RISD 2154 STEEL STRUCTURES Topic 1: Intro to Steel Design Erik Anders Nelson, PE, SE RISD 2154 Fall 2014 New Science of Strong Materials - JE Gordon Great Division in Technology: Metals and Non – Metals Decreasing Costs* and Improvements of iron and steel in the 18th and 19th century “perhaps the most important event in history” Future non-metals (carbon crystals / ceramics) may have higher strength and will resemble “improved versions of wood and bone” using sophisticated techniques RISD 2154 Fall 2014 1 Strength depends on Molecular Scale We know pretty well how things behave, but not necessarily why Engineering have little “why” why knowledge, knowledge we only have “how” how – we reviewed materials and wrote down testing data in unreadable books. With things like “strength” and “stiffness” properties we have been pretty good and predicting behavior and scaling up to large scale proportioning of structures. We need a good background in “The Science of Elasticity” - which requires us to abandon our instinctive knowledge of materials materials. It is the preconceptions that cause the difficulty in learning something new….can you answer objectively the difference between chocolate and caramel? RISD 2154 Fall 2014 Microstructure – Intro Chemistry Solids are held together by chemical and physical bonds Strength is related to these chemical bonds (so classical physics and chemestry) but also related to new concepts such as dislocations and stress concenrations. There are about 100 different types of atoms, and 20-30 of those effect materials related to us. Each kind of atom is called an element. Iron (Fe) and Carbon (C) are homogenous atoms that can be combined to create entirely different types of stuff (Steel). Or chlorine (poisines green gas) with sodium (a metal) creates NaCl or table salt. Elements/atoms form “matter” RISD 2154 Fall 2014 2 Does Steel Meet these Assumptions? RISD 2154 Fall 2014 Crystal Lattices RISD 2154 Fall 2014 3 Engineering Assumptions RISD 2154 Fall 2014 1650 Robert Hooke “as the deflection, so is the force.“ P = kx RISD 2154 Fall 2014 4 Force and Displacement F = kx (Tension) RISD 2154 Fall 2014 Force to Stress Force = F = 10 kips or 10,000 lbs Stress = F/A = 2 ksi or 2000 psi (or KN/m2 or ksf Elongation to Strain Elongation = x (in units of length like inches) Strain, e = x/L = unit less = change in length over original length RISD 2154 Fall 2014 5 Stress – Strain Curve of Caramel and Chocolate RISD 2154 Fall 2014 Deformations Under Axial Loading From Hooke’s Law: E E P AE From the definition of strain: L Equating and solving for the deformation, PL AE RISD 2154 Fall 2014 6 RISD 2154 Fall 2014 RISD 2154 Fall 2014 7 RISD 2154 Fall 2014 Consider L0 = 100 inch long tension member. ∆Yield ∆Onset of Strain Hardening ∆Peak Load = approx. 0.002(100) = approx. 0 0.02(100) 02(100) = approx. 0.20(100) = 0.2” = 2” = 20” Excessive deformations defines “Failure” for tension member yielding. Limit to FyAg. RISD 2154 Fall 2014 16 8 Design of Structural Systems Key design elements – Load calculation – Proportioning elements to resist loads – Refine elements for serviceability • • • • Deflection Interaction with architectural and mechanical systems Vibration Occupant-specific requirements RISD 2154 Fall 2014 Approx Engineering Properties of Materials Steel – Maximum stress: 50,000 lb/in2 ( can be 270,000) – Maximum strain: 0.003 – Modulus M d l off elasticity: l ti it 29 29,000,000 000 000 lb/i lb/in2 Concrete – Maximum stress: 5,000 lb/in2 (can be 12000) – Maximum strain: 0.004 – Modulus of elasticity: 3,600,000 lb/in2 Wood Values depend on wood grade grade. – Maximum Stress: 1200 lb/in2 – Modulus of elasticity: 1,400,000 lb/in2 RISD 2154 Fall 2014 9 RISD 2154 Fall 2014 Types of Loads Reference: ASCE 7-10 “Loads on Buildings and Other Structures” Dead Loads – Weight of structure – Partition loads Live Loads – Occupant load – Snow Lateral Loads – Seismic – Wind RISD 2154 Fall 2014 10 Dead Load (Self Weight) RISD 2154 Fall 2014 RISD 2154 Fall 2014 11 ASCE-7 RISD 2154 Fall 2014 Common Live Loads RISD 2154 Fall 2014 12 Snow Loads Basic snow load – 30 psf throughout RI, 30-40 psf in MA, up to 90 psf in NH, ME, VT Drift – Can add up to 150 psf additional snow load – At low roof/high roof areas Unbalance loading RISD 2154 Fall 2014 ASCE-7 - Ground Snow Load, Pg RISD 2154 Fall 2014 13 Load Notation RISD 2154 Fall 2014 Apply Dead and Live Loads RISD 2154 Fall 2014 14 Presentation Based on Spec Chapters Chapter A - General Chapter B - Design Info Chapter C - Stability Chapter D - Tension Chapter E - Compression Chapter F - Flexure Chapter G - Shear Chapter H - Combined Chapter J - Connections RISD 2154 Fall 2014 RISD 2154 Fall 2014 15 RISD 2154 Fall 2014 RISD 2154 Fall 2014 16 RISD 2154 Fall 2014 RISD 2154 Fall 2014 17 New ASD and LRFD Allowable Strength Design (henceforth, NEW ASD) approach. Although the NEW ASD bears a similar name to the older Allowable Stress Design (henceforth, OLD ASD), the two are not to be confused LRFD hasn’t changed. RISD 2154 Fall 2014 2010 ASD Combinations RISD 2154 Fall 2014 18 2010 LRFD Combinations RISD 2154 Fall 2014 ASD and LRFD Load Combinations D+L 1.2D + 1.6L RISD 2154 Fall 2014 19 AISC Steel Design Manuals 1963 AISC ASD 6th Edition 1969 AISC ASD 7th Edition 1978 AISC ASD 8th Edition 1989 AISC ASD 9th Edition ASD = actual stress values are compared to the AISC allowable stress values 1986 AISC LRFD 1st Edition 1993 AISC LRFD 2nd Edition 1999 AISC LRFD 3rd Edition LRFD = ultimate forces and moments are compared to the AISC limiting forces and moments capacity 2005 AISC 13rd Edition 2010 AISC 14th Edition LRFD and “New New ASD” ASD RISD 2154 Fall 2014 Limit State Expressions, Demand < Capacity RISD 2154 Fall 2014 20 Strength Design vs. Allowable Stress Design Allowable Stress Design or Working Stress Design, ASD-89 Load and Resistance Factor Design or Ultimate Design, LRFD ASD-05 / 10 Loads Service Ultimate 1.2D + 1.6L Service Useable p y Capacity Some fraction of yyield (Ex. 0.66 FyS) Full strength, incl. postyyield (Ex. 0.9FyZ) Full strength, incl. post-yield p y (Ex. FyZ/1.67) D+L Allowable Strength Design D+L RISD 2154 Fall 2014 Compare ASD to LRFD ASD 1.0D + 1.0L LRFD 1.2D + 1.6L OLD ASD LRFD 0.6Fy × (1.5) = 0.9Fy (LRFD) 0.5Fu × (1.5) = 0.75Fu (LRFD) NEW ASD LRFD Fy /1.67 × (1.5) = 0.9Fy (LRFD) Fu /2 × (1.5) = 0.75Fu (LRFD) ASD × 1.5 = LRFD RISD 2154 Fall 2014 21 L/D Calibration RISD 2154 Fall 2014 22
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