Wood construction systems

1. Wood construction systems GSD 6204: Building TechnologyHarvard Graduate School of Design

2. Construction system:a comprehensive method for organizing the assembly of building materials

3. “Open” systems vs. “Closed” systems: • Open systems • evolve over time • Closed systems • use a defined “kit of parts”

4. “Open” systems vs. “Closed” systems: • Open systems • evolve over time • fewer defining features • Closed systems • use a defined “kit of parts” • totalizing approach to element control

5. “Open” systems vs. “Closed”systems: • Open systems • evolve over time • fewer defining features • accept foreign elements • Closed systems • use a defined “kit of parts” • totalizing approach to element control • do not readily adapt to new features

6. “Open” systems vs. “Closed” systems: • Open systems • evolve over time • fewer defining features • accept foreign elements • associated with vernacular • Closed systems • use a defined “kit of parts” • totalizing approach to element control • do not readily adapt to new features • synthetic design approach

7. Traditional wood construction monolithic construction Modern wood construction layered, heterogeneous construction

8. Traditional wood construction monolithic construction individual elements have multiple functions Modern wood construction layered, heterogeneous construction increasing specialization of materials and roles

9. A modern layered approach to wood construction calls for more specialized elements, each with a role to play: • structural roles (vertical and horizontal bearing, bracing) • insulation (thermal, acoustic) • water- and weatherproofing • humidity control • ventilation • finish surface

10. Different wood species used for different roles, based on inherent properties of each: 1. Structure: spruce, pine, fir (most common in USA) based on high strength per weight, dimensional stability

11. Different wood species used for different roles, based on inherent properties of each: 1. Structure: spruce, pine, fir (most common in USA) based on high strength per weight, dimensional stability 2. Exterior enclosure: cedar, cypress, redwood, mahogany, teak and other tropical woods based on resistance to water and rot (trees which grow in wet environments), dimensional stability, workability

12. Different wood species used for different roles, based on inherent properties of each: 1. Structure: spruce, pine, fir (most common in USA) based on high strength per weight, dimensional stability 2. Exterior enclosure: cedar, cypress, redwood, mahogany, teak and other tropical woods based on resistance to water and rot (trees which grow in wet environments), dimensional stability, workability 3. Interior furnishings and finishes: various woods, especially hardwoods, in both solid and veneer applications based on durability (hardness) and appearance

13. Five construction issues specific to all wood systems: 1. Directionality -- strength varies depending on direction of grain relative to stresses (greater resistance in axis of grain). Engineered products such as plywood typically minimize this difference. 2. Dimensional stability -- dimensions vary with water content (15 times more pronounced across grain than longitudinally); sufficient drying necessary prior to use in construction; ventilation of cavity spaces essential. 3. Water damage -- risk relative to porosity of surface; end grain particularly susceptible to damage; chemical pressure treatment reduces risk. 4. Insect damage -- risk depends on region, species, protection of surface; chemical pressure treatment reduces risk. 5. Flammability -- safety risk more pronounced with lighter structural members; fireblocking for cavity walls.

14. Criteria for judging environmental performance: • 1. Replenishable natural resource: • managed forests; • material supplies are traceable and certifiable ; • FSC (Forestry Stewardship Council) = internationally recognized authority.

15. Criteria for judging environmental performance: • 1. Replenishable natural resource: • managed forests; • material supplies are traceable and certifiable ; • FSC (Forestry Stewardship Council) = internationally recognized authority. • 2. Adaptability over lifespan: • ease of adding or renovating structures using surgical rather than comprehensive demolition.

16. Criteria for judging environmental performance: • 1. Replenishable natural resource: • managed forests; • material supplies are traceable and certifiable; • FSC (Forestry Stewardship Council) = internationally recognized authority. • 2. Adaptability over lifespan: • ease of adding or renovating structures using surgical rather than comprehensive demolition. • 3. Production (or embodied) energy: • includes energy expended milling, drying, treating, and thermo-laminating; • includes transportation costs; • productionenergy costs still extremely low compared to other standard construction materials (concrete or metals).

17. Criteria for judging environmental performance: • 4. Pollutants -- • synthetic additives may pose problems of outgassing (formaldehyde = carcinogen); • heavy metals used in pressure treatment can pollute local soil and water.

18. Criteria for judging environmental performance: • 4. Pollutants -- • synthetic additives may pose problems of outgassing (formaldehyde = carcinogen); • heavy metals used in pressure treatment can pollute local soil and water. • 5. Recyclability -- • heavy timber: readily re-used in same form; • lighter members: strand-board, pulp, cellulose, other secondary products; • laminating additives and chemical treatments pose problems for recycling.

19. Engineered wood products: primary spanning members Laminated beam (“Glu-Lam”) * composite of dimensional lumber stock (such as 2x6, 2x8) to create large beams; * substitutes for, surpasses capacity of large timbers (difficult to obtain today from single tree).

20. Engineered wood products: primary spanning members LVL (laminated veneer lumber) * composite of thin layers of wood, similar in cross-section to a very thick plywood; * replaces conventional beams typically in the range of 2x10 to 6x16.

21. Engineered wood products: secondary spanning members Wood I-Joist * solid or laminated flanges top and bottom, connected by OSB web; * replaces traditional 2x joists, favored for light weight, low deflection, workability.

22. Engineered wood products: diaphragm panels Plywood * panel made by laminating thin veneers of wood (core typically rotary cut); * layers alternate direction of grain to resist shear in two directions. Fiberboards (MDF: medium density fiberboard) * panel core material made from sawdust-sized wood fibers and binding resins; * considerable density and weight; * more vulnerable to water damage than plywoods

23. Engineered wood products: diaphragm panels OSB (oriented strand board) * panel made from recycled wood scraps, shards, and laminating resins; * fibers oriented to create uniform tensile strength in all directions; * economical replacement for non-finish grade plywood

24. Engineered wood products: diaphragm panels Agrifiber board * experimental, eco-friendly products that do not depend on formaldehyde for fabrication; * core materials include straw, grains, and grasses

25. Engineered wood products: composite members Box beam * prefabricated sandwich floor or roof section (plywood + joist + plywood); * outer layers of plywood (OSB, planks) make interior joists more efficient, thinner; * cavity space between joists typically fitted with insulation, reducing on-site labor

26. Engineered wood products: composite members SIP (structural insulated panel) * similar to box beam construction, but without interior joists (OSB + insulation + OSB); * shear strength of foam insulation fulfills structural role of “web” between sandwich layers; * excellent insulation values (no cold-bridging interior members); * may also be used as bearing wall construction -- for limited loads.