Chapter 3 Loads on Buildings

Loads on Buildings

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

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)

Loads on Building Elements

Gravity Loads

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

Roof Dead load Calculated over horizontal projected area of roof.

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.

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)

Lateral Loads

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)

Racking & the diagonal brace

John Hancock Center, Chicago

Sears Tower, Chicago

Tornadoes Low probability, high potential for destruction

Tornado-prone regions in US

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

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

Basic Wind Speed Map of US

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

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

Wind pressure on a pitched roof

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

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

Topography

Site Exposure Categories

Enclosure classification

Enclosure classification

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

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

Earthquakes Ground shaking Landslides Surface fractures Soil liquifaction Tsunamis Fires

Map of major tectonic plates

Fault, focus & epicenter of an earthquake

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

Location of seismic activity in US

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

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

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

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

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

Lightweight structures resist earthquake forces well

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

Tornado damage: Bank One Building Fort Worth, TX

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

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

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

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

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

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

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 10 + 40 = 50 psf

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 0.125 = 0.125 ft3 (note that 1 ½ in. = 0.125 ft) Weight of 1 sq ft of concrete topping = 0.125 x 100 = 12.5 lb Hence, the total dead load of the floor is 10 + 12.5 = 22.5 psf