Formulating a Korea Industrial Standard for light-frame wood construction

Transcription

1 Formulating a Korea Industrial Standard for light-frame wood construction Jang, Sang Sik 1, Dong Ho Kim 2 ABSTRACT Light-frame wood construction has been widely recognized by Korean people since it was introduced to Korean market about ten years ago. Even though North American technique of light-frame wood construction has been transferred to Korea and become the principal standard for wood construction, a new Standard appropriate to Korean environment is needed. Therefore, Korean Society of Civil Engineers started committee activities last year in cooperation with Korea Wood Construction Association to write a proposal for Building Code on engineering design of wood structures. At the same time with the Building Code formulation, a Korea Industrial Standard (KS) for conventional light-frame construction will be formulated in this year. Light-frame wood construction is the most popular technique for residential wood housing at the present time in Korea. The technique is almost the same as the current Standard for light-frame wood construction applied in North America but some specific points are somewhat different from North American Standard. Therefore, this study was initiated to formulate a KS for light-frame wood construction applicable to wood construction industry in Korea. Korea Industrial Standard (KS) for light-frame wood construction is composed of 10 chapters and 2 appendices. Chapters include Introduction, Materials, Structures and Methods, Connection, Structural Calculation, Fire Safety, Thermal Conservation, Sound Insulation, Preservative Treatment and Building Equipment. At the end of the Standard, span tables for floor and ceiling joists and rafters and fire-resistance testing method are attached as appendices. INTRODUCTION Light-frame wood construction was introduced to Korea about ten years ago. During the time, North American technique of light-frame wood construction has been transferred to Korea and become the principal standard for wood construction. So far, there was no building code or standard for light-frame wood construction in Korea. Therefore, most builders have adopted Uniform Building Code (UBC) of North America as a basic rule for their construction projects in Korea. Recently, researchers and builders in Korea felt the need for a Korean Standard for the design of wood structures including light-frame wood buildings. Therefore, Korean Society of Civil Engineers started committee activities last year in cooperation with Korea Wood Construction Association to write a proposal for Building Code on engineering design of wood structures. The first draft for this Building Code has been submitted to the Ministry of Construction and Transportation of Korean government at the end of This draft will be reviewed by the Construction Council which is composed of professional engineers and architects, and professors. After reviewing by the Construction Council, public hearing will be scheduled for the proposed Building Code. This process will take about six to twelve months before the Building Code will be effective. At the same time with the Building Code formulation, a Korea Industrial Standard (KS) for conventional light-frame construction will be formulated in this year. Light-frame wood construction is the most popular technique for residential wood housing at the present time in Korea. The technique is almost the same as the current Standard for light-frame wood construction applied in North America. However, some specific points are somewhat different from North American Standard. Therefore, this study was initiated from the need for our own Standard on light-frame wood construction in Korea. CONTENT Korea Industrial Standard (KS) for light-frame wood construction is composed of 10 chapters and 2 appendices. Chapters 1 Professor, Dept. of Forest Products, Chungnam National University, Taejon , Korea 2 Administrative Official, Industry & Energy Agency for Technology and Standards, Kwacheon , Korea

2 include Introduction, Materials, Structures and Methods, Connection, Structural Calculation, Fire Safety, Thermal Conservation, Sound Insulation, Preservative Treatment and Building Equipment. At the end of the Standard, span tables for floor and ceiling joists and rafters and fire-resistance testing method are attached as appendices. Introduction 1. [Objective] This is aimed to lay down a technical standard for light-frame wood construction. 2. [Scope] This code is applied to light-frame wood construction, equal to or less than 3 stories, and having a floor area less than or equal to 3,000m 2. This includes, but not limited to multi-family housing, dormitory, facilities for elder and children, small-scale commercial building and public building, and motels and condominiums. 3. [Definition of terms] The following definitions are included: Light-frame wood construction, Dimension lumber, Beam and stringer, Post and timber, Structural panel, Span, Diaphragm, Shearwall, Covering(Sheathing), Bearing wall, Header, Crawl space, Bracing, and Story. Materials 4. [Structural lumber] 1) Quality of structural lumber used for light-frame wood construction shall conform to No. 2 grade of dimension lumber defined in KS F ) Allowable properties for structural lumber are given in Table 1 for normal load duration, and the values in Table 1 can be multiplied by 1.5 for load duration equal to or shorter than 10 minutes. 5. [Structural panel] Quality of sheathing materials which are installed as a covering of the frames for floor, wall, and roof shall conform to the standard given in Table 2. This requirement is limited to the items related to structural safety. 6. [Fastener] Quality of nails and screws which are used for structural purposes at floor, wall and roof assembly shall conform to KS D 3553, KS F 3514, KS D 7052 and KS B 1056, and galvanized products shall be used at those parts exposed to weather or moisture. 7. [Other materials] Other materials which are not defined in this Chapter but used for the structural purposes shall conform to appropriate KS or shall be approved by the Ministry of Construction and Transportation. Structures and Methods 8. [Foundation and sill plate] 1) Foundation wall shall be constructed by concrete, reinforced concrete, or cement block structures having thickness greater than 1.5 times the thickness of the first floor wall or 150mm, and extended to the depth deeper than the frost line. 2) Footing shall be concrete or reinforced concrete structure. Footing shall be installed for all buildings, and thickness and width of footing shall be at least the same as the thickness of foundation wall and twice of it, respectively. 3) Sill plate shall be tied to foundation by using anchor bolts or other fasteners having similar structural properties to anchor bolt. Anchor bolts shall have the minimum diameter of 12mm and be at least 230mm long, and the head of bolt shall be embedded in the foundation wall to the depth equal to or greater than 180mm. 4) Anchor bolts shall be located within 150mm from opening or end of each sill plate and at least two bolts shall be installed for each sill plate. Spacing between anchor bolts shall be equal to or less than 1.8m. 5) Sill plate shall be pressure-treated lumber with the following minimum retention of wood preservatives defined in KS M 1701: 3.5 kg/m 3 for CCA, 6.0 kg/m 3 for CCFZ, and 2.6 kg/m 3 for ACQ. 9. [Floor] 1) Dimension lumber having cross sections greater than 38mm 180mm shall be used for floor joist. 2) Floor joist shall be installed conforming to the span length as given in span tables in Appendix A. When lumbers greater than 38mm 235mm are used for floor joist (except when double joists are installed or span length is less than 4.5m), blocking shall be installed with space equal to or less than 2.4m. 3) Notching shall not be present at bottom side of the central one-third portion of horizontal structural members such as floor joists and girder. 4) Space between floor joists shall be less than mm. 5) Openings in floor shall be reinforced by members having the same size as floor joist. 6) When there is no bearing wall below the bearing wall of the 2 nd or 3 rd floor, floor joist below the bearing wall shall be reinforced. 7) Thickness floor sheathing shall be equal to or greater than 18mm. 8) Each members composing floor system shall be connected according to nailing schedule given in Table 3. 9) Adhesives conforming to KS M 3702 or KS M 3735 shall be used between floor sheathing and joists. 10) Maximum deflection of floor structure shall not exceed the values given in Table 4.

3 10. [Bearing and shear wall] 1) Bearing and shear wall shall be installed to resist effectively against vertical and lateral loads applied to the building except when columns are installed to support vertical loads on the basis of structural calculation. 2) Lumber having size equal to or greater than 38mm 140mm shall be used for the bearing and shear wall for the 1 st floor of 3 story buildings. 3) Thickness of sheathing for shear wall shall be equal to or greater than 12mm. 4) Distance between bearing or shear walls shall be equal to or less than 12m and the floor area bordered by bearing or shear walls shall be equal to or less than 40m 2 (60m 2 when floor joists are connected firmly to double plate and appropriate blockings are used). 5) At the corner of exterior wall, at least one shear wall of length greater than 900mm shall be installed. However, opening of length less than 2m can be installed at both sides of the corner when the corner is reinforced by a method providing strength equivalent to the above requirement. 6) Spaces between studs in bearing and shear walls shall follow the values given in Table 5. 7) At the corner of bearing and shear walls, at least three studs shall be used, and connected firmly with each other. 8) Double plate shall be installed on top of the bearing or shear walls to connect each other. 9) Width of an opening in bearing or shear walls shall be less than 4m and the sum of widths of the openings shall be limited to the values given in Table 6. 10) Headers in bearing walls, supported by trimmers of the same size as studs used in the wall, shall be installed on top of openings having width greater than 900mm according to Span Tables in Appendix A. 11) All the components of wall shall be connected to each other according to nailing schedule given in Table 3. 12) Maximum deflection of wall structures shall not exceed the values given in Table [Roof and ceiling] 1) Framing members used for roof and ceiling shall be installed according to Span Tables in Appendix A. 2) Space between ceiling joists and between rafters shall be less than mm. 3) Collar tie shall be installed between rafters except when ceiling joists are installed. 4) Trusses shall be designed and installed so that they can support all the applied external loads. 5) Rafters and trusses shall be connected firmly to double plate by using connectors. 6) Thickness of roof sheathing shall be equal to or greater than 12mm. 7) All the components of roof and ceiling shall be connected to each other according to the nailing schedule given in Table 3. 8) Width of opening at roof shall be less than 2m, and sum of widths of all the openings at one roof shall be less than 1/2 of the length of the roof. 9) Headers supported by trimmers of the same size as studs used in the roof openings shall be installed on top of openings having width greater than 900mm according to Span Tables in Appendix A. 10) Maximum deflection of roof and ceiling structures shall not exceed the values given in Table [Stairs] 1) Stairs shall be structurally safe, and proper headroom shall be secured above the stairs. 2) For interior stairs, thickness of tread and riser shall be equal to or greater than 38mm and 20mm, respectively, and the size of stringer shall be equal to or greater than 38mm 235mm. 3) Exterior stairs for 3 story building shall be constructed by noncombustable materials and, for buildings equal to or less than 2 story, 38mm thick lumber can be used as members of exterior stairs except riser for which 20mm thick lumber can be used. 4) Dimensions of each part of a stair shall be as given in Table 7. 5) Evacuation stairs for multi-family housing shall be installed when the number of units is greater than 6, and dimensions of excavation stair shall be as given in Table 8. 6) Stairs in multi-family housing shall not be spiral shape. Connections 13. [Connection] 1) Connections shall be designed so that connectors do not deform excessively and shear, tensile and bending stresses exceeding the strength of connectors are not developed. 2) Nailing schedule in Table 3 is minimum requirements and, in case of necessity, nailing shall be reinforced by structural calculation.

4 3) In connections using fasteners other than nail, structural calculation shall be performed by using the allowable stresses given by the manufacturers of the fasteners and higher stresses than these shall not be developed in service. Structural Calculation 14. [Structural calculation] 1) This code is based on allowable stress design methodology and wood structures not following the requirements of this code shall be proved to be safe by structural calculations according to Ordinance 53, Structural Design Standard for Buildings or Structural Calculation Standard for Wood Structures of the Ministry of Construction and Transportation. 2) Actual stresses applied to structural members of a building shall not exceed the allowable stresses given in Table 1. 3) In service, each structural member of a building shall not show excessive deflection and maximum deflection of important structural components of a building shall not exceed the values given in Table 4. Fire Safety 15. [Fire-resisting structure] 1) In a multi-family building, wall, floor, beam and roof (when fire-resistive material is used for finishing of ceiling right under the roof structure, the ceiling is considered as a part of roof structure) shall be constructed by fire-resisting structures approved by the Ministry of Construction and Transportation(Bulletin ) or by the structures having fire-resistance greater than the requirements given in Table 9 when tested by the method given in Appendix B of this Standard. Floor shall be constructed by one of the following methods and roof shall be covered by noncombustable material. 1 Gypsum board of thickness equal to or greater than 12mm on top of plywood, solid wood, OSB or PB of thickness equal to or thicker than 18mm (will be called as floor covering). 2 Light weight foam concrete of thickness equal to or greater than 12mm on top of floor covering 3 Mortar (including concrete, light-weight concrete and cinder concrete) of thickness equal to or greater than 12mm on top of floor covering 4 Plaster of thickness equal to or greater than 12mm on top of floor covering 5 Wood decking of thickness equal to or greater than 38mm (including wood floor covering) 2) Stairs (except interior stairs of one unit) shall be one of the following structures. 1 Structures defined in Number 7 of Article 3, clause 3, the Enforcement Ordinance of Korean Building Code. 2 Important parts of stairs shall be wood thicker than 65mm. 3 The bottom side of important parts of stairs shall be covered by the following fire-resistant materials when wood having thickness between 35mm and 65mm is used to build those parts: a) Reinforced gypsum board of thickness equal to or greater than 15mm. b) Double gypsum boards of thickness equal to or greater than 9.5mm. c) Gypsum board of thickness equal to or greater than 15mm plus rockwool (thickness equal to or greater than 50mm and density equal to or greater than 40kg/m 2 ) 3) Structural parts except those above-mentioned shall be protected by the following. 1 Joints of fire-resistant covers shall be protected to prevent fire penetration. 2 In walls other than fire-resistant structures, fire stop shall be installed with space equal to or less than 3m. 3 In joints and connections between wall and floor, wall and roof, and stair and floor which are not fire-resistant structure, fire stops shall be installed to prevent fire penetration. 4 When lights, vents, electrical outlets, switch boxes, etc. are to be installed on fire-resistant cover, those parts shall be protected effectively to prevent fire penetration. 16. [Fire-protection of opening in exterior walls] 1) In the openings of exterior walls where danger of fire burning exists, fire- resistant door of grade 1(fire-resistance of 1 hour) or 2(fire-resistance of 20 minutes) shall be used according to Article 57, Clause 4, Enforcement Ordinance of Korean Building Code. 17. [Fire separation] 1) For buildings having total floor area exceeding 1000m 2 (when fire extinguishing equipments such as sprinkler are installed, this limit can be increased to 2000m 2 ), fire-resistant wall and fire-resistant door of grade 1 shall be installed to separate the building into areas equal to less than 1000m 2. In this case, fire-resistance of the separating wall shall be equal to or greater than 2 hours. 2) When living room is located at basement or 3 rd floor, residential rooms (limited to buildings of 2 story or higher),

5 spaces having no vent, stair area, passageway of elevator, duct space, or other similar areas shall be separated from the other area (excluding corridor, balcony, or other similar area exposed directly to outdoor) by wall or floor having fireresistance equal to or greater than 1 hour, or by fire-resistant door of grade 1 or 2. In this case, interior of the wall (excluding wall of height less than 1.2m from floor) or ceiling (roof when there is no ceiling) shall be finished by noncombustable or quasi-noncombustable materials. 3) When part of a building is the one defined in Article 40, Clause 1 of the Korean Building Code, that part shall be separated from the other part of the building by wall and floor of above-mentioned fire-resistant structures or by fireresistant door of grade 1. 4) Border walls between units in multi-family housings, classrooms of schools, wards of hospitals, rooms of sleeping accommodations, and partition walls between rooms of dormitories shall be constructed by fire-resistant structures and extended up to the bottom of roof or ceiling. 5) When corridor having wooden roof structure is installed to connect to other building (limited to fire-resistive building of floor area equal to or greater than 200m 2 ) in a building of total floor area greater than 200m 2, and the length of the roof exceeds 4m, it shall be separated by walls of fire-resistant structures. 6) Fire-proof doors which are used for fire separation shall be kept as being closed, or shall be closed automatically when smoke is spread or temperature is increased. 7) When wiring or plumbing are installed through fire separation structures, noncombustable materials shall be used within 1m of both sides of the penetration points and the space between pipe and surrounding structure shall be filled with noncombustable materials except plumbing inserted in fire- resistant pipe shaft. 8) When air passageway for vent, heating or air-conditioning passes through fire separation, fire-resisting damper conforming to Article 28, Clause 2, Number 3 of Enforcement Regulation of Korean Building Code shall be installed in that area. 18. [Evacuation] 1) Regulations for installations of corridors, stairs, exits, and ways for evacuation and fire extinguishing in a plot shall follow Articles 33, 34, 39 and 42 of the Enforcement Ordinance of Korean Building Code. 2) In addition to the above, a passageway or open space of width equal to or greater than 3m shall be installed around the building except the followings. 1 Each unit is separated by floors and walls of fire-resistant structures, and fire-resistant doors of grade 1 or 2. 2 Main corridor connected to ground level shall be opened to outside of the building (including the one having openings to outside) 3 Balcony and other structures that can be used for evacuation shall be installed in each unit. 4 When fire penetration from the upper floor of the opening is possible, blind or balcony made of the following materials shall be installed on top of openings to prevent fire penetration. a) Noncombustable materials b) Other materials that covered by fire-proof material at bottom Thermal Conservation 19. [Thermal conservation] 1) When constructing a building, the following thermal conservation guideline shall be applied. 1 Ceiling or roof of living room of the top floor, floor of living room of the bottom floor (including floors of direct contact with exterior), and exterior walls and windows of multi-family housing shall be constructed based on thermal conductivity values given in Table10. 2 When hot water heating system is installed separately for each unit in multi-family housing, thermal conductivity of floor of the living room shall be less than 1.0. Sound Insulation 20. [Sound insulation] 1) Border walls and partition walls of the following shall be sound-resistant as well as fire-resistant, and shall be extended up to the bottom of the roof or floor of the upper floor. 1 Border walls (excluding the part bordering balcony) and floors between units in multi-family housing 2 Partition walls and floors between classrooms in schools, wards in hospitals, guest rooms in sleeping accommodations, and bedrooms in dormitories. 3 Walls of residential area bordering elevator passageway 2) All the sound-proof structures shall conform to the values given in Table 11.

6 Preservative Treatment 21 [Preservative treatment of wood] 1) Water-proof sheet or other similar materials shall be used between sill plate and foundation wall, and sheathing and exterior finishing. 2) Important structural members contacting directly with ground or concrete foundation shall be pressure treated with preservatives and other necessary measures shall be applied to protect damage from termite or other insects. 3) Connectors that are easy to corrode or to be in moist condition for long time shall be protected by necessary anticorrosive agent or made of galvanized or stainless steel. Building Equipment 22. [Electrical wiring] Wires installed in buildings shall follow the regulations given in Chapters 202, 203, 204, 206 and 213 of the Regulatory Article for Electrical Equipment. 23. [Water supply and heating] 1) Plumbing installed for water supply and heating in buildings shall be made of copper and shall be checked about leakage by pressure test before installation. REFERENCES American Forest & Paper Association National Design Specification. International Conference of Building Officials Uniform Building Code. International Code Council International Building Code, Final Draft. International Code Council International Residential Code, Final Draft. Japanese Ministry of Construction Standard for Wooden 3 Story Multi-Family Housings. Japanese Ministry of Construction Standard for Light Frame Wood Construction. National Research Council Canada National Building Code of Canada. Standards New Zealand Timber Structures Standard. NZS Standards New Zealand Code of Practice for Light Timber Frame Buildings not required specific design. NZS 3604.

7 Table 1. Allowable properties for structural lumber (Unit: N/ mm2 ) Species group Species Grade F b F t F c F c F v E Larch Pine White pine Cedar Sheathing for Exterior wall Larch, Douglas-fir Red pine, White cedar Hemlock, Fir White pine, Spruce, Radiata pine Cedar No 1 No 2 No 3 No 1 No 2 No 3 No 1 No 2 No 3 No 1 No 2 No Table 2. Kind and quality of structural panel Application Kind Standard Wall sheathing except above-mentioned application Floor or roof Sheathing Structural plywood OSB PB Hardboard Wood-cement composite Above-mentioned materials Gypsum board Structural plywood OSB PB ,250 10,780 9,310 10,290 8,820 8,330 8,330 7,350 6,860 8,330 6,860 5,880 No 1 of KS F 3113 OSB of KS F 3104 Type 200, 150, , and of the KS F 3113 S35 or T45 of KS F C of KS F 4030 Above-mentioned standards (including No 2 of KS F 3113) Gypsum board (KS F 3504) No 1 an No 2 of KS F 3113 OSB of the KS F 3104 Types of 200, 150, , , P and M of KS F 3104 Note) *L = Span length Table 4. Maximum allowable deflection for structures Structure Live loads Total loads Roof L * /360 L/240 Floor L/240 L/180 Wall L/100 - Table 5. Maximum spaces between studs Materials (mm) Space between studs in bearing wall (mm) 1) 1 story building 1) 1 st floor of 2 story building 2) 2 nd floor of 2 story building 2) 2 nd floor of 3 story building 3) 3 rd floor of 3 story building 1) 1 st floor of 3 story building

8 Table 4. Nailing schedule No. Connection Nailing * Nailing method Size and Number of nails 1) Joist to sill or girder Toe nailing 65mm nail, 3 2) Bridging to joist, Toe nailing 65mm nail, 2 3) Bottom plate to joist or blocking Face nailing 90mm 400mm o.c. 4) Top plate to stud End nailing 90mm nail, 2 5) Stud to bottom plate Toe nailing or End nailing 65mm nail, 4 or 90mm nail, 2 6) Double studs Face nailing 90mm 600mm o.c. 7) Double plate Face nailing 90mm 400mm o.c. 8) Top plate, laps and intersections Face nailing 90mm nail, 2 9) Continuous header, two pieces 90mm 400mm o.c. along each edge 10) Ceiling joists to plate Toe nailing 65mm nail, 3 11) Continuous header to studs Toe nailing 65mm nail, 4 12) Ceiling joists, laps over partitions Face nailing 90mm nail, 3 13) Ceiling joists to parallel rafters Face nailing 90mm nail, 3 14) Rafter to plate Toe nailing 65mm nail, 3 15) Built-up corner studs 90mm 600mm o.c. 100mm 800mm o.c. at top and 16) Built-up girder and beams bottom and staggered 2-100mm nails at ends and at each splices 17) Decking of thickness 38mm 2-90mm nail at each support (galvanized nail) Sheathing to framing(floor, roof and wall): 18) Thickness < 13mm 50mm galvanized nail Thickness 13-25mm 65mm galvanized nail Thickness >25mm 75mm galvanized nail Structural panel siding to framing: 19) Thickness < 13mm 50mm galvanized nail Thickness 13mm 65mm galvanized nail * When kind of nails are not defined, general wire nails can be used. Table 6. Maximum allowable rate of opening in bearing wall Story 1 story building 2 story building 3 story building 1 st story 75% 60% 40% 2 nd story - 75% 60% 3 rd story % Table 7. Dimensions of stairs (Unit: %) Stair Kind Width of stair Max. height of riser Min. width of tread Multi-family housing 120cm 23cm 15cm Other housing 75cm

9 Table 8. Stair dimensions for multi-family housing Kind Width Max. height of riser Min. width of tread Interior stair 120cm 20cm 24cm Exterior stair 90cm Table 9. Required fire-resistance Building components Bearing wall Exterior Combustable part Wall Wall Other part except above Partition wall Column Floor Beam Roof Note 1) Fire-resistant structure is not required for floor of the lowest story. Fire-resistance 1/2 hr 1/2 hr Table 10. Required thermal conductivity for building components Building components Mid South Cheju island Exterior wall, Floor of the bottom story Ceiling or roof of the top story Exterior wall of multi-family housing Exterior window Note 1) Mid: Seoul, Kyoungki-do, Incheon, Chungchyoungbuk-do, Kangwon-do 2) South: Chungchyoungnam-do, Taejeon, Cheonlabuk-do, Kwangju, Cheonlanam-do, Kyoungsangbuk-do, Kyoungsangnam-do, Taegoo, Pusan Central frequency(hz) ,000 Table 11. Required sound insulation for wall STC(dB) Note 1) Testing method is given in KS F ) STC grading method is given in ISO 717-Part 1 and frequency ranging from 125Hz to 4,000Hz is applied. Appendix A - Span Tables for floor joist, ceiling joist, rafters and headers. Appendix B - Fire-resistance testing method.