Building Construction Related to the Fire

Service

Chapter 2 — Structural Fire Resistance and Building

Classifications

Learning Objective 1

Define fire resistance.

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Fire Resistance

• Function of the properties of all materials used; includes combustibility, thermal conductivity, chemical composition, density, and dimensions

• Ability of a structural assembly to maintain load-bearing and structural integrity under fire conditions

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Fire Resistance

• Ability of walls, partitions, and ceilings to act as a fire barrier

• Fire resistance rating– Expressed in hours and fractions of

hours– Incorporated into construction

classifications and building codes

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Learning Objective 2

Discuss methods of determining fire resistance and the limitations of each method.

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Determination of Fire Resistance

• Conducting standard fire resistance testing in a laboratory

• Performing analytical calculations to determine the resistance to a standard fire test exposure

• Employing analytical structural fire engineering design methods based on real fire exposure characteristics

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Fire Resistance Test Method

• Laboratory test is most commonly used method

• Component subject to heat of a standard fire in test furnace

• Primary points of failure for test• Fire resistance ratings expressed in

standard intervals

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(Continued)

Fire Resistance Test Method

• Factors impacting fire resistance ratings from standard test

• Standard fire test evaluates ability of structural assemblies to carry a structural load and act as a fire barrier

• Information NOT provided by test• Assemblies may be penetrated

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(Continued)

Fire Resistance Test Method

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Learning Objective 3

Identify fire testing organizations and discuss the significance of fire test results.

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Fire Testing Organizations

• Testing for fire resistance ratings is beyond capability of local fire and building departments

• Several laboratories are equipped to conduct testing

• Some organizations use furnaces primarily for research and product development

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Fire Test Results

• Published by testing laboratories

• Deviation from materials or dimensions specified alters test results

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Learning Objective 4

Recognize the role of analysis in determining fire resistance.

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Analytical Calculation of Fire Resistance

• Mathematical equations have been developed to predict behavior of materials under test conditions

• ASCE/SFPE 29 provides methods for calculating fire resistance ratings

• Limited to use with certain materials

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Analytical Design Using Real Fire Exposures

• NFPA® 251 test– Most commonly used– May be too severe or not severe

enough

• Ratings determined analytically using a different time-temperature curve must be interpreted cautiously

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Noncombustible Materials

• Building codes contain explicit criteria for determining combustible material

• Incapable of supporting combustion under normal circumstances

• Most common test for determining combustibility – ASTM E 136

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Learning Objective 5

Discuss the basic building classifications as they relate to fire resistance.

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Classifications

• Type I, Fire-Resistive• Type II, Noncombustible or

protected noncombustible• Type III, Exterior protected• Type IV, Heavy timber• Type V, Wood frame

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(Continued)

Classifications

• In building codes, based on materials used in construction and hourly fire resistance ratings required for structural components

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NFPA® 220

• Details requirements for each classification and subclassification

• Designated by three-digit number code– 1st – Fire resistance rating in hours of

exterior bearing walls– 2nd – Fire resistance rating of structural

frames or columns and girders supporting loads greater than one floor

– 3rd – Fire resistance rating of floor construction

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International Building Code® (IBC®)

• Similar to NFPA® 220• Requirements for individual

structural members differ• Building codes use construction

types and occupancy in conjunction with sprinkler systems and separations to establish limits on heights and areas of buildings

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Type I Construction

• Structural membersare noncombustible construction with a specified fire resistance

• Combustible material typically permitted in certain components

• Most common methods of construction

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Courtesy of McKinney (TX) Fire Department

Type II Construction

• Noncombustible – Can be either protected or unprotected

• IBC® divides into two subclassifications

• Material other than steel can be used

• Building codes allow combustible material similar to that in Type I

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(Continued)

Type II Construction

• Unprotected, noncombustible building cannot be expected to provide structural stability under fire conditions

• Point at which unprotected members will fail depends on several factors

• Protected, noncombustible structural fire protection is similar to Type I

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Type III Construction

• “Ordinary construction”• Exterior walls frequently masonry,

but can be any noncombustible material with required fire resistance

• Interior members– Partially or wholly combustible– Protected or unprotected

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(Continued)

Type III Construction

• IBC® divides into two subclassifications

• When fire resistance is required, most common is plaster (older) or gypsum board (newer)

• Wood dimensions permitted to be smaller than Type IV

• Fundamental fire concern – Concealed spaces

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Type IV Construction

• “Mill construction”• Difference from Type III• Used in older factories,

mills, and warehouses• Primary fire hazard –

Massive amount of fuel from large structural members

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Courtesy of McKinney (TX) Fire Department

Type V Construction

• All structural components may be combustible

• Two subclassifications• Differences from Type

III• Threat to adjacent

structures if heavily involved in fire

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Mixed Construction

• Some buildings are allowed a mixed construction type

• May result in special challenges for emergency responders

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Learning Objective 6

Discuss the concept of fire load and its impact on building construction types.

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Fire Load

• Maximum heat released if all available fuel in a building was consumed

• Product of weight of combustibles multiplied by their heat of combustion

• Expressed in lb/sq ft (kg/sq m)• Varies depending on heat of

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Fire Load and Construction Type

• Buildings with combustible structural components have greater fire load than Type I and II

• Fire load does not translate into equivalent structural load

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Learning Objective 7

Explain occupancy classifications as they relate to fire risks.

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Occupancy Classifications

• Assign building occupancies into groups with broadly similar fire risks

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IBC® Classifications

• Group A – Assembly

• Group B – Business• Group E –

Education• Group F – Factories• Group H – High

Hazard

• Group I – Institutional

• Group M – Mercantile

• Group R – Residential

• Group S – Storage• Group U – Utility

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International Building Code®

• Considerable variation of hazards within groups, so further divided to describe occupancy groups with more similar characteristics

• Total of 26 subgroups• Separate provision for one- and

two-family dwellings not more than three stories high

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NFPA® Classifications

• Assembly• Educational• Day care• Health care• Ambulatory

health care• Detention and

correctional

• Residential• Residential board

and care• Mercantile• Business• Industrial• Storage

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Mixed Occupancies

• Buildings may contain more than one occupancy classification

• Codes require fire-resistive separations between various occupancies

• Requirements depend on local code• Reduction in occupancy separation

may be permitted if building is sprinklered

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Change of Occupancy

• Frequently occurs• Can create problems• May require expensive renovations

that owner not willing to pay

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Summary

• Fire behavior in buildings is determined largely by construction materials and structural fire resistance.

• Structural fire resistance is determined most often through laboratory testing.

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(Continued)

Summary

• Building codes classify construction into types based on construction material, structural fire resistance, and occupancy.

• Occupancies within individual occupancy groups present roughly similar fire risk factors.

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Review Questions

1.What is fire resistance?2.What is the value of standard fire tests?3.What is a noncombustible material?4.What considerations do firefighters need to take into account when they encounter dropped ceilings?

(Continued)

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Review Questions

5.What are the major occupancy classifications contained in the International Building Code® (IBC®)?

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