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1、CHAPTER 13 SOME CURRENT DESIGN TRENDS Metal building systems represent one of the youngest and most dynamic sectors of the construction industry. New materials and design applications of metal buildings continue to emerge, expanding the architects palette of choices. This chapter examines some lates
2、t trends in specifying metal buildings, a few winning design solutions, and factors that further increase competitiveness of pre-engineered buildings. 13.1FACADE SYSTEMS: MANSARDS AND CANOPIES As metal building systems expand their acceptance into commercial, institutional, and community environment
3、s, the old bland, utilitarian look of metal-sheathed gable buildings gives way to more interesting and diverse design solutions. Visual interest can be added not only by the wall materials discussed in Chap. 7 but also by various facade treatments ranging from basic canopies to sophisti- cated fasci
4、a panels. 13.1.1Canopies A functional and aesthetically pleasing canopy is perhaps the most common facade treatment. The simplest way to build a canopy is to provide a cantilevered extension of the primary frame at the eave level and to continue the roof framing onto the canopy (Fig. 13.1). The eave
5、-line canopy is most appropriate for continuous and wideup to 10 ftcanopy coverage that extends the full length of the building. For this solution to work, the building exterior must be visually compatible with exposed cantilevered rafter framing, which stays in full view even when soffit panels cov
6、er the underside of the canopys roofing. A more refined option is a flush-framing canopy, where all the framing is hidden from view (Fig. 13.2a). At sidewalls, parallel to roof purlins, this canopy is supported by a special cantilevered canopy rafter at each column (Fig. 13.2b), while at the endwall
7、s roof purlins can simply be extended past the wall line (Fig. 13.2c). A flush-framing canopy, while sleek in design, is limited to the maximum width of 3 to 5 ft, or about one-half to one-third that of the eave-line canopy. For an even more sophisticated treatment, a bullnose canopy can be specifie
8、d (Fig. 13.3). A bull- nose canopy looks and functions best when located some distance below the roof level and attached to a contrasting wall material. Rather than being supported by a roof extension, it is carried by closely spaced frames attached to the building wall, with hat or channel steel me
9、mbers in between. Obviously, the supporting wall must be strong enough and is best made of masonry or concrete, 377 although it is possible to reinforce a metal-panel structure for this purpose. The panels for the bull- nose should be curved, as discussed in the next section. The spacing of support
10、frames is controlled by the flexural capacity of hat and channel sections. Structural properties of some representative hat and channel sections produced by MBCI are given in Fig. 13.4. The gage of MBCI sections is rather thin; similar sections made of thicker metal are available from other manufact
11、urers. 13.1.2Fascias and Mansards Fascia and mansard panels look so natural on pre-engineered buildings that one might forget to spec- ify and detail them separately. A vertical fascia and parapet panel, the most common kind, is com- monly supported by its own moment-resisting frame rigidly attached
12、 to the primary building framing. The primary frame has to be designed for an additional loading from the fascia. Some com- mon details of this solution are shown in Fig. 13.5. Mansard-style fascia panels require only some modifications of the vertical panel details (Fig. 13.6), but a completely dif
13、ferent type of framing is needed for a so-called double-curve eyebrow panel (Fig. 13.7). A curved fascia in combination with contrasting wall panels has helped transform what could have been a basic pre-engineered building into a modern-looking office (Fig. 13.8). For an even more adventurous design
14、, a triple-step curved fascia (Fig. 13.9) can add spice to almost any building. A note of caution: Mansards and parapets may look great on metal buildings, but they should be specified with a full understanding of the potential dangers involved. Unlike free-draining gable roofs, the interior gutters
15、 can, and do, get plugged up with ice or debris. It is imperative that such systems are supplemented with overflow scuppers or storm drains to remove any standing water that otherwise can overload the roof framing. In cold climates, drifted snow can pile up against the parapets and overload the purl
16、ins in the exterior bays. As discussed in Chap. 10, failure in a single bay can propagate throughout the building and result in a total loss. The author has investigated a metal building with a parapet in which this scenario has in fact been played out, while none of the surrounding parapet-free bui
17、ld- ings collapsed. In general, it is better to avoid using parapets in metal buildings located in snow regions. 378CHAPTER THIRTEEN FIGURE 13.1Eave-line canopy with exposed rafter. (Ceco Building Systems.) SOME CURRENT DESIGN TRENDS379 FIGURE 13.2Flush-framing canopy with soffit panel: (a) overall
18、appearance; (b) sidewall section; (c) endwall section. (Metallic Building Systems.) 13.2CURVED PANELS As the illustrations demonstrate, well-proportioned curved panels can make an excellent visual impression. These panels have become extremely popular since 1985 when Curveline, Inc., of Ontario, Cal
19、ifornia, brought into the United States the crimp-curving method of panel bending, first developed and patented in the Netherlands. During crimp-curving, metal panels are being incrementally pushed and pulled into rounded forms in a computer-controlled process.1Today, curved panels are specified not
20、 only as fascias, mansards, and canopies but also as walkway roofs, decorative column covers, equipment screens, roof transitions, and even as curved formwork for concrete. Curveline, Inc., remains the industry leader, offering the widest range of products available for curving. The company can form
21、 panels 2 to 30 ft long, with a maximum width of 5 ft. Panel depth can range from 3?4to 6 in, the thickness from 0.016 to 0.052 in (29 to 18 gage).2In addition to a single-curve configuration, Curveline can produce complex and breathtaking multiple and S curves (Fig. 13.10). Exposed fastener panels
22、are most suitable for curving, although some concealed fastener products can also be curved. The process of crimp-curving approximates true curvature by means of many short chords, a look that some dislike. Where a smoother line is desired, the “chorded” look of crimp-curved panel ribs can be avoide
23、d if the panels are turned with their flat parts, rather than the ribs, facing the outside. Each manufacturer of curved panels has its own standards for the minimum bending radii. Typically, the deeper the panel, the larger is the radius. Panels made of thin materials, especially high-strength steel
24、, normally require a bigger bending radius. 380CHAPTER THIRTEEN FIGURE 13.3Bullnose canopy with soffit. (Centria.) SOME CURRENT DESIGN TRENDS381 FIGURE 13.4Section properties of hat and channel sections. (MBCI.) 382CHAPTER THIRTEEN FIGURE 13.5Vertical fascias. (Metallic Building Systems.) SOME CURRE
25、NT DESIGN TRENDS383 FIGURE 13.6Mansard panels. (Metallic Building Systems.) The steel composition most favorable for curving, according to Curveline, is ASTM A 446 grade D carbon steel G-90 with a tensile strength of 50,000 lb/in2.3Panels made of galvanized steel, alu- minum, and stainless steel may
26、 be curved. Curved panels are structurally more efficient than the straight ones and can often be made of thinner metal, affording some material savings. For a continuous support, curved girts and purlins conforming to the panels outline can be produced at the same source. Wherever curves follow str
27、aight panel runs, as in building corners, a separate curved piece may or may not be required, depending on the supplier. According to Curveline, Inc., a separate curved connector piece is usually not needed, and the curve can be built into an end of the straight panel. For both aesthetic and functio
28、nal reasons, an extra joint is just as well avoided. A notable exception is the mitered corner (Fig. 13.11), which turns out better if shop-fabricated separately. When factory curving is not practical, field curving is possible. Some companies, such as Berridge Manufacturing of Houston, Tex., offer
29、both roll-forming and curving of the panels on-site. Alternatively, a rounded corner may be obtained without crimp-curving if the panel is bent parallel to the ribs, a relatively easy operation. While curved panels are visually attractive, the panel finish might be severely compromised during curvin
30、g. Some fabricators that had gotten into the curving business during the 1980s could 384CHAPTER THIRTEEN FIGURE 13.7Double-curve eyebrow fascia. (Centria.) not overcome the technical difficulties and survive. To this date, some major manufacturers, such as Butler, not only do not offer curved panels
31、 themselves but also advise against curving their products by others. For the same reason, many architects avoid specifying curved panels in corro- sive climates. Before specifying crimp-curved panels, designers should contact some local fabricators engaged in this business to inquire about availabl
32、e panel profiles, finishes, bending radii, and product warranties. It is instructive to view some of their past projects, preferably at least several years old, to look for signs of corrosion. During inspection, one should look for any incomplete bending and dimpling of panels, for proper curving of
33、 all the trim pieces, and for acceptability of tolerances. In addition to the firms mentioned above, some other companies involved in production of curved panels include ATAS Aluminum Corp. of Allentown, Pennsylvania; Floline Architectural Systems of St. Louis, Missouri; Petersen Aluminum Corp. of E
34、lk Grove Village, Illinois; Centria of Moon Township, Pennsylvania; and BHP Steel Building Products USA, Inc., of West Sacramento, California. 13.3STEEL-FRAMED HOUSES Always looking for new opportunities, the metal building industry has begun supplying pre-engi- neered framing for residential constr
35、uction at a spectacular pace. According to AISI, 13,000 steel- framed houses were built in this country in 1993, compared to only 500 built in the two prior years. In 1994, 40,000 steel homes were expected to be built in North America.4 Historically, steel has been prohibitively expensive for reside
36、ntial applications, but with wood prices escalating sharply in the early 1990s, steel suddenly became cost-competitive. Apart from SOME CURRENT DESIGN TRENDS385 FIGURE 13.8Curved fascia adds interest to an office building. (Curveline, Inc.) the price trends, which might prove transient, steel has so
37、me real advantages over wood: It is non- combustible, dimensionally stable, does not warp or rot, and is unaffected by termites. The major disadvantage of steel is its poor thermal properties. To be sure, houses of steel have been tried before. Peter Naylors “portable iron houses,” described in Chap
38、. 1, were offered for California Gold Rush fortune seekers as far back as the mid- nineteenth century. A century later, after World War II, the U.S. government granted a loan to Lustron Corp. of Columbus, Ohio, to build homes of steel. The Lustron Homes were made of steel framing and sheathed with p
39、orcelain-coated steel exterior panels. Even the interior partitions and 386CHAPTER THIRTEEN FIGURE 13.9Triple-step curved fascia. (Centria.) ceilings were made of steel. According to a web site devoted to Lustron Homes, these houses were produced in 1949 and 1950; they retailed for approximately $70
40、00. There are three methods of building the house of steel. The first is to simply substitute steel studs and joists for wood, essentially following traditional construction of studs and joists spaced 16 or 24 in on centers. Everything elseroofing, siding, doors, windowsstays the same as in a wood-f
41、ramed house. This method allows for an easy framing conversion to steel in both standard and custom-designed houses; it is undoubtedly used in most steel-framed houses. The second method of framing is panelized construction: The structure is built from preassem- bled steel-stud wall panels and roof
42、trusses. Both studs and trusses are spaced 32 to 68 in on centers, with hat-section subgirts and subpurlins similar to those in Fig. 13.4 spanning in between. Despite the claims of efficiency, this system is rather complex structurally and may require more bracing and anchorage than others.5It is un
43、familiar to both traditional house builders and pre-engineered build- ing erectors and can introduce a lot of confusion at the jobsite. SOME CURRENT DESIGN TRENDS387 FIGURE 13.10Complex curves grace the U.S. Space Camp in Huntsville, Alabama. (Curveline, Inc.) The third method is to construct a true
44、, if small, pre-engineered building, complete with the usual main frames, girts, purlins, rod bracing, and metal roofing. The bay spacing of such pre-engineered structures ranges between 6 and 10 ft. A rigid-frame gable building can be quite appropriate for a large contemporary residence with an ope
45、n floor plan and cathedral ceilings. The system, however, is unfamiliar to most residential designers and builders. It also differs so much from the traditional construction that to present it as a framing substitution for already-designed projects is difficult. Furthermore, the commonly available c
46、ommercial-style components of metal building systems such as doors, windows, siding, and roofing might not be in line with the owners expectations. Since few homeowners dream of a house sheathed in metal siding, either a brick or a wood exterior is desir- able. These traditional finishes normally ne
47、ed to be backed by 3?4-in-thick plywood sheathing, which can span about 4 ft between the metal supports. The problem of thermal bridging can be solved by applying rigid insulation to the outside of steel studs. Polyisocyanurate insulation offers excellent insulation value (see Chap. 8) and is availa
48、ble in the form of insulating sheathing. Rigid insulation can also be incorporated in the EIFS exteriors, as dis- cussed in Chap. 7. A typical high-quality exterior wall may consist of 6-in steel studs covered with 3?4-in plywood and 1-in insulating board coated with an EIFS finish. The studs may be
49、 filled with 6-in fiberglass insulation covered with a heavy-gage plastic vapor retarder and 5?8-in drywall.6 Those interested in designing and building metal houses can subscribe to Metal Home Digest, a magazine mentioned in Chap. 2, Sec. 2.10. 13.4COMPUTERIZATION OF THE INDUSTRY In one word, what has helped to transfer “pre-engineered” designs of old into the modern metal building systems? Computers! Heavy reliance on these machines has allowed metal building manu- 388CHAPTER THIRTEEN FIGURE 13.11Mitered corner. (Centria.) facturers to discard the old menu of a few predesigned build