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CE 496 Introduction to Structural Design

CE 496 Introduction to Structural Design. Winter 2011 Howard Lum. February 17 , 2011 Agenda. Loads Tributary Area Dead Load Live Load & LL Reduction Wind Load Seismic Load Steel Tension, Compression and Flexural Design Concrete Basics Q&A. Loads.

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CE 496 Introduction to Structural Design

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  1. CE 496Introduction to Structural Design Winter 2011 Howard Lum

  2. February 17, 2011Agenda • Loads • Tributary Area • Dead Load • Live Load & LL Reduction • Wind Load • Seismic Load • Steel Tension, Compression and Flexural Design • Concrete Basics • Q&A

  3. Loads • CBC Chapter 16 provides the requirements • Dead Loads: • Wt. of Steel = 490 pcf • Wt. of Concrete = 150 pcf • Wt. of Masonry = 115 pcf • Density of Water = 62.4 pcf • Density of Wood = 40 pcf

  4. Live Loads • Live Loads depend on the group of occupancy • Live Loads: See CBC Table 1607.1 (2 pages) • For the County project, special conditions include: • Handrail design:Lateral Load = 50 plf • Vehicle Barrier:Lateral Load = 6,000 lb

  5. Beam A Trib. Width for Beam A PLAN Loads are given in pressure (psf) To convert psf to uniform load W (pounds/ft): W (#/ft) = Load (psf) x Tributary Width Example: LL=60 psf, beams spaced at 12 ft on center, span = 40 ft wLL= (60)*12 = 720 lb/ft AT = 12 * 40 = 480 SF

  6. Beam & Column Loads Beam Tributary Area Column Tributary Area

  7. Live Load Reduction • Floor Live Load Reduction – CBC 1607.9 • Method 1 (1607.9.1) • LL Reduction if KLL AT >= 400 SF • L = Lo (0.25 + 15/√KLL AT) • Lo: unreduced LL (psf) • KLL: 1-4 (Table 1607.9.1) • AT Tributary Area (sq ft) • LL > 100 psf shall not be reduced EXCEPT: • Member supports 2 or more floors • LL – no reduction in public assembly areas

  8. LL Distribution • DL always applies to the entire structure • LL applies to areas of maximum stress: • Simple Span Beam • Overhang Beam LL DL LL DL LL DL

  9. Load Combinations • CBC Section 1605 • Strength Design: U <= Ф*(Strength) • U = 1.2 (D+F) + 1.6 (L+H) • U = 1.2 D + 1.6 W + 0.5 L • U = 0.9 D + 1.6 W + 1.6H • Working Stress Design: • D + L < (Allowable stress) – Not used • Strength Design is based on probabilistic approach of loads and strength variance • Working Stress Design is based on conventional elastic stress less than allowable values

  10. Deflection Limits • Allowable deflections in CBC Table 1604.3 • Max Deflection = L/360 or L/240 where: • L = beam span • Floor LL: L/360 Floor DL+LL: L/240 • If L=10 ft, max allow defl. = L/240 = 0.5 inch • Use working loads for all deflection calculations L

  11. Wind Load • CBC Section 1609 and ASCE 7-05 Ch 6 • Simplified method: Ps = λ *Kzt *I * ps30 where: • λ is height/exposure factor • Kzt is topgraphic factor (Eq 6-3) • I is importance factor • Ps30 is pressure at 30 ft, I=1 • Other structures ASCE 6.5.15 F = qzGCfAf where qz = velocity pressure, G = 0.85 (rigid structure), Cf from Fig. 6-21, Af = proj. area

  12. Wind Load • Importance Factor I is based on Occupancy Table 1-1 and Table 6-1 • California: basic wind speed = 85 mph • Exposure B, C, D as defined in CBC 1609.4.3 • City of Long Beach has special wind provisions based on geographical locations

  13. General approachEarthquake Load Design • Maximum Considered Earthquake (MCE) • 2% probability of exceedance in 50 years ( or 2,500 years return period) • RP = 1/Pe = 1/(0.02/50) = 2500 • All MCE’s are given in the CBC with the latest update in USGS website • http://earthquake.usgs.gov/hazards/ • Spectral Accelerations characterized by: • Ss (short T) and S1 (long T) • TL (transition T) – CBC Fig. 22-16

  14. SS (short T)MCE Acceleration for T=0.2 sec

  15. S1 (Long T)MCE Acceleration for T=1 sec

  16. Site Class per CBC

  17. Earthquake Load • Site Class Modifications to MCE • Site Class (A-F) determined by Table 1613.5.2 • Hard rock to soft soil • Fa = Short Period Mod. Factor - 1613.5.3(1) • Fv = Long Period Mod. Factor - 1613.5.3(2) • Design Earthquake: • SDS = 2/3 * Fa * Ss • SD1 = 2/3 * Fv * S1

  18. Earthquake behavior of Structures

  19. Effective Seismic Weight • W (in seismic analysis): • Weight (DL) of the Diaphragm • Weight (DL) of the Exterior Walls • + 25% floor LL for Storage Areas • +10 psf floor LL for Partitions • + weight of permanent equipment • + 20% of flat roof snow load (> 30 psf) • Reference: ASCE 12.7.2 and 12.14.8.1

  20. Tributary Weights

  21. Base Shear (ASCE 12.8) • V= Cs * W: • Cs = SDS/ (R/I) • Max Cs = SD1/T(R/I) or SD1TL/T^2(R/I) • Min Cs = 0.01 • Min Cs = 0.5S1/(R/I) for S1 > 0.6 • T calculation: • Ta = Ct * hnx • Min. T = Cu * Ta where Cu from Table 12.8-1 • Reference: ASCE 12.8.1 to 12.8.3

  22. TABLE 11.6-1 SEISMIC DESIGN CATEGORY BASED ON SHORT PERIOD RESPONSE ACCELERATION PARAMETER Value of SDS Occupancy Category I or II III IV SDS < 0.167 A A A 0.167 ≤ SDS < 0.33 B B C 0.33 ≤ SDS < 0.50 C C D 0.50 ≤ SDS D DD

  23. Vertical distribution of Forces • Reference: ASCE 12.8.3 • Fx = Cvx * V • where Cvx = wxhxk/∑wihik W V

  24. Seismic Load Combination • Load combination w/seismic • Strength Design (12.4.2.3): • (1.2 + 0.2 SDS)D + *QE + L + 0.2S • (0.9 – 0.2 SDS)D + *QE + 1.6H • Check both downward seismic and uplift seismic forces in combination with dead load and live load • Note: L can be 0.5L (if Lo <= 100psf)

  25. Steel Properties Fu Fy Es=29,000 ksi 1

  26. Steel: Availability of ASTM Grades 58 58 50

  27. Tension Members Fracture Yield • Strength Design: Pu <= t * Pn • Tensile Capacity: t * Pn • Failure Modes: • Deformation at yield (gross area) t = 0.90, Pn = Fy*Ag • Fracture at tensile strength (net area) t = 0.75, Pn = Fu*Ae

  28. Compression – Stability controls • Elastic Buckling Stress (Euler) given in AISC Eq E3-4 • Fe: critical compressive stress above which column buckles • Fe is independent of Fy or Fu • Fe = π2*E/(KL/r)2 • Fcr = 0.877*Fe K=1.0

  29. Compression – Strength Controls • KL/r <=4.71 √E/Fy, column strength governs (not stability) • Fcr = [0.658 (Fy/Fe) ]*Fy where Fe = π2*E/(KL/r)2

  30. AISC Compression E3 Fy=50 ksi Fe (Euler Critical Stress) Fy=36 ksi Fcr Fcr=(0.658Fy/Fe)*Fy Fcr=0.877Fe KL/r

  31. Double Sym. Beam Design Compression Flange AISC Chapter F b*Mn = 0.9*Mn Mn is dependent on lateral unbraced length Lb of the compression flange Lateral-torsional buckling governs design if Lb>Lp

  32. Lb - Unbraced Length Lb is independent of the span length Lb can be 0 if the compression flange is continuously braced Example: Span = 50 ft, Lb = 25 ft

  33. Steel Beam Design Curve

  34. Concrete Design ACI 318  Mn >= Mu where  is 0.9  Vn =  (Vc +Vs) >= Vu where  is 0.75  Pn>= Pu where  is 0.65 to 0.90 f’c: 28-day compressive strength (3000 – 8000 psi) Fy: Yield strength of reinforcement (60 ksi)

  35. Questions & Answers

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