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CHAPTER 3

Loads in Buildings

Introduction

Loads in Buildings

Two Main Classifications of Loads 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 kilo-pounds (lbs, kips)

Loads on Building Elements

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Gravity Loads

Dead Loads

Loads in Buildings

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 Loads

Loads in Buildings

Live Load

¨  Load whose magnitude and placement changes with time. ¤ Floor live load

n Varies with occupancy type n Generally calculated as uniform loads in psf, except in special

cases (parking garage - loads concentrated on car tires)

¤ Roof live load (generally 20 psf) n  Includes weight of repair personnel and temporary storage. n  If the roof snow load is greater than the live load, it is used

instead.

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Rain Load

Loads in Buildings

Rain Load

¨  Occur as a result of accidental accumulation of melted snow or rainwater ¤ Most significant in long-span, relatively low slope

roof. These roofs require: n Adequately stiff roof assemblies n Minimum slope: 1/4 inch per 1 ft n Secondary drains (parapet roofs)

Lateral Loads

Wind Load Basics

Loads in Buildings

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 and the Diagonal Brace

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Diagonal Bracing in Buildings

¨  John Hancock Center, Chicago

Diagonal Bracing in Buildings

¨  New York Times Building, New York City

Low probability, High potential for destruction

Tornados Tornado-Prone Regions in the U.S.

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 700

years ¨  Measured at 33 ft (10 m) above ground

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Basic Wind Speed Map of the U.S. Induced Pressure and Suction

Wind always exerts pressure perpendicular to the building surface.

Wind Pressures on a Pitched Roof

Factors That Affect Wind Loads

Loads in Buildings

Factors That Affect Wind Loads

¨  Height above ground ¨  Exposure classification of the site ¨  Enclosure classification of the building

Site Exposure Categories

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Ballooning of Building

Ballooning of a building with a relatively large opening in one wall. Such a building is referred to as a partially enclosed building. A building that is not partially enclosed is called an enclosed building . Most buildings belong to the enclosed building category.

Roof Snow Load

Loads in Buildings

Roof Snow Load

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

Ground Snow Loads

Earthquake Load

Loads in Buildings

Earthquakes

¨  Ground shaking ¨  Landslides ¨  Surface fractures ¨  Soil liquefaction ¨  Tsunamis ¨  Fires

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Map of Major Tectonic Plates Fault, Focus, and Epicenter

Frequency of Earthquake Occurrence Seismic Activity in the U.S.

Map of the United States showing approximate variation of seismic activity. The darker the area, the greater the seismic activity. White areas have little or no seismic activity.

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 Affect Earthquake Loads

Loads in Buildings

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Factors That Affect Earthquake Loads

¨  Ground motion ¨  Building’s mass and ductility of the structural frame ¨  Type of soil ¨  Building occupancy’s risk category

Ground Movement & Deformation

¨  Total earthquake load: ¤  Inertial force created

in a building as a result of ground acceleration

Wind vs. Earthquake Resistance of Buildings

Loads in Buildings

Earthquake Resistance (Ductility)

¨  A tent resists earthquake forces admirably due to its light weight and its ability to absorb deformations.

Wind vs. Earthquake

Earthquake Wind

Acts on building and 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

Design for whichever causes the worst effect (greater stresses)

Tornado Damage

¨  Bank One Building Fort Worth, TX

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Earthquake Damage

¨  Unreinforced masonry panels in buildings with concrete structural frames. PRINCIPLES IN PRACTICE:

Dead Load and Live Load Estimation

Loads in Buildings

Live Load and Dead Load Estimation

¨  Tributary area of a building component: ¤ Area or areas of a building which contribute load to a

component

Live Load and Dead Load Estimation

Live Load and Dead Load Estimation Example 1:

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Example 1 – cont’d Example 2:

Example 2 – cont’d Example 3:

Example 4: