced)
T = As × fy = 1,473 × 275 = 405,075 N = 405.1 kN
Balanced axial load:
Pb = Cc + Cs − T = 1,356.0 + 378.8 − 405.1 = 1,329.7 kN
Balanced moment (about centroid at 250 mm from top):
Mb = Cc(250 − a/2) + Cs(250 − 65) + T(435 − 250)
Mb = 1,356.0(250 − 126.75) + 378.8(185) + 405.1(185)
Mb = 1,356.0(123.25) + 70,078 + 74,944
Mb = 167,127 + 70,078 + 74,944 = 312,149 kN·mm = 312.1 kN·m
Design values:
φPb = 0.65 × 1,329.7 = 864.3 kN
φMb = 0.65 × 312.1 = 202.9 kN·m
Note that at the balanced point, the tension steel strain equals εy exactly by definition. The compression steel strain (0.002347) exceeded εy, so compression steel is at fy. When f'c = 21 MPa is small, the 0.85f'c subtraction from compression steel stress (to avoid double-counting concrete area) makes a noticeable difference. Always check this subtraction.
Exam preparation tips
- Memorize the two combined multipliers cold: tied = 0.52 (0.80×0.65) and spiral = 0.6375 (0.85×0.75). These appear in nearly every column problem.
- Always use net concrete area (Ag − Ast) in the concrete term of Po. Writing 0.85f'cAg instead of 0.85f'c(Ag−Ast) is the single most common column error on the PRC board exam.
- Know the four key reinforcement limits without reference: ρg = 1% min, 8% max; 4 bars min for tied, 6 bars min for spiral; ties: smallest of 16db(long), 48db(tie), least column dimension.
- For the spiral ratio formula, always check: Is fyt ≤ 700 MPa? Cap it if not. Use Ach (to outside of spiral) not the net core area.
- Commit the balanced depth formula to memory: cb = 600d/(600 + fy) in SI units (fy in MPa). This works because Es = 200,000 MPa so εy = fy/200,000 and the formula derives from strain compatibility.
- On the interaction diagram: the balanced point gives maximum Mn, not minimum safety. Points above balanced are compression-controlled (φ = 0.65 tied); below are tension-controlled (φ → 0.90).
- For slenderness: check braced vs. unbraced frame FIRST. Use klu/r ≤ 22 for unbraced and ≤ 34−12(M1/M2) for braced. If the problem does not specify frame type, ask yourself whether shear walls or bracing are mentioned.
- When the problem asks for 'design axial capacity' vs. 'nominal axial strength,' these are different: nominal = Po, design = φPn,max = (cap)(φ)Po.
- In multiple-choice items where the answer choices differ only slightly (e.g., 2779 vs 2780 kN), carry at least 4 significant figures throughout your calculation before rounding.
- For reverse design (find column size given Pu), express Ast = ρg × Ag, substitute into Po, set φPn,max = Pu, solve for Ag, then get h = √Ag for square column. Round h UP, then verify.
- The 23% capacity advantage of spiral over tied columns is a useful sanity check: if your spiral result is not about 22–23% higher than the equivalent tied result, recheck your work.
- In board exam time management: column problems with interaction diagrams typically take 8–12 minutes. If you spend more than 15 minutes on one, move on and return. Pure axial capacity problems should take 3–5 minutes maximum.
Diagrams
RC Columns — Complete Concept Map
A comprehensive mind map of all RC column topics covered in this chapter, showing the hierarchical relationships between classification, capacity formulas, reinforcement requirements, the interaction diagram, and slenderness effects. Use this as a study roadmap before and after reviewing each concept.
Step-by-Step Column Axial Capacity Design Procedure
A decision-tree flowchart showing the complete step-by-step procedure for computing the design axial capacity of an RC column. The flowchart guides the user from identifying the column type through computing Po, applying the correct cap and φ, checking steel ratio limits, and determining if slenderness effects are needed.
Axial-Moment Interaction Diagram — Key Points and Zones
A flowchart-style diagram illustrating the four key points on the P-M interaction curve (pure axial, zero tension, balanced, and pure flexure), the compression-controlled and tension-controlled zones with their respective φ factors, and the design adequacy check. The balanced point as the location of maximum moment is highlighted.
Slenderness Classification and Moment Magnification Procedure
A complete decision flowchart for slenderness classification and the moment magnification method. Starting from computing klu/r, the chart guides through checking against the appropriate limit, computing Cm and δns, applying the magnifier to M2, and checking against the minimum moment requirement. This covers all the steps needed for board exam problems on slender columns.
Historical Development of Column Design Provisions
A timeline showing the historical development of RC column design provisions in the Philippines, from the enactment of RA 544 in 1966 through the current NSCP 2015. This provides context for understanding why certain provisions exist and how they have evolved.
Distribution of Column Topics in PRC Board Exam (Typical)
A pie chart showing the approximate distribution of RC column examination questions in the PRC Civil Engineer Licensure Examination based on historical board exam patterns. Axial capacity computation (35%) and interaction diagrams (25%) together account for 60% of column questions, confirming where to focus study time.
In summary
Reinforced concrete columns are among the most critical structural members in any building, and their correct design requires mastering several interrelated concepts: the nominal axial strength formula Po with its net concrete area, the distinct cap factors and φ values for tied (0.52 combined) versus spiral (0.6375 combined) columns, the longitudinal and transverse reinforcement limits mandated by NSCP 2015, the construction of the axial–moment interaction diagram with its key points (pure axial, balanced, and pure flexure), and the moment magnification procedure for slender columns.
For PRC board examinees, the highest-yield study priorities are: (1) memorizing the combined multipliers 0.52 (tied) and 0.6375 (spiral) without hesitation, (2) always computing net concrete area (Ag − Ast), (3) applying cb = 600d/(600+fy) for the balanced condition, (4) checking steel ratio limits 1%–8%, and (5) identifying frame type before applying slenderness limits. The ~23% capacity advantage of spiral columns over tied columns is a useful numerical benchmark.
Mastery of these concepts, demonstrated through consistent practice of board-style problems in SI units with proper citation of NSCP 2015 provisions, will prepare you to answer column questions confidently and efficiently within the time constraints of the PRC Civil Engineer Licensure Examination. Approach each problem systematically: identify column type, compute areas, apply the correct formula, and verify reinforcement limits — every time.