1. Chapter 3
Loads on Buildings

2. Loads on Buildings

3. Loads on Buildings
Gravity loads caused by gravitational pull, act vertically.
Lateral loads caused by wind and earthquakes, horizontally.

4. Units of Measurement Kilo-pounds (kips) = 1,000 pounds
Distributed over a surface, pounds per square foot (psf)
Distributed over a linear element (beam), pounds per foot (lb/ft, kips/ft)
Load on a column, pounds or kilopounds (lbs, kips)

5. Loads on Building Elements

6. Gravity Loads

7. Dead load Self-weight of the building - does not vary over time.
Weights of materials and components
Estimated with greater certainty than other types of loads
Calculated based on material volume X density

8. Roof Dead load
Calculated over horizontal projected area of roof.

9. Live Load Load whose magnitude and placement changes with time.
Floor live load
varies with occupancy type
Generally calculated as uniform loads in psf, except in special cases (parking garage - loads concentrated on car tires)
Roof live load (generally 20 psf)
Includes weight of repair personnel and temporary storage.
If the roof snow load is greater than the live load, it is used instead.

10. Rain Load
Occurs as a result of accidental accumulation of melted snow or rainwater
Most significant in long-span, relatively low slope roof. These roofs require:
Adequately stiff roof assemblies
Minimum slope: 1/4 inch per 1 ft
Secondary drains (parapet roofs)

11. Lateral Loads

12. Wind Load Basics Primarily horizontal
Also exert an upward force on flat and low-slope roofs
Loads are resisted by
Anchorage to foundation
Wind bracing elements (stiffening elements)

13. Racking & the diagonal brace

14. John Hancock Center, Chicago

15. Sears Tower, Chicago

16. Tornadoes Low probability, high potential for destruction

17. Tornado-prone regions in US

18. Hurricanes Rotational winds up to 150 mph
Form at sea in warm areas (over warm water)
In the US and its Territories, defined as coastal regions where basic wind speed exceeds 90 mph

19. Design (Basic) Wind Speed
Averaged over a 3 second interval
Highest peak 3-second gust speed during past 50 years
Measured at 33 ft (10 m) above ground

20. Basic Wind Speed Map of US

21. Wind Direction
Assume wind comes from all directions, even though it may not.

22. Induced Pressure & Suction: Wind always exerts pressure perpendicular to the building surface.

23. Wind pressure on a pitched roof

24. Facts about wind loads
Wind load is expressed in terms of wind pressure (psf)
Wind pressure is proportional to the square of wind speed
Simplified equation works for low rise buildings:
Wind pressure on a building component is the difference between inside air pressure and outside air pressure

25. Other Factors That Affect Wind Loads
Wind speed increases with height above ground
Wind pressure is dependent upon exposure classification
Wind at the top of a hill is higher than on flat land
Internal pressure varies with building enclosure shape and air tightness
Important building occupancies designed for 100-year recurrence interval

26. Topography

27. Site Exposure Categories

28. Enclosure classification

29. Enclosure classification

30. Roof Snow Load Ground snow load Roof slope
Wind exposure classification
Warm roof or cold roof
Building importance

31. Excerpt from Table 3.2: Approximate Ground Snow Loads for Selected Locations
Snow Load (psf)
Dallas, Texas 3
Seattle Washington 18
Spokane Washington 42
Charleston, W. VA.
Madison, Wisconsin 35

32. Earthquakes Ground shaking Landslides Surface fractures
Soil liquifaction
Tsunamis
Fires

33. Map of major tectonic plates

34. Fault, focus & epicenter of an earthquake

35. Worldwide Frequency of Earthquake Occurrence
Descriptor
Richter magnitude
Average frequency per year
Great
≥8.0
0 - 1
Major
7 – 7.9 17
Strong
6 – 6.9
134
Moderate
5 – 5.9
1,319
Light
4 – 4.9
13,000
Minor
3 – 3.9
130,000
Very minor
2 – 2.9
1,300,000

36. Location of seismic activity in US

37. Richter Scale Generally used measure of the intensity of an earthquake
Total energy released b an earthquake (E) is proportional to Richter magnitude (R)
E  101.5R
A magnitude 6.5 and above is considered significant

38. Factors that determine earthquake loads
Ground acceleration
Building’s mass and ductility of structural frame
Type of soil
Importance of building

39. Deformation due to ground movement
Total earthquake load:
inertial force created in a building as a result of ground acceleration

40. Site Class Site Class A Hard Rock Site Class B Rock
Site Class C Soft Rock
Site Class D Stiff Soil
Site Class E Soft Soil
Site Class F Liquefiable Soil

41. Seismic Use Group Group III Most important or most hazardous buildings
Important or hazardous buildings
Group I
Normal buildings – buildings that do not belong to Group III or Group II

42. Lightweight structures resist earthquake forces well

43. Wind vs. Earthquake: design for whichever causes the worst effect (greater stresses)
Acts on building & its contents
Acts on building enclosure
Structure must remain intact in the event of an intense earthquake, even though it may be permanently deformed
Structure suffers no permanent damage under worst conditions

44. Tornado damage: Bank One Building Fort Worth, TX

45. Earthquake damage: unreinforced masonry panels in buildings with concrete structural frames

46. Principles in Practice: Live Load and Dead Load Estimation
Tributary area of a building component
Area or areas of a building which contribute load to a component

47. Principles in Practice: Live Load and Dead Load Estimation
Tributary area of beam A

48. Principles in Practice: Live Load and Dead Load Estimation
Tributary area of a column

49. Principles in Practice: Live Load and Dead Load Estimation
Example 1: Estimating dead load on a beam

50. Principles in Practice: Live Load and Dead Load Estimation
Example 1: Estimating dead load on a beam
Solution

51. Principles in Practice: Live Load and Dead Load Estimation
Example 3: Estimating total gravity load of typical wood frame residential floor
Solution:
The dead load of the floor is 10 psf, Example 2
The live load on a residential floor is 40 psf, Table 3.1
Hence, the total gravity load (dead load + live load) is = 50 psf

52. Principles in Practice: Live Load and Dead Load Estimation
Example 4: Estimating dead load of wood frame residential floor with concrete topping
Solution:
The dead load of a typical residential wood floor is 10 psf, Example 2
Volume of 1 sq ft of concrete topping = 1.0 x = ft3
(note that 1 ½ in. = ft)
Weight of 1 sq ft of concrete topping = x 100 = 12.5 lb
Hence, the total dead load of the floor is = 22.5 psf