Architecture 314 Structures I Characteristics of a Force Loads on Structures Load Types Dead Live Wind Snow Earthquake Load Combinations ASD LRFD University of Michigan, TCAUP Structures I Slide 1 of 27 Dead Load The weight of material by density: by area: University of Michigan, TCAUP Structures I Slide 2 of 27
Dead Load Weight of material Weight of structure Weight of permanent fixtures University of Michigan, TCAUP Structures I Slide 3 of 27 Dead Load Floor framing Joist selfweight Joist weight on floor on center space in inches (o.c.) Wood density by species: ca. 25 50 PCF University of Michigan, TCAUP Structures I Slide 4 of 27
Live Load Live Load (on projected area) Floor live load Occupancy 40 PSF to ~250 PSF Roof live load Construction Maintenance 12 PSF to 20 PSF (depending on area and slope) University of Michigan, TCAUP Structures I Slide 5 of 27 Live Load Floor University of Michigan, TCAUP Structures I Slide 6 of 27
Roof Live Load Minimum L r 20 PSF between 12 PSF and L r = 20 R 1 R 2 1 for A t 200 ft2(18.58 m 2 ) R 1 = 1.2 0.001A t for 200 ft 2 < A t < 600 ft 2 0.6 for A t 600 ft 2 (55.74 m 2 ) where A t = tributary area in ft 2 (m 2 ) supported by any structural member and 1 for F 4 R 2 = 1.2 0.05 F for 4 < F < 12 0.6 for F 12 where, for a pitched roof, F = number of inches of rise per ft and, for an arch or dome, F = rise-to span ratio multiplied by 32. University of Michigan, TCAUP Structures I Slide 7 of 27 Wind Load Minimum force 10 psf (ASCE-7 6.1.4.1) Basic pressure equation ASCE equation Sec. 6.5.10 eq.6-15 Velocity V is in MPH 0.00256 accounts for air density/2 and conversions Wind Load (normal to surface) Pressure load + pressing load - suction load University of Michigan, TCAUP Structures I Slide 8 of 27
Wind wind tunnel testing Boundary Layer Wind Tunnel University of Michigan, TCAUP Structures I Slide 9 of 27 Wind CFD Computational Fluid Dynamics University of Michigan, TCAUP Structures I Slide 10 of 27
Snow Load Snow Load (on projected length) Ground Snow p g by region Ann Arbor 25 psf Flat Roof p f = 70% p g x condition factors (exposure, thermal, importance) Sloped Roof p s = p f x slope factors Balanced (full roof) Un-balanced University of Michigan, TCAUP Structures I Slide 11 of 27 Snow Load Ground Snow p g by region Ann Arbor 25 psf Flat Roof p f = 70% p g x condition factors (exposure, thermal, importance) Sloped Roof p s = p f x slope factors Balanced (full roof) Un-balanced University of Michigan, TCAUP Structures I Slide 12 of 27
Snow Load Drift Loading Windward Leeward Sawtooth University of Michigan, TCAUP Structures I Slide 13 of 27 Earthquake Loads Ground Motion Measurement Amplification Building Resistance University of Michigan, TCAUP Structures I Slide 14 of 27
Geologic Background Plate Tectonics Geologic Faults University of Michigan, TCAUP Structures I Slide 15 of 27 Geologic Background Fault Location Focus (hypocenter) Epicenter University of Michigan, TCAUP Structures I Slide 16 of 27
Ground Failure Landslides Liquefaction Subsidence University of Michigan, TCAUP Structures I Slide 17 of 27 Ground Motion Primary P wave hits first pressure hammer Secondary (Shear) S wave back and forth adds to P wave University of Michigan, TCAUP Structures I Slide 18 of 27
Ground Motion Acceleration Measured in g s (1 g = 32 ft/sec2) 0.001 g limit of perception 0.1 g 0.2 g 0.5 g weak construction fails hard to stand up very sever for earthquake San Francisco, 1906 approximately 0.7g University of Michigan, TCAUP Structures I Slide 19 of 27 Measurement Magnitude Richter scale 0 to ~9.5 Size of the wave Accounts for distance attenuation Logarithmic (base 10) Intensity Modified Mercalli scale I to XII Relates to effects Includes duration Differs with location University of Michigan, TCAUP Structures I Slide 20 of 27
Measurement Intensity Modified Mercalli scale I to XII Relates to effects Includes duration Differs with location University of Michigan, TCAUP Structures I Slide 21 of 27 Characteristics of Period Frequency Cycles / second (Hz) Period Inverse of frequency Fundamental Period measure in seconds approx. = stories/10 Soil approx. 0.5 ~ 2.0 University of Michigan, TCAUP Structures I Slide 22 of 27
Amplification Fundamental Period Modes Modal shapes Modal frequency Resonance Response Spectrum Fundamental period of soil 0.4 to 1.5 (or 2.0) Harder is shorter (rock) Softer is longer (soil) University of Michigan, TCAUP Structures I Slide 23 of 27 Building Resistance Damping Material Partitions Ductility connections Strength and stiffness drift University of Michigan, TCAUP Structures I Slide 24 of 27
Provisions and Codes National Earthquake Hazards Reduction Program (NEHRP) not a code but provisions ASCE 7 Section 9 code based on NEHRP University of Michigan, TCAUP Structures I Slide 25 of 27 Load Combinations Load Types Dead Load - D Roof Live Load - Lr Floor Live Load - L Snow Load - S Wind Load - W Earthquake - E Load Combinations Allowable Stress Design (ASD) Not factored D + L D + (Lr or S) D + 0.75 L + 0.75 (Lr or S) D + (W or 0.7 E) Strength Design (LRFD) With gamma ( ) safety factors 1.4 D 1.2 D + 1.6 Lr + 0.5(Lr or S) 1.2 D + 1.6(Lr or S) + (L or 0.8W) 1.2 D + 1.6W + L + 0.5(Lr or S) 1.2 D + 1.6E + L + 0.2S University of Michigan, TCAUP Structures I Slide 26 of 27