CE 406 – Structural Steel Design

CE 406 – Structural Steel DesignA. DESIGN REFERENCES:International Building Code, 2009 EditionASCE 7-10 - Minimum Design Loads for Buildings and Other StructuresManual of Steel Construction – 14th Edition, American Institute of SteelConstructionBuilding Code Requirements for Reinforced Concrete (ACI 318-08),American Concrete Institute.Building Code Requirements for Masonry Structures (ACI 530-08) andSpecifications for Masonry Structures (ACI 530.1-08), American ConcreteInstituteASTM Standards, American Society for Testing and MaterialsStandard Specification for Open Web Joists, K-Series, LH-Series and JoistGirders, Steel Joist InstituteSDI Specifications and Commentaries for Non-Composite Steel Form Deck andSteel Roof Deck, Steel Deck InstituteB. DESIGN LOADS:All portions of the structure shall be designed in accordance with the following:VERTICAL LOADSROOFDead Loads: (estimated before design, verified after)(Self-weight and weights of supported equip & building materials)Typical roof: Single ply membrane attached withallowance for built-up roof Insulation Deck Framing (estimate) Mech. & Elect. (HVAC & lights) Misc. Hanging (ceiling & sprk) Chapter 25 psf5 psf2 psf6 psf5 psf5 psfTotal DL 28 psfLive Loads: (variable – location and magnitude)Live Load (non-reduced)LL 20 psfPage 1

CE 406 – Structural Steel DesignELEVATED FLOOR SLABSDead Loads: (estimated before design, verified after)(Self-weight and weights of supported equip & building materials)3" NWT Concrete on Form Deck 36 psfFraming (estimate) 10 psfMech. & Elect. (HVAC & lights) 5 psfMisc. Hanging (Sprinklers, Ceiling etc.) 5 psfFloor DLLive Loads:(Floor LL based on Occupancy)Office AreasPublic Rooms and LobbiesEquipment and Mechanical Areas 56 psf 50 psf100 psf150 psf SLAB-ON-GRADELive Loads: (Industrial Floor)Uniform Live Load 500 psfActual Wheel Loads for a Forklift capable ofcarrying 2 (two) 2,000 pound palletsConcentrated Rack Storage post loads(see Rack Layout)ENVIRONMENTAL LOADS (based on location) (usually lateral)Snow Loads:Ground Snow Load 10 psfCe 1.0 Exposure Factor (Exposure C, Partially Exposed)Ct 1.0 Thermal FactorI 1.2 Importance FactorWind:Per International Building CodeBasic Wind Speed, V 90 mphUse Factor, I (Essential Facility) 1.15Seismic:Ss 0.409S1 0.139Site Class DSeismic Use Group ISeismic Design Category DResponse Modification Coefficient, R 5 (both directions)(Other Loads :)(Traffic, Impact, Longitudinal, Soil, Thermal, Fluid, Blast )Chapter 2Page 2

CE 406 – Structural Steel DesignDesign Philosophies(AISC Manual 2-8 to 2-12)“Limit States” Design PhilosophyStrengthServiceabilityStrength – Safety Related – (Flexural, Axial strength, Buckling, Fatigue,Fracture, Yielding) Focus of AISC specificationServiceability – Performance – (Concerns use & occupancy, deflection,vibration, cracking, deterioration)Allowable Strength Design (ASD)Members are sized to support Service (Unfactored) loads based on Allowable strengthRnΩTypical:RWhere:Ra Allowable StrengthRn Nominal Strength according to Specification provisionΩ Factor of Safety given by Specification,Typical range – 1.50 to 2.00aExamples:Nominal moment strengthMManΩF.O.S. 1.67 (bending)Actual Moment (Unfactored)Chapter 2Nominal axial compressive strengthPnPa ΩF.O.S. 1.67 (compression)Actual Axial Force (Unfactored)Page 3

CE 406 – Structural Steel DesignAllowable StrengthΣR Ri aRnΩRequired Strength determined from ASD Load CombinationsLoad Combination EquationsD dead loadL live load due to occupancyLr roof live loadS snow loadR rain load exclusive of the ponding contributionW wind loadE earthquake loadASD Equations1. D2. D L(floor)3. D (Lr or S or R)(roof)4. D 0.75L 0.75(Lr or S or R)(column)5. D (0.6W or 0.7E)6a. D 0.75L 0.75(0.6W) 0.75(Lr or S or R)6b. D 0.75L 0.75(0.7E) 0.75(S)7. 0.6D 0.6W8. 0.6D 0.7ENote 1. 0.6W and 0.7E reduction of wind and earthquake force whichis already factored or ultimateNote 2. 0.75 in equations 4. and 6. – Combination of time-variableloads (0.75 not taken on dead load)Ref:Chapter 2ASCE 7-10 – p. 8Steel Manual-14th ed – p. 2-11Text – p. 57Page 4

CE 406 – Structural Steel DesignLoad & Resistance Factor Design (LRFD)Members are sized to support Factored loads based on Reduced Material strengthTypical:RuRnNominal strengthλQi i“Load” factor, typ. 1.0:Rn“Resistance” factor, typ. 1.0Working (service) loadAccounts for uncertainties in material strength, dimensions,workmanship (fabrication/ erection stresses), residual stresses(uneven cooling), also, the consequence of failure.λ:iAccounts for uncertainties in estimating magnitude (D 1.2,L 1.6) Dead estimate more accurate the live load estimate.Environmental loads – Act of God/Nature, difficult to predict Earthquakeand Wind (already ultimate/factored load), W E 1.0 (formerly 1.6)LRFD Equations1.2.3.4.5.6.7.1.4D1.2D 1.6L 0.5(Lr or S or R)1.2D 1.6(Lr or S or R) (L* or 0.5W)1.2D 1.0W L* 0.5(Lr or S or R)1.2D 1.0E L* 0.2S0.9D 1.0W0.9D 1.0ERef:(only dead load)(floor/column)(roof/column)(column/roof)ASCE 7-10 – p. 7Steel Manual-14th ed – p. 2-10Text – p. 53* The load factor on L in combinations (3.), (4.), and (5.) is taken as 1.0 for floors inplaces of public assembly, for live loads in excess of 100 psf and for parking garage liveload. The live factor is permitted to equal 0.5 for other live loads.Downward load (gravity)the maximum gravity load.is positive ( ). Load combinations 1. through 5.account forUpward load (uplift)is negative (-). Load combinations 6. and 7. are used to accountfor the possibilities of uplift.Chapter 2Page 5

CE 406 – Structural Steel DesignExample #1 (Example 2-1, p. 55, text)W24x55 @ 8’-0”D 50 psf (not including self-weight)L 80 psfFind Governing Load, lbs / ft or plfD 55 plf (50 psf) 8 ft 455 plfL (80 psf) 8 ft 640 plf1. U 1.4 (D) 1.4 (455) --------------------------------- 637 plf2. U 1.2 (D) 1.6 (L) 0.5 (Lr or S or R) 1.2 (455) 1.6 (640) 0.5 (0) ------------------- 1570 plf –controlsExample #2D 20 psfS 30 psfW 32 psf (assume suction – upward only)Determine U:1. U 1.4 (D) 1.4 (20) ----------------------------------------------- 28 psf2. U 1.2 (D) 1.6 (L) 0.5 (Lr or S or R) 1.2 (20) 1.6 (0) 0.5 (30) ----------------------------------- 39 psf3. U 1.2 (D) 1.6 (Lr or S or R) (0.5 L or 0.5 W) 1.2 (20) 1.6 (30) 0.5 (-32) --------------------------------- 56 psfor 1.2 (20) 1.6 (30) ----------------------------------------------- 72 psf4. U 1.2 (D) 1.0 (W) 0.5 L 0.5 (Lr or S or R) 1.2 (20) 1.0 (-32) 0.5 (0) 0.5 (30) ----------------------- 7 psfor 1.2 (20) 0.5 (30) ----------------------------------------------- 39 psf5. U 1.2 (D) 1.0 (E) 0.5 (L) 0.2 (S) 1.2 (20) 1.0 (0) 0.5 (0) 0.2 (30) ------------------------- 30 psf6. U 0.9 (D) 1.0 (W) 0.9 (20) 1.0 (-32) --------------------------------------------- -14 psf7. U 0.9 (D) 1.0 (E) 0.9 (20) 1.0 (0) ------------------------------------------------- 18 psfThe roof should be designed for:Chapter 272 psf (downward) 3. (1.2D 1.6S)-14 psf (upward) 6. (0.9D 1.0W)Page 6

Girders, Steel Joist Institute SDI Specifications and Commentaries for Non-Composite Steel Form Deck and Steel Roof Deck, Steel Deck Institute B. DESIGN LOADS: All portions of the structure shall be designed in accordance with the following: VERTICAL LOADS R