SKYSCRAPER LOADS

1
SKYSCRAPER LOADS

• Reference Minimum Design Loads for Buildings and
  Other Structures ASCE 7-05

2
GRAVITY DEAD LOADS

• Reinforced Concrete 150 pcf
• Lightweight Concrete 110 pcf
• Structural Steel 490 pcf
• Timber 32 pcf
• Glass 160 pcf
• Rain 62.4 pcf
• Snow 10 pcf

3
GRAVITY LIVE LOADS

• Offices 50 psf
• Hotel/Residential 40 psf
• Lobbies 100 psf
• Libraries 150 psf
• Schools 40 psf
• Stadiums 100 psf

4
LATERAL WIND LOAD

• Bernoulli's Equation
• state 1 wind before it hits building, p 0
• state 2 air after it hits building, v 0
• Both states have same elevation y

5
AIR AT 60 DEGREES (F)

• p pressure in psf
• v velocity in mph

6
ASCE 7-05 FORMULA

• h height
• Exposure
• closely spaced obstructions 7.0 1200ft
• scattered obstructions 9.5 900ft
• open terrain, unobstructed 11.5 700ft

7
DESIGN WIND SPEED

• Salt Lake City, Chicago 90mph
• New York 100mph
• Miami, Hong Kong 150 mph

8
WIND LOADING IS COMPLEX

• Pressure on windward side
• Suction on leeward side
• Drag and Lift
• Vortex Shedding -- vibration
• Acceleration due to sudden gusts
• Wind tunnel tests are performed for most
  skyscrapers

9
LATERAL SEISMIC LOADING
10
MULTISTORY BUILDING

• Sum of story forces base shear V

11
ASCE 7-05 STORY FORCES

• weight of ith story (no live load)
• height of ith story from the ground
• If , k 1 (T building period)
  
• If , k 2 (T of stories/15)
• Otherwise, k 0.75T/2

12
ASCE 7-05 BASE SHEAR

• M total mass of building
• W total weight of building Mg

13
DUCTILITY FACTOR R

• R 8 for special eccentrically braced frame
• R 1.5 for ordinary concrete building
• All other buildings somewhere in between
• It is assumed that the maximum inelastic
  displacement equals maximum elastic displacement

14
SPECTRAL ACCELERATION Sa
15
PEAK DESIGN ACCELERATION

• Flat part of design spectrum
• High seismicity (California, Beijing) 1.4g
• Medium Seismicity (Utah) 0.9g
• Low seismicity 0.5g
• Also depends on soil conditions at site

16
SEISMIC LOADING IS COMPLEX

• Inertial effects
• Damping effects
• Inelastic effects
• Site effects
• Higher modes
• Uncertainty
• Most skyscrapers analyzed under multiple records
• Some are modeled on a shake table