the adaptability of two-by-four wood framing … · the adaptability of two-by-four wood framing...

Adaptables2006, TU/e, International Conference On Adaptable Building Structures Eindhoven [The Netherlands] 03-05 July 2006

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The Adaptability of Two-by-Four Wood Framing Construction

Li-Chu Lin National Kaohsiung First University of Scinece and Technology No.1, University Road, Yuanchau, Kaohsiung, Taiwan, R.O.C. [email protected]

KEYWORDS Open building, Sustainable construction, Wood construction, Light wood framing ABSTRACT Building for living adaptation requires spatial flexibility and constructional openness. Spatial flexibility deals with dimensional coordination by ‘grid’ and ‘zone’ [Wang 1997], constructional openness deals with level separation and interface decomposibility and recomposibility [Lin 2002]. Spatial flexibility is the job of designers, its method had been fully developed by J. N. Habraken, while constructional openness involves designers, manufacturers, contractors and even users, there are a lot of technical issues remained. For the two-by-four wood framing construction, the above-mentioned two criteria of constructional openness can not be reached at the same time because of its unique structural system. The deveopment of the two-by-four wood framing construction represents a history of immigration and industrialization. It has a great deal of advantages in design and construction, such as low-tech, light-weight, handy, fast erection and human-feeling, and wood is revalued as green material today, but its inherent limitations for living adaptation in a sustainable way is not negligible; The bearing wall structure discourages wide openings, studs align in the framing confuses the relocation of bearing walls, and the rapidly innovated M/E building equipment entangle with the structure. In a word, although its constructional interface is relatively open, the intergration of building level with infill level unavoidably causes all the problems for adaptation. In this study such problems are discussed, but it is very difficult to find out solutions for improvement. A new module to deal with those problems was tested but failed, minor suggestions are made : (1) Adopting 2x6 as one stud system may simplify the dimensional coordination on structural level and to accommondate to the grid of infill level at the same time. (2) Consolidating mechanical and electrical systems into fewer locations with shafts and troughes so that notching and boring of the structural framing could be minimized, and the rearrangement of facility lines for adaptation could be easier. 1. Background Wood construction has a long history in human settlement and is recognized as a kind of “green building” today. Due to the progress of wood protection measures, and the rediscovery of wood property of structural protection from fire, wood material for construction is considered as not only healthy, comfortable but also energy conserving, resource reusable and recyclable, and earthquake tolerable.

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The Adaptability of Two-by-Four Wood Framing Construction, Li-Chu Lin

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But a “green bulding” may not be a “sustainable building” in terms of the model published on CIB Agenda 21 in 1998 to keep balance in three dimensions: environment, economy and the social-cultural. Green building is in favor of the environment but has little to do with the social-cultural. In which open building can play a significant role especially for the social-cultrual inheritance and transformation through living adaptation in bulding life cycle. In Taiwan, green building has become a popular term in public and private sectors. Recently the two-by-four platform wood framing construction system was promoted by Taiwan government in the name of “green building”. Although this constructional system is prevalent in North America today and Japan opened the door to it after Kobe earthquake in 1995, it is still alien to local professions. In terms of living adaptation, there are several technical and cultural barriers worth furthur study. 2. Two-by-four wood framing construction The construction is structured by a two-by-four system framing, which is also called light wood framing or stick-framing. Since Professor George Snow developed the balloon framing in 1833, the structural system has transformed to platform framing and become the most prevalent construction method in North America today. Therefore, this study focuses on the platform framing construction. Instead of full-height wall framing members for two-story construction, platform framing features the construction of each floor on top of the one beneath. 2.1 Structural system The two-by-four system framimg is composed by rafters, joists and closely spaced studs, which are generally 2 in. x 4 in. or 2 in. x 6 in. nom. in section, spaced a maximum 16 in.(40 cm) or 24 in.(60 cm) on center, combined with sheathing to form a structure to resist lateral loads or racking. It is acturally a bearing wall structural system. Two factors are contributing to the strength and stiffness: load sharing and composite action [CWC website 2006]. Load sharing means that alternate paths of load transfer become available when the primary path fails. Thus, the structure is not prone to sudden collapse. Composite action is the reinforcement that sheathing and fasteners make to the lumber members. See ‘Fig. 1’ below.

Figure 1. Typical platform framing (source: http://www.hometips.com)

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Bracing is one of the most critical structural elements to resist horizontal forces such as wind and earthquake (the seismic ground motion). The stick type lateral bracing (i.e. nominal 1’x4’ let-in bracing) is commonly used but gradually replaced by the “continuous wood structural panel sheathing” because of its superior structural performance that allows narrower minimum width requirement by the US building code. “Braced wall panel”, termed by the US building code (IBC and IRC) in 2003, consists of the wall panel (e.g. plywood or OSB1 Rated Sheathing), the framing and the fasteners. And multiple braced wall panels align to form a “braced wall line” or “shear wall line”(see Fig. 2). These shear wall lines should form a right-angle intersection plan in a building, and a braced wall panel or shear wall segment shall be placed at each exterior building corner, every two panels or segments shall not exceed 25 ft (7.6 m) from their center to center. See ‘Fig.2’ below. Besides, the building dimension, i.e. length and width, shall not exceed 80 ft (24 m). Single spans of floor framing members shall not greater than 26 ft (7.9 m). Headers shall be provided over all wall openings, and they shall be supported by wall studs, hangers or anchors.

Figure 2. Braced wall panel and braced wall line (Source: http://www.apawood.org) 2.2 Jointing method Platform framing is erected with sticks, which are jointed mainly by nails and metal connectors. There is almost no mortise-and-tenon joint technique involed. See ‘Fig.3 and 4’ and [Table 1]. As to nails, placing the right length of nails and in right directions is critical.

1 OSB: oriented strand board

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Figure 3. Typical lateral framing connections (Source: AF&PA 2001 ) Adaquate connections between roof, ceiling, wall and floor assemblies shall be provided to transfer lateral forces acting perpendicular to the wall surface.

Figure 4. Jointing methods (Source: http://www.strongtie.com/products)

Interface Function Jointing Product Spacing Wood blocks, wedges, nail stoppers Sealing Silicon, strips, trim Linking (connectors) Caps, bases, hangers, holdowns

Straps, ties, anchors: tiedowns Fixing Nails, anchor bolts,Srews,

Toothed plates, anchoring adhesives Strengthening Enhancers, bracers, post bases, joist-hangers

Table 1. Jointing materials used in wood framing construction. 3. Adaptability analysis With a view to examining how open the two-by-four building system can afford for change, the common or typical ways of change in daily life have to be clarified and focused. Five situations of making change or adaptation were observed [Lin & Wang 2000]:

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(1) Changing floor plan To rearrange space layout for different uses, the renovation construction may involve removing the existing walls and erecting new walls. Floor, ceiling and facility lines (i.e. ducts, pipes, outlets, switches, etc.) will be modified accordingly. (2) Changing infill elements To change style or spatial atmosphere, infill elements may be reconfigured in size, shape, color or material. For instance, a wood Dutch window in living room is enlarged to be a metal French door. (3) Expanding spaces To enlarge the existing interior space by covering sundeck, cantilevering or attaching new structures, the renovation construction may involve removing the existing walls and erecting new walls and windows or doors. Floor, ceiling and facility lines (i.e. ducts, pipes, outlets, switches, etc.) will also be modified accordingly. (4) Maintaining existing functions To renew the functions of building elments or equipments, the renovation construction may involve removing the existing one and install a new one with the same size and shape. (5) Upgrading functions To improve the existing functions of building elments or equipments, the renovation construction may involve removing the existing one and install a new one with different size, shape or material. Except the fourth one maintaining existing functions, which only involves on-site work of replacement, nothing else will be affected in a noticable way although partial deconstruction may be indispensable. The other four situations of renovation construction work will directly affect the structural framing. That means, these adjustments or adaptations will be made mostly on building level for the two-by-four building system. 3.1 Criteria of Adaptability Building for living adaptation requires spatial flexibility and constructional openness. Spatial flexibility involves dimensional coordination with grid and zone, which had been fully developed by Habraken [Wang 1997], therefore, it is not an issue to be discussed here. While constructional openness involves level separation [Habraken 1998] and interface decomposibility and recomposibility [Lin 2002]. The latter requires components of generic shape, joints detachable and working process simple enough to DIY (Do It Yourself). See ‘Fig. 5’ as below.

Figure 5. Criteria of building adaptation 3.2 Constructional adaptability examination

Grid (Dimensional Coordination) Spatial Flexibility

Zone (Spatial Composition) Adaptability

Level Separation Constructional Openness Interface Decomposibility and Recomposibility

•••• components of generic shape •••• joints detachable •••• working process simple enough to DIY

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But for the two-by-four wood framing construction, these two criteria of constructional openness can not be reached at the same time because of its unique structural system. Although it is in favor of DIY which deals with lightweight, small-sized components, easily handled tools and simple working process, plus the component shape and jointing method are applicable for interface decomposition and recompodition, its building system can hardly distinguish the building level with the infill level. There are three major constraints observed and discussed as follows. (1) Bearing walls should line up with their supports, which cause difficulties to relocate wall studs. And the rules of shear wall line restrict the

change of wall.

When bearing wall perpendicular to joist, its supporting bearing wall below should not be offset a distance equal to the depth of the joist, that is 45° [AF&PA's WFCM/ANSI Canvass Committee 2001]. See ‘Fig. 6’ below. Thus, bearing walls must be stacked closely above one another, which bothers the removal or relocation of upper or lower floor wall because they are hidden or blocked by the platform so that it is not easy to find the exact positions of the studs above or below.

There are two factors related to this problem; One is the dual stud spacing system, that is 16 in. (40 cm) max. for 2x4 studs o.c. and 24 in. (60 cm) max. for 2x6 studs o.c., they can hardly line up with each other vertically. But their common divisor is 8 in. (20 cm), and the offset is always 8 in. (20 cm) which is shorter than the depth of joist (10-12 in. or 25-30 cm), so they can work together without exceeding the allowable offset when at the same wall line. Nonetheless, 2x6 framing is better for load bearing and environmental-proof materials installation, it is more welcome today. In this case, 2x6 as one stud system may simplify the dimensional coordination and structural coordination.

Figure 6. Bearing walls should line up with their supports. (Source: AF&PA 2001 )

A new and only one module of 18 in. (or 45 cm) stud spacing was tested in this study, due to the limitations on opening, that is the requirement of additional full-height jack studs, the dimansional coordination is not applicable.

The other one is the “shear wall lines”, which should be overlaped or within the allowable offset among floors. Since the platform interrupts the linkage of studs and blocks the visual connection, careful investigation and confirmation of the locations of the lines before renovation are critical.

(2) Openings such as windows, doors on the bearing walls are limited in size and location. No openeing is allowed on shear walls (i.e. braced wall panels). No matter what is the stick type lateral bracing (i.e. 1’x4’ let-in bracing) or the continuous wood structural panel sheathing on the bearing wall, a header above the opening should be inserted to carry the load of the interrupted studs above, and the header should be supported by full-height jack studs (also called king studs) at each end. See ‘Fig. 7’. Although the diagonal let-in bracing could be changed to K-shape bracing when an opening on the wall is needed, the size and location of the opening is restricted. See ‘Fig. 8’ below.

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Figure 7. An opening with header and king studs (Source:http://www.taunton.com/finehomebuilding)

Figure 8. Changing diagonal let-in bracing to K-shape bracing when making an opening (Source: http://www.tdi.state.tx.us/)

For a wide opening such as a garage door, the narrow return walls on its sides are among the weakest points in a house because they are inherently difficult to be braced properly against high lateral loads. A minimum 32 in. width for garage return wall is required by the Uniform Building Code, and its width can be reduced to 24in. even 16 in. when carefully engineered as a shear wall [Utterback 2000]. Although there are some advanced bracing products available in the market, they work more like prefabricated columns instead of on-site stick-framing walls. See ‘Fig. 9’ below.

Figure 9. Advanced braced wall panal (Source: www.hardyframe.com & www.simpsonstrongwall.com) (3) Conduits, pipes and ducts penetrating floor joists or wall studs are limited in size and location,

meanwhile these facility lines and fixtures are inserted into the cavaties of the framing thus entangled with the framing. Which add difficulties to renovations. Consolidating mechanical and electrical systems into fewer locations with shafts and troughes may help relatively.

For joist notching and boring, the maximum joist notch is one-fourth of its depth at the end and one-sixth of its depth in the outer thirds of the span. The maximum diameter of a hole is one-third the depth of the joist and minimum 2 in. from the top or bottom edge. No notching in the middle one-third of the joist span. As for the stud, maximum notching is 25% of its depth for bearing wall and 40% for nonbearing wall. Maximum boring is 40% of its depth for bearing wall and 60% for nonbearing wall . See ‘Figs. 10’ as below.

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Figure 10. Limitations for notching and boring (Source: http://www.taunton.com/finehomebuilding) With a view to minimizing the notching and boring on the framing, and to disentangling the facility lines with the framing, systematically gathering and organizing the electrical , plumbing and HVAC lines should be taken into account. Shafts and troughes may contain those lines in a consolidated way, but they are inevitably fixed by the framing, thus the capacity for change is limited. 3.3 Other challenges (1) New market trend Recent observations found that large windows on exterior walls become possible in climatic design due to the progress of glass performance, and the open-view atmosphere these windows provide seems more welcome and fashionable. Therefore, wider opening and more opening become a challenge to the two-by-four framing construction, especially on the ground floor, such as the french doors in living room or dining room and wider windows in other rooms. These demands would inherently weaken the bearing wall structure and the auxiliary strengthening methods would become more complicated. See ‘Fig. 11’ below.

Figure 11. Wide opening with narrow braced wall panels (Source: APA publication ) (2) Cultural difference The extent of familiarity with a building system may also affect the free will of adaptation. In the regions where people like Taiwanese are used to post-and-beam concrete construction or timber-frame related construction, which has a long history and the idea of post-and-beam has been deeply rooted in Chinese culture, people know very little of the two-by-four wood framing construction, even in the professional circle. Therefore, general education before officially adopting this alien construction

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method is absolutely necessary. On-site practice without comprehensive instructions may lead risks on making mistakes when doing adaptation work. (3) Inconsistent measring system In the regions where metric system is prevalent, English system may confuse the teams in practice and cause inconvenience. Therfore, adoption of internationally recognized dimensional system is critical in open market. Besides, although nails are mechanical fasteners, and the detachment of nails from sticks is not very difficult, when there are too many nails jointed togather and in different directions, it becomes a work load for decomposition and recomposition of renovation. 4. Conclusion The deveopment of the two-by-four wood framing construction represents a history of immigration and industrialization. Although it has a great deal of advantages in design and construction, such as low-tech, light-weight, handy, fast erection and human-feeling, and wood is revalued as green material today, its intrinsic limitations for living adaptation in a sustainable way is not negligible; The bearing wall structure discourages wide openings, studs align in the framing confuses the relocation of bearing walls, and the rapidly innovated M/E building equipment entangle with the structure. In a word, although its constructional interface is relatively open, the intergration of building level with infill level unavoidably causes all the problems for adaptation. The trial to find out the responsive solutions is basicly failed, only a couple of minor suggestions are concluded as the following: (1) Adopting 2x6 as one stud system may simplify the dimensional coordination on structural level and to accommondate to the grid of the lower level (i.e. infill level) at the same time. 24 in. (60 cm) spacing as a module for door, window and wall cabinets, etc. (2) Consolidating mechanical and electrical systems into fewer locations with shafts and troughes so that notching and boring of the structural framing could be minimized, and the rearrangement of facility lines for adaptation could be easier. Besides, adoption of internationally recognized dimensional system, that is metric system, is critical in open market. 5. References AF&PA's WFCM/ANSI Canvass Committee, 2001, Wood Frame Construction Manual for One- and

Two-Family Dwellings, American Forest & Paper Association. Canadian Wood Council, ‘Features of light framing’,

http://www.cwc.ca/applications/light_framing/features.php, 2006-03-10. Cohen, D., Mckay, S., Brock,L., Cole, R. & Prion, H. 1996, ‘Wood construction in Japan Past and

present’. Forest Production Journal, vol.46, pp.18-24. Foliente, G.C. 2000, ‘History of Timber Construction’, Wood Structures: A Global Forum on the

treatment, Conservation and Repair of Cultural Heritage, ASTM STP 1351, PA: ASTM, pp.5-22.

Graubner, W. 1992, Encyclopedia of Wood Joints, The Taunton Press. Habraken, N.J. 1998, The Structure of the Ordinary, The MIT Press.

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Lin, L-C & Wang, M-H. 2000, ‘Technological Change of Infill Construction for Continuous Customization’, Proceedings of Continuous Customization in Housing, Tokyo: OBT2000 Organizing Committee, pp. 229-236.

Lin, L-C. 2002, Architectural Construction Theory of Open Interface, Ph.D thesis, N. Cheng-Kung Univ., Taiwan.

Newman, M. 1995, Design and Construction of Wood-Framed Buildings, McGraw-Hill, Inc. Russell, B. 1981, Building Systems, Industrialization and Architecture, John Wiley & Sons. Sparkes, A.J. 1968, ‘The Strength of Mortise and Tenon Joints’, FIRA Tech. Report No. 33. Spence, W.P. 1998, Construction Materials, Methods and Techniques, Delmar Publishers. Utterback, D. 2000, ‘Common Engineering Problems in Frame Construction’. Fine Homebuilding, no.

128, The Taunton Press, pp.110-115. Wang, M-H(Chinese ed.). 1997, Variation: The Systematic Design of Supports (Habraken, N.J.), N. Cheng-Kung Univ., Taiwan.

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