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1、CHAPTER 5 SECONDARY FRAMING: GIRTS AND PURLINS 5.1INTRODUCTION Secondary structural members span the distance between the primary building frames of metal build- ing systems. They play a complex role that extends beyond supporting roof and wall covering and carrying exterior loads to main frames. Se
2、condary structurals, as these members are sometimes called, may serve as flange bracing for primary framing and may function as a part of the buildings lateral loadresisting system. Roof secondary members, known as purlins, often form an essential part of horizontal roof diaphragms; wall secondary m
3、embers, known as girts, are frequently found in wall bracing assemblies. A third type of secondary framing, known by the names of eave strut, eave purlin, or eave girt, acts as part purlin and part girtits top flange supports roof panels, its web, wall siding (Fig. 5.1). Girts, purlins, and eave str
4、uts exhibit similar structural behavior. Since most secondary members normally encountered in metal building systems are made of cold-formed steel, our discussion starts with some relevant issues in design of cold-formed steel structures. 5.2DESIGN OF COLD-FORMED FRAMING As mentioned in Chap. 2, the
5、 main design standard for cold-formed framing is Specification for the Design of Cold-Formed Steel Structural Members by American Iron and Steel Institute (AISI).1The Specification, Commentary, Design Examples, and other information constitute the AISI Manual.2 The first edition of the Specification
6、 appeared in 1946, with subsequent editions following in 1960, 1968, 1980, 1986, 1989 (by Addendum), 1996, 1999, and 2000 (the last two by Supplement). The LRFD-based Specification was first issued in 1991.3 In 2002, the title was changed to North American Specification for the Design of Cold-Formed
7、 Structural Members,4to reflect the fact that many of the Specifications provisions apply not only to the United States, but also to Canada and Mexico. The provisions common to all three countries are included in the main body of the document; the country-specific items are placed in the Appendix. T
8、he users of the 2002 Specification have a choice of ASD, LRFD, and LSDLimit States Design formats. (The LSD design approach is widely used outside the United States.) As can be imagined, the combined Specification does not look any simpler than its notoriously complex predecessors. The changes betwe
9、en various editions are substantial, a fact that reflects on the continuing research in this area of steel design. Since the Specification provisions are so fluid, framing manu- facturers are challenged to comply with the latest requirements. Unfortunately, some have fallen behind, still using the p
10、revious editions. Anyone who has ever attempted to design a light-gage member following the Specification pro- visions probably realized how tedious and complex the process was. This fact helps explain why 91 cold-formed steel framing is rarely designed in most structural engineering offices. When s
11、uch framing is needed, one of two things tends to happen to the engineers: They either uncritically rely on the suppliers literature or simply avoid any cold-formed design at all by specifying hot-rolled steel members and hoping for a contractor to make the substitution and to submit the required ca
12、lculations. In this chapter, we limit our immersion into the actual Specification formulas that could easily have become obsolete by the time you read this book. Instead, we point out but a few salient concepts. What makes cold-formed steel design so time-consuming? First, materials suitable for col
13、d form- ing are usually quite thin and thus susceptible to local deformations under load. (Remember how easy it is to dent a tin can?) This mode of failure is of much less concern in the design of thicker hot- rolled members. These local deformations can take two forms: local and distortional buckli
14、ng. The nature of distortional buckling (Fig. 5.2a) is not very well understood, at least not as well as that of local buckling (Fig. 5.2b). In local buckling, some part of the compression flange and the web buck- les when the stresses reach a certain limit; that part then ceases to carry its share
15、of the load. In dis- tortional buckling, the compression flange and the adjacent stiffening lip move away from the original position as a unit, also weakening the section. Research on distortional buckling proceeds at a brisk pace, with some important work done by Bambach et al.5and Schafer and Peco
16、z,6among others. Second, the flanges of light-gage sections cannot be assumed to be under a uniform stress distribu- tion, as the flanges of an I beam might be (the shear lag phenomenon). To account for both the local buckling and the shear lag,the Specification utilizes a concept of “effective desi
17、gn width,”in which only certain parts of the section are considered effective in resisting compressive stresses (Fig. 5.3). This concept is pivotal for stress analysis and deflection calculations performed for cold-formed members. 92CHAPTER FIVE FIGURE 5.2Local deformations of cold-formed Z sections
18、 in flexure, with top flange in compression: (a) distortional buck- ling; (b) local buckling. (After Refs. 5 and 6.) FIGURE 5.1Typical eave strut. SECONDARY FRAMING: GIRTS AND PURLINS93 FIGURE 5.3Effective width concept for C and Z sections (shaded areas are considered ineffective). FIGURE 5.4Purlin
19、 movement from lateral buckling. (LGSI.) The effective design width depends on the stress in the member, which, naturally, cannot be com- puted until some section properties are assumed first. Because of this “vicious circle,” a few design iterations are needed. A common simplified yet conservative
20、procedure for the effective width cal- culations assumes the level of stress to be the maximum allowable. Another complication caused by a nonuniform stress distribution across thin, often nonsymmetrical, sections is their lack of torsional stability. Light-gage compression and flexural members can
21、fail in torsional-flexural buckling mode by simultaneous twisting and bending, a failure that can occur at relatively low levels of stress. In plan, purlins that buckle laterally are displaced from their original positions as shown in Fig. 5.4. The maximum lateral displacement typically occurs in th
22、e middle of the span. Torsional-flexural buckling can be prevented by keeping the compressive stresses very low or by plenty of bracing, as discussed later in this chapter. The complexities of light-gage member design do not stop at flexural and compression calcula- tions. Tedious shear calculations
23、 are often accompanied by even more cumbersome web crippling checks. To be sure, web crippling failures occur in hot-rolled steel members too, but light-gage sec- tions are incomparably more susceptible. Web crippling failures such as that shown in Fig. 5.5 are most likely to occur at supports, wher
24、e shear stresses are at their maximum. Web crippling stresses are additive to bending stresses, and a combination of both needs to be investigated. Whenever web crippling stresses are excessive, bearing stiffeners are required at supports, in which case it is common to assume that the total reaction
25、 force is transferred directly through the stiffener into the primary framing, neglecting any structural contribution of the members web. A small gap might even be left under the flange of a girt or purlin. The stiffeners are usually made of clip angles, plates, or channel pieces. In Fig. 5.6, the l
26、oad is transmitted from the web of a Z purlin via screws or bolts to the clip-angle stiffener and then from the stiffener to the rafter. Some other clip designs, which not only help the purlin resist web crippling stresses but also stabilize it laterally, are described later in the chapter (Sec. 5.5
27、.5). The Specification recognizes the fact that analytical methods of establishing load-carrying capac- ities of some cold-formed structural framing may not always be available or practical and allows determination of structural performance by load testing for such cases. The testing procedure is 94
28、CHAPTER FIVE FIGURE 5.5Web crippling. FIGURE 5.6Bearing clip angle acting as web stiffener. described in the Specification section entitled “Tests for Special Cases.” In the 1986 edition, the test criteria were relatively clear. Specifically, the member or assembly being tested should have been able
29、 to carry twice the live load plus 1.5 times the dead load (the strength test) and not to distort excessively under 1.5 times the live load plus 1.0 dead load (the deflection test). The values of the effective section modulus and moment of inertia were established based on the measurements of strain
30、s and deflections. The test results applied only to the specimen being tested. If the testing was intended to apply to the whole class of sections, as it usually was, the material properties such as yield strength were measured and the test results adjusted by the ratio of the nominal to actual stre
31、ngth of the steel. For example, if the nominal yield strength of the steel was 55 ksi but the actual was measured at 60 ksi, the test results were reduced by the ratio of 55/60 ? 0.917. Otherwise, they would overstate the capacity of similar members made from steel with a yield strength lower than 6
32、0 ksi. The 1996 and later editions derive the allowable design strength of the member or assembly as the average value of all the test results divided by a factor of safety. The latter is equal to 1.6 divided by the resistance factor, which requires some computations to be determined. 5.3COLD-FORMED
33、 STEEL PURLINS 5.3.1Available Sizes and Shapes Cold-formed C and Z purlins are the workhorses of the industry. Configurations of these members have originated at the bending pressthey represent the two basic ways to bend a sheet of metal into a section with a web and two flanges. Light-gage purlins
34、of 8 to 12 in in depth can span 25 to 30 ft, and even more, depending on the loading, material thickness, and deflection criteria. Purlin spacing is dictated by the load-carrying capacity of the roof panels; a 5-ft spacing is common. Appendix B includes section properties for purlin sizes offered by
35、 some manufacturers. Cold-formed purlins are normally made of high-strength steel. Uncoated cold-formed members, still in the majority, usually conform to ASTM A 570 or A 607. Occasionally, galvanized purlins are provided. The old designation for galvanized members,ASTM A 446, has been replaced with
36、 a new ASTM Standard Specification A 653.7The new standard includes the designations of zinc coating, G60 and G90, which used to be a part of a separate standard, ASTM A 525. (The latter has been replaced by ASTM A 924, which now covers all kinds of metal coatings applied by a hot-dip process.) For
37、the products of structural quality (SQ), three grades33, 40, and 80are available, corresponding to the old grades A, C, and E of ASTM A 446. For example, ASTM A 653 SQ grade 40 with coating designation G60 takes the place of the old ASTM A 446 grade C with G60 coating. The minimum yield strength for
38、 steel sections 16 gage and heavier is normally specified as 55,000 psi, although the Light Gage Structural Institute (LGSI) bases its load tables8on a minimum yield strength of 57,000 psi. How is it possible that LGSI can use a higher strength of steel than most manufacturers for the same material
39、specification? The ASTM specifications define the minimum yield strength of steel, but the actual strength is often higher. It may be possible to justify using the 57 ksi, rather than 55 ksi, yield strength, if a credible program of inspection and material testing is maintained, and only the steel w
40、ith a minimum actual strength of 57 ksi is allowed for use. This is what LGSI does, although this practice is not followed in structural steel design. Similarly, LGSI member companies have adopted slightly different section properties for their cold-formed sections than those of most metal building
41、systems manufacturers (Fig. 5.7). LGSI products try to optimize flange and lip sizes. Cold-formed purlins can be designed as simple-span or continuous members. The beneficial effects, as well as the disadvantages, of continuous framing are explained in Chap. 3. The concept of continuous Z purlins wa
42、s introduced in 1961 by Stran-Steel Corp., already mentioned in Chap. 1 as a pioneer of pre-engineered buildings. (Prior to that invention, manufacturers used simple-span cold-formed sections or bar joists.) Cold-formed purlins can be made continuous by overlapping and SECONDARY FRAMING: GIRTS AND P
43、URLINS95 fastening. Light-gage C sections can be easily lapped back to back; theoretically, Z sections can be nested one inside another. In reality, however, the traditional equal-flange Z sections of thicker gages might be difficult to nest. Zamecnik9observes in his investigation of a warehouse wit
44、h the notice- ably distorted Z purlins that it is “impossible to nest the two sectionswithout bending the web of the lower purlin away from the bottom flange,” a situation that contributes to undesirable rotation of the purlins at the supports (see Fig. 5.8). Noteworthy, LGSI Z sections have flanges
45、 of slightly unequal width to facilitate splicing and provide better fit. 96CHAPTER FIVE FIGURE 5.7Typical C and Z girt and purlin sections: (a) used by major metal building manufac- turers; (b) offered by LGSI members. A reminder to specifiers: LGSI sections should not be forced on the metal buildi
46、ng systems man- ufacturers or specified indiscriminately, since the manufacturers have their production lines geared toward their own standard members. Please investigate the availability first. Also, local steel erectors might not be familiar with LGSI sections and therefore might not be aware of t
47、he need to turn every other purlin upside down, as is needed to achieve the benefits of unequal-flange design. Erectors need to be educated on the benefits of using unequal-flange sections and on their installation techniques. 5.3.2Design for Continuity To achieve some degree of continuity, cold-for
48、med sections are lapped and bolted together for a dis- tance of at least 2 ft; i.e., each member extends past the support by at least 1 ft (Fig. 5.9). The degree of continuity may be increased with a longer lap distance, albeit at a cost of the extra material used in the lap. Some research10indicate
49、s that load capacity of Z purlins continues to increase until the length of the lap approaches one-half of the span, while other research11suggests that the limit is much smaller than that. SECONDARY FRAMING: GIRTS AND PURLINS97 FIGURE 5.8Forcing Z purlins of identical size one inside another at the splice causes their rotation at support. (After Zamecnik, Ref. 9.) A purlin can be attached to rafters in various ways, depending on the magnitude of crippling stress in the purlins web. A simple bolting through the member flanges is acceptable if the web crip- pling stress is not cr