Building Construction Related to the Fire

Service

Chapter 3 — The Way Buildings are Built: Structural Design

Features

Learning Objective 1

Explain the various loads exerted on a building resulting from environmental sources.

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Load and Gravity

• Load – Any effect or force that a structure must resist

• Gravity– Force on building through weight of

building components and contents– Snow, ice, or water on building adds

weight– Total weight of building exerts force on

soil beneath itBuilding Construction

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Wind

• Basic effects– Direct pressure– Aerodynamic drag– Negative pressure

• Secondary effects– Rocking effect– Vibration– Clean-off effect

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Calculating Direct Pressure of Wind

p = Cv(2)v = velocityp = static pressureC = numerical constant (0.00256) that accounts for air mass and assumptions of building behavior

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Earthquakes

• Slippage between tectonic plates of earth’s surface

• Subjects buildings to seismic forces• Occur most frequently in fault zones• Seismic loads far more complex

than those for wind• Vibrational motion can be 3-

dimensionalBuilding Construction

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(Continued)

Earthquakes

• Torsional, resonant forces• Force magnitude depends on factors• Most significant force is horizontal

motion• Buildings with geometric irregularities

more susceptible to earthquake damage• Building codes require stronger seismic

bracing for buildings with larger occupancies

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(Continued)

Earthquakes

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(Continued)

Earthquakes

• Design methods to protect against forces– Increase stiffness using symmetrical

shear walls and cross bracing– Continuous structures with high

degree of redundancy in structural frames

– Damping mechanisms– Base isolation

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Soil Pressure

• Exerts horizontal pressure against foundation

• Can only be estimated

• Magnitude depends on several factors

• Active or passive

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Determining Soil Pressure

p = Cwhp = pressureh = depth of soilw = density of soilC = numerical constant that depends on the physical properties of the soil

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Other Forces

• Temperature• Vibration• Shrinkage

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Learning Objective 2

Distinguish between the classifications of loads based on origin and movement.

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Live Loads and Dead Loads

• Classification of types of forces resulting from gravity

• Dead – Weight of any permanent part of a building

• Live – Any load that is not fixed or permanent

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(Continued)

Live Loads and Dead Loads

• Distribution of loads– Uniformly distributed loads– Concentrated load

• Snow loads• Water loads

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Static and Dynamic Loads

• Static – Steady or applied gradually– Dead loads– Many live loads

• Dynamic – Involves motion– Wind, earthquakes, vibration, falling

objects– Capable of delivering energy in

addition to weight

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Structural Equilibrium and Reactions

• Equilibrium – When support provided by structural system is equal to the applied loads

• Reactions – Forces that resist the applied loads

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Learning Objective 3

Recognize and discuss the internal forces resulting from the loads and forces applied to a structural member.

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Interior Forces Createdby Exterior Loads

• Tension – Pull material apart• Compression – Squeeze material• Shear – Slide one plane past an

adjacent plane• Strength of materials varies with

direction of interior forces

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(Continued)

Interior Forces Createdby Exterior Loads

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(Continued)

Interior Forces Createdby Exterior Loads

• Stress – Quantity used to evaluate magnitude of interior forces

• Factors of safety• Stresses typically occur in

combination within a member• Keeping stresses within allowable

values determines shape and size of structural members

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Exterior Loads Classified by Manner Applied

• Axial – Applied to center of cross section of structural member and perpendicular to that cross section

• Eccentric – Perpendicular to the cross section of the structural member but does not pass through the center of the cross section

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(Continued)

Exterior Loads Classified by Manner Applied

• Torsional – Offset from center of cross section of the structural member and at an angle to or in the same plane as the cross section

• Loads change under fire conditions due to thermal energy released

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(Continued)

Exterior Loads Classified by Manner Applied

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Learning Objective 4

Describe the basic structural components.

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Beams

• Simply supported• Cantilever• Continuous• Restrained• Primary design consideration –

Ability to resist bending from applied loads

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(Continued)

Courtesy of Ed Prendergast

Beams

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Columns

• Designed to support an axial compressive load

• Thin columns fail by buckling

• Short, squatty columns fail by crushing

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Arches

• Interior stresses primarily compressive

• Produce inclined forces at end supports

• Carry loads across a distance

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Courtesy of Donny Howard

Cables

• Flexible members used to support roofs and brace tents and restrain pneumatic structures

• When used to support loads over distance, will assume shape of parabola

• Stresses are tension

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Trusses

• Framed units made up of group of triangles in one plane

• If loads applied at only point of intersection of truss members, only compressive or tensile stresses

• If has curved top chord, subjected to bending forces

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(Continued)

Trusses

• Provide inherently rigid frame• Potential for early failure under

adverse conditions; failure of any portion of top or bottom chords results in failure of truss

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(Continued)

Trusses

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Space Frames

• Truss structures developed in three dimension

• Suited to support uniformly distributed loads

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Courtesy of Ed Prendergast

Truss Connectors

• Wood truss – Pins, bolts, gusset plates, adhesives, brackets, metal straps

• Steel trusses – Steel gusset plates, rivets, welds

• If connector fails, truss will fail

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Learning Objective 5

Describe the basic structural systems.

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Structural Bearing Walls

• Span elements such as beams, trusses, and precast concrete slabs

• Usually exterior walls with interior support system of columns and beams

• Provide lateral support along direction of the wall

• Subjected to compressive loadsBuilding Construction

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Frame Structural Systems

• Walls enclose frame but provide no structural support

• Steel stud wall framing• Post and beam construction• Rigid frames• Truss frames• Slab and column

frames

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Courtesy of Ed Prendergast

Shell and Membrane Systems

• Consist primarily of enclosing surface with stresses resulting from applied loads occur within the surface

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Courtesy of Ed Prendergast

Membrane Structure

• Thin stretched flexible material• Addressed in building codes if life

of 180 days or more• Addressed in fire codes if life of

180 days or less• Cannot resist compressive forces• Advantages

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Shell Structure

• Rigid three-dimensional structures having a thickness that is small compared to other structural materials

• Regular geometric shapes• Most commonly constructed of

concrete

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Summary

• Forces exerted on buildings arise from many sources and determine how a building’s structural system is designed.

• A variety of structural components are available to support the applied loads.

• Under fire conditions, loads and stresses are subject to change in magnitude and direction.

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Building Construction

Review Questions

1.What is a load?2.What factors determine the magnitude of the forces developed within a building during an earthquake?3.What is a dead load?

(Continued)

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Review Questions

4.How do dynamic loads differ from static loads?5.What is a membrane structure?

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