# Mass Timber Construction

## Mass Timber Products

### Product Types
| Product | Description | Typical Use |
|---------|-------------|-------------|
| CLT | Cross-laminated timber | Floors, walls, roofs |
| Glulam | Glued laminated timber | Beams, columns |
| NLT | Nail-laminated timber | Floors, decks |
| DLT | Dowel-laminated timber | Floors, decks |
| MPP | Mass plywood panels | Floors, walls |
| LVL | Laminated veneer lumber | Beams, headers |
| PSL | Parallel strand lumber | Columns, beams |

### CLT Properties
| Property | Typical Value |
|----------|---------------|
| Density | 30-35 pcf |
| Bending strength | 2,000-3,000 psi |
| Compression parallel | 1,500-2,500 psi |
| Shear | 200-400 psi |
| MOE | 1.2-1.6 × 10⁶ psi |

### CLT Layer Configurations
| Layers | Thickness Range | Application |
|--------|-----------------|-------------|
| 3-ply | 4"-5" | Walls, light floors |
| 5-ply | 5"-7" | Floors, roofs |
| 7-ply | 8"-12" | Heavy floors, long spans |
| 9-ply | 12"-16" | Long spans, heavy loads |

### Glulam Grades
| Grade | Appearance | Application |
|-------|------------|-------------|
| Industrial | Utilitarian | Hidden structure |
| Architectural | Limited defects | Exposed, painted |
| Premium | Minimal defects | Exposed, clear finish |
| Custom | Specified | Special applications |

---

## Code Compliance

### IBC 2021 Mass Timber Types
| Type | Stories | Area (sf) | Fire Rating |
|------|---------|-----------|-------------|
| Type IV-A | Up to 18 | Unlimited | 3-hour |
| Type IV-B | Up to 12 | Varies | 2-hour |
| Type IV-C | Up to 9 | Varies | 2-hour |
| Type IV-HT | Per table | Per table | 1-hour (HT) |

### Fire Resistance
| Rating | Char Depth | Notes |
|--------|------------|-------|
| 1-hour | ~1.5" | Char rate 1.5"/hr |
| 2-hour | ~3" | Additional layers |
| 3-hour | ~4.5" | Requires protection |

### Char Rate Calculation
```
Char depth = 1.5 × (time in hours) × 1.2 (factor of safety)

Example: 2-hour rating
Char = 1.5 × 2 × 1.2 = 3.6" required sacrificial wood
```

### Protection Requirements
| Type | Floor/Roof | Wall |
|------|------------|------|
| IV-A | 2" gypsum | 2" gypsum |
| IV-B | 1" gypsum | 1" gypsum |
| IV-C | Exposed OK | Exposed OK |
| IV-HT | Exposed OK | Exposed OK |

### Sprinkler Requirements
| Type | Requirement |
|------|-------------|
| IV-A | NFPA 13 throughout |
| IV-B | NFPA 13 throughout |
| IV-C | NFPA 13 throughout |
| IV-HT | Per occupancy |

---

## Structural Design

### Span Capabilities
| Element | Efficient Span | Maximum Span |
|---------|----------------|--------------|
| CLT floor (5-ply) | 15-20' | 25' |
| CLT floor (7-ply) | 20-25' | 30' |
| Glulam beam | 30-60' | 100'+ |
| Glulam column | Any height | Per slenderness |

### Connection Types
| Type | Application | Capacity |
|------|-------------|----------|
| Self-tapping screws | CLT-to-CLT | Light loads |
| Through bolts | Heavy connections | High loads |
| Steel knife plates | Beam-column | High loads |
| Concealed brackets | Architecturally exposed | Moderate |
| Bearing seats | Simple support | Varies |
| Post-tensioning | Seismic | Very high |

### Typical Connections
| Connection | Common Hardware |
|------------|-----------------|
| CLT panel-to-panel | Surface splines, screws |
| CLT to glulam beam | Steel angles, screws |
| Glulam beam-column | Concealed brackets |
| Column base | Steel base plate, bolts |
| Lateral | Hold-downs, straps |

### Lateral Systems
| System | Application |
|--------|-------------|
| CLT shear walls | Primary lateral |
| CLT diaphragms | Floor/roof |
| Moment frames | Open floor plans |
| Braced frames | Industrial |
| Core walls | Tall buildings |

### Diaphragm Design
| Element | Consideration |
|---------|---------------|
| Panel joints | Shear transfer |
| Chord forces | Perimeter members |
| Collector forces | Transfer to shear walls |
| Openings | Reinforcement at edges |

---

## Moisture Protection

### Moisture Content
| Stage | Target MC | Maximum |
|-------|-----------|---------|
| Manufacturing | 12% ± 3% | 15% |
| Delivery | 12% ± 3% | 19% |
| Installation | < 16% | 19% |
| In-service | 8-14% | 19% |

### Protection Strategies
| Phase | Strategy |
|-------|----------|
| Transport | Wrap, covered trucks |
| Storage | Elevated, covered |
| Installation | Rapid enclosure |
| Permanent | Roof, wall assembly |

### Critical Details
| Location | Protection |
|----------|------------|
| Roof parapet | Cap flashing, cant |
| Balcony | Waterproof membrane |
| Ground contact | 6" min clearance |
| Exterior columns | Steel base, flashing |
| Window openings | Sill pan, drainage |

### Monitoring
| Method | Application |
|--------|-------------|
| Moisture sensors | Embedded in CLT |
| Visual inspection | Regular checking |
| Pin moisture meter | Spot checks |
| Relative humidity | Ambient monitoring |

---

## Acoustic Performance

### CLT Sound Transmission
| Assembly | STC | IIC |
|----------|-----|-----|
| 5-ply CLT bare | 35-40 | 25-35 |
| CLT + topping | 50-55 | 45-55 |
| CLT + ceiling | 55-60 | 50-60 |
| CLT + both | 60-65 | 55-65 |

### Improvement Strategies
| Strategy | STC Gain | IIC Gain |
|----------|----------|----------|
| Concrete topping (2") | +10 | +15 |
| Gypcrete topping | +5 | +15 |
| Resilient ceiling | +10 | +10 |
| Acoustic mat | +5 | +15 |
| Mass layer | +5 | +5 |

### Typical Assemblies
| Assembly | Components | STC/IIC |
|----------|------------|---------|
| Basic | CLT + mat + topping | 50/55 |
| Enhanced | CLT + mat + topping + ceiling | 60/60 |
| Premium | CLT + isolation + topping + ceiling | 65/65 |

---

## Thermal Performance

### CLT Thermal Resistance
| Thickness | R-Value |
|-----------|---------|
| 4" | ~4 |
| 6" | ~6 |
| 8" | ~8 |

### Insulation Strategies
| Location | Type | R-Value |
|----------|------|---------|
| Exterior | Continuous | R-15 to R-30 |
| Cavity | Mineral wool | R-13 to R-21 |
| Roof | Above deck | R-30 to R-60 |

### Thermal Mass Benefits
| Climate | Benefit |
|---------|---------|
| Hot-dry | Night cooling |
| Cold | Heat retention |
| Moderate | Temperature stability |

### Air Barrier
| Location | Strategy |
|----------|----------|
| CLT joints | Tape, sealant |
| Panel-to-wall | Continuous membrane |
| Penetrations | Seal all holes |

---

## Sustainability

### Carbon Benefits
| Factor | Impact |
|--------|--------|
| Sequestered carbon | ~1 ton CO2/m³ wood |
| Avoided emissions | Lower than concrete/steel |
| Forest management | Sustainable sourcing |
| End of life | Reuse, recycle, bioenergy |

### Certifications
| Certification | Focus |
|---------------|-------|
| FSC | Sustainable forestry |
| PEFC | Sustainable forestry |
| SFI | North American forests |
| EPD | Environmental data |

### Comparison to Conventional
| Factor | Mass Timber | Concrete | Steel |
|--------|-------------|----------|-------|
| Embodied carbon | Low | High | Medium |
| Weight | Light | Heavy | Light |
| Construction speed | Fast | Slow | Fast |
| Local sourcing | Variable | Common | Variable |

---

## Construction Process

### Prefabrication
| Element | Shop Work |
|---------|-----------|
| CLT panels | Cut to size, openings |
| Glulam | Cut, drilled, hardware |
| Connections | Pre-installed where possible |
| MEP | Coordinated penetrations |

### Installation Sequence
```
1. Foundation/podium complete
2. Columns erected (crane)
3. Beams placed and connected
4. CLT floor panels placed
5. CLT walls erected
6. Repeat for each floor
7. Roof structure
8. Enclosure (critical path)
```

### Crane Requirements
| Element | Weight | Crane Capacity |
|---------|--------|----------------|
| CLT panel | 50-150 psf | 10-30 tons |
| Glulam beam | 35 pcf | Varies |
| Column | 35 pcf | Varies |
| Pick capacity | Per lift plan | 1.25× factor |

### Schedule Advantages
| Factor | Savings |
|--------|---------|
| Prefabrication | 20-40% faster |
| Dry construction | Weather tolerance |
| Fewer trades | Less coordination |
| Reduced finishing | Exposed structure |

---

## Detailing

### Panel Joints
| Type | Application |
|------|-------------|
| Butt joint | Non-structural |
| Spline | Shear transfer |
| Half-lap | Panel continuity |
| Step joint | Out-of-plane |

### Penetrations
| Type | Approach |
|------|----------|
| Small (< 6") | Core after install |
| Medium (6-12") | Pre-cut in shop |
| Large (> 12") | Reinforcement required |
| MEP | Coordinate with shop drawings |

### Exposed Finish
| Finish | Application |
|--------|-------------|
| Sanding | Smooth surface |
| Planing | Remove handling marks |
| Oil | Natural appearance |
| Stain | Color enhancement |
| Paint | Full coverage |
| Clear coat | Protection |

### Edge Conditions
| Condition | Detail |
|-----------|--------|
| Exposed edge | Sand, finish |
| Wall-to-floor | Base trim |
| Ceiling | Reveal, trim |
| Column | Wrap or expose |

---

## Cost Considerations

### Cost Factors
| Factor | Impact |
|--------|--------|
| Panel size | Larger = more efficient |
| Connection complexity | Simpler = lower cost |
| Fire rating | Higher = more material |
| Finish quality | Exposed = premium |
| Transportation | Distance matters |

### Cost Comparison (2024)
| System | $/SF Structure |
|--------|----------------|
| Mass timber | $50-80 |
| Steel frame | $40-60 |
| Concrete | $45-70 |

### Value Engineering
| Strategy | Savings |
|----------|---------|
| Optimize panel layout | 5-10% |
| Standard connections | 10-15% |
| Hybrid structure | Variable |
| Reduce exposed area | 5-10% |

---

## Hybrid Systems

### Mass Timber + Concrete
| Application | Benefit |
|-------------|---------|
| Concrete core | Lateral, fire stairs |
| Concrete podium | Parking, retail |
| Concrete topping | Acoustics, mass |

### Mass Timber + Steel
| Application | Benefit |
|-------------|---------|
| Steel connections | High capacity |
| Steel moment frames | Open plans |
| Steel beams | Long spans |

### Mass Timber + Light Frame
| Application | Benefit |
|-------------|---------|
| Light frame partitions | Flexibility |
| Light frame infill | Cost savings |
| CLT shear walls | Lateral only |

---

## Resources

### Design Standards
| Standard | Content |
|----------|---------|
| NDS | Timber design |
| ANSI/APA PRG 320 | CLT standard |
| AWC | Glulam, connections |
| CLT Handbook | Design guide |

### Software
| Tool | Use |
|------|-----|
| RFEM/RSTAB | Structural analysis |
| Revit | BIM modeling |
| Cadwork | Timber detailing |
| hsbCAD | Timber production |
