Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 1
Wood Types: Balloon framing (rare) Platform framing Heavy timber framing Advantages: The only renewable material Warm, natural appearance Low energy required Easy to work, low cost Light weight = low seismic forces Challenges: Combustible Termite attacks Decays in variable humidity Limited height and floor area Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 2
Rupture Length Rupture length is the maximum length a bar of constant cross section can be suspended without rupture under it s weight in tension (compression for concrete & masonry) Rapture length defines the efficiency of material as strength / weight ratio: R = F / γ where R = rupture length F = breaking strength γ = specific gravity (self weight) The graph data is partly based on a study of the Institute of Light weight Structures, University Stuttgart, Germany. Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 3
Energy use For simple beam of Aluminum [Aluminium] Concrete [Béton] Steel [Acier] Wood [Bois] demonstrates wood requires much less energy! Wood Study by EPFL (Ecole Polytechnique Federale de Lausanne) Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 4
Wood properties 1 Effect of fiber direction on shrinkage and deformation 2 Flat grain: strong patterns, large deformation 3 Vertical grain: fine patterns, small deformation 4 Plainsawing: flat grain, minimal waist 5 Quartersawing: vertical grain, some waist 6 Possible pattern: vertical grain,minimal waist 7 Possible pattern: vertical grain, minimal waist A Length shrinkage 0.2% 1 B Radial shrinkage 2.5% 1 C Tangential shrinkage 5% 1 D Center cut causes concave deformation E Vertical grain cut causes minimal distortion F Diagonal grain cut causes rhomboidal distortion. G Flat grain cut causes maximum distortion. 1 Typical softwood shrinkage @ 20% moisture reduction Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 5
Heavy timber framing Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 6
Timber residence Architect: Thomas Herzog rods resist lateral load Gamble house Pasadena Architects: Green and Green Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 7
Timber residence Architect: Marquies and Stoller Engineer: Eric Elsesser rods resist lateral load Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 8
Heavy Timber A Joists support floor or roof deck B Planks supported by beams C Single beam require device to connect to column D Twin beams bolted to column allow pipes, etc. to pass between E Post F Cross-bracing resists lateral load Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 9
Post, beam, joist framing Post / beam framing: 1,2 Flush joint Requires metal connector Ducts and pipes can t pass between beams 3,4 Twin beam / single post Simple bolted connection of twin beams to post Ducts and pipes can pass between beams 5,6 Single beam and twin post Less buckling strength than a single post Simple bolted joint of single beam to twin posts Joist framing: 1,2 Flush joists require joist hangers 3-6 Top joists Simple framing Allows ducts between joists Expose beams below ceiling Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 10
Post cap and base Post cap and base are essential for seismic safety To prevent beam from slipping off the post 1 Post cap 2 Post cap alternate 3 T-post-beam connector 4 L-post-beam connector 5 Post base with u-strap 6 Post base alternate Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 11
Beam anchor Beam anchors are essential for seismic safety to prevent beam from slipping off the wall 1 Steel bracket bolted to beam, anchor bolt into wall 2 Anchor bolt directly through beam into wall 3 Axon of 1 above 4 Axon of 2 above 5 Bracket with 2 anchor bolts 6 Concealed T-bracket Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 12
Beam support Beam hangers are required fro flush framing 1 Beam hanger with single flange 2 Beam hanger with twin flange 3 Heavy-duty twin beam hanger 4 Heavy-duty single beam hanger 5 Concealed beam hanger 6 Angle brackets secure beam to wall plate Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 13
Balloon framing Balloon framing, invented by George W Snow, was first used for St. Mary's Church, built 1833 in Chicago. Balloon framing was a pejorative term, used by critics to make fun of its light weight. Platform framing, which evolved from balloon framing, replaced it largely today. Yet less vertical shrinkage than platform framing makes balloon framing preferred for veneer walls. Studs extend two stories with blocking as fire stops. Floor joists, usually 2x12 at 16 in, rest on 1x4 ribbons. Plywood or other wall sheathing, resists lateral load. Plywood also provides floor and roof decks. Blocking of joists resist rotation and supports plywood edges to transfer shear. Joist and stud spacing may also be 12 or 24 in. Plywood and gypsum board panels 48 (4 ) wide match 2, 3, or 4 joist or stud spaces of 24, 16 or 12, respectively. A Joist at 16", alternately 12 or 24" B Blocking under plywood panel edges C Double plates overlap at corners and splices D Stud, 2x6, spaced 16", alternately 24" E Fire blocking at floor level F Ribbon, 1x4, support floor joists G Anchor bolt, 1/2 at 4 max. H Sole plate, min. 6 above soil I Concrete foundation Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 14
Platform framing Platform framing, which derived from and largely replaced balloon framing tody, is widely used for low-rise residential structures, due to economy & flexibility. 2x4 studs @ 16 reach from platform to platform. Double top plates overlap at corners and splices. Plywood sheathing, nailed to studs, resists lateral loads. Joists, usually 2x12 @ 16 support plywood platforms. Blocking resists joist buckling and supports plywood panel edges to transfer shear. Plywood and gypsum board panels 48 (4 ) wide match 2, 3, or 4 joist or stud spaces of 24, 16 or 12, respectively. Maximum height allowed: 3 stories (4 with fire sprinklers) A Joists, 2x12 or 2x10 @ 16", 24, or 12 o.c.. B Blocking C Double top plates overlap at corners and splices D Studs, 2x4, 2x6, or 3x4 @ 16 or 24 o. c. E Bottom plates F Double plates supporting joists G Anchor bolt, 1/2 @ 6 o. c. H Sole plate, min. 6" above soil I Concrete foundation Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 15
Wall intersections Wall intersections are joined by overlapping twin top plates nailed together with two face nails. Multiple studs provide nailing surfaces for sheathing. 1 Wall corner 2 Wall intersection between studs 3 Wall intersection at studs A Double top plate B Stud C Top plates at corner joined with two 16d face nails D 1x6 let-in braces before sheathing is applied E Metal braces before sheathing is applied F Corner studs provide nailing surface for sheathing G Sole plate H Anchor bolt spaced maximum 4 ft I Blocking and shim J Double studs provide nailing surface for sheathing Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 16
Wall openings Wall openings have a header supported by trimmers. Small opening headers may be twin joists. Large openings may need a beam header. Openings extending to the ceiling require joist hangers. Metal straps join header to rim joist to transfer lateral load to shear walls. 1 Conventional door and window openings 2 Window openings flush with floor or roof joists A Header, twin joist or single beam B Trimmer supports header C Four 10d nails join header to stud D Twin sill plates E Cripple studs F Header beam flush with joists G Joist hanger H Metal straps connect header to rim joist to transfer I Post lateral load to shear walls Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 17
Joist bracing Bridging resists joist buckling only Blocking resists joist buckling and transfers shear at plywood panel edges. Blocking doubles allowable plywood diaphragm shear. Tongue-and-groove plywood with nails through tongue transfers shear without blocking. Floor openings usually have twin joists around edges, face-nailed with 16d nails at 6 in to transfer shear. For large openings beams are better than twin joists. 1 Bridging 2 Blocking 3 Floor opening A Bridging resists rotation of joists only B Double plate overlap C Blocking resists joist buckling transfers shear D Twin joists at floor opening E Joist hangers Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 18
Joist and truss joist Standard joists of solid soft wood are most common. Yet increasing scarcity of wood and a need for longer spans prompted development of prefab joists. Various joists, made from economical scrap, provide for longer spans and consistent quality. These joists have depths of nominally 5 up to 24 in. For long spans, truss joists are used. 1 Joist of solid wood 2 Composite particle board joist with laminated flange 3 Laminated joist 4 I-joist of plywood web and laminated flanges 5 I-joist of plywood web and double flanges 6 Box joist of double plywood web and single flanges 7 Truss joist of metal web bars bolted to double flanges 8 Truss joist with bars joined by metal gusset gang-nails 9 Truss joist of twin flange bars nailed to single web bar Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 19
Roof truss Roof trusses usually have bars joined by gusset plates or by bolt and split ring. Gusset plate trusses have all bars in the same plane. Split ring trusses need bars to overlap at joints. Closely spaced trusses usually support directly a deck of plywood or planks without purlins. Widely spaced trusses require purlins to support decks. 1 Truss with gusset plate joints 2 Gusset plate for nails on both sides of truss 3 Gang-nail gusset plate on both sides of truss 4 Truss with bolt and split ring joints 5 Split-ring joint A B Split ring resists shear stress Bolt ties split-ring assembly together Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 20
Plywood diaphragm Plywood diaphragms nailed to joists transfer lateral load to shear walls. Blocking between joists transfers shear from panel to panel. 1-2 Continuing joints normal to load require few nails. 3-4 Continuing joints parallel to load require more nails 5-6 Continuing joints both ways require most nails 7 Case 1 axon 8 Blocking, spaced 8 ft rather than 4 ft 9 Blocking at panel edges transfers shear 10 Tongue-and-grove joint, nailed at 7" transfers shear A Assumed load direction B Joists C Blocking D Plywood panels E Nail Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 21
Plywood shear wall Plywood must be nailed to wood framing to resist lateral shear of wind and seismic forces. 1. Plywood shear wall 2. Plywood shear wall with horizontal boards 3. Max. shear wall aspect ratio 1:3.5 (Los Angeles 1:2 since Northridge EQ) 4. Plywood nail spacing A Blocking transfers shear B Nail C Plywood sheathing D Hold-down resists overturning E Nail spacing at panel edges (max. 6 ) F Nail spacing at other studs (max. 12 ) Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 22
Wall straps and hold-downs Wall straps splice drag struts that transfer lateral load to shear walls. They may also secure upper floor shear walls to shear walls below to resist overturning. Hold-downs tie both ends of shear walls to footing to resist overturning 1 Metal strap 1 ties shear wall to lower floor to resist overturning 2 Metal strap in drag strut transfers lateral load to shear walls 3 Twin hold-downs 1 Tie shear wall to lower floor to resist overturning 4 Hold-down 1 Ties shear wall to the foundation to resist overturning 1 Required at both ends of shear walls Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 23
Metal anchors Metal anchors connect roofs and floors to shear walls. Floors and roofs typically are nailed to blocking or rim joists that rest on twin top plates. Plywood panels usually reach only the top plates and leave a gap in shear transfer from floor to floor. Metal anchors provide the missing connection. Full height plywood panels need no metal anchors. 1 Exterior wall with metal anchors 2 Interior wall with metal anchors 3 Exterior wall without need for metal anchors 4 Interior wall without need for metal anchors A Blocking (or rim joist) at exterior shear wall B Metal anchor with 12-8d nails resists 500# shear C Extend of plywood panel to top of plate only D Blocking of interior shear wall E Metal anchor F Possible shear wall above floor G Plywood panel nailed to top of blocking or rim joist provides load pass without need for metal anchors H Floor joist I Plywood nailed to top joist requires no metal anchor Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 24
Joist support Beams may support floor and roof joists two ways, flush support and top support. Flush support joins beam and joist with flush top Requires joist hangers Is more labor intensive Provides narrow joist support Conceals beam within floor depth Prevents ducts and pipes to pass between joists Top support joist supported on top of beam Needs no hardware to connect Is less labor intensive to install Provides wider support Exposes the beam below the ceiling; Allows ducts and pipes to pass between joists 1 Flush support with joist hanger 2 Flush support on steel beam with wood ledger 3 Flush support with double flange joist hanger 4 Wood ledger on bottom flange of steel beam 5 Top support overlapped on wood beam 6 Top support overlapped on wood ledger of steel beam Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 25
Asymmetric support Joists on only one side of beam rotate the beam. Such beams should be restraint against rotation 1 Asymmetrical joist support rotates the beam 2 Hold-down ties beam to joist to resist rotation 3 Metal strap ties beam to joist to resist rotation Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 26
Fire and sound rating 1 One hour fire rated wall 2 Two-hour fire rated wall 3 One hour fire rated floor 4 Two-hour fire rated floor 5 Sound rated wall of about 60 STC 6 Sound rated floor of about 60 IIC and 60 STC A Wall studs. B Gypsum board, 5/8" (16 mm), type X (fire rated) C Gypsum board, 2 layers 5/8" type X each side D Cellular concrete, 1.5 E Plywood sub-floor, 5/8 or 3/4 F Joists. G Plywood sub floor, 1" (25 mm) H Gypsum boards, 2 layers 5/8" type X I Resilient furring channels, spaced 24" J Double studs with 1" sound gap K Glass fiber insulation L Cellular concrete, 1.5" with carpet and pad floor M Plate to transfer shear over sound gap in plywood N Sound gap in plywood to prevent sound transmission IIC = Impact Insulation Class STC = Sound Transmission Class Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 27
Wall erection Plywood shear wall resists lateral wind and seismic loads Woodframe house Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 28
Town homes, Beverly Hills Architect: G G Schierle Engineer: W C Minn The wood platform framing over concrete garage is common for residential buildings. CMU shear walls at garage, plywood shear walls above. The concrete slab, 12 thick for 30 span, provides required 3-hour fire rating between garage and residential units Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 29
Terrace Homes, Hermosa Beach Architect: Schierle Engineer: Robert Metha Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 30
Terrace Homes Hermosa Beach The design philosophy to adapt building to the site not site to building, minimized grading and retaining walls. A 14/24 ft module allows shear walls aligned vertically. Each two-story unit has two terraces for outdoor living. The terraces provide open space that allowed 33 units at a lot zoned for only 25 units by conventional planning. Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 31
Terrace Homes Hermosa Beach Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 32
Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 33
Park City Village Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 34
Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 35
Wood structures Copyright G G Schierle, 2001-02 press Esc to end, for next, for previous slide 36