Building Construction Related to the Fire Service 4th Edition

Building Construction Related to the Fire Service 4th Edition
Chapter 3 — Structural Design Features of Buildings

Terminal Objective Describe various forces and loads placed upon buildings and how these affect structural components and systems

Enabling Objectives 1. Explain various forces, stresses, and loads exerted on the structural design features of a building. 2. Describe common load-bearing structural components. 3. Identify commonly encountered composite structural systems. 4. Identify Florida’s criteria for designation as an approved Nationally Recognized Testing Laboratory. (FL Objective)

Enabling Objectives 5. Recognize commonly used internet websites for most NRTL’s. (FL Objective) 6. Identify Florida Building Code Section 721 as having procedures to determine fire resistance. (FL Objective) 7. Explain additional residential occupancies. (FL Objective) 8. Describe lightweight truss markings as covered in F.S and FAC 69A (FL Objective)

Structural supports allow a building to withstand common forces.
To calculate the structural supports needed, loads are categorized and calculated The direction of the forces from loads acting on the interior of structural members is expressed as stress

The forces within a structural support system that resist applied loads are referred to as reactions R. A uniformly applied load on a beam will stress each of the supports equally

Equilibrium exists when a structural support system can support a load equal to or greater than the applied loads. Reactions R must equal or surpass the applied loads L to achieve structural equilibrium

Loss of equilibrium can lead to partial or total collapse
Equilibrium exists when a structural support system can support a load equal to or greater than the applied loads. Loss of equilibrium can lead to partial or total collapse A common type of reaction is a bending moment within a horizontal structural component loaded vertically When vertical load exceeds component strength, the component will bend and possibly fail

The force of gravity is the most common load imposed on a structure.
Gravity-related loads are from the weight of a structure’s: Components Contents Occupancy Activity

Additional forces added to the structure will increase the load supported by a building’s supports.
Natural Building-related Shrinkage Vibration External temperature changes may cause peripheral structural members to expand and contract Temperature As lumber dries over time, dimensions of wooden structural components shrink

Stresses within a material are classified according to the direction of the force.
The direction of interior stresses is important because material properties have unique tolerances. Tension – Pulls material apart Compression – Squeezes Material Shear – Slides one plane of a material past an adjacent plane

Structural members may be constructed in specific shapes and sizes to control multiple stresses.
Structural members may be shaped, arranged, and supported in specific ways to manage the expected stresses

Loads applied to the exterior of a structural member create internal stresses within the member.
Based on the placement of the load External forces often can be visually identified and evaluated Interior forces must be calculated

Unique stresses are caused by external load alignments.
Axial loads are applied along the center of the cross- section, and may evenly load the component Eccentric loads are applied to one side of the cross-section, and may bend the component Torsional loads are applied at an angle to the cross-section, and may twist the component

For a structure to remain intact, the total stresses within members must be lower than the material’s failure point. Failure Point of the Material Maximum Supportable Design Stress Minimum Required Strength Factor of Safety This allowance adds tolerance for variations in the properties of the construction materials, workmanship, and live and dead loads.

Structural failure of one component may compromise others
Failure due to stress may include visible indicators, such as cracking, crumbling, bending, and breaking. Structural failure of one component may compromise others Courtesy of West Allis (WI) Fire Department.

Any change can increase the probability of structural failure
Loads may shift slowly over time, or more quickly when affected by large-magnitude forces. Loads must be anticipated and supported for a structure to remain upright during its working life Any change can increase the probability of structural failure

A dead load is fixed in location and quantifiable.
Dead loads include any structural elements that do not change significantly over the lifetime of the structure

A live load is not fixed or permanent.
Live loads can change significantly at relatively short intervals

A live load is not fixed or permanent.
Actual weight and distribution is often not precisely quantifiable Varies by occupancy Building codes specify minimum live loads for different occupancies Actual live loads that exceed code requirements must be used in the design calculations

Loads applied to buildings may be classified according to the rate of speed at which they are applied to a structure. Static Loads Steady or are applied gradually Can reasonably predict that the constant force needed to be supported will equal the weight of the object Dynamic Loads Involve motion and are capable of delivering energy greatly in excess of the weight of the object involved May cause structural failure

Dynamic loads involve motion, including impact from falling objects.
Dynamic loads may include impact from: Wind Moving Vehicles Earthquakes Vibration Falling Objects Emergency/Maintenance Work

Dynamic loads involve motion, including impact from falling objects.
Kinetic Energy of a Falling Object E = 1/2mv² E = Kinetic energy M = Mass of an object V = Velocity

For a dynamic load to stop moving, the surface it impacts must absorb the kinetic energy.
Dynamic loads may cause structural failure or loss of resiliency Whether the surface can withstand the dynamic load depends on: Design strength Energy-absorbing properties of the materials used to support the surface

Loads may be widely distributed or contained in a small area.
Differences in structural loads of the same magnitude can be significant Concentrated Load Uniformly Distributed Load

Concentrated loads produce highly localized forces and non-uniform loads in the supporting structural members. Concentrated loads include heavy machinery, such as an industrial paper cutter.

Concentrated loads produce highly localized forces and non-uniform loads in the supporting structural members. Structural supports must be designed to accommodate the anticipated loads When a concentrated load is known and exceeds the uniform load values in the code, the minimum support needed must be used in the design calculations

Rain and snow are live loads.
Roof features facilitate drainage Ponding can occur on large flat roofs Rain Roof accumulation depends on the slope/shape of the roof and effects of adjacent structures Amount expected on the ground is used in calculating the snow load on a roof Building codes contain requirements for snow loads Snow load calculated for the roof may be lower than live loads used for floors Snow

Water from firefighting operations can add an additional live load to a building.
Dewatering operations may be necessary since a water depth of 3 inches (75 mm) adds a static load of 21 pounds per square foot (1 kPa) Dynamic Static

Air is a gas that has mass
The kinetic energy of air manifests as wind that presents a force that can be calculated. Air is a gas that has mass Kinetic Energy of Wind E = 1/2mv² E = Kinetic energy M = Mass of an object V = Velocity

Factors such as wind speed and direction may influence the overall effect of wind.
Effects of Wind Direct Pressure – Straight-line winds apply force Drag – Wind flowing around an object may catch along a building’s surface Negative pressure – Suction effect on downwind side of a building

Effects of Wind Rocking – Building sways in a back-and-forth motion Vibration – Wind passing over a surface may shake the surface Clean-off – Dislodge or move objects

Wind Load Considerations When Designing Buildings Direct pressure is the primary consideration Wind velocity Static air pressure Assume wind direction perpendicular to the building wall Wind pressure increases with increases in wind velocity

Building design can influence the effects of wind.
Smooth contours in a building will shed wind forces more easily than contour features that catch the wind.

Building design can influence the effects of wind.
Complex Factors Engineers must Account For Building height Surrounding terrain Adjacent urban development Common Locations where Wind Forces are Dangerous against Walls with Insufficient Supports Construction sites Demolition sites Fire-damaged buildings

Seismic forces apply the most complicated load that must be accommodated by structural design.
Illustration courtesy of U.S. Geological Survey (USGS) Department of the Interior Seismic maps of the United States show areas that are susceptible to earthquake activity.

Seismic forces apply the most complicated load that must be accommodated by structural design.
Forces result from movement between tectonic plates along a fault line or zone As plates move and slip, they produce vibrations (waves) known as earthquakes Explosive detonations may cause a seismic effect similar to earthquakes

Earthquake-prone areas are mapped by organizations
Earthquakes can occur anywhere on Earth, but some places have a higher probability of seismic events. Earthquake-prone areas are mapped by organizations United States Geological Survey National Geographic Reproduced with the permission of Natural Resources Canada 2015.

All model building codes provide seismic maps and design provisions.
Designs are based on duration and magnitude of seismic forces Where expected seismic loads are different than wind or gravitational loads, design plan must accommodate more stringent requirements Per code, some buildings require stronger bracing than seismic map indicates Other buildings must be designed for greater seismic loads because they are essential for community recovery

Seismic load is the application of forces caused by earthquakes.
Overall effect of seismic load on a structure depends on acceleration of the ground beneath the building more than the total movement Seismic loads may be far more complex than wind loads

Movement of the ground beneath a building can be three-dimensional.
Lateral Torsional Resonant

Lateral Loads Create horizontal motion that is the most significant force generated by an earthquake Inertia holds upper portion of building in initial position as lower portion moves with the ground Shear stress develops between upper and lower portions of building Low buildings less susceptible

Torsional Loads applied to the structural member that is twisted by seismic motion Resonant Some buildings are affected differently due to the resonance of the earthquake and building features

Earthquake Video #1 Source: YouTube

Earthquake Video #2 Source: YouTube

Accommodations for seismic forces may be included in new construction or retrofitted to existing buildings. Seismic Expansion Joints Damping Mechanisms Base Isolation A building’s architecture influences the degree by which it is affected by seismic activity

Produces damaging forces at the sections’ junction
Expansion joints can be added to increase the flexibility of a connection. Tall and shorter sections in a building will respond differently to seismic vibrations Produces damaging forces at the sections’ junction

Damping mechanisms absorb resonant energy as the structure begins to move.
Typically installed at the connections between columns and beams Operate on a principle similar to mechanical door equipment

Base isolation isolates the building from the horizontal movement of the earth’s surface.
Courtesy of San Diego County Sheriff’s Department Elastomeric bearings used in base isolation prevent some seismic force from travelling into a building.

Change resonance of the building
Base isolation isolates the building from the horizontal movement of the earth’s surface. Base Isolation Shear Systems Elastomeric bearings placed in a layer between the building and the foundation Change resonance of the building Sliding Systems Special plates that slide on each other to isolate the building from horizontal shear force

Structural stiffening is used to harden a structure against expected loads.
Shear Walls and Cross Bracing Effective against ground motions with a relatively long vibrational period Must be symmetrically located to prevent torsional forces May not be suitable for all locations Courtesy of Tanya Hoover

Structural Support Redundancy
Structural stiffening is used to harden a structure against expected loads. Structural Support Redundancy Redundant structural members support the entire system, making collapse less likely to occur Uses continuous joints that have a greater ability to absorb energy

Soil exerts a lateral load against a foundation and must be evaluated in the design process.
Loads associated with soils are estimated based on historical data provided via building codes.

The magnitude of the soil pressure load depends on the soil’s properties.
Types of Soil Pressure Active Passive Simple Soil Pressure Formula P = CWH P = Pressure C = Depth of Soil W= Density of Soil C = Numerical Constant ( C depends on properties of the soil)

The magnitude of the soil pressure load depends on the soil’s properties.
Active Soil Pressure Pressure exerted by soil against a foundation Soil assumed to behave similarly to a fluid Pressure range from zero at the top of a foundation wall to maximum pressure at the base Passive Soil Pressure Force of foundation against soil Firefighters should recognize the importance of foundation shifts over time

Sand content in soil is particularly relevant when surveying sites with high seismic activity.
Sandy soil usually has some cohesive properties during seismic activity Soil liquifaction occurs when sandy soil near water sources is looser and saturated with water Entrapped water prevents sand particles from moving closer together Reducing the ability of the soil to support a structure

REVIEW QUESTION What common forces, stresses, and loads may impact the structural design of a building?

Structural support components work in tandem within a support system.
Beams Columns Arches Cables Trusses Space Frames Enable the capability of larger structures to withstand: Their own weight Expected loads

Materials used in beams include:
A beam is a structural member that carries loads perpendicular to its longitudinal dimension. Primary design consideration is ability to resist being deformed from the applied loads Materials used in beams include: Steel Wood Reinforced concrete

A beam is a structural member that carries loads perpendicular to its longitudinal dimension.
Supported beneath both ends Free to rotate Simply Supported Rigidly supported at each end May retain its load-bearing ability longer Restrained Supported on one end Must support a vertical load Resists bending stresses Cantilever Similar to cantilever but with additional support Overhanging May span several vertical supports Continuous

Different portions of a beam carries different stresses.
Top Flange — Carries compressive stresses Web — Known as the neutral axis because the tension and compression stresses are zero, but the neutral axis is the maximum point of shear stress Bottom Flange — carries tensile stresses

Beams may be shaped to maximize the ability of the beam to carry the expected load.
Increases the efficient use of material Reduces weight of the beam Some thickness can be removed from the web without greatly affecting the strength of the beam

Many beams are constructed in the shape of the capital letter “I”.
Top and bottom flanges carry most of the load resisting bending stress Web and flanges are usually affixed to each other Stresses can be calculated mathematically Tall beams are capable of supporting greater loads than short beams Courtesy of Dave Coombs

Columns are structural members designed to support an axial load.
Stresses created within a column are primarily compressive Materials used include wood, steel, cast iron, concrete, and masonry Not primarily designed to withstand bending stresses Likely to fail if support beneath the column or beams attached to the column shift out of alignment

An arch is a curved structural member with primarily compressive interior stresses.
Arches produce inclined forces at their end supports, which the supports must resist.

Arches are used to carry loads across a distance.
Often used as support for roofs and entrances in masonry buildings Materials used include masonry, steel, concrete, and laminated wood Bending stresses may develop if supports at the ends of the arches shift

Arches are sometimes designed with hinges to accommodate minor adjustments.
Hinges may replace a keystone to allow an arch to move under specific circumstances.

Cables are used to support loads.
Stresses in a cable are tension stresses Straight, but may assume curved shape supporting loads Usually made of steel or aluminum

True truss made only of straight members
Trusses are framed structural units made up of a group of triangles in one plane. True truss made only of straight members Triangles provide a rigid frame but adding a diagonal brace creates a stronger assembly

Trusses may be arranged in a wide variety of styles.
Typical truss spans are 22 to 70 feet (7 m to 21m) Truss spans in modern construction may exceed 100 feet (30 m)

Most trusses are prefabricated and use less material and weight than a comparable beam.
Materials used in trusses include wood, steel, combination of wood and steel Lightweight wood or metal is commonly used in floor construction Lightweight wood used in roof construction Lightweight steel used for floor and roof construction in fire-resistive/ noncombustible construction

Lightweight trusses can be used to support a roof.
Courtesy of McKinney (TX) Fire Department

NOTE Roof trusses that are even lighter in weight than steel bar joists are available. In effect, these trusses are made of galvanized steel studs or channels similar to them, and assembled with self-drilling screws.

The material used in the truss members will affect the materials used in the connections.
Connectors Used with Steel Trusses Steel gusset plates Rivets Welds Connectors Used with Wood Trusses Pins or bolts Gusset plates Structural adhesives Brackets Metal straps

Load a truss at the intersections of web members.
Loading a truss at the strongest points (at the intersection of web members) will apply compressive or tensile stresses to top and bottom beams of the truss

Failure of either results in failure of the truss
Loads applied to the truss between the intersection point of the members will create bending stresses. Trusses may fail under adverse conditions because the stresses in the top and bottom chords balance each other Failure of either results in failure of the truss

Connectors in truss assemblies are a critical factor in the strength of the truss.
Failure of a Connector Failure of the Truss Quality control in manufacturing can affect the behavior of trusses under fire conditions

Space frames are three-dimensional truss structures.
Economic use of materials Support uniformly distributed loads Design is more complicated Courtesy of Ed Prendergast

REVIEW QUESTION What components may enable structures to withstand their own weight plus the expected loads?

Individual structural components have value when assembled into a system that will support a building. Almost infinite number of structural designs Necessity and economics result in commonly encountered structural systems Each system has advantages and disadvantages

Individual structural components have value when assembled into a system that will support a building. Structural Bearing Walls Frame Structural Systems Membrane and Shell Systems

Common Materials Used in Bearing Walls
Load-bearing walls carry compressive loads and provide lateral support to the structure. Common Materials Used in Bearing Walls Concrete Blocks Brick Stone Solid Wood Concrete Panels Load-bearing walls are commonly placed at exterior of the structure, but may include internal walls

Bearing walls may be constructed as a continuous barrier or interrupted for door and window openings. Bearing Wall Structures Use Walls to Support Spanning Elements Beams Trusses Precast Concrete Slabs

A frame structural system provides a building with support.
Walls may provide lateral stiffness, but no structural support In the fire service, the term frame construction often refers to a wood-frame building, but other materials may be used Components of a frame may be constructed using a series of trusses

There are different types of structural frame construction.
Steel Stud Walls Closely spaced vertical steel studs connected by top and bottom horizontal members Studs placed 12 to 16 inches (300 to 400 mm) apart Diagonal bracing is often provided for stability May be covered with paneling and sheathing 2 inch x 4 inch (50 mm x 100 mm) wood studs

Post and Beam Construction

Post and Beam Construction Uses a series of vertical elements (posts) to support horizontal elements (beams) that carry transverse loads Requires addition of other members to withstand lateral loads Materials used include masonry for the posts, steel/precast concrete for the posts and beams Distinctive characteristic is the spacing of vertical posts and cross-sectional dimension of the members

Rigid Frame Courtesy of the McKinney (TX) Fire Department

Rigid Frames Characterized by columns and beams reinforced to transmit bending stress through the joints Materials used in rigid frames include steel, laminated wood, and reinforced concrete Peak of roof is usually provided with a hinged connection Joints will be the last portion of the assembly to fail under fire conditions

Slab and Column A common concrete framing system uses concrete floor slaps supported by concrete columns Horizontal systems used to support floor loads include wood and metal decks supported by beams and columns Intersection between slab and column is usually reinforced

NOTE Slab and column framing is different than slab and beam framing. More information on the latter is covered in Chapter 10.

Surface systems consist primarily of an enclosing, waterproof surface.
Membrane Structure Courtesy of the McKinney (TX) Fire Department

Membrane structures are similar to fabric tents, but are intended to be permanent structures.
Building codes address membrane structures with a life of 180 days or more Fire codes address those used for less than 180 days

Recent designs use polytetraflouroethylene (PTFE) coated glass fiber
Membrane structures are similar to fabric tents, but are intended to be permanent structures. Recent designs use polytetraflouroethylene (PTFE) coated glass fiber Fabrics cannot resist compressive forces so frameworks must support the fabric Types of Frames Cables/masts Tubular Solid

Membrane structures possess several design advantages over traditional construction.
Fabrics weigh less than other roof systems Can usually be erected in less time than a rigid structural system Fabrics flex and absorb some of the stresses caused by seismic and wind forces In U.S. and Canada, frames are usually steel (sometimes aluminum), but other materials include wood and concrete

Shell structures are rigid, three-dimensional structures having thin components.
Most commonly constructed of concrete Lend themselves to regular geometric shapes Courtesy of Ed Prendergast

REVIEW QUESTION What are types of commonly encountered composite structural systems?

Florida Objectives - Criteria for designation as an approved NRTL
Florida Administrative Code 69A Click HERE to view the webpage, then click on “View Rule” and read the document

Florida Objective – Common NRTL Websites
US Dept of Labor has the current list of NRTL’s HERE, but here are a few examples: UL, BACL, CSA, Curtis-Straus, FM Approvals, NSF International

Florida Objectives – Determining Fire Resistance
Florida Building Code Section 721 identifies Prescriptive Fire Resistance for: Structural Parts Walls and Partitions Floor and roof systems

Florida Objective – Lightweight Truss Markings
Required per F.S and FAC 69A Click on the references to view

Summary Buildings must withstand a wide variety of forces, loads, and stresses. Firefighters must have a basic understanding of forces acting on structures so they can be aware of structural hazards and collapse dangers.

Summary Under fire conditions, the loads and stresses exerted on a structural system are subject to change in magnitude and direction resulting in structural failure.

Summary A structural engineer will apply calculations and requirements by model building codes to design supports. While determining the supports required, a variety of structural components may be used, because these components support loads in different ways.

Building Construction Related to the Fire Service 4th Edition
Chapter 3 — Structural Design Features of Buildings

Building Construction Related to the Fire Service 4th Edition

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