1. Concepts of Construction
Chapter 2
Concepts of Construction

2. Objectives
Identify the variety of forces to which buildings are subjected.
Identify the different types of loads and how they are applied to buildings.

3. Objectives
Define safety factor and explain how it affects the choice of materials used in a building assembly.
Identify composite materials and explain how the combination of two different materials affects structural elements.

4. Objectives
Identify and define key types of structural members such as beams, trusses, columns, walls, roofs, and arches.
Identify the various types of structural connections and explain how they affect transmission of loads and behave in a fire.

5. Introduction Ages of different construction Law of physics
New technologies and techniques
Community
Know how buildings will react to fire
Law of physics
Forces of nature and man
Can’t rely soley on learning from fire-ground experience

6. Gravity Exerts a continued force on a building
Buildings appear to be sitting on foundations, but are under great stresses
The gravity resistance system

7. 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

8. Stress and Strain Stress Strain
Measured in pounds per square inch (psi)
KIP = 1000 pounds
Strain
Actual percentage of elongation (deformation)

9. Four Types of Forces Compression Tension Torsion Shear
© Brandon Holmes/ShutterStock, Inc.

10. Dead Loads
The weight of the building itself and any equipment permanently attached
Added dead load
© Donald King/AP Images

11. Live Loads Any load other than a dead load Concentrated load
Assumes even distribution
Concentrated load
Accounts for large heavy objects in a building
Higher psf than live load requirements

12. Live Loads Added live loads Any non-dead load added after construction
Flat-roof buildings often are unable to withstand added live loads
© Design Pics Inc./Alamy Images.

13. Impact Loads Loads delivered quickly Lateral impact loads
Example: September 11, 2001, jetliner attacks
Lateral impact loads
Courtesy of Glenn Corbett.

14. Impact Loads Modifications due to terrorism:
Increased building setback
Bollards and barriers
Security windows
© Jay Lazarin/iStockphoto

15. Static and Repeated Loads
Static loads
Applied slowly and remain constant (a heavy safe)
Repeated loads
Applied intermittently

16. Wind Loads A force applied by the wind.
Small wood buildings are susceptible.
Masonry buildings are less susceptible.
Courtesy of Glenn Corbett.

17. Wind Loads Bracing Shear walls resist lateral loads. Braced frames
Moment frames
Hurricane bracing
Courtesy of IIBHS - The Insurance Institute for Business & Home Safety

18. Wind Loads Key factors High-rise buildings must factor in wind forces.
K-bracing
Portal bracing
© Medioimages/Photodisc/Thinkstock

19. Wind Loads Key factors (cont’d)
Diaphragm floors are designed to stiffen the building against wind.
Masonry walls do double duty as shear walls.
High-rise buildings take wind load on the exterior walls.

20. Wind Loads Key factors (cont’d) Megastructure Tube construction
Core construction
Vierendeel trusses

21. Concentrated Loads Heavy loads located at one point in a building
A steel beam resting on a masonry wall
Courtesy of the estate of Francis L. Brannigan.

22. Concentrated Loads Wall columns may carry a concentrated load weight.
Piers
Short columns used to level floors
Can create a gap between the floor and related arch

23. Axial Load A force that passes through the centroid of the section
Courtesy of Glenn Corbett.

24. Axial Load Centroid = center point
Eccentric load = force perpendicular to the plane of the section
Courtesy of Glenn Corbett.

25. Fire Loads
Represent 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

26. Suspended Loads Suspended from a ceiling or other structure Tie rod
Suspended high-rise buildings use suspended beams.

27. 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.

28. Composite Materials Steel and concrete combined
Can be constructed with bar-joist trusses
Brick and block-composite wall
© Graham Prentice/ShutterStock, Inc.
© Remi Cauzid/ShutterStock, Inc.

29. Structural Elements Beams, columns, arches, and walls
Differ on how they support and transfer weight
Structural frames

30. Beams Transmit forces perpendicular to the reaction points Deflection
Neutral axis
Stiffness

31. Cable Cable is an ideal beam. Cable-supported roofs
Truss shapes over mass

32. Carrying Capacity and Depth of Beams
Capacity increases as the square of the depth.
Increases only in direct proportion to increases in width

33. Types of Beams Simple Continuous Fixed Overhanging Bracket
Joist, steel joist, and bar joist
Girder

34. Types of Beams Built-up girder Spandrel girder Lintel Grillage
© Meredith Lamb/ShutterStock, Inc.

35. Types of Beams Cantilever beam Needle beam Suspended beam
Transfer beam
© Raymond Forbes/age fotostock

36. Beam Loading Distribution of loads along a beam
May be concentrated in one area
Create excessive beam loading
Courtesy of FEMA.

37. Reaction and Bending Reaction Bending moment
Result of force exerted by a beam on a support
Bending moment
Load that will bend or break the beam

38. Trusses
A triangle or group of triangles arranged in a single plane such that loads applied at the intersection points will cause only direct stresses
Only support axial loads
Separate compressive and tensile stresses

39. Trusses Components Struts, ties, and panel points = the web Chords

40. Trusses
Various types of trusses

41. Compression and Tension
Top chord is in compression
Bottom chord is in tension
Cantilevering reverses this

42. Truss Principles
Doubling the length of a beam decreases its carrying capacity by half.
Inverted king post truss
Queen post truss

43. Problems with Trusses
If one element of a truss fails, the entire truss will fail.
Failure of one truss causes multiple failures.
Problems include:
Rising roofs
Unforeseen defects
Defective design and poor maintenance

44. Truss Failures in Fire Loss of a gusset plate could pull plate apart.
Truss can burn through at panel points.
Room partitions in apartment houses support falling trusses.

45. Columns Transmit a compressive force along a straight path
Struts or rakers
Bent
Bay
Pillar
© S.Borisov/ShutterStock, Inc.

46. Load-Carrying Capacity in Columns
Decrease in load-carrying capacity proportionately
Columns lose strength by the square of the change in length.

47. Shapes of Columns
Most efficient shape distributes material equally around the axis as far as possible from cylinder’s center.
Difficult to attach beams to round columns

48. Wooden Columns
Smoothed-off tree trunks
Almost always hollow

49. I-Beams vs. H-Beams Steel beams Steel columns I-shaped H-shaped
Box shaped
Cylindrical

50. Types of Columns Piers Long, slender columns Intermediate columns
Short, squat columns
Fail by crushing
Long, slender columns
Buckle
Intermediate columns
Fail in either manner

51. Euler’s Law Columns Very long, thin columns
The addition of even a single atom over the critical load can cause sudden buckling and collapse.
Formula: Pc = (π2EI)/L2

52. Euler’s Law Columns
By bracing a column you can increase the load-carrying capacity.
As a column gets longer, its load-carrying capacity decreases.

53. Temporary Bracing Used in incomplete structures
Also known as guy bracing
May not be adequate to resist high winds or other unexpected loads

54. Walls
Transmit the compressive forces applied along the top or received at any point
Main divisions of walls:
Load-bearing
Non-load-bearing

55. Walls Types of walls: Cross Veneer
Courtesy of the estate of Francis L. Brannigan.

56. Walls Types of walls (cont’d):
Composite: composed of two or more masonry materials that react together under load.
Courtesy of the estate of Francis L. Brannigan.

57. Walls Types of walls (cont’d): Panel Curtain Party
Courtesy of Glenn Corbett.

58. Walls Types of walls (cont’d): Fire Partition Demising Rubble masonry
Wythe 
Courtesy of Achim Hering.

59. Cantilever Walls Cantilever wall
Severe winds may topple a freestanding wall
Eccentric loading
Precast concrete tilt slab walls
Courtesy of the estate of Francis L. Brannigan.

60. Wall Bracing Serpentine Buttresses Pilasters Wall columns
Cavity or hollow
Courtesy of Glenn Corbett.

61. Wall Breaching Homogenous walls acts as one unit
A load coming down the wall to a window or doorway does not follow a vertical path; it splits and passes on both sides of the opening.

62. Roofs Vital to the stability of the structure Roof damage
Can cause wall collapse

63. Roofs
Different roof styles.

64. Arches Combine the function of the beam and column
Tend to push outward at the base
Voussoir
Courtesy of Glenn Corbett.

65. Rigid Frames Derived from the arch Steel rigid frames
Industrial and commercial buildings
Wooden rigid frames
Churches

66. Shells and Domes Shell Dome Thin, curved plate Concrete
A shell and three-dimensional arch
Geodesic domes
Courtesy of Glenn Corbett.

67. 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

68. Foundations Deliver loads to the ground Foundation materials differ.
Almost all are concrete.

69. Connections Transfer the load from one structural element to another
Only as strong as weakest link

70. Connections Pinned Rigid-framed building
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.

71. Connections Monolithic concrete Wet joint Plastic design
© angelo gilardelli/ShutterStock, Inc.

72. Failure of Connections
Masonry walls shift outward.
Temporary field bolting of steel fails.
Steel connectors rust.
Concrete disintegrates.
Sand-lime mortar is water soluble.

73. Failure of Connections
Gravity connections
Gusset plates
Gang nails
© Photodisc

74. Failure of Connections
A fire cut at the end of a beam.
A wall anchor box (bottom) and a post cap (top).

75. Overhanging and Drop-in Beams
Economical
No support from below

76. Spliced Beams Long wooden beams are not always available
Shorter beams are spliced together with metal connectors to produce the desired length.
Connectors may fail when heated sufficiently.

77. Summary All buildings are subject to one enemy: gravity.
A load in a building works to destroy the gravity resistance system of the structure.
Specific terms are used to describe the types of different loads and the way they are placed on a building.

78. Summary
The safety factor represents the ratio of the strength of the material just before failure to the safe working stress.
At times, two materials are combined to take advantage of the best characteristics of each, making a composite material.

79. Summary Buildings are made up of structural elements.
The manner in which loads are transmitted from the point of application of the ground affects a building’s stability.
Connections are a vital part of a structure’s gravity resistance.