The Aluminum Design Manual (ADM), published by the Aluminum Association, is updated every five years, and the 2015 ADM is now available. Part I of the ADM is the Specification for Aluminum Structures (SAS), which provides rules for determining the strength of aluminum structural components and is the primary component of the manual. Since compliance with the specification is required by the International Building Code, which is used throughout the United States, changes to the specification affect most building applications of aluminum in the U.S. Furthermore, its provisions are used by other groups such as the American Welding Society and the American Association of State Highway and Transportation Officials in their standards.
The SAS was completely reorganized in 2010 for the first time since its initial 1967 publication. The 2015 SAS retains the same organization as the 2010 specification, paralleling the 2010 AISC Specification for Structural Steel Buildings. However, numerous changes in the specifications provisions were made for its 2015 edition, and this article highlights significant changes.
Shear Ultimate Strength and Compressive Yield Strength The shear ultimate strengths Fsu and compressive yield strengths Fcy are no longer listed by alloy-temper. Instead, Table A.3.1 states that the shear ultimate strength is 60 percent of the tensile ultimate strength and the compressive yield strength is 90 percent of the tensile yield strength for unwelded H tempers and 100 percent of the tensile yield strength for others.
Modulus of Elasticity The modulus of elasticity is no longer tabulated by alloy, but rather is set in Table A.3.1 as 10,100 ksi for all alloys addressed by the SAS. Similarly, the shear modulus is 3,800 ksi for all alloys.
Photo: TGB Partnership
Wrought Product Strengths and Specifications A single table (Table A.3.3) now shows the ASTM specification, thickness, unwelded and welded tensile yield strengths (Fty, Ftyw), unwelded and welded tensile ultimate strengths (Ftu, Ftuw), and tension coefficient kt for each alloy-temper-product addressed by the specification. 1060-H12 and -H14 sheet, plate, and drawn tube and 2219-T87 plate were added to the specification.
Weld Filler Strengths Weld filler strengths were moved from Section J.2 to Section A.3. Only tensile strengths are now provided, rather than tensile and shear strengths shown in the 2010 specification. (Shear strengths are now taken as 0.6 times the tensile strength.) The strength for 4047 filler was added.
Alternate Method for Determining Flexural Strength The alternate method for determining flexural strength, now called the Direct Strength Method, was moved to Section B.5.5.5, and modified so the strength of compact sections is the plastic moment and the inelastic local buckling strength transitions from the plastic moment to the post-buckling strength.
Effective Net Area for Members in Axial Tension Section D.3.2 was changed so that the effective net area is no less than the net area of the connected elements.
Member Buckling Strength for Axial Compression Section E.2 was changed to transition the inelastic buckling strength from the compressive yield strength to the elastic buckling strength. The 2010 and 2015 buckling strengths are compared in Figure 1.
Yield and Rupture Strengths for Flexure The yield limit state strength (plastic moment) for wrought products was revised to the plastic section modulus times the compressive yield strength (ZFcy), with the plastic modulus limited to 1.5 times the section modulus. The rupture limit state moment (ultimate moment) was revised to the plastic section modulus times the tensile ultimate strength (ZFtu/kt), with the plastic modulus limited to 1.5. This is a change from earlier specification editions, which used only part of the plastic modulus and were limited to specific cross-sectional shapes.
Lateral-Torsional Buckling (LTB) Strength LTB strengths were revised so that the inelastic buckling strength is the plastic moment when the unbraced length is zero (Figure 2). The LTB strength of shapes symmetric about the bending axis was revised for better accuracy by explicitly including the warping constant (Cw) term rather than approximating it. The LTB strength of singly symmetric shapes unsymmetric about the bending axis and with the moment of inertia of the compression flange greater than the moment of inertia of the tension flange (Iyc > Iyt) can no longer be taken as the LTB strength of the shape as though both flanges were the same as the compression flange. (This practice, while allowed by previous specification editions, is unconservative.) Finally, the LTB strength expression was written so that the LTB strength can be determined from the elastic lateral-torsional buckling moment Me. This allows the use of software to determine the LTB strength from the elastic LTB strength in the same manner as the local buckling strength.
Shear Provisions addressing the shear rupture limit state (G.1), shear in webs supported on one edge only (G.3), and shear in rods (G.5) were added.
PJP Groove Welds The strength of PJP groove welds was reduced to reflect the notch effect of partial penetration by factoring the size of the weld by 0.6 (Table J.2.2). This is a significant decrease in PJP groove weld strength compared with earlier specification editions.
Fillet Weld Strength The method of computing the strength of fillet welds was changed from using a tabular value for shear strength by filler alloy to using 0.6 times the tabular value for tensile strength times 0.85 to account for stress concentration effects associated with fillet welds (Table J.2.2).
Bolts, Rivets, and Screws Provisions prescribing material requirements for non-aluminum bolts, rivets, and screws were dropped. The requirements for slip-critical bolted connections were updated to match the 2009 RCSC Specification for Structural Joints Using High Strength Bolts.
Pins A new Section (J.6) was added for pins, addressing for the first time hole size, shear strength, and flexural strength.
Contact with Dissimilar Metals The requirements for contact with dissimilar metals were broadened in Section M.7.1 to address metals in addition to steel.
Quality Control and Quality Assurance Chapter N was added to address quality control, performed by fabricators and erectors, and quality assurance, done by other parties.
Changes to the specification were reflected in the other parts of the Aluminum Design Manual, especially in the Part VI Design Aids tables of buckling constants, resistance and safety factors, allowable stresses, and weld strengths. Guidelines for Aluminum Sheet Metal Work in Building Construction, previously a separate Aluminum Association publication, was added as Part VIII of the manual.
As the Specification for Aluminum Structures approaches its 50th anniversary in 2017, it remains the source of virtually all aluminum structural design requirements in the United States and is more accurate, extensive, and user-friendly than ever before.
J. Randolph Kissell, P.E., co-founded the TGB Partnership, an engineering firm specializing in aluminum structures. He has been involved in the design, fabrication, erection, and inspection of aluminum structures since 1978, including 12 years as the engineering manager of Conservatek, an aluminum space frame manufacturer and erector. He co-authored Aluminum Structures A Guide to Their Specifications and Design, published by John Wiley and now in its 2nd edition, and co-holds numerous U.S. patents for aluminum structures. Kissell is the secretary of the Engineering and Design Task Force of the Aluminum Association, responsible for the Specification for Aluminum Structures.