Structural vs Nonstructural Members Roger LaBoube, Ph.D., P.E. Wei-Wen Yu Center for Cold-Formed Steel Structures
Behavior of Cold-Formed Steel Members
STEEL DESIGN SPECIFICATIONS Type of steel Specification Cold-Formed, Carbon Steel Cold-Formed, Stainless Steel Hot-Rolled AISI ASCE AISC What is unique about cold-formed steel? 1-3
KEY CHARACTERISTICS OF COLD-FORMED STEEL STRUCTURES Shapes are cold formed from flat sheets or plates Original mechanical properties of steel are changed in certain areas of the cross section due to the cold forming There are some standardized shapes Thin material Predominant failure mode influenced by local buckling and by the post-buckling strength increase 1-4
Structural Member Design Based On AISI S100-12, North American Specification for the Design of Cold-Formed Steel Structural Members 1-5
Applicable in North America Canada Mexico United States 1-6
DESIGN PHILOSOPHIES FOR COLD-FORMED STEEL DESIGN ASD Allowable Strength Design LRFD Load and Resistance Factor Design 1-7
ASD STRENGTH REQUIREMENTS (Section A4.1.1) R R n / R = Required strength (load effect) R n = Nominal strength (ability to resist load) = Factor of safety R n / Design strength (available strength) 1-8
LRFD STRENGTH REQUIREMENTS (Section A5.1.1) R u R n R u = Required strength R n = Nominal strength = Resistance factor R n = Design strength (available strength) 1-9
DESIGN ECONOMY OF LRFD VS. ASD: BEAMS 1-10
Limits of Applicability (Section A1.2) Nominal strength and stiffness shall be determined by Chapters A through G and Appendices A and B. As an alternate: Determine design strength or stiffness by tests and Chapter F Determine design strength or stiffness by rational analysis. Direct Strength Method (Appendix 1) Second Order Analysis (Appendix 2) 1-11
TYPICAL APPROVED STEELS (Section A2.1) 17 Approved Steels in Section A2.1. ASTM A653 Steel Sheet, Zinc Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process F y = 33 to 50 ksi F u = 45 to 70 ksi F y = 80 ksi F u = 82 ksi ASTM A792 Steel Sheet, 55% Aluminum-Zinc Alloy- Coated by the Hot-Dip Process (Galvalume) F y = 33 to 50 ksi F u = 45 to 70 ksi F y = 80 ksi F u = 82 ksi 1-12
TYPICAL APPROVED STEELS (Section A2.1) ASTM A1011 Steel Sheet, Hot-Rolled, Carbon, Structural, High Strength Low-Alloy with improved Formability ASTM A1003 Steel Sheet, Carbon, Metallic- and Nonmetallic-Coated for Cold Formed Framing Members (Replaced A653 for framing members) 1-13
OTHER STRUCTURAL QUALITY STEELS (Section A2.2) Maximum Thickness 1 inch Published Material Specification with Specified Mechanical and Chemical Properties Minimum Ductility Requirements of Section A2.3 1-14
REQUIRED DUCTILITY (Section A2.3.1) F u /F y 1.08 Elongation 10% (two-inch gage length) 7% (eight-inch gage length) 1-15
LOW DUCTILITY STEELS (Section A2.3.2) ASTM A653, A792, A875 and A1008, Grade 80 May be used for deck and panel profiles Limits on F y and F u Alternatively, suitability shall be determined by tests. Exceptions: Multi-web configurations (flexural strength) & Closed box sections (compression strength) 1-16
EFFECT OF COLD FORMING PROCESS 1-17
MAJOR COLD-FORMED STEEL DESIGN CONCEPT Effective Design Width Stiffened Elements Partially Stiffened Elements Unstiffened Elements 1-18
UNIFORMLY COMPRESSED UNSTIFFENED ELEMENTS 1-19
The interaction of local and overall buckling is determined by the following: Overall buckling capacity is determined by use of the full properties. Local buckling is reflected by the use of the effective cross section. 1-20
Local and Overall Buckling Interaction For a Column: P n = F n A e For a Beam: M n = F n S e 1-21
UNIFORMLY COMPRESSED UNSTIFFENED ELEMENTS 1-22
EFFECT OF LOCAL BUCKLING ON BEAM SECTION 1-23
EFFECT OF LOCAL BUCKLING ON COLUMN SECTION ult Ineffecitve Areas f = F y A eff Effective Section 1-24
Structural vs. Nonstructural Member: What does Nonstructural Really Mean? Same basic design issues apply
Structural Member Curtain Wall Axial Load Bearing Floor joist
Nonstructural Members Ceiling Fascia Interior walls
Prior to 2015 IBC what did the building code tell us about a nonstructural member?
Code Definition ASTM C645 Standard for Nonstructural Steel Framing Members: A member in a steel framed wall system which is limited to a transverse (out-of-plane) nominal load of not more than 10 lb/ft 2 (0.48 kpa), a superimposed nominal axial load, exclusive of sheathing materials, of not more than 100 lb/ft (1.46 kn/m), or a superimposed nominal axial load of not more than 200 lbs (0.89 kn). This is a stud definition, what about ceiling or fascia framing, which could be bending alone and no sheathing bracing.
Code Definition North American Specification for the Design of Cold-Formed Steel Structural Members (AISI S100): An interior partition wall stud in a composite steel framed interior wall system with sheathing attached to both flanges and that is limited to a transverse (out-of-plane) nominal load of not more than 10 lb/ft 2 (0.48 kpa), a superimposed nominal axial load, exclusive of sheathing materials, of not more than 100 lb/ft (1.46 kn/m), or a superimposed nominal axial load of not more than 200 lbs (0.89 kn). This is a stud definition, what about ceiling or fascia framing, which could be bending alone and no sheathing bracing.
Industry Load Tables 5 psf, 7.5 psf and 10 psf for full unreduced load. These loads are stipulated as live loads in the building code, not wind loads.
Design Approaches Sheathing braced design Composite (considers properties of the sheathing) Per AC86, Ω = 1.5 All-steel design Non-composite (steel properties only) Per S100, Ω = 1.67
Design Approaches Sheathing braced design Composite (considers properties of the sheathing) Acceptance Criteria for Cold-Formed Steel Framing Members Interior Nonloading-Bearing Wall Assemblies (ICC-ES AC86 )
Design Approach All-steel design Non-composite (steel properties only) Sheathing serves as a brace Develop M y for example (no overall buckling) Unbraced compression Flanges. Also not a wall stud
ASTM C645 Stipulates Minimum Properties including thickness Effective Properties are computed using AISI S100. Note minimum thickness limit.
Flange Flat-Width-to-Thickness Ratio (Section B1.1) (a) Maximum Flat-Width-to-Thickness Ratios (1) Stiffened Compression Elements (Edge Stiffeners) Simple lip (w/t 60) Section B1.1(a)(1)
Why 0.0188 minimum for design? ASTM C645 Table 2: Flange width is 1.25 (w/t =60) Yield stress is 33 ksi Safety factor is 1.67
Confusion in the market place today. Why? 1. Definitions explicitly for walls 2. Differing Safety factor 3. EQ sections
Equivalent Nonstructural Member An equivalent nonstructural member is a member that meets the performance requirements of the building code but does not have the same dimensional and/or thickness characteristics as Standard Products defined in ASTM C 645.
As thin as 0.015 Equivalent Thickness Stud F y greater than 33 ksi But, AISI w/t 60 Manufacturers used AISI rational design approach.
Thickness minimums no longer apply Effective Properties are computed using AISI S100 What is an Equivalent Thickness? ASTM C645
Code Defined (Enabled) Equivalent Thickness? ASTM C645
AISI has attempted to address the confusion in the marketplace
NORTH AMERICAN STANDARD FOR COLD-FORMED STEEL FRAMING NONSTRUCTURAL MEMBERS AISI S220-11 (Adopted in 2015 IBC) The design and installation of cold-formed steel nonstructural members in buildings shall be in accordance with the provisions of this standard.
Scope This standard applies to nonstructural members that comply with the following: Not limited to 1) Member is in a steel-framed system that is limited to a transverse (out-of-plane) load of not more than 10 lb/ft 2 (0.48 kpa). Exception: Pressurized air plenums, ceilings and elevator shaft enclosures are permitted to have a load of not more than 15 lb/ft 2 (0.72 kpa). 2) Member is in a steel-framed system that is limited to a superimposed axial load, exclusive of sheathing materials, of not more than 100 lb/ft (1.46 kn/m). 3) Member is limited to a superimposed axial load of not more than 200 lbs (0.89 kn).
Nonstructural Member. A member in a steel-framed system that is not part of the gravity load resisting system, lateral force resisting system or building envelope. New Definition!!
Scope Where there is a conflict between this standard and other reference documents, the requirements contained within this standard shall govern.
Material ASTM A1003/A1003M-11 Type NS In the past, only 33 ksi yield stress Now 33, 40, 50, 60, 70, 80 ksi Why all of the yield stress values? To reflect the EQ sections in the marketplace
Corrosion Protection ASTM A653 G 40 or equivalent
Product Minimum base steel thickness not less than 95% of the design thickness (consistent with S100) To receive sheathing, the minimum flange width shall be 1-1/4 inch. For track, the minimum flange width shall be 1 inch Designator: 600S125-17 Manufacturing tolerance tabulated
Member Design Non-composite assembly design (all-steel design) Composite assembly design (considers the influence of the sheathing)
Non-Composite Assembly Design (i) Chapters A through E of AISI S100 [CSA S136] with (but must meet w/t = 60 limit): Ω N = 0.9 Ω (this will be about 1.5) Φ N = 1.1 Φ Ω and Φ are taken from AISI S100 Premise is that the consequence of failure for a nonstructural member is less than for a structural member and, consequently, this standard permits a lower reliability for nonstructural members.
Non-Composite Assembly Design (Do not meet w/t = 60 limit) (ii) Chapter F of AISI S100 [CSA S136] with: β o = 1.6 (Target Reliability Index if structural member 2.5 per AISI S100) Ω = Safety factor per Section F1.2 of AISI S100 = 1.6/ (this will be about 1.5) Φ = Resistance factor per Section F1.1(b) of AISI S100
Non-Composite Assembly Design (Do not meet w/t = 60 limit) (ii) continued: If Section A1.2(b) of AISI S100 (i.e. rational analysis) is utilized then supplementary tests are permitted to be performed and Chapter F of AISI S100 is permitted to be employed for determination of Φ or Ω (will be about 1.5).
Composite Assembly Design Design based on the tests undertaken and safety factor evaluated in accordance with Chapter F of this standard. Chapter F: Tests, when required shall be in accordance with an approved test method and Section F1 of AISI S100 with β o = 1.6.
Connection Design Connections shall be designed in accordance with AISI S100 or testing in accordance with Section F1 of AISI S100, and the requirements of Chapter D of this standard.
D1.1 Screw Connection Screw fasteners for steel-to-steel connections shall be in compliance with ASTM C1513 or an approved design or approved design standard. Use of a larger than specified screw size shall be permitted, providing that the design and installation is in accordance with the minimum spacing and edge distance requirements.
D1.2 Screw Connections Installation: Minimum of 3 exposed threads No permanent separation between plies Minimum 3 exposed threads shall protrude through steel 1-58
D1.3 Screw Connections Stripped Screws: Stripped screw fasteners in direct tension are considered ineffective Stripped screw fasteners in shear may be considered effective (not more than 25% of the total number considered effective) 1-59
D1.4 Screw Connections Spacing: Provides for an allowance if the spacing is less than 3 times screw diameter, as specified by AISI S100: Specification If spacing is greater than 2 times screw diameter, screws can be considered 80% effective 1-60
D1.5 Gypsum Board Gypsum board shall be attached to coldformed steel framing in accordance with the applicable building code or an approved design standard. Screw fasteners for gypsum board to steel connections shall be in compliance with ASTM C954, ASTM C1002, or ASTM C1513, as applicable, with a bugle head style.
E. Miscellaneous E1 Utilities Holes Plumbing Electrical E2 Insulation
F. Testing (S220-15) Composite Assemblies Bridging Connectors Screw Penetration
F1. Composite Assemblies Tests when required shall be in accordance with AISI S916 or an approved test method Section F1 of AISI S100 with β o = 1.6 applies for safety factor determination
Test Standards ICC AC86 has been the recognized test method (not a consensus standard) AISI S916, Test Standard for Cold-Formed Steel Framing Nonstructural Interior Partition Walls with Gypsum Board (2015 edition)
AISI S916 - Scope Applies to performance test methods for the determination of the strength and stiffness of nonstructural interior partition wall assemblies subject to uniform pressure loads up to 15 psf.
AISI S916 Limiting Heights Partition wall assembly flexural strength End reaction strength Partition wall assembly stiffness
Tests to be Performed
F2 Bridging Connector Tests when required shall be in accordance with AISI S915 or an approved test method Section F1 of AISI S100 applies for safety factor determination
F3 Screw Penetration Test to demonstrate the capability to pull the head of the screw below the surface of the gypsum board in 2 seconds or less without screw spin out Test protocol in Appendix 1 of S220
CFSEI Tech Note W105-13 Design of Nonstructural Members Provides an introduction to AISI S220 Illustrative example problems
AISI Structural Framing Standards www.aisistandards.org General: AISI S200: General Provisions AISI S201: Product Standard AISI S202: Code of Standard Practice Design Standards: AISI S210: Floor and Roof System Design AISI S211: Wall Stud Design AISI S212: Header Design AISI S213: Lateral Design AISI S214: Truss Design Prescriptive Methods: AISI S230: Prescriptive Method for One and Two Family Dwellings 1-72
Wei-Wen Yu Center for Cold-Formed Steel Structure laboube@mst.edu, 573-341-4481
Questions?