RSJ vs UB vs UC Beam Comparison Table – Which Steel Section Is Best for Your Build?

Understanding the differences between RSJ (Rolled Steel Joist), UB (Universal Beam), and UC (Universal Column) sections is crucial for optimal structural design. While all are steel I-sections, each has distinct characteristics that make them suitable for different applications. This comprehensive guide explains when to use each type and howto select the right section for your project.

What Are the Different Section Types?

RSJ (Rolled Steel Joist)

Traditional UK designation for I-beams with:

• Narrower flanges relative to depth
• Tapered flanges (thicker at web, thinner at edges)
• Older terminology, often interchangeable with UB in modern usage
• Classic designation: e.g., 203×133×25

Best for: Hidden installations, standard residential work, where narrow width beneficial

UB (Universal Beam)

Modern standardized I-sections with:

• Wider flanges than RSJ for same depth
• Parallel flanges (uniform thickness)
• Optimized for bending resistance
• Designation: e.g., 305×165×40 UB

Best for: Long spans, heavy loads, commercial construction, better lateral stability

UC (Universal Column)

Heavier sections designed primarily for columns:

• Very wide flanges (depth ≈ width, creating nearly square profile)
• Thick webs and flanges
• Excellent for axial (vertical) loads
• Designation: e.g., 254×254×73 UC

Best for: Columns, posts, situations requiring biaxial bending resistance

Detailed Comparison Table

Key Dimensional Differences

Feature	RSJ	UB	UC
Flange width	Narrow (40-60% of depth)	Medium-Wide (50-70% of depth)	Very Wide (80-100% of depth)
Flange shape	Tapered	Parallel	Parallel
Web thickness	Thinner	Medium	Thicker
Weight efficiency	Good for beams	Optimized for beams	Optimized for columns
Lateral stability	Lower	Higher	Highest

Performance Characteristics

Characteristic	RSJ	UB	UC
Bending strength (major axis)	Good	Excellent	Good
Bending strength (minor axis)	Lower	Moderate	Excellent
Axial load capacity	Moderate	Moderate	Excellent
Lateral-torsional buckling resistance	Lower	Good	Excellent
Deflection control	Good (depends on depth)	Excellent	Good
Connection simplicity	Good	Excellent (parallel flanges easier)	Excellent

Cost Comparison

Per Ton Basis (2026 UK prices):

• RSJ: £800-1,100/ton
• UB: £850-1,150/ton
• UC: £900-1,200/ton

Per Meter for Similar Capacity:

• RSJ: Often cheapest for residential beams
• UB: Competitive for long spans
• UC: Most expensive for beam applications (not optimized for this use)

Value analysis: Choose based on application, not just price per ton.

When to Use Each Section Type

Use RSJ/Narrow UB When:

1. Concealed Installations Narrower flanges easier to box in with plasterboard or conceal in walls.

Example: Domestic knockthrough where beam will be hidden in ceiling void – 203×133×25 RSJ easier to conceal than wider UB.

2. Matching Existing Structure Older buildings often have RSJ sections – matching makes connections simpler.

3. Standard Residential Spans (2-5m) Adequate performance for typical domestic loads without over-engineering.

4. Limited Headroom When depth is constrained but lateral restraint is good (floor joists provide restraint).

5. Budget-Conscious Projects RSJ often most economical for straightforward residential beam applications.

Use Universal Beam (UB) When:

1. Long Spans (>5m) Superior section properties provide better efficiency for longer spans.

Example: 6m garage opening – 305×165×40 UB more efficient than trying to achieve with narrower RSJ.

2. Heavy Loads UB sections optimized for maximum bending resistance per kg of steel.

Example: Commercial premises with multiple floors above.

3. Limited Lateral Restraint Wider flanges provide better resistance to lateral-torsional buckling when top flange not continuously restrained.

Example: Roof beam with only occasional purlin connections.

4. Point Loads Better web capacity and flange stiffness handle concentrated loads.

Example: Supporting columns or other beams at discrete locations.

5. Modern Construction Standards UB designations standard in current engineering practice and steel tables.

Use Universal Column (UC) When:

1. Vertical Loads (Columns/Posts) Primary application – supporting loads axially (vertically).

Example: Steel column supporting multiple floor beams, transferring loads to foundation.

2. Biaxial Bending Situations where bending occurs in more than one direction.

Example: Corner column supporting beams from two directions.

3. Short, Heavily Loaded Beams When span/depth ratio <10 and loads very heavy.

Example: Transfer beam over basement opening in commercial building.

4. Portal Frame Columns Industrial buildings requiring moment-resisting connections.

5. Situations Requiring Maximum Cross-Sectional Area Fire protection, corrosion resistance (more metal = longer failure time).

Practical Examples with Calculations

Example 1: Domestic Knockthrough (4m span)

Requirements:

• Span: 4.0m
• Load: 10 kN/m
• Residential bedroom above
• Standard ceiling height

Option A: RSJ – 203×133×30

• Cost: £340 (4.3m × £79/m)
• Weight: 129kg
• Section modulus: 245 cm³
• Deflection: ~8mm ✓
• Boxing dimension: 133mm width
• Best choice: Easiest to conceal, adequate capacity

Option B: UB – 203×203×46

• Cost: £460 (4.3m × £107/m)
• Weight: 198kg
• Section modulus: 410 cm³
• Deflection: ~5mm ✓
• Boxing dimension: 203mm width
• Over-engineered: Unnecessary capacity, harder to conceal

Winner: RSJ – simpler, cheaper, adequate

Example 2: Commercial Mezzanine (7m span)

Requirements:

• Span: 7.0m
• Load: 15 kN/m
• Office floor above
• Limited intermediate support possible

Option A: RSJ – 254×146×43

• At 7m span: Deflection excessive (~35mm, limit ~19mm) ✗
• Fails deflection check

Option B: UB – 356×171×51

• Section modulus: 876 cm³ -I = 19,610 cm⁴
• Deflection: ~18mm ✓
• Cost: £520 (7.3m × £71/m)
• Works: Meets all criteria

Option C: UB – 406×178×54

• Even better deflection control
• Cost: £540 (7.3m × £74/m)
• Marginal extra cost for better performance

Winner: UB required – RSJ sections inadequate for this long span

Example 3: Steel Column

Requirements:

• Height: 3.0m
• Axial load: 200 kN
• Lateral restraint: Top and bottom only
• Must resist minor bending from eccentricity

Option A: RSJ – 203×133×30

• Narrow flanges = poor buckling resistance
• Slenderness ratio too high ✗
• Not suitable for column application

Option B: UB – 203×203×46

• Better than RSJ but still not optimized
• Marginal capacity ✓ but not ideal

Option C: UC – 203×203×52

• Square profile ideal for column
• Excellent buckling resistance
• Handles biaxial bending
• Cost: Similar to UB option
• Best choice: Purpose-designed for columns

Winner: UC – correct section type for application

Technical Differences Explained

Flange Geometry Impact

Tapered Flanges (RSJ):

• Advantages: Slightly better stress distribution at web-flange junction
• Disadvantages: More complex connection design, harder to weld

Parallel Flanges (UB/UC):

• Advantages: Simplified connections, easier fabrication, standard bolt positioning
• Disadvantages: Slight stress concentration at web-flange junction (negligible in practice)

Modern preference: Parallel flanges (UB/UC) for ease of fabrication

Section Efficiency

Bending Efficiency (Zx per kg):

1. UB: Best (optimized flange size and placement)
2. RSJ: Good (traditional proportions)
3. UC: Lower (excess material for beam applications)

Compression Efficiency (Area per kg):

1. UC: Best (stocky profile)
2. UB: Good
3. RSJ: Lower (narrow flanges)

Use section optimized for primary load type

Buckling Resistance

Lateral-Torsional Buckling (beams without full lateral restraint):

Ranking (best to worst):

1. UC (wide flanges + stocky web)
2. UB (wide flanges)
3. RSJ (narrow flanges)

Example: Unrestrained 5m roof purlin resisting top flange buckling:

• 203×203 UC can span ~20% further than 203×133 RSJ of same weight
• Wider flanges dramatically improve buckling resistance

Load Distribution

Point Load Resistance:

Web capacity (ability to support concentrated loads without local crushing):

• UC: Thickest webs, best
• UB: Medium webs, good
• RSJ: Thinner webs, adequate with bearing stiffeners

Flange stiffness (resistance to local bending under concentrated loads):

• UC: Widest and thickest flanges, excellent
• UB: Wide flanges, very good
• RSJ: Narrow flanges, requires load spreading

Modern UK Practice

Current Trends

Residential:

• Small projects: Still commonly specify RSJ (e.g., “203×133×30 RSJ”)
• Larger projects: Increasingly use UB designation (e.g., “254×146×37 UB”)
• Both acceptable – often used interchangeably

Commercial/Industrial:

• Almost exclusively UB/UC designations
• Better standardization across international projects
• Easier connection to European standards (IPE, HE sections)

Steel Supplier Catalogs

Most UK suppliers stock:

• RSJ sections: 152-305mm depths, narrower flanges
• UB sections: 127-914mm depths, full range
• UC sections: 152-356mm depths for common column sizes

Availability: UB generally better stock availability in larger sizes.

Conversion Guide: RSJ to UB Equivalents

Common equivalents (approximately):

RSJ Designation	Nearest UB Equivalent	Notes
203×133×25	203×133×25 UB	Often identical
203×133×30	203×133×30 UB	Often identical
254×146×31	254×146×31 UB	Often identical
254×146×37	254×146×37 UB	Often identical

Note: In practice, many “RSJ” designations in modern steel actually refer to parallel-flange UB sections. The terminology persists even though the profiles have evolved.

Cost-Benefit Analysis

When Extra Cost of UB is Justified:

Scenario: 6m span, 12 kN/m load

Narrow section (RSJ-type):

• 254×146×43: Marginal deflection compliance
• Cost: £465
• Problem: Bouncy floor feel, marginal Building Control acceptance

Wider section (UB):

• 305×165×40 UB: Excellent deflection control
• Cost: £510
• Benefit: Superior floor feel, easy Building Control approval

Extra cost: £45 Value: Better performance, happier occupants, no comebacks

Verdict: Worth it!

When Cheaper RSJ is Adequate:

Scenario: 3m span, 8 kN/m, fully restrained by floor joists

RSJ option:

• 203×133×30: Adequate strength and deflection
• Cost: £255
• Installed in 215mm partition wall

UB alternative:

• 203×203×46: Over-capacity, harder to conceal
• Cost: £355
• Requires 250mm partition (wastes floor space)

Saving: £100 Trade-off: None – RSJ is better choice here

Verdict: Use RSJ!

Specification Language

Correct Terminology for Engineers/Suppliers

Good specification: “254×146×37 UB, grade S275, to BS EN 10025”

Acceptable variation: “254×146×37 RSJ, grade S275” (supplier will understand)

Ambiguous (avoid): “254mm steel beam” (insufficient detail)

For UC sections: “203×203×52 UC” (always use UC designation for columns – never call these RSJ or UB)

Conclusion

Choose section type based on application:

RSJ (narrow UB):

• Standard residential beams
• Short to medium spans (<5m)
• Concealed installations
• Budget-conscious projects

Universal Beam (UB):

• Commercial construction
• Long spans (>5m)
• Heavy loads or limited restraint
• Modern engineering standards

Universal Column (UC):

• Columns and vertical supports
• Biaxial loading
• Best buckling resistance
• Portal frames

Remember: Many modern “RSJ” specifications actually refer to parallel-flange UB sections. The most important factor is selecting a section with adequate strength, stiffness, and buckling resistance for your specific application, verified by a structural engineer.

Disclaimer: Section selection must be verified by a chartered structural engineer. This guide provides general comparison only and does not replace professional engineering design or Building Regulations compliance.