1996 LRFD Manual for Engineered Wood Construction

More documents

Recommendations

Info

GUIDELINELRFD MANUALTO LRFDFORFORENGINEEREDSTRUCTURALWOODCOMPOSITECONSTRUCTIONLUMBER 5pletely biodegradable (over time) when exposed to wetconditions. It is the goal of designers to detail their structureso that the wood remains serviceable for the life ofthat structure.Good construction details prevent deterioration ofwood frame structures. When such details are ignored,decay or termite damage may occur. Naturally durable orpressure treated wood assures satisfactory performanceunder adverse conditions.Decay is caused by fungi which are low forms of plantlife that feed on wood. For fungi to attack wood in serviceall the following conditions must be present: (1)temperature in the range of 35 to 100 o F, (2) adequate supplyof oxygen, and (3) wood moisture content in excess of20%.The foregoing requirements for growth of fungi indicatea method to prevent decay in structures. Temperature,except in arctic climates, is impractical to control. Lackof sufficient oxygen to support decay occurs only whenwood is completely below the ground water line or continuouslysubmerged in fresh water. Control of moisturecontent of wood is a practical and effective method forprevention of decay.The subterranean termite is an insect which attacks incolonies and derives its nourishment from cellulosic materialssuch as wood, fabric, paper and fiber board. Thetermite may attack wood frame structures above the groundby means of shelter tubes attached to foundation walls,piers and other members in contact with the ground. However,only under conditions which permit the insect toestablish and maintain contact with soil moisture, is acolony able to penetrate and consume wood in service.Thus, a barrier separating wood from earth, supplementedby inspection, is a practical and effective method for preventingdamage by termites.Principles of Good ConstructionProtection of wood frame structures to provide maximumservice-life involves three methods of control whichcan be handled by proper design and construction. Oneor more of the following methods may be employed: (1)control moisture content of wood, (2) provide effectivetermite barriers, (3) use naturally durable or preservativelytreated wood.Wood construction maintained at a moisture contentof 20% or less will not decay. Optimum conditions fordecay occur when the moisture content is above 25%. Itshould be stressed that when wood is protected from wateror from vapor condensation, and exposed to normalatmospheric conditions such as exist inside buildings andoutdoors, its moisture content rarely exceeds 15%. Therefore,moisture content control by means of accepted designand construction details is a simple and practical methodof providing protection against decay.While moisture control also contributes to preventionof subterranean termite attack, the primary control methodrequires use of effective barriers supplemented by periodicinspection. Termite barriers are provided by the useof accepted construction practices which drive termitesinto the open where shelter tubes can be detected by inspectionand destroyed.Wood frame structures provided with a recognizedbarrier supplemented by periodic inspection can be permanentlyensured against subterranean termite attack.Architectural consideration or use exposures (swimmingpools, marine structures, wet process industries,ground contact, unusual climatic conditions) may not permitmoisture or termite control by design and construction techniquesalone. Also, experience in certain geographicalregions may indicate the need for greater protection. Underthese circumstances naturally durable wood of certainspecies may be used, or wood may be pressure treatedwith preservatives to prevent decay and termite damage.General Recommendations for Good ConstructionRecommendations provided for good construction willassure basic resistance to decay. Due to climatic conditionsor geographical location, additional control measuresmay be required in some buildings or structures.Control of decay or termite attack is accomplishedprimarily through application of four fundamental constructionpractices:1. Positive site and building drainage.2. Adequate separation of wood elements from knownmoisture sources to (a) prevent excessive absorption,(b) allow for periodic inspection, and (c) provide thenecessary physical barrier for termite protection.3. Use of naturally durable or pressure treated wood whereindicated. See product-specific recommendations regardingproper procedures for preservative treatmentof that product.4. Ventilation and condensation control in enclosed spaces.These construction practices eliminate the danger ofdecay or subterranean termite damage. They also serveto control damage from other insects present in limitedgeographical areas.1.3.3 Designing for Fire SafetyThe model building codes in the U.S. cover virtuallyevery safety-related topic related to the construction ofbuildings, and fire-related issues comprise a surprisingly1INTRODUCTIONAMERICAN FOREST & PAPER ASSOCIATION
6 INTRODUCTIONlarge portion of the model codes. Designing for fire safetyis a complex and multifaceted issue. The following informationprovides an overview of the subject:To provide fire safety in any structure, many approachesare considered. This involves a combination of(1) preventing fire occurrence, (2) controlling fire growth,and (3) providing protection to life and property. All needsystematic attention to provide a high degree of economicalfire safety. The building design professional can controlfire growth within the structure by generating plans thatinclude features such as protecting occupants, confiningfire in compartment areas, and incorporating fire suppressionand smoke or heat venting devices at critical locations.Controlling construction features to facilitate rapidegress, protection of occupants in given areas, and preventingfire growth or spread are regulated by codes as afunction of building occupancy. If the design professionalrationally blends protection solutions for these items withthe potential use of a fire-suppression system (sprinklers,for example), economical fire protection can be achieved.Although attention could be given to all protectiontechniques available to the building design professional,the scope here is limited to the provisions that prevent firegrowth and limit the fire to compartments of origin.PlanningGenerating the plans for a building of prescribed occupancyis a challenge because of the varying requirementsof three major regional building codes: Building OfficialsConference of America (BOCA), International Conferenceof Building Officials (ICBO), and Southern Building CodeCongress International, Inc. (SBCCI). As a first step, theauthority having jurisdiction where a proposed buildingis to be constructed must be consulted for the requirementsof the specific design project. This normallyconcerns the type of construction desired as well as allowablebuilding areas and heights for each constructiontype.Building construction is generally classified into typessuch as wood frame, noncombustible wall-wood joist, andheavy timber. Wood frame construction is defined as havingexterior walls, bearing walls, partitions, floors androofs of wood stud and joist framing of 2-in. nominal dimension.These are divided into two subclasses that areeither protected or unprotected construction. Protectedconstruction calls for having load-bearing assemblies ofone-hour fire endurance.Noncombustible wall-wood joist types of constructionhave exterior walls of noncombustible materials androofs, floors, and interior walls and partitions of woodframe. As in wood frame construction, these are dividedinto two subclasses that are either protected or unprotected.Heavy timber construction includes exterior walls ofnoncombustible materials and columns, floors, roofs, andinterior partitions of wood of a minimum size, as follows:Table 1.3-1. Minimum Sizes to Qualifyas Heavy Timber ConstructionMaterialRoof decking:Lumberor wood structuralpanelsFloor decking:Lumberor flooringor wood structuralpanelsRoof framing:Floor framing:Columns:Minimum size(nominal size or thickness)2 in. thicknessl-l/8 in. thickness3 in. thickness1 in. thickness1/2 in. thickness4 by 6 in.6 by 10 in.8 by 8 in. (supporting floors)6 by 8 in. (supporting roofs)Noncombustible construction is generally required tobe of noncombustible materials having fire-endurance ratingsof up to 4 hours, depending on the size and locationof the building. Some circumstances provide for the useof wood in the walls of noncombustible wall-wood joistand noncombustible types of construction. For example,the Uniform Building Code allows the use of fire-retardanttreated wood framing for nonbearing walls if such wallsare more than 5 feet from the property line. Exceptions to theuse of noncombustible materials in the noncombustible-typebuildings are sometimes made for heavy timber members.Heavy timber members can often be used for roof membersmore than 25 feet above the floor, balcony, or galleryin one-story noncombustible-type buildings.Besides having protected and unprotected subclassesfor each building type, increases in floor area and heightof the building are allowed when sprinkler protection systemsare included. For example, protected wood frameeducational occupancies can be increased from two to threestories in height because of the presence of sprinklers.Also, the floor area in the first two stories may be doubledor even tripled under some conditions.AMERICAN WOOD COUNCIL
Page 2 and 3: 1996 EDITIONLRFDLOAD AND RESISTANCE
Page 4 and 5: TABLE OF CONTENTSChapter/TitlePageC
Page 6 and 7: iNotationAGross areaA nNet area, ne
Page 8 and 9: iiiP uCompressive or bearing force
Page 10 and 11: vGlossaryAdjusted resistance. The r
Page 12 and 13: viithickness (timbers), green shall
Page 14 and 15: ixPrefabricated wood I-joists. Pre-
Page 16 and 17: LRFD MANUAL FOR ENGINEERED WOOD CON
Page 18 and 19: GUIDELINELRFD MANUALTO LRFDFORFOREN
Page 70 and 71:
GUIDELINELRFD MANUALTO LRFDFORFOREN
Page 72 and 73:
Page 74 and 75:
Page 76 and 77:
Page 78 and 79:
Page 80 and 81:
Page 82 and 83:
Page 84 and 85:
Page 86 and 87:
Page 88 and 89:
Page 90 and 91:
Page 92 and 93:
Page 94 and 95:
LRFD MANUAL FOR ENGINEERED WOOD CON
Page 96 and 97:
Page 98 and 99:
Page 100 and 101:
Page 102 and 103:
Page 104 and 105:
Page 106 and 107:
Page 108 and 109:
Page 110 and 111:
Page 112 and 113:
Page 114 and 115:
Page 116 and 117:
Page 118 and 119:
Page 120 and 121:
Page 122 and 123:
Page 124 and 125:
show all

1996 LRFD Manual for Engineered Wood Construction

Create successful ePaper yourself

Delete template?

Save as template?