Steel Spans & Columns — Creative Design Agent Knowledge Base
W-Shape Beam Span Capacities (Residential Floor/Roof Loading)
Floor Beams (40 PSF live + 15 PSF dead = 55 PSF total)
| Section | Depth | Weight | Fy (ksi) | Max Span (strength) | Max Span (L/360 deflection) | Governing |
|---|
| W8×10 | 7.89" | 10 plf | 50 | 12' | 10' | Deflection |
| W8×18 | 8.14" | 18 plf | 50 | 18' | 14' | Deflection |
| W10×12 | 9.87" | 12 plf | 50 | 15' | 13' | Deflection |
| W10×22 | 10.17" | 22 plf | 50 | 22' | 17' | Deflection |
| W12×16 | 11.99" | 16 plf | 50 | 19' | 17' | Deflection |
| W12×26 | 12.22" | 26 plf | 50 | 28' | 20' | Deflection |
| W14×22 | 13.74" | 22 plf | 50 | 25' | 21' | Deflection |
| W14×30 | 13.84" | 30 plf | 50 | 32' | 24' | Deflection |
| W16×26 | 15.69" | 26 plf | 50 | 30' | 26' | Deflection |
| W16×36 | 15.86" | 36 plf | 50 | 38' | 30' | Deflection |
| W18×35 | 17.70" | 35 plf | 50 | 38' | 32' | Deflection |
| W21×44 | 20.66" | 44 plf | 50 | 46' | 38' | Deflection |
| W24×55 | 23.57" | 55 plf | 50 | 52' | 45' | Deflection |
ML Systems Standard: W12×26 at 20' Bay
- Why W12×26: Spans the 20' bay with ~0% overstress, deflection ≈ L/360 exactly
- Depth advantage: 12" depth fits within typical floor/ceiling sandwich
- HSLA upgrade: Same section in HSLA 65 ksi → strength increases 30%, but deflection unchanged (same E = 29,000 ksi, same I)
- Key insight: In residential, deflection almost ALWAYS governs over strength. HSLA's higher Fy helps with strength but NOT with stiffness. To reduce deflection, you need deeper sections (more I), not stronger steel.
Roof Beams (Lower Loading — 20 PSF live + 15 PSF dead)
Spans increase ~20-30% vs floor beams due to lighter loading. W10×22 easily handles 20' roof spans.
HSLA vs A36 — When It Matters
| Property | A36 | A572 Gr 50 | HSLA 60 | HSLA 80 |
|---|
| Fy (yield) | 36 ksi | 50 ksi | 60 ksi | 80 ksi |
| Fu (ultimate) | 58 ksi | 65 ksi | 70 ksi | 90 ksi |
| E (modulus) | 29,000 ksi | 29,000 ksi | 29,000 ksi | 29,000 ksi |
| Strength ratio vs A36 | 1.0× | 1.39× | 1.67× | 2.22× |
| Weight savings | — | 15-25% | 25-35% | 35-45% |
| Cost premium | — | ~5% | ~15% | ~25% |
| Weldability | Excellent | Good | Fair (preheat) | Poor (avoid) |
| DfD preference | Low | Good | Best | Bolted-only |
HSLA Decision Matrix
- Use HSLA 60: When strength governs (columns, short-span beams, connections under high load)
- Use A572 Gr 50: Default for most beams (deflection governs anyway, cheapest option)
- Use HSLA 80: Only for columns where axial load is extreme and weight savings justify cost
- Never field-weld HSLA 60+: Heat-affected zone (HAZ) degrades the micro-alloyed grain structure. DfD = bolted connections = no welding = perfect HSLA match.
Steel Columns
HSS (Hollow Structural Section) — Square/Rectangular Tubes
| Section | Size | Wall | Area | Weight | Axial Capacity (Fy=46, KL=10') | Use Case |
|---|
| HSS 3×3×1/4 | 3" sq | 0.25" | 2.59 in² | 9.42 plf | ~45 kips | Light partition support |
| HSS 4×4×1/4 | 4" sq | 0.25" | 3.37 in² | 12.21 plf | ~85 kips | Standard residential |
| HSS 4×4×3/8 | 4" sq | 0.375" | 4.78 in² | 17.27 plf | ~120 kips | Heavy residential |
| HSS 6×6×1/4 | 6" sq | 0.25" | 5.24 in² | 19.02 plf | ~155 kips | Multi-story residential |
| HSS 6×6×3/8 | 6" sq | 0.375" | 7.58 in² | 27.48 plf | ~225 kips | Commercial/heavy |
| HSS 8×8×3/8 | 8" sq | 0.375" | 10.4 in² | 37.69 plf | ~340 kips | Large spans/loads |
W-Shape Columns (When Moment Connection Needed)
| Section | Depth | bf | Weight | Axial (KL=10') | When to Use |
|---|
| W6×15 | 5.99" | 5.99" | 15 plf | ~90 kips | Light moment frame |
| W8×24 | 7.93" | 6.50" | 24 plf | ~165 kips | Standard moment frame |
| W10×33 | 9.73" | 7.96" | 33 plf | ~240 kips | ML Systems preferred |
| W12×40 | 11.94" | 8.01" | 40 plf | ~315 kips | Heavy moment frame |
| W14×48 | 13.79" | 8.03" | 48 plf | ~400 kips | Large open spans |
Column Selection Logic
Default: HSS 4×4×1/4 — clean square profile, easy to wrap with insulation, hides in 2×6 wall
Multi-story or heavy load: HSS 6×6×1/4 — still fits in wall, handles 2-3 story tributary
Moment frame needed (open floor plan): W10×33 — wide flange for bolted end-plate connection
Exposed/architectural: HSS round (pipe) or weathering steel HSS for visual effect
Corner columns: Can downsize — tributary area is 1/4 of interior column
Euler Buckling & Effective Length
Pcr = π²EI / (KL)²
K values:
Fixed-fixed: K = 0.65 (both ends moment-connected)
Fixed-pinned: K = 0.80 (one end moment, one end pin)
Pinned-pinned: K = 1.00 (both ends pinned — typical residential)
Fixed-free: K = 2.10 (cantilever column — AVOID)
Practical: For a pinned-pinned HSS 4×4×1/4 column:
- 8' height: capacity ~95 kips
- 10' height: capacity ~85 kips
- 12' height: capacity ~70 kips
- 14' height: capacity ~55 kips (getting borderline for interior columns)
Rule of thumb: Every 2' of additional height costs ~15% of column capacity.
Base Plate Design
Sizing Formula
Base plate area ≥ Factored axial load / (0.85 × f'c × φ)
Where:
f'c = concrete strength (typically 4,000 PSI)
φ = 0.65 (bearing on concrete)
Typical ML Systems Base Plates
| Column | Base Plate | Thickness | Anchor Bolts | Footing Size |
|---|
| HSS 4×4 | 8"×8" | 1/2" | (4) 5/8" A307 | 24"×24"×12" |
| HSS 6×6 | 10"×10" | 5/8" | (4) 3/4" A307 | 30"×30"×14" |
| W10×33 | 12"×12" | 3/4" | (4) 3/4" A325 | 36"×36"×16" |
| Moment base | 14"×14" | 1" | (6) 7/8" A325 | 42"×42"×18" |
DfD Base Plate Detail
- Stub plate cast into foundation at pour — flush with top of slab
- Column base plate bolted to stub plate with A325 bolts
- At deconstruction: unbolt column from stub plate, crane-lift column away
- Stub plate remains in foundation for Cycle 2 reuse
- Critical: Stub plates sized for N+2 column loads (multi-cycle over-engineering)
Bolt Specifications
A325 vs A490
| Property | A325 (Group A) | A490 (Group B) |
|---|
| Tensile strength | 120 ksi | 150 ksi |
| Proof load | 85 ksi | 120 ksi |
| Shear capacity (per bolt, 3/4") | ~15.9 kips | ~19.9 kips |
| Bearing capacity | Governed by plate thickness | Same |
| Installation | Snug-tight or turn-of-nut | Turn-of-nut or TC bolts |
| Reuse | Yes (DfD compatible) | No (ASTM prohibits reuse) |
| Galvanizing | Available (hot-dip) | NOT available |
| ML Systems choice | Primary — all connections | Avoid (non-reusable) |
Bolt Sizing for Residential Steel
| Bolt Diameter | Shear (A325-N) | Typical Use |
|---|
| 1/2" | 7.07 kips | Light connections, hangers |
| 5/8" | 11.0 kips | Beam-to-beam, secondary |
| 3/4" | 15.9 kips | Standard ML Systems connection |
| 7/8" | 21.6 kips | Moment connections |
| 1" | 28.3 kips | Heavy moment, base plates |
Bolt Pattern Rules
- Minimum edge distance: 1.5× bolt diameter from edge of plate
- Minimum spacing: 3× bolt diameter center-to-center
- Standard pattern: 3" gauge, 3" pitch for 3/4" bolts
- Even number of bolts per connection (2, 4, 6) — never odd
Connection Types
1. Simple Shear (Pinned) — Most Common
- What: Beam sits on column, transfers vertical load only, allows rotation
- Hardware: Clip angle, single plate ("shear tab"), or seated connection
- Bolts: 2-4 bolts through web
- Use: Standard beam-to-column where no lateral resistance needed
- DfD: Excellent — unbolt clip angle, crane-lift beam away
2. Bolted End-Plate Moment Connection — ML Systems Preferred
- What: Steel plate welded (shop only) to beam end, field-bolted to column flange
- Capacity: Transfers moment (bending) + shear — rigid frame behavior
- Bolts: 4-8 bolts through end plate into column flange
- Use: Open floor plans, lateral resistance, cantilevers
- DfD: Good — all field connections are bolted, shop welds are permanent but stay with the beam
- Advantage over field-welded: No HAZ concerns with HSLA, faster erection, reversible
3. Seated Connection (Knife/Unstiffened)
- What: Angle or tee bracket bolted to column, beam flange bears on seat
- Use: Light loads, easy erection (beam sets on shelf like a bookshelf)
- DfD: Excellent — very simple to reverse
4. Moment Connection Types (By Rigidity)
| Type | Rigidity | Bolts | Best For |
|---|
| Extended end plate | Full moment | 6-8 | Primary lateral system |
| Flush end plate | Partial moment | 4-6 | Secondary frames, moderate spans |
| Top-and-seat angle | Partial moment | 4 | Light moment, wind bracing |
| Shear tab only | Pinned (zero moment) | 2-3 | Gravity-only connections |
Deflection — The Real Design Driver
Why Deflection Governs in Residential
In residential construction with typical spans (15-25'), the beam almost always has enough STRENGTH but too much DEFLECTION. This is because:
Low loads: Residential = 40-50 PSF live vs 80-100 PSF commercial
Long spans: Homeowners want open plans = longer beams
Strict limits: L/360 for floor live load = 0.67" max deflection over 20'
Human perception: People feel floor bounce above L/360; plaster cracks above L/240
Deflection Formulas (Simply Supported)
Uniform load: δ = 5wL⁴ / (384EI)
Point load center: δ = PL³ / (48EI)
Two point loads: δ = Pa(3L² − 4a²) / (24EI) [loads at distance 'a' from supports]
Reducing Deflection (Without Changing Span)
| Strategy | Effect | Cost | Notes |
|---|
| Deeper section (↑I) | Best — I grows as d³ | Low | W12 vs W10 = ~70% more I |
| Heavier section (same depth) | Moderate | Low | Thicker flanges = more I |
| Composite action (concrete on steel) | ~2× stiffness | Moderate | Shear studs to precast — BUT kills DfD |
| Cambering | Visual fix only | Low | Pre-bend beam upward = looks flat under load |
| Continuous spans | ~60% less deflection | Design complexity | Beam runs over interior column |
| Moment connections | ~20-40% less | Moderate | End fixity reduces midspan deflection |
ML Systems Deflection Strategy
First choice: Size up the beam (W12×26 → W14×30 if needed)
Second choice: Add intermediate column (split 40' span into two 20' spans)
Third choice: Moment connections at beam ends (partial fixity)
NEVER: Composite with shear studs — destroys DfD reversibility
Cambering: Use for spans > 25' where some deflection is inevitable
Advanced Beam Concepts
Castellated Beams (Hexagonal Web Openings)
- Standard W-shape cut in zigzag pattern, offset, and re-welded → ~50% deeper beam with hexagonal holes
- Depth increase: 1.5× original depth at same weight
- Stiffness increase: ~2.5× due to depth³ relationship
- MEP routing: Ducts, pipes, conduit pass through web openings
- Span increase: 20-30% longer spans vs parent section
- DfD note: Re-welding is shop-only; field connections still bolted
- Use case: Long-span living areas where MEP needs to run through structure
Cellular Beams (Circular Web Openings)
- Same concept as castellated but with round holes — smoother aesthetics
- Better for exposed steel — clean circular openings look intentional
- Slightly less structural efficiency than hex openings
- Use case: Exposed ceiling in loft/industrial aesthetic homes
Vierendeel Trusses (Rectangular Openings)
- Frame-like truss with NO diagonal web members — only verticals
- Large rectangular openings between chords — perfect for windows, doorways, MEP
- Less efficient than diagonal trusses (bending in chords, not just axial)
- Use case: "Wall of glass" — Vierendeel spans above a window wall, carrying roof/floor load while allowing full-height glazing
- Connection: Moment connections at all chord-to-vertical joints (rigid frame behavior)
Stub Girders
- Short beam stubs welded to bottom chord, full-depth openings between stubs
- Use case: Very long spans (30-45') with integrated MEP zones
- Rare in residential — more common in commercial
Cantilever Design
Rules
- Maximum cantilever: ≤ L/3 of back span (structural) or ≤ L/4 (comfort)
- Example: 20' back span → max 6.7' cantilever (structural), 5' recommended
- Deflection at tip: δ = PL³/(3EI) — deflection grows as L³, so small increases in length = large deflection increases
- Uplift at back support: Cantilever creates net uplift — must be anchored or counter-weighted
Cantilever Connection
- Requires moment connection at the support point (not a simple pin)
- Bolted end-plate moment connection = DfD compatible
- Back span beam must be heavier than cantilever (counterbalance + load path)
- Floor vibration: Cantilevers are inherently bouncy — target natural frequency > 8 Hz
ML Systems Cantilever Applications
| Feature | Typical Cantilever | Beam | Notes |
|---|
| Bay window bump-out | 2-3' | W10×22 | Minimal — well within limits |
| Covered entry porch | 4-6' | W12×26 | Standard — moment at wall line |
| Balcony | 4-8' | W14×30 | Need railing load (200 PLF) at tip |
| Dramatic overhang | 8-12' | W16×36+ | Requires engineering review, heavy moment connection |
Floor Vibration Control
Why It Matters
Steel-framed residential floors can feel "bouncy" compared to heavy wood/concrete floors because steel is lighter (less mass to dampen vibration).
Natural Frequency Target
fn = 0.18√(g/δ) ≈ 0.18√(386/δ)
Where δ = instantaneous deflection in inches under design load
Target: fn > 8 Hz (imperceptible to occupants)
Strategies for ML Systems
Precast hollow-core on steel: Heavy plank (65 PSF) provides mass → excellent vibration damping
Deeper beams: Stiffer = higher natural frequency
Shorter spans: 20' bay grid is already good (vs 30'+ commercial)
Avoid lightweight wood joist floors on long steel spans — most common vibration complaint
Neoprene bearing pads between precast and steel: slight damping effect (bonus of DfD detail)
Portal Frames (Rigid Frames for Open Walls)
What They Do
A portal frame is a moment-connected beam-column assembly that provides lateral stability WITHOUT diagonal bracing or shear walls. This means you can have a completely open wall (glass, garage door, folding wall) and still resist wind/seismic loads.
ML Systems Application
- Open-plan living: Remove an entire wall line → portal frame at column grid carries lateral loads
- Garage without shear wall: Steel portal frame around garage door opening
- Indoor-outdoor living: Folding glass wall panels with portal frame above
Sizing (Typical Residential)
| Opening Width | Column | Beam | Connection |
|---|
| 12' | W8×24 | W10×22 | Flush end plate (4 bolts) |
| 16' | W10×33 | W12×26 | Extended end plate (6 bolts) |
| 20' | W10×49 | W14×30 | Extended end plate (8 bolts) |
| 24'+ | W12×53 | W16×36 | PE-designed moment connection |
Quick Reference — "What Size Steel Do I Need?"
Homeowner Asks → Engineer Answers
| Request | Span | Beam | Column | Connection |
|---|
| "Remove a wall" (bearing) | 12-16' | W10×22 | HSS 4×4 | Shear tab |
| "Open floor plan" (20' clear) | 20' | W12×26 | HSS 4×4 or W10×33 | Moment if lateral |
| "Great room" (25-30') | 25-30' | W14×30 to W16×36 | HSS 6×6 | Shear + bracing |
| "Loft/mezzanine" | 15-20' | W10×22 | HSS 4×4 | Shear tab |
| "Cantilever balcony" (6') | 6' out, 18'+ back | W12×26 | W10×33 | Moment |
| "Wall of glass" | 16-24' | W12×26 to W14×30 | W10×33 | Portal frame |
| "Column-free garage" (22') | 22' | W14×30 | HSS 6×6 | Portal frame |
| "Rooftop deck" | 20' | W12×26 | Same as below | Check uplift |