Structural Loading — Design Architect Knowledge Base
Load Types
Dead Loads (D) — Self-Weight
| Assembly | PSF | |----------|-----| | Residential floor (wood frame + subfloor + finish) | 10–15 | | Precast hollow-core plank (8") | 55–65 | | Precast + recovered oak overlay (3/4") | 58–68 | | Exterior wood stud wall (2×6 + sheathing + siding) | 8–12 | | Interior partition (2×4 + GWB both sides) | 6–8 | | Asphalt shingle roof (shingles + sheathing + trusses) | 10–12 | | Steel beam W12×26 | 26 plf | | Steel column W10×26 | 26 plf | | Fiber cement panel facade | 3–5 | | Continuous rigid insulation (1" polyiso) | 0.2 |
Live Loads (L) — IRC/IBC Required
| Use | PSF | Code Reference | |-----|-----|----------------| | Residential floor (living areas) | 40 | IRC R301.5 | | Sleeping rooms | 30 | IRC R301.5 | | Uninhabitable attic (no storage) | 10 | IRC R301.5 | | Uninhabitable attic (limited storage) | 20 | IRC R301.5 | | Habitable attic with fixed stairs | 30 | IRC R301.5 | | Decks and balconies | 40 | IRC R301.5 | | Guardrails (concentrated) | 200 lbs | IRC R301.5 | | Roof live load (maintenance) | 20 | IBC 1607.12 | | Roof live load (slope > 4:12) | 15 | IBC 1607.12 |
Snow Loads (S) — Rhode Island
| Location | Ground Snow (pg) PSF | |----------|---------------------| | Coastal RI (Newport, Narragansett) | 25–35 | | Central RI (Cranston, Warwick) | 30–40 | | Northern RI (Woonsocket, Burrillville) | 35–40 | | Reference | IBC Table 1608.1, ASCE 7-22 Ch 7 |
Roof snow load: pf = 0.7 × Ce × Ct × Is × pg
- Ce = exposure factor (0.8 windswept, 1.0 normal, 1.2 sheltered)
- Ct = thermal factor (1.0 heated, 1.1 unheated)
- Is = importance factor (1.0 residential)
Wind Loads — Rhode Island
| Parameter | Value | |-----------|-------| | Basic wind speed (Risk Category II) | 115–130 mph | | Coastal exposed areas | Up to 130–140 mph | | Exposure category (typical residential) | B or C | | Reference | ASCE 7-16 Table 1609.3 |
Main wind force resisting system (MWFRS): Directional procedure per ASCE 7 Ch 27-28. For residential ≤ 60' mean roof height, simplified procedure (Ch 28) typically applies.
Seismic Loads — Rhode Island
| Parameter | Value | |-----------|-------| | Seismic Design Category | A (minimal) | | Ss (short period) | ≤ 0.15g | | S1 (1-second period) | ≤ 0.04g | | Consequence | Exempt from detailed seismic analysis | | Prescriptive connections | Acceptable |
Load Combinations
LRFD (Strength Design)
ASD (Allowable Stress Design)
Load Path — Gravity
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Roof shingles → Sheathing → Trusses (top chord)
↓
Truss reactions → Top plate (bearing wall or steel beam)
↓
Steel beam → Bolted end-plate connection → Steel column
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Column base plate → Anchor bolts → Grade beam / Spread footing
↓
Soil (bearing capacity 2,000–2,500 PSF typical RI)
Load Path — Lateral (Wind)
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Wind pressure on facade panels
↓
Panel clips → Steel column flanges (bolted DfD)
↓
Column moment connection → Steel beam (moment frame action)
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Base plate → Anchor bolts → Foundation (overturning resistance)
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Soil passive pressure + footing weight (sliding resistance)
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Tributary Area Calculations
For a steel beam supporting joists from both sides:
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Tributary width = (span_north / 2) + (span_south / 2)
Linear load (plf) = (D + L) psf × tributary width (ft)
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For a column (interior):
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Tributary area = bay_width × bay_depth
Column axial load = (D + L) psf × tributary area (sf) × number of floors
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Example — ML Systems 20' × 20' bay, 2-story:
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D = 15 psf (floor) + 12 psf (roof) = 27 psf per floor
L = 40 psf (floor) + 30 psf (roof snow)
Tributary area = 20' × 20' = 400 sf
Interior column load (LRFD):
Floor: 1.2(15×400) + 1.6(40×400) = 7,200 + 25,600 = 32,800 lbs
Roof: 1.2(12×400) + 0.5(30×400) = 5,760 + 6,000 = 11,760 lbs
Total: 44,560 lbs ≈ 44.6 kips
Corner column (1/4 tributary): ≈ 11.1 kips
Edge column (1/2 tributary): ≈ 22.3 kips
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Deflection Limits
| Condition | Limit | 20' Span Max Deflection |
|-----------|-------|------------------------|
| Floor live load (GWB ceiling) | L/360 | 0.67" |
| Floor total load | L/240 | 1.00" |
| Roof live load | L/360 | 0.67" |
| Roof total load | L/180 | 1.33" |
| Tile/stone flooring | L/720 | 0.33" |
| Precast hollow-core (typical) | L/360 | 0.67" |
Formula: δ = 5wL⁴ / (384EI) for uniform load
- w = load per unit length
- L = span
- E = modulus of elasticity
- I = moment of inertia
Steel Properties — ML Steel vs A36
| Property | A36 | HSLA 60 ksi | HSLA 80 ksi |
|----------|-----|-------------|-------------|
| Yield strength (Fy) | 36 ksi | 60 ksi | 80 ksi |
| Tensile strength (Fu) | 58 ksi | 75 ksi | 90 ksi |
| Modulus of elasticity (E) | 29,000 ksi | 29,000 ksi | 29,000 ksi |
| Strength ratio vs A36 | 1.0× | 1.67× | 2.22× |
| Cost premium vs A36 | — | ~15% | ~24% |
| Weight savings (same capacity) | — | 25–35% | 35–50% |
| Source | Mill | ML recycled auto | ML recycled auto |
Key insight: E is identical across all steel grades — deflection depends on geometry (I), not strength (Fy). Higher-strength steel allows lighter SECTIONS but the moment of inertia may be lower, so deflection must be checked independently of strength.
Foundation Design
Soil Bearing — Rhode Island
| Soil Type | Allowable Bearing (PSF) |
|-----------|------------------------|
| Bedrock | 12,000+ |
| Dense gravel/sand (compacted) | 3,000–4,000 |
| Medium sand, glacial till | 2,000–3,000 |
| Stiff clay | 2,000 |
| Soft clay/silt | 1,000–1,500 |
| RI typical (prescriptive) | 2,000 |
Frost Depth
| Location | Frost Depth |
|----------|-------------|
| Most of RI | 40" |
| Block Island (New Shoreham) | 30" |
| Practical design | 42"–48" (safety margin) |
Spread Footing Sizing
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Required area = Column load (service) / Allowable soil bearing
Example: 45 kips / 2,000 psf = 22.5 sf → 4'-9" × 4'-9" pad
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Multi-Cycle Over-Engineering
- Size footings for N+2 future stories above current design
- Embed stub plates in grade beams (A36 plate, A325 anchor bolts)
- Upfront cost premium: 5–10%
- Avoids future underpinning: saves 20–30% on expansion
- Column base plates designed for future axial load + moment