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Design for prefabrication
Ser
-
NA7110
~ 2 1 h 8
no.
2 3c. 2
National Research Council
Canada
Division of Building Research
H O U S I N G N O T E N O . 2 3
DKSllrGM
FOR
RRIEFARRIICATIOM
ANALYZED
Reprinted from Canadian Builder,
Vol.
XIV,
No. 12, December 1964, p. 24-26
Ottawa, December 1964
The technical aspects of house de- sign can often be modified to facili- tate factory production, and this note presents some of these aspects as a guide to the builder and designer of mass produced houses. It is not con- cerned with either aesthetic or layout considerations of mass house design. Most of the technical considerations in prefabricated house design are common to all house design. Builders and designers should be familiar with the fundamentals of building for the Canadian climate, and publications are available as introductions to the
fundamentals.
The most critical detail affecting prefabrication is the through
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wall joint. All building involves joints bc- tween materials, but these usually ex- tend through only one or two layers in any one area (except at windows, doors, etc.). As the prefab evolves to the more completely prefinished pack- age that it should be, joints are more often required to extend from inside to outside between the enclosure com- ponent. N o layers are added to cover up. Many prefabricated houses have suffered from wind and rain infiltra- tion through joints of faulty design or fit, or with dried-out caulking. The following approach can allow easily assembled through-wall joints of ade- quate performancc at the most reason- able cost:(1) Provide a joint seal near the warm face of the enclosure, allow the outer portion to be relatively loose, and incorporate a widened chamber in the outer portion. This detail should confine interior water vapour to a warm area (above the dew point) and allow any vapour that may pass the seal to escape freely, thus preventing condensation in the joint. Further, the detail follows the "rain-screen" principle dcveloped and widely used in Scandinavia, to pre- vent rain infiltration. This is ex- plained in Canadian Building Digest No. 40, "Rain Penetration and Its Control" by G. K. Garden, published by the DBR/NRC. In low buildings, local deviations from this type of de- tail are not usually harmful because wind pressures are low, but the cham- ber should then always lead to suit- able drain points.
(2) For the seal itself, gaskets are preferable to caulkings. Low-cost oil- base caulkings are time consuming to
FIGURE 3
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Canadian prefabrication: A town house complex (Minto Cor struction Ltd.)Design
for prefabrication
apply and they later dry out andcrack; the durable elastomers take even longer to apply and they are very costly. If the joint is narrow at the seal and movement is limited (as is normal in small buildings) even a very simple and inexpensive gasket of multi-folded polyethylene film may be fully adequate. Tightening of the joint cannat squeeze it out; it will not deteriorate in concealed positions; and it can follow joint movement while re- sisting the low wind pressures on low buildings.
(3) The structural requirements of panel joints for low buildings are not at all severe. Wind racking produces surprisingly little shear between large panels (4 by 8 ft. or larger) in build- ings of one or two stories. Both cal- culations and tests show that nailing (or equivalent) of panels to floor and roof plates can be more than ade- quate, with no structural connections in the vertical joints.
Figure 1 shows some simple joints that illustrate the above points, and also provide for alignment of the abutting panels. The approach fits wood-frame construction just as well as it fits more sophisticated new sys- tems.
Folded Polyethylene
Film Gasket
No matter how simplified the joint, it still costs money to do it well in the factory and in the field, and it al- ways remains the critical area of po- tential trouble. Throughout the mod- ern history of prefabrication, most de- velopers have first proposed a system based on small, repeated panels with many joints, but the more successful ones have nearly always turned to much larger wall panels as their busi- ness developed. Small panels do al- low flexible design with standard panels. They can be produced to
stock since their use is not dependent on the buyer's choice of a particular house. And they are readily handled on site by one or two men, without the use of special equipment. But their disadvantages are equally clear and ~rsually critical. They require a high footage of joints, increasing the problem of providing at competitive costs protection against all elements, while giving pleasing appearance and protected, durable edges.
In contrast, the "room wall" or "whole wall" approach offers the fol- lowing advantages inherent in larger panels. Joints are minimized, occur- ing only at large windows, doors, o r corners, just as with traditional con- struction. Costs are reduced. With nearly every production process it is cheaper to make something large than to make it small; every start and stop and every edging is costly. Further, the large panels are better suited to the provision of complete interior and exterior finish and the incorporation of wiring.
Large panels do allow less flexibil- ity of design with standard parts than do small panels, but many prefabrica- tors have shown that more than ade- quate variety can be produced with- out decreasing the production effici- ency (Figure 2). The better prefab- ricators have evolved to effective use of what might be called an "opening- and
-
fillers" approach. Appropriate doors and windows and other features are chosen from a family of standard "opening" panels of wall height and located wherever desired around the house perimeter. Solid wall "filler" panels are then made up of a stand- ard construction varying only inlength, the length determined by the location of the "opening" panels. In this way the relatively difficult and high c o s t components (windows, doors, patio doors, and feature panels) are of standard dimensions and man- ufacture, with dimensions usually centred on modular grid lines. Then the lower cost solid wall sections can readily be made to varying lengtks, with dimensions backed off the mod- ular grid to fit between the "opening" panels. While simplifying design and manufacture, this approach has inci- dentally promoted a pleasing coher- ence of design in some large projects, and several design awards winners in Canada have used this system.
The role of modular co-ordination in prefabrication in Canada deserves some discussion. It is used as a plan- ning and layout tool by most prefab- ricators, using a minor module of 4 in. and a major module of 16, 24 or 48 in. Applied to the "openings and fillers" approach, it allows rapid yet flexible layout and production, and also minimizes waste in framing or sheet materials. These modest bug significant advantages are the main ones gained by the use of "modular" in a "closed system", i.e., where com- ponents are sized and detailed for use with themselves and other known components, to form a "system" build- ing. Most prefabricators produce closed systems, and so modular co- ordination is relegated to this minor role.
Proponents of "modular" contend that its main advantages can only be obtained in "open systems", where one man's components can fit with others' components to form anyone's
Gasket
for Alignment
Open and Closed b Tongue and Groove Joint i n Wood C ) Batten Joint
FIGURE 2
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Canadian prefabrication: Some single houses. (Minto Construction Ltd.)building. Ideally, the widespread use of one adopted discipline of modular co-ordination could reduce the num- ber of sizes of building components, resulting in much higher production of any one size, reducing both manu- facturing and distribution costs.
"Open systems" will not become a part of the building scene for some time since they depend on the resolu- tion of many questions: sizes, types and shapes of joints, tolerances, devia- tions, position of the element or joint with respect to the grid line, and al- lowance for the thickness of compo- nents at corners and intersections.
The strong trend to the use of large wall sections, the development of simple and sound details for through- wall joints, and the acceptance of bold exterior design following the "open- ings and fillers" approach, all encour- age the evolution toward more com- plete prefinished component building. The advantages of more complete factory content in the prefab package were discussed in the first note in this series.
Other shell components can equal the value of exterior walls in their shop content and as proportions of
the house costs, and in their adapta- tion to prefinishing. The trend to low slope roofs and "cathedral" ceilings allows the elimination of ceilings as separate structures, so that single panels can act as combined roof-ceil- ing components. These can meet the basic criterion for prefabrication: they can be compact enough and in- clude enough material, labour and finishing, to be worth making in the shop.
Partitions too can be more com- plete and can be satisfactory in much slimmer thicknesses than normal frame partitions, if sound transmis- sion through the partition and vibra- tion from the closing of doors is con- trolled. The tightness of the panel joints is at least as important as the mass of the panel itself in controlling sound transmission. To control door closing shock, it is best to let the door assembly be relatively independent of the partition by providing rigid fram- ing from floor to ceiling.
Large prefinished sections are more easily damaged if the prefabricator attempts to handle them in traditional fashion on the job. This brings up a very basic point in the philosophy of
prefabrication. Should the prefab package be simple and rough, adapted to manual handling by typical build- ers "working in the mud", or should it include the optimum amount of shop finishing with the recognition that it needs a distinct approach to site assembly, including trained crews and proper equipment? Only the lat- ter approach seems to hold real po- tential for truly competitive factory house production. The use of truck- mounted boom cranes should be ex- plored fully as Canadian firms pro- gress toward far more complete pre- finishing of their large panel systems. One company found that such equip- ment reduced damage to prefinished sections by 80% to 90% while dou- bling the speed of erection.
Such equipment also allows the pre- fabricator to adapt readily to the pro- duction of multiple family houses (Figure 3). Some are now producing two- and three
