AAMA2 wall fasten

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This Technical Information Report is a consensus document developed by the Engineering Design Standards Task Group of the Architectural Window, Curtain Wall and Store Front Division of the American Architectural Manufacturer's Association (AAMA). Twenty-two professional engineers and ten designers representing twenty-two companies and associations were members of the task group.

This report is published as a service to those with the responsibility for design, specification, manufacture and installation of metal curtain wall systems.

While this report is believed to be accurate, the information should not be used or relied upon without competent professional examination and verification of its accuracy, suitability and applicability. Publication of the material contained in this report is not a warranty of any kind whatsoever, expressed or implied, on the part of AAMA, nor is it a representation that the information is suitable for any general or particular use, or of freedom from infringement of any patent or patents. Anyone making use of this information assumes all liability arising from such use.

The design professional should be aware that standards, specifications and technical data developed by other organizations, incorporated by reference in this report, may be modified or amended from time to time subsequent to the printing of this report.

Prior to the publication of this Technical Information Report no single body of information existed which provided the guidance needed for analyzing fasteners used in metal curtain wall construction. The Aluminum Association (AA) provided specifications and guidance for aluminum. The American Institute of Steel Construction (AISC) and the American Iron and Steel Institute (AISI) provided similar information for steel. Other information required to adequately design and analyze fasteners was found scattered among a number of organizations including the American National Standards Institute (ANSI), the American Society for Testing and Materials (ASTM), the Industrial Fasteners Institute (IFI) and the Society of Automotive Engineers (SAE). In some instances the larger users of industrial fasteners, such as the General Services Administration, developed their own standards for fastener analysis. The results of this were often guidelines tailored only to meet the specific needs of the individual users.

In recognition of this situation the Curtain Wall Engineering Design Standards Task Group of the Architectural Window, Curtain Wall and Store Front Division of AAMA undertook the task of developing this report. The report brings together in one document the data and guidance essential for selection of fasteners needed in metal curtain wall framing and anchoring systems. The information is also applicable to storefronts, sloped glazing, skylights and other architectural metal applications.

AAMA welcomes comments and suggestions relative to this report with the objective of improving and expanding the next edition when it is published.

1.0 INTRODUCTION TO METAL CURTAIN WALL FASTENERS

The purpose of this Technical Information Report is to provide metal curtain wall designers with the data necessary to select fasteners for curtain wall framing members and components, and for anchoring the curtain wall to the building structure.

Technical information and data assembled in this report were drawn from a number of organizations. The relevant publications of these organizations are listed under Section 2, 'Applicable Documents.'

Uniform coarse machine threaded fasteners and spaced threaded fasteners are covered in this report. The Unified Thread Series are generally used in either clear holes with mating nuts or in tapped holes. Thread cutting screws with machine threads are used to cut their own threads in pre-drilled holes. Spaced threaded fasteners, on the other hand, are generally used only as tapping screws. This subject is covered in detail in Section 6, 'Fastener Load Tables Commentary.'

Metric fasteners are not addressed in this document, but the design parameters included apply equally well to metric fasteners. Metals used in fasteners, on which the data in this report is based, include two groups of carbon steel and five groups of stainless steel alloys. The use of aluminum fasteners is not recommended for curtain wall anchoring systems and no data on aluminum fasteners is included. Carbon steel fasteners shall be plated or coated in accordance with the specifications in Section 4, 'Protection Against Corrosion.'

Tables giving allowable tension, shear and bearing loads for fourteen different fastener sizes, for carbon steel and stainless steel alloys, are included in this report. The four sizes at the small end of the size range, in ascending order, are designated #6-32, #8-32, #10-24 and #12-24. For fasteners designated in this manner the number preceding the hyphen is related to the fastener diameter. For larger size fasteners the number preceding the hyphen is the nominal diameter in inches and/or a fraction thereof. The ten larger size fasteners range from 1/4-20 through 1-8. In both designation systems the number following the hyphen is the number of threads per inch. Equations needed to calculate the allowable loads are included with the tables.

2.0 APPLICABLE DOCUMENTS

ALUMINUM ASSOCIATION (AA) "Specifications for Aluminum Structures," Fifth Edition AMERICAN INSTITUTE OF STEEL CONSTRUCTION (AISC)

"Manual of Steel Construction, Allowable Stress Design," Ninth Edition

AMERICAN IRON AND STEEL INSTITUTE (AISI) "Specification for the Design of Cold-Formed Steel Structural Members," August 19, 1986 Edition "Stainless Steel Cold-Formed Structural Design Manual," 1974 Edition

AMERICAN NATIONAL STANDARDS INSTITUTE (ANSI)

ANSI/ASME B1.1, "Unified Inch Screw Threads" (UN and UNR Forms) AMERICAN SOCIETY FOR TESTING AND MATERIALS (ASTM)

Volume 15.08, "Annual B ook of ASTM Standards, Fasteners"

ASTM A 143, "Recommended Practice for Safeguarding Against Embrittlement of Hot-Dip Galvanized Structural Steel Products and Procedure for Detecting Embrittlement"

ASTM A 153, "Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware"

ASTM A 307, "Specification for Carbon Steel Bolts and Studs, 60,000 psi Tensile Strength"

ASTM A 325, "Specification for High-Strength Bolts for Structural Steel Joints"

ASTM A 449, "Specification for Quenched and Tempered Steel Bolts and Studs"

ASTM A 490, "Specification for Heat-Treated Steel Structural Bolts, 150 ksi Minimum Tensile Strength" ASTM A 563, "Specification for Carbon and Alloy Steel

ASTM B 201, "Practice for Testing Chromate Coatings on Zinc and Cadmium Surfaces"

ASTM 456, "Specification for Electrodeposited Coatings of Copper Plus Nickel Plus Chromium and Nickel Plus Chromium"

ASTM 633, "Specification for Electrodeposited Coatings of Zinc on Iron and Steel"

ASTM B 695, "Specification for Coatings of Zinc Mechanically Deposited on Iron and Steel"

ASTM B 696, "Specification for Coatings of Cadmium Mechanically Deposited"

ASTM 766, "Specification for Electrodeposited Coatings of Cadmium"

ASTM F 593, "Specification for Stainless Steel B olts, Hex Cap Screws and Studs"

ASTM F 594, "Specification for Stainless Steel Nuts" ASTM F 606, "Method for Conducting Tests to Determine the Mechanical Properties of Externally and Internally Threaded Fasteners, Washers and Rivets" INDUSTRIAL FASTENERS INSTITUTE (IFI) "Fastener Standards," Sixth Edition "Manufacturer's Capability Guide"

INDUSTRIAL PRESS, INC.

"Machinery's Handbook"

MCGRAW-HILL BOOK COMPANY

"Marks' Mechanical Engineers Handbook" SOCIETY OF AUTOMOTIVE ENGINEERS (SAE) "Standards for Grades 2 and 5 Steel"

3.0 QUALITY CONTROL

The selection and use of proper fasteners is critically important to the safe and satisfactory performance of curtain wall systems. This report provides the information necessary to select the proper fasteners and to specify them. Beyond this point, however, it is essential that the manufacturers of fasteners maintain excellent quality control procedures in their plants to ensure that their products meet the specifications for which they are designed. The purchasers, too, must have means for determining that they are, in fact, getting fasteners that meet their specifications. Unfortunately, there has been a plague of inferior bolts being sold in this country which have been fraudulently identified as bolts having quality which they do not possess. Inadvertent use of such inferior bolts could have disastrous results.

The problem with inferior fasteners on the market has been serious during the past few years. Many fasteners may be found to be substandard mechanically and dimensionally when checked even though marked as high performance grades. Protective coatings on fasteners may also be a problem. As a result of more stringent environmental requirements and tightening economic pressures, fewer manufacturers are applying adequate coatings. The quality and thickness of protective coatings in today's market, particularly on low price fasteners, is somewhat unreliable. In order to be certain that the fastener needed to meet design criteria is provided, the designer must not only specify fastener size and type, he must also specify material, minimum mechanical properties, thickness and type of protective coating required. See the suggested Fastener Specification Checklist, Section 13.0, for items to be included in fastener specifications.

ASTM standards give the chemical and mechanical requirements for the steels used in fasteners. In addition, they set forth requirements which the purchaser of fasteners may specify for the quality control procedures to be followed in connection with his order. These include shipment lot testing, source inspection, alloy control, heat control, permeability, manufacturer's identification and material identification. ASTM F 606 sets forth in detail the test methods for determining the mechanical properties of externally and internally threaded fasteners. Appropriate reference to these standards can provide the basis for reliable quality assurance programs.

4.0 PROTECTION AGAINST CORROSION

It is essential that fasteners have adequate protection against corrosion. If such protection is not provided, failures in connections may ultimately occur. Curtain wall framing systems may contain and channel considerable amounts of water both from rain and condensation of water vapor. This would tend to accelerate corrosive action where satisfactory protection did not exist and this would be the case with unprotected carbon steel fasteners. In addition to corrosion resulting from atmospheric conditions and moisture, protection shall also be provided against galvanic corrosion which occurs when dissimilar metals are in contact in the presence of moisture. To protect against both types of corrosive action carbon steel fasteners plated in accordance with the specifications listed below or stainless steel fasteners are recommended for use with aluminum curtain wall systems.

Stainless steel fasteners come in a variety of alloy types. All stainless steel alloys referenced in this report have good resistance to corrosion. However, some of these alloys have better resistance than others. Type 316, for example, has a higher resistance than Type 304. It may seem that specifying the higher resistance and types of stainless steel for all fasteners would address these concerns with corrosion. Unfortunately, this is not necessarily true. Some fastener designs are not manufactured in all types of stainless steel because of the need for hardening heads or points, or because of the capacities of the screw machines used to manufacture fasteners. The higher resistance types of stainless steel generally cannot have the finishes applied which match anodized framing without resorting to painting. Painting of screw heads is expensive and of dubious durability. Many types of fasteners are only available in stainless steels having lower resistance to corrosion. Small order quantities, less than 100,000 fasteners per run, may also limit the availability of the fastener desired or greatly increase its cost. The specifier and purchaser must be aware of these matters and make the best compromise possible, all factors considered, in the selection of the fasteners.

Carbon steel fasteners may be plated with zinc, cadmium, nickel or chrome to provide adequate resistance to corrosion. The severity of the service conditions, to which the fasteners will be exposed, must be considered in the specification. For zinc and cadmium coatings the following specifications are recommended:

Zinc plated fasteners shall meet the requirements of ASTM B 633 for Class FE/ZN 5, 5μm coating thickness, service condition SC 1 (mild), with Type III finish meeting corrosion resistance requirements after a 12-hour salt spray test. Zinc plated fasteners shall meet the requirements of ASTM B 633 for Class FE/ZN 8, 8μm coating thickness, service condition SC 2 (moderate), with Type II finish meeting corrosion resistance requirements after a 96-hour salt spray test.

Mechanically deposited zinc coated fasteners shall meet the requirements of ASTM B 695 for Class 5 coating, 5 μm thick with Type II finish, or Class 8 coating, 8 μm thick with Type II finish. B oth Class 5 and Class 8 coatings shall meet the corrosion resistance requirements after a 72-hour salt spray test. (Thicker coatings meeting this ASTM standard are available if required.)

Cadmium plated fasteners shall meet the requirements of ASTM B 766 for Class 5, 5 μm thick, Type III coating meeting corrosion resistance requirements after a 12-hour salt spray test.

Cadmium plated fasteners shall meet the requirements of ASTM B 766 for Class 8, 8 μm thick, Type II coating meeting corrosion resistance requirements after a 96-hour salt spray test.

Mechanically deposited cadmium coated fasteners shall meet the requirements of ASTM B 696 for Class 5 coating, 5 μm thick with Type II finish, or Class 8 coating, 8 μm thick with Type II finish. Class 5 coatings with Type II finish shall meet the corrosion resistance requirements after a 72-hour salt spray test. Class 8 coatings with Type II finish shall meet the corrosion resistance requirements after a 96-hour salt spray test. (12 μm coatings meeting this ASTM standard are available if required.)

An advantage of mechanical deposition is that it does not produce hydrogen embrittlement in hardened steel during the coating process.

Type II and Type III finishes for zinc and cadmium receive supplementary colored chromate treatments. These supplementary treatments produce a bright or semi-bright continuous, protective conversion coating of uniform color which retards the formation of white corrosion products caused by exposure to stagnant water, moist atmosphere or stagnant environments containing organic vapors. Colors produced can range from yellow through bronze and olive-drab to brown and black. The salt spray test used to evaluate these treatments shall be conducted in accordance with ASTM B 201.

The performance of both zinc and cadmium coatings depends largely on their coating thickness and the kind of environment to which they are exposed. Without proof of satisfactory correlation, accelerated tests such as the salt

other environments, nor will the tests serve as comparative measures of the corrosion protection afforded by the two different metals. Thus the superiority shown by cadmium coatings over zinc coatings of equal thickness in the standard salt spray test cannot be construed as proof that this will hold true in all atmospheric environments.

The following specification is recommended for nickel or chrome plated fasteners: Nickel or chromium plated fasteners shall meet the requirements of ASTM B 456.

Zinc coatings may also be applied by the hot-dip process (Galvanizing). For such coatings the following specifications are recommended:

Zinc coating applied by the hot-dip process shall meet the requirements of ASTM A 153. For Class C hardware, which includes threaded fasteners over 9 mm (3/8 in) in diameter, minimum weight of coating on surface, 40 mg/cm2 (1.25 oz/ft2) For Class D hardware, which includes threaded fasteners 9 mm (3/8 in) and under in diameter, minimum weight of coating on surface, 30 mg/cm2 (1.00 oz/ft2).

B ased on mathematical calculations, 30 mg/cm2 (1.00 oz/ft2) corresponds to an average thickness of 0.04 mm (1.7 mil).

Hydrogen Embrittlement is a condition of low ductility in metals resulting from the absorption of hydrogen during the manufacturing process. High strength bolts and screws in the 1.03 GPa (150 ksi) range and higher are particularly subject to embrittlement if hydrogen is permitted to remain in the steel and the steel is subjected to high tensile stress. The hazard caused by hydrogen embrittlement is the unpredictable failure which may occur to a fastener under high tensile load. Results of such failure could be disastrous.

Acid pickling and alkaline cleaning prior to the application of protective metallic coatings generate hydrogen which can be absorbed in the fasteners and if not removed can be trapped by the coatings. Also, hydrogen as a by-product of electroplating can be generated and trapped in the plating.

The mechanism of hydrogen embrittlement failure is believed to be due to the migration of hydrogen into microscopic cracks when a sufficient load is applied to a fastener. This causes internal pressures and microscopic ruptures in the stressed areas. This action continues under repeated or constant high tension loads and eventually leads to a failure of the fastener. Hydrogen embrittlement is non-corrosion related and is often mistaken as the cause of failure when a corrosion process is active and the true cause of failure is hydrogen assisted stress corrosion cracking. For hot-dip galvanized steel fasteners, hydrogen can be absorbed during the pickling process. Heating to 150°C (300°F) after pickling and before galvanizing, in most cases, results in expulsion of the hydrogen absorbed during pickling. Reference may be made to ASTM A 143 for more information on the subject of embrittlement of hot dip galvanized structural steel products. In practice, hydrogen embrittlement of galvanized steel is usually of concern only if the steel exceeds approximately 1.03 GPa (150 ksi) in ultimate tensile strength. ASTM provides specifications for galvanizing A 325 bolts but galvanizing of A 490 bolts is not permitted.

Stress Corrosion is the effect of corrosion on a metal which is under stress. When metals are under stress the effect of corrosion can be much more severe than when metals are not stressed. This is true for metals subjected to constant high tension stresses as well as metals subjected to cycling stresses which cause fatigue. Stress corrosion failures can occur shortly after the load is applied but may not occur for months or years later. Such failures occur without warning. It is believed that when corrosion occurs microscopic cracks develop in the high stress areas. The combined effects of stress and corrosion cause the crack to grow inwardly which reduces the cross-sectional area. Eventually, when the cross-sectional area can no longer support the load, the fastener breaks. The rate of failure depends on the level of stress, the corrosive conditions and the metallurgical properties of the fasteners. Hydrogen Assisted Stress Corrosion is similar to stress corrosion. It takes place when stress corrosion cracking is accelerated by the presence of hydrogen which is generated in a service application by a galvanic couple, for example, between aluminum and iron, in the presence of water. Even fasteners which might resist stress corrosion cracking alone can fail if service generated hydrogen is diffused into the surface of the fastener.

Stress Embrittlement is similar to hydrogen embrittlement and like hydrogen embrittlement it is non-corrosion related. Hydrogen generated through the service environment, not in manufacture, causes stress embrittlement. For example, hydrogen can be absorbed into the surface of an uncoated fastener when caustic substances, such as soap and detergents, come in contact with nitrates and silicates. Metals most susceptible to stress embrittlement are steels heat-treated to high strength levels and with high carbon content.

In carbon steel fasteners, the higher the hardness, the greater the chance of stress corrosion, hydrogen embrittlement and stress embrittlement.

This review of hydrogen embrittlement, stress corrosion, hydrogen assisted stress corrosion and stress embrittlement has been presented to point out how dangerous failures may occur in high strength steel

strengths of 1.03 GPa (150 ksi) and greater are most susceptible. Reliable fasteners depend on carefully controlled manufacturing processes which reduce to a minimum the chance of hydrogen embrittlement. Designs for curtain wall anchoring systems must take into account the stresses for which fasteners must be selected and the coatings to be employed in order to eliminate problems due to galvanic action and stress corrosion. ASTM standards and technical literature of reputable manufacturers provide valuable information on these subjects.

Other significant factors, described in the following paragraphs, must be taken into consideration when galvanized high-strength bolts and nuts are to be used. Reduction of Mechanical Properties. The heat treatment temperatures for certain types of high-strength bolts, Type 2 A 325 for example, is in the range of the molten zinc temperatures for hot-dip galvanizing, and, therefore, there is a potential for diminishing the heat treated mechanical properties by the galvanizing process. For this reason, AISC Specifications require that such fasteners be tension tested after galvanizing to check the mechanical properties.

Nut Stripping Strength. Hot-dip galvanizing affects the stripping strength of the nut/bolt assembly because to accommodate the relatively thick zinc coating on bolt threads it is usual practice to tap the nut oversize. This overtapping results in a reduction in the amount of engagement between the steel portions of the male and female threads with a consequent approximately 25% reduction in stripping strength. Only the stronger hardened nuts have adequate strength to meet specification requirements with the reduction due to overtapping.

Torque Involved in Tightening. Hot-dip galvanizing both increases the friction between the bolt and nut threads and also makes the torque induced tension much more variable. Lower torque and more consistent results are provided if the nuts are lubricated. Refer to ASTM A 325 for specifications and ASTM A 563 for testing requirements.

Shipping Requirements. Galvanized bolts and nuts are to be treated as assemblies and shipped together. Purchase of galvanized bolts and galvanized nuts from separate sources is not recommended because the amount of over-tapping appropriate for the bolt and the testing and application of lubricant would cease to be under the control of a single supplier. In that case the responsibility for proper performance of the nut/bolt assembly would become obscure.

5.0 PREVENTION OF FASTENER LOOSENING

There are many devices designed to keep the fasteners commonly used in curtain wall framing from loosening or turning out due to thermal movements, building movements, wind forces or vibration. Those commonly used are the various types of lock washers including pyramidal, internal tooth, external tooth, helical spring, serrated flanges and SEMS assemblies. Also used, to a lesser degree, are locking devices such as nylon patches, plastic screw inserts, rolled threads and dissimilar numbers of threads per inch for fasteners and their nuts or tapped holes. These devices can effectively prevent loosening of fasteners due to building movements and vibration induced by wind or other causes. Appropriate devices should be selected for the specific applications in which they will be used.

Another important criterion for choosing a locking device is its torque limiting ability. Where fasteners are used in extruded aluminum screw chases there is a tendency for the threads in the aluminum to strip if too much torque is applied to the steel fastener. However, if a lock washer is used, especially a toothed lock washer, the friction between the steel washer teeth and the softer aluminum surface is usually great enough to cause the fastener to tighten before stripping of the aluminum chase occurs. If a torque specification is given for a particular fastener application, it is important that the specification be followed to prevent stripping.

Not all fasteners in a framing system require locking devices to resist vibration or torque limiting devices. Generally those fasteners which would be considered main structural fasteners or anchors in curtain wall applications, and those which attach moving parts to the framing require the consideration of these types of devices. Fasteners which hold shear blocks in place, perimeter fasteners for windows and storefronts and those which hold light trim in place do not require locking or torque limiting devices.

The sources of fastener vibration are basically two, wind and machinery. Vibrations induced by changes in wind pressure tend to be of low amplitude and rather long cycle times. Vibrations induced by machinery will tend to be of greater amplitude and of much higher frequency. Most curtain wall framing applications do not encounter vibration sources other than those induced by the wind. Machinery induced vibrations, though of infrequent occurrence, are serious in nature and should be carefully analyzed. It will be assumed that only wind induced vibrations occur in the framing connections described herein.

6.0 FASTENER LOAD TABLES COMMENTARY

Fastener Load Tables provide data for evaluating the loaded performance of various size fasteners and fastener metals. The performance of the metals being fastened must be determined separately. The values given are for quality fasteners in round clearance holes or tapped holes as noted. When specifying fasteners, the designer, in addition to specifying loaded performance, must specify fastener quality, corrosion resistance and minimum mechanical properties. Specification of these items is usually done by appropriate reference to ASTM or other recognized standards.

The two general types of fasteners described

in this report have either machine threads or spaced threads. Thread angle of both types

of threads is 60 degrees. Machine threaded fasteners have threads which are closely spaced in accordance with the diameter/pitch combinations of the Unified Coarse Thread Series (UNC), as shown in Figures 1 (external threads), 4 (external threads) and 5 (internal threads). The form

of Unified Threads is specified in ANSI/ASME B1.1, Unified Inch Screws Threads (UN and UNR Forms). Fasteners with spaced threads, as shown in Figure 2, have an expanded thread pitch which results

in the spaced threaded fastener having fewer threads per inch than a fastener with machine threads of the same diameter.

As mentioned in the Introduction, Unified Coarse Machine Threaded Fasteners are generally used in either clear holes with mating nuts or in tapped holes. Thread cutting screws with machine threads are used to cut their own threads in pre-drilled holes. These screws carry tensile and shear loads. Spaced threaded fasteners are generally used only as tapping screws. Most thread forming screws and some thread cutting screws have spaced threads. Like fasteners with machined threads, these fasteners carry tensile and shear loads. However, due to the small number of threads per inch, spaced threaded fasteners have smaller effective tensile and shear areas than machine threaded fasteners of the same nominal diameter. Also, less thread engagement means that a spaced threaded fastener will have lower pullout resistance threads. However, this is not always true for thin materials. To provide conservative values, the tensile and shear loads for fasteners with spaced threads are based on a minimum cross-sectional area. This area is found by using the minimum minor diameter and neglects any additional strength provided by the threads.

The minimum material thickness to equal the allowable tension for spaced thread fasteners as shown in Tables 23 through 31 is based on the TSA (I) and N for Unified Coarse Threads from Table 4. Based on

a series of pullout strength tests made with 8-18, 10-16, 12-14 and 1/4-

14 screws in different thicknesses of aluminum alloy 6063-T5, it has been determined that the values of pullout or material which appear in the tables are conservative.

Thorough testing has shown that fasteners with Unified Threads fail in tension at loads corresponding to those of unthreaded parts with diameters approximately midway between their pitch diameters and minor diameters. The area determined by this diameter is known as the Tensile Stress Area. It is calculated by the following equation: EQUATION 1

For tension connections in tapped holes, be sure that the fastener length is sufficient to ensure full thread engagement.

The geometric shear area for fasteners with Unified Threads is equal to the area of a circle with a diameter equal to the basic minor diameter of the external thread. This area, known as the Thread Root Area, is calculated by Equation 2. Refer to the Appendix for a derivation of this equation.

EQUATION 2

The tensile stress area as calculated by Equation 1 is used to determine the allowable tension loads for Unified Coarse Thread Steel Screws below 15 mm (5/8 in) in diameter and for stainless steel screws sizes 6-32 through 1 inch.

The Thread Root Area as calculated by Equation 2 is used to determine the allowable shear loads for Unified Coarse Thread Steel Screws below 15 mm (5/8 in) in diameter and for stainless steel screws sizes 6-32 through 1 inch.

For steel screws 15 mm (5/8 in) through 25 mm (1 in) nominal diameters the AISC recommendations are followed. These recommendations call for a tensile stress area and shear stress area based on the nominal diameter as shown in Equation 3.

1.2269 2

Thread Root Area = A(R) = 0.7854 D -

sq. in.

0.9743 2

Tensile Stress Area = A(S) = 0.7854 D -

sq. in. Tensile Stress Area = Shear Stress Area = 0.7854D2 in2

Inasmuch as the use of 0.6F y appears overly conservative for fastener metals with low yield-to-ultimate ratios, and not sufficiently conservative for fasteners with high yield-to-ultimate ratios, the 0.6F y limit has been replaced by the lesser of

in the computation of the tensile and shear allowable stresses given in the load tables for steel fasteners. These limits produce tensile and shear values which are less than those based on the AISI Stainless Steel Cold-Formed Design Manual (1974) and less than values that would be obtained from the AISC provisions for larger bolts. The 2.5 factor of safety on ultimate is consistent with the 1986 AISI Cold-Formed Steel Specification value and exceeds the 2.34 factor of safety currently used for aluminum bolts. An exception to this is made for the 15 mm to 25 mm (5/8 in to 1 in) steel screws in Tables 5 and 6, and for the 12 mm to 25 mm (1/2 in to 1 in) steel screws in Tables 7 and 8. For these screws the AISC values for allowable tensile and allowable shear stresses are used. Using the nominal area, the allowable tensile stress F t = 0.33F u and the allowable shear stress F v = 0.17F u . Tables are based on threads in the shear plane.

Fasteners subjected to combined tensile and shear loads are limited by the following interaction equation:

EQUATION 4

This equation applies to all fasteners regardless of size.

The allowable bearing area for fasteners is the nominal diameter multiplied by the length in bearing, except that for countersunk bolts and screws one half the depth of the countersink is deducted from the length.

Allowable bearing for steel in standard round or short-slotted holes for two or more fasteners is determined by Equation 5. Values calculated from this equation are used in the load tables. For allowable bearing for single fasteners use Equation 9 on page 10.

EQUATION 5

The bearing strength of steel fasteners in slotted holes perpendicular to the walls should be reduced below the bearing strength in round holes in accordance with the requirements given in the section on 'Allowable B earing at B olt Holes,' subsections 'Minimum Spacing for Steel' and 'Minimum Edge Distance for Steel', Page 10.

Allowable bearing for aluminum in standard round or short-slotted holes is determined by Equation 6.

F by is the minimum bearing yield strength for the aluminum alloy. 1.65 is the safety factor. Allowable bearing stresses for aluminum may be found in Table 5.1.1a in The Aluminum Association's, "Specifications for Aluminum Structures." The bearing strength of aluminum for fasteners in slotted holes perpendicular to the walls should be reduced to 2/3 of the bearing strength in round holes. Refer to Page 11 for information on bolt spacing and edge distance.

Also included in the load tables for fasteners with Unified Threads are values for minimum material thickness to equal the tensile capacity of the fastener. Reference should be made to sample calculations on page 19 and to Table 4 for the equations and methods of calculation used to determine these thicknesses.

The pullout resistance of self-tapping screws in screw slots should be reduced to 3/4 of the pullout resistance in tapped round holes. The Aluminum Association provides some guidelines for the design of screw slots. These are shown on Figure 3, page 13. However, these no longer represent industry standards inasmuch as manufacturers with knowledge and experience in extrusion design generally design their own screw slots to meet the needs of the application. The length of a screw used in a screw slot is found to be 4/3 of the chart value for round tapped holes plus 6 mm (1/4 in) plus the thickness of the material the screw passes through before entering the screw slot. The shear strength of a fastener in a screw slot should be equivalent to that in a round hole provided that it is not in the direction of the opening of the screw slot.

Fasteners which are used in screw chases, that is, between two parallel walls with either extruded or tapped screw threads as shown in Figures 6a and 6b, must be carefully evaluated. In general, bearing strength against the extruded side walls may be assumed to be equivalent to that for a fastener in a slotted hole. The pullout strength of the fastener in a screw chase would be the pullout strength of the fastener in a tapped hole multiplied by the percent engagement in the screw chase as given in Equation 14. The shear strength of a fastener in a screw chase should be considered to be equal to that of a fastener in a round hole if the direction of load application is perpendicular to the walls of the screw chase. The shear strength of a fastener in a screw chase parallel to the walls of the chase is reduced by the shear factor given in Equation 13. In all cases, consideration should be given to the use of a safety factor which accounts for mechanical property variations, extrusion tolerances and fabrication tolerances which may run to the extreme end of their range.

The allowable stresses in the load tables have not been increased by 1/3 for wind loads in calculating the values for the loads shown. The decision on whether or not to use the higher loads is left to the discretion of the responsible engineer.

Allowable Bearing = 1.2(F u ) (D) (t) lbf f v 2 f t F v +

F t

F by

Allowable Bearing = (D) (t) lbf

F t = 0.75F y or F t = 0.40F u , and

F Y F u F v = 0.75 √3or F v = 0.40 √3

H - 0.866 P

FAS T EN ER C /L

0.5P

P -I /N

H /8

H /6

MAJO R D IAME TER /2PIT C H DIA MET ER /2

MIN O R D IA METE R /2

N - T H R E AD S/IN C H 60°

FIGURE 1: Unified Coarse Threads, External

H /8

N - T H R E A D S /IN C H

P ITC H D IAME TE R /2

MIN O R D IAME TE R /2

P -I /N

FA S TE N ER C /L

M A JO R D IA M E T ER /2

60°

FIGURE 2: Spaced Threads, External

7.0 ALLOWABLE BEARING AT BOLT HOLES

Minimum Spacing and Minimum Edge Distance

Allowable Bearing at Bolt Holes for Steel

On the projected area of fasteners in shear connections with the end distance in the line of force not less than 1.5 nominal diameter, D, and the distance center to center of fasteners not less than 3D:

In standard or short-slotted holes with two or more fasteners in the line of force:

F b = 1.2F u

Where: F b = Allowable Bearing Stress

F u = Ultimate Tensile Stress

EQUATION 7

In long-slotted holes with the axis of the slot perpendicular to the direction of the load and with two or more fasteners in the line of force:

F b = 1.0F u

EQUATION 8

On the projected area of the fastener closest to the edge in standard or short-slotted holes with the edge distance less than 1.5D and in all connections with a single fastener in the line of force:

EQUATION 9

Where L e = Distance from the free edge to the centerline of the fastener.

If 1.2F u is the desired bearing stress for a single fastener in the line of force, then L e must not be less than 2.4D. Minimum Spacing for Steel

Along a line of transmitted forces, the distance between centers of standard holes shall be not less than 3D when F b is determined by Equations 7 or 8.

Minimum Edge Distance for Steel

Along a line of transmitted force, with two or more fasteners in the direction of the force, the distance from the center of a standard hole to the edge of the connected part shall be not less than 1.5D when F b is determined by Equations 7 or 8.

Where deformation around a hole is not a design consideration, where oversized and slotted holes are involved, and where conditions differ from those described in the foregoing paragraphs, reference should be made to Part 5, Pages 5-74 and 5-75 of the Ninth Edition, "AISC Manual of Steel Construction; Allowable Stress Design," for the procedures to be followed in determining the allowable bearing stress, minimum Allowable Bearing at Bolt Holes for Aluminum

For Standard round holes:

Where: F b = Allowable

earing

Stress

F by = Minimum Bearing Yield Stress

1.65

Safety

Factor EQUATION 10

This value shall be used for a ratio of edge distance to

hole diameter of 2 or greater. For smaller ratios this allowable stress shall be multiplied by the ratio:

Edge distance is the distance from the center of the fastener to the edge of the material in the direction of the applied load. Edge distance shall not be less than 1.5D.

For slotted holes:

EQUATION 11

This allowable stress is equal to 2/3 of the allowable bearing stress on fasteners in standard round holes.

Minimum Spacing for Aluminum

Minimum distance between bolt centers shall be 2-1/2 times the nominal bolt diameter.

Minimum Edge Distance for Aluminum

The distance from the center of the bolt to the edge of the sheet or shape toward which the pressure is directed shall be twice the nominal diameter of the bolt when stress is computed by Equation 10 or 11.

For further information on bearing loads, spacing and edge distances reference should be made to Section 5, 'Mechanical Connections,' of The Aluminum Association's Publication #30, "Specifications for Aluminum Structures," Fifth Edition. Allowable bearing stresses for a number of aluminum alloys will be found in Table 5.1.1a of this section.

F by

F b =

1.65

F by

F b =

2.48

Edge Distance

2 (Hole Diameter)

F u

F b = (L e)

F u

8.0 STANDARD AND SLOTTED BOLT HOLES

It is recommended that holes for bolts not exceed the sizes specified in Table 1 for friction connections. Slots longer than these dimensions may be used for expansion or anchor alignment purposes with appropriate engineering analysis or testing.

MAXIMUM SIZE OF BOLT HOLES, INCHES

Nominal Bolt Diameter, d in. Standard Hole Diameter, d in. Oversized Hole Diameter, d in. Short-Slotted Hole

Dimensions in.

Long-Slotted Hole Dimensions in.

1 1 1 1 1 1 <1/

2 d + 32 d +

16 d + 32

by d + 4 d +32 by 2 - 2 2

1 1 1 1 1 1 d + 16 d +

8 d + 16

by d + 4 d +16 by 2 - 2 2

TABLE 1

Standard holes shall be used in bolted connections, except that oversized and slotted holes may be used as approved by the designer. The length of slotted holes shall be normal to the direction of the shear load. Washers or back-up plates shall be installed over oversized or short-slotted holes in an outer ply unless suitable performance is demonstrated by load tests in accordance with Section F of AISI specification entitled, "Design of Cold-Formed Steel Structural Members," August 19, 1986 Edition.

9.0 PULLOUT STRENGTH

Pullout strength must be sufficient to resist the allowable tension of the fastener used in a tension connection. It depends on the metal alloy being fastened, the allowable shear stress of the metal, the fastener size and number of threads per inch (N), the internal thread stripping area (TSA(I)), and the length of fastener engagement (LE). Refer to Table 4 for dimensional information on Unified Coarse Threads. The length of engagement necessary to conservatively provide this pullout strength is given by the following equation:

EQUATION 12

F su is the shear ultimate strength for aluminum. Values for F su are given in Table 3.3.1a of The Aluminum Association's Manual, "Specifications for Aluminum Structures."

The length of engagement calculated by this equation also equals the minimum thickness (MT) required for adequate pullout resistance. 1/N has been added to increase the thickness in order to compensate for the fact that threads at each side of the plate or sheet being fastened diminish from full cross-sectional area (a thread's length in from the plate or sheet surface) to zero area at the surface. This added thickness also helps to resist local cupping or bending at the fastener location. The values given for minimum thickness in the load tables for both Unified Coarse Threads and spaced threads are calculated from Equation 12. In the case of spaced threads, however, the allowable tension is calculated from the minor diameter area but the number of threads used in the calculation is the same as that used for Unified Threads. This procedure gives values which by test have been shown to be conservative. Values less than those shown in the tables should not be used without engineering calculations and/or tests which demonstrate their acceptability.

The effectiveness of the length of engagement beyond the minimum required diminishes as the length increases. Little, if any, added strength is gained by exceeding a length equal to twice the nominal thread diameter. For spaced threads the effective length of fasteners with tapered points begins at the point of full diameter threads.

Allowable Tension 1

LE = (Allowable Shear Stress) TSA (I) (N) +

0.4F u 0.75F y

Allowable Shear Stress for Steel = √3 or √3

, whichever is less

Allowable Shear Stress for Aluminum = 0.4F su

10.0 SCREW SLOTS

From The Aluminum Association's, "Drafting Standards for Aluminum Extruded and Tubular Products"

B **1/2 A

D ia. A

W all 'T '

R m in - 1/2 A

R n om - 1/2 W all 'T '

70° Ma x.

62° N o m.

N om W a ll - 1.3 m m (0.050 in ) or 12 W all 'T ' w hich eve r

is g re ate r.

TY PIC A L LO C AT IO N AW AY FR O M C O R N ER

TY PIC A L C O RN E R LO C ATIO N

TY PE F SELF-TA P PING S C R E W

SELF TAPPING, SCREW TYPE F

SCREW O D Inches

DIAM ETER Inches

4-40*6-32*8-3210-2412-24

14 x 20

*N ot recom m ended for incorporation on inside w all of ho llow or sem i-hollow shapes.

**The reco m m e nd e d loca tion for scre w slo ts o n th e in sid e ho llow o r sem i-h ollo w sha pe s is at the corne rs. W h e n not located at corners, D im ension "B " m ust be at least 6 m m (1/4 in).

4-48*6-40*8-3610-3212-28

4 x 28

0.1120.1380.1640.1900.2160.250

0.099 ± 0.006 0.120 ± 0.0060.147 ± 0.0070.169 ± 0.0070.190 ± 0.0070.228 ± 0.007

11.0 SLIDING FRICTION IN SCREW CHASE

EQUATION 13

c = 1/2 the angle between the faces of a thread, degrees F = Tensile force exerte

d by tightening screw, in screw

chase, lb.

f = Coefficient of friction. For mild steel on aluminum f

= 0.47.

P = Pitch of screw, in.1/N = Pitch, in. R = Major radius of screw thread, in. r = Minor radius of screw thread, in.

R e = Ratio of area of screw thread engagement in screw

chase from Equation 14.

r m = Mean radius of screw thread, in.

= Ultimate lateral frictional resistance to sliding of a screw in a screw chase parallel to walls of chase, lb. Shear factor for determining resistance of screw in screw

chase parallel to walls of chase, lb. T = Torque on screw or bolt = Lrm, lb-in. Equation 13 is based on external threads as shown in Figure 4. The ultimate lateral frictional resistance to sliding as given by this equation is used to determine the shear strength of a screw in a screw chase when loaded parallel to the walls of the screw chase. Equation 13 is expressed in terms of the torque; the major, mean and minor radii of the screw; the pitch of the screw; and the coefficient of friction between the fastener metal and the aluminum extrusion. To determine an allowable design value, divide SF by a suitable safety factor. A safety factor of 2.34 is recommended. For derivation of Equation 13, see the Appendix. Figure 4 conforms to ANSI/ASME B1.1

17H/24

H /6

P /2

H /8

5H /8

H /8

P /2

H /4

FIGURE 4: External Threads FIGURE 5: Internal Threads

F(P±[2 r m f sec c])

SF =

(2 r m ±[Pf sec c])

T ￥[(24) (R-r)]2 + (8.5P)2

R e r m

P±(2 r m f) (24) (R-r) ￥[(24) (R-r)]2 + (8.5P)2

SF =

(2 r m )±(Pf)

(24) (R-r)

50 ￥(0.539) + (0.181) (0.325) 0.110 (0.05) + (0.325) 0.734 ￥(0.539) + (0.181) SF = (0.691) - (0.0235) 0.734

(147.7)(0.4257) (0.664) = 94.7 lbs 94.7

Design Value = 2.34

= 40.5 lbs

Sample Calculation for 1/4-20 Screw:

R = 0.125

in r = 0.0944 in r m = 0.110 in P = 0.05 in T = 50 lb-in f =

0.47

2 π r m = 2 π (0.110) = 0.691 2 π r m f = (0.691 (0.47) = 0.325 Pf = (0.05) (0.47) = 0.0235 24 (R – r) = 24 (0.125 – 0.0944) = 0.734

[24 (R – r)]2 = (0.734)2 = 0.539 (8.5P)2 = [(8.5) (0.05)]2 = 0.181

R e = 0.325 from sample calculation for Equation 14.

Substituting foregoing values in Equation 13 to find the ultimate lateral frictional resistance:

To determine the design value, divide SF by the recommended 2.34 safety factor.

a A e

R 2

180 - sin a

R e =

At = (R 2 – r 2)

2cos -1

R r 180 -sin 2cos -1 R R e =

R 2

(R 2 – r 2)

12.0 SCREW ENGAGEMENT IN SCREW CHASE

A e /2

l r

R l

FIGURE 6a

EQUATION 14 (See Appendix for derivation of Equation 14.)

Sample calculation for 1/4 - 20 screw:

R = 0.125 in, r = 0.0944 in

r 0.0944 a = 2cos -1

R =

2cos -1 0.125

= 2cos -1 (0.755)

= 81.9 degrees

[81.9]

180 -sin[81.9]

R e = R 2

(R 2 – r 2)

1.429 – 0.990

= (0.125)2 [(0.125)2 – (0.0944)2

]

0.439

= 0.0156 0.0211

= 0.325

or 32.5% thread engagement

= Angle defining limits of screw engagement, in screw chase, degrees

A e = Total area of screw thread engagement in screw chase, sq. in.

At = Thread area of fastener = π (R 2

– r 2

) sq. in. R = Major radius of screw thread, in. r

= Minor radius of screw thread, in.

R e = Ratio of engaged thread area to total thread area in screw chase = A e /At 2l = Length of engagement, in.

13.0 FASTENER SPECIFICATION CHECK LIST

A. MECHANICAL PROPERTIES

1. Description (including drawing)

a. Size

b. Length

c. Head Style

d. Thread Type

e. Point Type

f. Special Features (i.e., undercut head)

g. Other

2. Metal

3. Minimum Yield Strength

4. Minimum Tensile Strength

5. Hardness

6. Other

B. FINISH

1. Clear or Natural

2. Colored

a. Painted

b. Burned

3. Other C. CORROSION PROTECTION

1. As Fabricated

2. Plated

(Refer to appropriate ASTM Standards. See page 3.)

a. Zinc

b. Cadmium

c. Nickel

d. Chromium

3. Black Oxide

4. Waxed

5. Other

D. Fastener Exposure

1. Outside Face of Building

2. Inside Exterior Cover But High Exposure

3. Inside Glazing Pocket

4. Behind Inner Seal Line

5. Visible Inside Building

14.0 FASTENER AVAILABILITY

This report contains load tables for fourteen different sizes of fasteners manufactured from a number of different carbon steel and stainless steel alloys. Types of fasteners included are Unified Coarse thread machine screws and bolts, and spaced thread self-tapping screws. Metric fasteners are not included in this standard. Recommended specifications for protective metallic coatings for carbon steel fasteners cover zinc, cadmium, nickel and chromium. The stainless steel alloys included have a range of corrosion resistant properties. The number of choices available to the designer, therefore, is staggeringly large.

Obviously, it is economically impossible for a fastener manufacturer or supplier to make available in stock all of the fastener types and sizes in all of the different alloys with all of the different protective coatings which may be specified. As pointed out in the, "Protection Against Corrosion," section of this report, many types of stainless steel fasteners are readily available only in alloys having lower resistance to corrosion than Type 316. SAE Grade 2 and Grade 5 carbon steel fasteners, while generally available in 6 mm (1/4 in) diameter and larger sizes, may not be readily available in screws less than 6 mm (1/4 in) diameter. On the other hand, structurally equivalent fasteners for the smaller screws made from commercial grades of steel wire are readily available. Such items as the type of threads, heads, points and lot size will further influence availability.

Fasteners which are commonly used are generally available from stock and can be reasonably purchased in small quantity orders. Fasteners are also available on a custom order basis. However, a custom order will usually require a large quantity of fasteners if a reasonable price is to be realized. Often the cost of fasteners in small quantity, custom orders could be so great as to economically rule out their use.

The designer of curtain wall systems must recognize these limitations in availability and make acceptable compromises in the selection of fasteners which will assure structural adequacy, effective resistance to corrosive actions, satisfactory over-all performance, and a cost which will not adversely affect the economic viability of the wall system.

Reference may be made to the Industrial Fasteners Institute publication, "Manufacturer's Capability Guide," for finding sources for fasteners and accessories. This guide contains a large list of fastener manufacturers, gives data on product capability, size range, and material. Materials include carbon and alloy steels, stainless steel and non-ferrous metals.

15.0 SAMPLE CALCULATIONS FOR LOAD TABLES

Stainless Steel Fastener, Alloy Groups 1, 2 and 3, Condition AF, 1/4-20 Screw

Nominal Thread Diameter Threads Per Inch D = 0.250 in N = 20

Minimum Ultimate Tensile Strength Minimum Tensile Yield Strength F u = 85,000 psi 0.40 F u = 34,000 psi

F y = 50,000 psi

0.75 F y = 37,500 psi 0.40 F u is the lesser allowable tensile stress and is, therefore, used to calculate allowable loads shown in the load tables. TABLE 9

Allowable Tension = 0.40F u [A (S)] lb = 0.40 (85,000) (0.0318) = 1,081 lbs

Allowable Shear (Double) = 2(550) = 1,100 lbs

Allowable Bearing 3 mm (1/8 in) A36 Steel = 1.2(F u )(D)(0.125) lbs (F u = 58,000 psi) = 1.2(58,000)(0.25)(0.125) = 2,175 lbs

NOTE: Limitations on minimum spacing and minimum edge distance on page 10.

Allowable Bearing 3 mm (1/8 in) 6063-T5 Aluminum = (F b )(D)(0.125)lbs (F b = 16,000psi)

= (16,000)(0.25)(0.125) = 500 lbs

NOTE: Limitations on minimum spacing and minimum edge distance on page 10.

Allowable Bearing 3 mm (1/8 in) 6063-T6 Aluminum = (F b )(D)(0.125)lbs. (F b = 24,000psi)

= (24,000)(0.25)(0.125) = 750 lbs. 0.9743 2

A(S) = Tensile Stress Area = 0.7854 D -

0.9743 2

= 0.7854

0.25 -

= 0.0318 sq. in.

0.40F u [A(R)]

Allowable Shear (Single) =

√3

0.40(85,000)(0.0280)

√3

= 550 lbs.

1.2269 2

A(R) = Thread Root Area = 0.7854 D -

1.2269 2

= 0.7854

0.25 -

= 0.0280 sq. in.

Thread Stripping Area, square inch per thread, Internal, TSA (I):

TSA(I) =

3.1416 (DSMIN)

+ 0.57735 (DSMIN – ENMAX) sq. in./thread

1 =

3.1416 (0.241)

+ 0.57735 (0.241 – 0.222) 0.027 sq. in./thread

Where: DSMIN = Minimum major diameter of external threads (See Table 4)

ENMAX = Maximum pitch diameter of internal threads (See Table 4)

Allowable Tension 1

Minimum Thickness, MT = [Allowable Shear Stress] [TSA (I)] [N] + N

in.

Allowable Tension 1

MT A36 Steel = 1

+ N (0.4) (58,000) √3

[TSA (I)] [N]

1081 1

= 1

20 (0.4) (58,000) √3 [0.027] [20]

= 0.199 in.

Allowable Tension 1

*MT6063-T5 Aluminum = [(0.4)(13,000)] [TSA (I)] [N] +

1081 1 =

[(0.4) (13,000)] [0.027] [20]

+ 20

= 0.435 in.

Allowable Tension 1

*MT6063-T6 Aluminum = [(0.4)(19,000)] [TSA (I)] [N] +

1081 1 =

[(0.4) (19,000)] [0.027] [20]

+ 20

= 0.313 in.

*NOTE: 90 MPa (13,000 psi) is the shear ultimate strength for 6063-T5 aluminum for thicknesses less than 12 mm (1/2 in). For thicknesses over 12 mm (1/2 in) use 83 MPa (12,000 psi). 131 MPa (19,000 psi) is the shear ultimate strength for 6063-T6 aluminum for all thicknesses.