Selections from ANSI/TPI National Design Standard for Metal Plate Connected Wood Truss Construction

Transcription

1 Selections from ANSI/TPI 1-00 National Design Standard for Metal Plate Connected Wood Truss Construction Page 1 6. ADJUSTMENTS TO DSIGN VALUES 6..1 Load Duration Factor (C D ) Design values shall be permitted to be adjusted for load duration conditions in accordance with this section, unless otherwise specified by local code requirements. The Building Designer shall be permitted to reduce the load duration factor when the expected load durations are greater than the assumed durations in Section Adjustments for load duration apply to all lumber and plate lateral resistance (tooth holding) design values, with the exception of modulus of elasticity (E) and compression perpendicular to grain (F c ) The adjustment for load duration shall be accomplished by multiplying the design value by the appropriate C D factor as follows: Permanent 0. Normal - 10 Years duration 1.00 Snow Months duration 1.15 Construction 7 Days duration 1.5 Wind & Earthquake 5-10 minutes 1.60 Impact *.0 * For FRT and pressure-preservative lumber and all connections subject to an impact load, the duration of load factor shall not exceed For combinations of loads with different durations, the load duration factor, C D, for the shortest duration load that is part of that load combination shall apply for that entire load combination. 6.. Repetitive Member Increase Repetitive member design values that are listed in the recognized lumber grading rules, or a 15 percent increase to F b and 10 percent increase to F c and F t for solid sawn lumber members to which structural wood sheathing is mechanically attached, or a 10 percent increase to F b, F c, and F t for solid sawn lumber members to which structural wood sheathing is not attached, shall only apply to chord members where three or more trusses are positioned side by side, are in contact, or are spaced no more than inches

2 on center and are joined by roof sheathing, flooring, gypsum, or other load distribution elements attached directly to the chords Single-ply and two-ply girder trusses are not permitted to use the repetitive member increases outlined in Section Pages CORROSIVE ENVIRONMENTS The following coatings are recognized as providing increased corrosion protection to metal connector plates: (a) Epoxy-Polyamide Primer (SSPC-Paint ) (b) Coal-Tar Epoxy-Polyamide Black or Dark Red Paint (SSPC-Paint 16) (c) Basic Zinc Chromate-Vinyl Butyral Wash Primer 9SSPC-Paint 7) and cold applied Asphaltic Mastic (Extra Thick Film) Paint (SSPC-Paint 1) (d) Post-plate-manufacture hot dip galvanizing per ASTM A Embedded metal connector plates shall be free of dirt and oil prior to coating application. If the coating is damaged prior to, or during truss installation, such damage shall be alleviated before accessibility is impeded Metal connector plates, including Types 0 and 16 stainless steel plates, shall not be exposed to swimming pool environments unless adequate provision is made to prevent stress corrosion cracking. In lieu of use of a stainless steel that is not susceptible to stress corrosion cracking (see commentary), trusses shall be separated from the pool environment by a vapor barrier and shall be separately ventilated from the pool environment. Commentary Pages CORROSIVE ENVIRONMENTS If the environment for the intended end use of the trusses is expected to result in wood moisture content exceeding 19 percent, unusually high temperatures for typical wood building construction, or unusual corrosion potential, the Building Designer is responsible for specifying these conditions on the structural design documents (see Section..). In these cases, a means for providing increased metal connector plate resistance to the corrosive environment must be specified by the Truss Designer, unless specified by the Building Designer. The decision of which option to use for increasing corrosion resistance, whether using one of the coatings recognized in Section or another acceptable coating or means as determined by the Building Designer, should be

3 made with respect to the particular conditions, i.e., the suitability for the specified environment, given the advantages and drawbacks of the various options The dual system of G60 hot dip galvanizing and proven barrier-coat painting options affords a synergistic improvement in protection of steel against corrosion - typically 50% greater than the simple addition of the protection provided by zinc and paint separately. In one independent study of corrosion resistance effectiveness, a number of painting options (treatments) were applied to standard galvanized G60 connector plates embedded into a splice joint. The combinations of various paints and G60 zinc were compared to control samples of truss plates that were regalvanized in accordance with ASTM A15 requirements, by subjecting all the plates to the same accelerated freeze/thaw and salt fog tests. Of the eight coating/wash primer combinations studied, those indicating the highest probability of lasting 70 or more years in an enclosed marine environment were epoxy, coal tar epoxy, and asphalt mastic with wash primer, and these coatings have been included in the Standard since These coating systems, as specified in (a)-(c), should be considered as an alternative corrosion protection option to the code criteria that allow double dipped galvanized or stainless steel metal connector plates used in Coastal High Hazard and Ocean Hazard Areas. Prior to application of the paint coatings referenced in Section 6.5.1, the surface of the galvanized G60 metal connector plates shall be solvent cleaned in accordance with SSPC-SP 1 Solvent Cleaning (16). Also, the paints shall be applied in accordance with SSPC-PA 1 Shop, Field, and Maintenance Painting. The study found that spray-applied coatings were not found to be practical for metal connector plate joint protection compared to brush-applied coatings, which provided unbroken coverage over the entire joint. Thus, the SSPC paints shall be brush applied, taking care to work the coatings into the slot openings and exposed underlying wood. Specifications for SSPC paints are found in the Steel Structures Painting Manual; the suitability and appropriate dry film thicknesses of the SSPC paints in (a)-(c) are as follows: (a) SSPC PAINT - Epoxy Polyamide Paint is suitable for exposures in SSPA rated environmental zones A (frequently wet by fresh water), B (frequently wet by salt water), A (chemical - acidic), B (chemical - neutral), and C (chemical - alkaline). SSPC Paint shall be brush applied to a dry film thickness of.5 mils at its thinnest point. (b) SSPC PAINT 16 - Cold Tar Epoxy-Polyamide Black (or Dark Red) Paint is suitable for exposures in SSPA rated environmental zones: A (frequently wet by fresh water), B (frequently wet by salt water), C (fresh water immersion), D (salt water immersion), A (chemical - acidic), B (chemical - neutral), and C (chemical - alkaline).

4 SSPC Paint 16 shall be brush applied to a dry film thickness of 8 mils at its thinnest point. (c) SSPC PAINT 7 - Basic Zinc Chromate-Vinyl Butyral Wash Primer and SSPC PAINT 1 - Cold Applied Asphalt Mastic (Extra Thick Film). SSPC Paint 1 is suitable for exposures in SSPA rated environmental zones A (frequently wet by fresh water), B (frequently wet by salt water), B (chemical -neutral), and C (chemical - alkaline). One primer coat of SSPC Paint 7 shall be brush applied to a dry film thickness of 0. mils. SSPC Paint 1 shall then be brush applied as a topcoat to a dry mil film thickness of 5 mils at its thinnest point. Unless otherwise specified, it is advisable that the purchaser or building official reserve the right to inspect the applied SSPC paint system in accordance with SSPC-PA Measurement of Dry Paint Thickness with Magnetic Gages (Non-Destructive). The study concluded that the paint coating systems over standard galvanized connector plates would be expected to out-perform the double galvanized metal connector plates in field use. However, other options may be more appropriate when ocean salts or corrosives are not present in the air, or when plates extend off of the wood, and regalvanizing is another option that is recognized for increasing corrosion protection. Although ASTM A15 regulates proper application of the regalvanizing (i.e., excess zinc is required to be spun off by centrifuge) and thus addresses such concerns relating to excess zinc, some reduction in tooth holding or other warnings may be appropriate with regalvanized plates due to the environments where regalvanized plates are likely to be specified, such as where there is the potential for increased moisture content, in which case a C M factor may be necessary Stress corrosion cracking (SCC) is an issue for components in swimming pool building atmospheres, which are safety-critical and load-bearing, but are not washed or cleaned frequently. Types 0 and 16 stainless steel have been found to fail due to SCC in highly aggressive chloride environments and have been affected by SCC in some swimming pools. More highly alloyed grades of austenitic stainless steel have a much greater degree of SCC resistance. Two grades have been tested and found to be resistant to SCC under laboratory conditions, namely 17 LMN and L. For a full copy of ANSI/TPI 1-00, National Design Standard for Metal Plate Connected Wood Truss Construction, contact: Truss Plate Institute 58 D Onofrio Drive Suite 00 Madison, WI /8-50

5 Truss Plate Institute ANSI/TPI 1-00 Appendix I (Non-mandatory) APPENDIX I (NON-MANDATORY) STANDARD LUMBER & TIMBER SIZES & PROPERTIES OF SECTIONS TABLE I-1 STANDARD NOMINAL & MINIMUM-DRESSED SIZES OF BOARDS, DIMENSION, AND TIMBERS (a) Item Nominal Inch Thicknesses Minimum Dressed Dry Green Dry Green Nominal Inch Inch mm Inch mm Face widths Minimum Dressed Inch mm Inch mm Boards b 1 1-¼ ¾ 1 1-¼ / 1-1/ 1-9/ ½ -½ 5-½ 6-½ 7-¼ 8-¼ 9-¼ 10-¼ 11-¼ 1-¼ 15-¼ /16-9/16-5/8 5-5/8 6-5/8 7-½ 8-½ 9-½ 10-½ ½ Dimension -½ -½ -½ /16-9/16-1/16-9/16-1/ ½ -½ 5-½ 7-¼ 9-¼ 11-¼ 1-¼ 15-¼ /16-9/16-5/8 5-5/8 7-½ 9-½ ½ Timbers 5 & Thicker ½ off 1 off ½ off 1 off 5 & Wider ½ off 1 off ½ off 1 off a Refer to Voluntary Product Standard PS-0-9 "American Softwood Lumber Standard" for definitions on dry and green lumber. b Boards less than the minimum thickness for nominal 1-inch but 5/8 (16mm) or greater thickness dry (11/ 16 inch [17mm] green) shall be regarded as ALS lumber, but such boards shall be marked to show the size and condition of seasoning at the time of dressing. They shall also be distinguished from nominal 1-inch boards on invoices and certificates. (*) Reproduced from Voluntary Product Standard PS-0-9 "American Softwood Lumber Standard" for definitions of dry and green lumber, National Institute of Standards & Technology, U.S. Dept. of Commerce, Technology Administration.