Ch 02 Concepts of Construction

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Concepts of Construction

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Objectives (1 of 2)

• Identify the variety of forces that buildings are subjected to

• Identify the different types of loads and how they are applied to buildings

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Objectives (2 of 2)

• Identify and define key different types of structural members such as columns, beams, and walls

• Identify the variety of different types of structural connections and how they behave in a fire

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Introduction

• How will the buildings in your community react to fire?

• Prefire inspection

• The laws of physics

• Knowledge gained from the field

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Gravity

• Exerts a continued force on a building

• The gravity resistance system

• Unintended loads imposed by fire

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Definition of Loads

• Forces or other actions that result from the weight of all building materials, occupants and their possessions, environmental effects, differential movement, and restrained dimensional changes

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Stress and Strain

• Stress

• Measured in pounds per square inch (psi)

• KIP = 1000 pounds

• Strain

• Actual percent of elongation (deformation)

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Three Types of Forces

• Compression

• Tension

• Torsion

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Dead Loads

• The weight of the building itself and any equipment permanently attached

• Added dead load

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Live Loads (1 of 2)

• Any load other than a dead load

• Assumes even distribution

• Concentrated load

• Accounts for large heavy objects in a building

• Larger than live load requirements

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Live Loads (2 of 2)

• Water trapped on a roof

• Firefighting equipment

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Impact Loads

• Loads delivered quickly

• Lateral impact loads

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Static and Repeated Loads

• Static loads

• Applied slowly and remain constant

• A heavy safe

• Repeated loads

• Applied intermittently

• A rolling bridge crane

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Wind Loads (1 of 3)

• A force applied by the wind• Small wood

buildings are susceptible

• Masonry buildings are less susceptible

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Wind Loads (2 of 3)

• Diaphragm Floors and Bracing

• Designed to stiffen the building against wind

• Diagonal braces

• K-bracing

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Wind Loads (3 of 3)

• Types of Bracing

• Portal bracing

• Masonry walls

• Tube construction 

• Core construction

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Concentrated Loads

• Heavy loads located at one point in a building

• A steel beam resting on a masonry wall

• Piers

• Short columns used to level floors

• Create a gap between the floor and related arch

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Axial Load

• A force that passes through the centroid of the section

• Centroid

• Eccentric load

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Fire Loads

• Represents the potential fuel available to a fire and the total amount of potential energy (heat) in the fuel

• Heat release rate (HRR)

• Q 

• Caloric value

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Suspended Loads

• Suspended from a ceiling or other structure

• Tie rod 

• Suspended high-rise buildings use suspended beams

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Safety Factor

• Represents the ratio of the strength of the material to the safe working stress

• Ultimate strength

• Design load is a fraction of the ultimate strength

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Composite Materials

• Steel and concrete combined

• Can be constructed with bar-joist trusses

• Brick and block-composite wall

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Structural Elements

• Beams, columns, arches, and walls

• Differ on how they support and transfer weight

• Structural frames

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Beams

• Transmits forces perpendicular to the reaction points

• Deflection

• Neutral axis

• Stiffness

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Carrying Capacity and Depth of Beams

• Increases as the square of the depth

• Increases only in direct proportion to increases in width

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Types of Beams (1 of 2)

• Simple • Continuous • Fixed• Overhanging• Bracket• Joist, steel joist, and bar joist • Girder

2

Types of Beams (2 of 2)

• Built-up girder • Spandrel girder • Lintel • Grillage • Cantilever beam • Suspended beam• Transfer beam

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Beam Loading

• Distribution of loads along a beam

• May be concentrated in one area

• Create excessive beam loading

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Reaction and Bending

• Reaction

• Result of force exerted by a beam on a support

• Bending moment

• Load that will bend or break the beam

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Columns• Transmit a

compressive force along a straight path• Struts or rakers• Bent• Bay• Pillar

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Decrease in Load-Carrying Capacity

• Decrease in load-carrying capacity proportionately

• 12-foot column vs. 6-foot column

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Shapes of Columns

• Most efficient

• Distributes the material equally around the axis

• As far as possible from the center of the cylinder

• Difficult to attach beams to round columns

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Wooden Columns

• Smoothed off tree trunks

• Almost always hollow

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I-Beams vs. H-Beams

• Steel beams

• I-shaped

• Steel columns

• H-shaped

• Box shaped

• Cylindrical

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Types of Columns

• Piers • Short, squat columns • Fail by crushing

• Long, slender columns• Buckle

• Intermediate columns • Fail in either manner

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Euler’s Law Columns

• Very long thin columns

• Critical load

• Formula: Pc = (π2EI)/L2

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Temporary Bracing

• Used in incomplete structures

• Also known as guy bracing

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Walls (1 of 3)

• Transmit the compressive forces applied along the top or received at any point

• Main divisions of walls:

• Load-bearing

• Non-load-bearing

• Cross

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Walls (2 of 3)

• Veneer • Composite • Header • Stretcher • Panel • Curtain • Party

2

Walls (3 of 3)

• Fire

• Partition

• Demising

• Rubble masonry

• Wythe 

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Cantilever Walls

• Severe winds

• May topple a free-standing wall

• Eccentric loading

• Precast concrete tilt slab walls

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Wall Bracing

• Serpentine

• Buttresses

• Pilasters

• Wall columns

• Cavity or hollow

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Wall Breaching

• Homogenous walls

• Act as one unit

• Obstructed load

• Window or door

• Does not follow a vertical path

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Roofs

• Vital to the stability of the structure

• Roof damage

• Can cause wall collapse

2

Arches

• Combine the function of the beam and column

• Tend to push outward at the base

• Voussoir

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Rigid Frames

• Derived from the arch

• Steel rigid frames

• Industrial and commercial buildings

• Wooden rigid frames

• Churches

2

Shells and Domes

• Shell

• Thin, curved plate

• Concrete

• Dome

• A shell and three dimensional arch

• Geodesic domes

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Transmission of Loads

• Transmission

• Spread from the point of application to the ground

• Must be continuous

• Any failure will lead to collapse

• Accurate knowledge of the ground

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Foundations

• Delivers loads to the ground

• Foundation materials differ

• Almost all are concrete

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Connections (1 of 3)

• Transfer the load from one structural element to another

• Only as strong as its weakest link

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Connections (2 of 3)

• Pinned

• Elements are connected by simple connectors such as bolts, rivets, or welded joints

• Rigid-framed building

• Connections are strong enough to reroute forces if a member is removed

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Connections (3 of 3)

• Monolithic concrete 

• Wet joint 

• Plastic design 

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Failure of Connections (1 of 2)

• Masonry walls shift outward

• Temporary field bolting of steel

• Ledger board 

• Steel connectors rust

• Concrete disintegrates

• Sand-lime mortar is water soluble.

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Failure of Connections (2 of 2)

• Gravity connections

• Gusset plates

• Gang nails

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Overhanging and Drop-in Beams

• Economical

• No support from below

2

Spliced Beams

• If beam not long enough

• Spliced with connectors

• Connectors fail in heat

2

Summary

• Gravity

• Gravity resistance system

• Specific terms

• Structural elements

• Composite material

• Connections