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Overview of Design Methods Allowable Stress Design or Working Stress Method

Overview of Design Methods Allowable Stress Design or Working Stress Method Factor of Safety= Allowable Stress -------------------------- Actual Stress NDS is based on the Allowable Stress Design Method

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Overview of Design Methods Allowable Stress Design or Working Stress Method

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  1. Overview of Design Methods • Allowable Stress Design or Working Stress Method • Factor of Safety= Allowable Stress • -------------------------- • Actual Stress • NDS is based on the Allowable Stress Design Method • (Which is primarily used in wood, masonry, steel, and concrete)

  2. Overview of Loads -Gravity and Lateral Loads • Gravity Loads • Dead Loads - Building Materials • Live Load- people • impact loads- percentage increase to live load • roof live loads- uniformly distributed • (loads imposed during construction and loads caused by • fire-fighting equipment) • (reroofing operations, HVAC installation and servicing) • snow loads- handled separately (uniformly distributed) • wind forces handled separately • special loads- hydrostatic uplift • temporary structures

  3. Lateral Loads • wind loads- uniformily distributed • seismic - horizontal loading • Lateral earth pressure • Load and Force Combinations (Text P. 2.76 & 2.77) • DL+FLL+RLL(or snow) • DL+FLL+WIND(OR SEISMIC) • DL+FLL+WIND+1/2 SNOW • DL+FLL+SNOW+1/2 WIND • DL+FLL+SNOW+SEISMIC • Notes: You either use roof live load or snow load, wind load • or seismic load, roof live loads are infrequent, roof live • loads is not combined with seismic or wind load

  4. Wood & Timber Structural Systems (Chapter 4 in Text) • Two Types of Lumber - Sawn Lumber and Glulam • Sawn Lumber is cut from the log • Glulam is a conglomeration of various wood member glued together into a larger wood member • Design Specification - NDS and WWPA • National Design Specification for Wood Construction- Is the • basic spec for the U.S. • Rule Writing Agencies - these agencies write the grading rules • for structural lumber • i.e. Western Wood Products Association(others listed on p.4.6)

  5. A. Grading of Lumber • Visually Graded Lumber - Wood members are graded by Visual Inspection • Machine Stress Rated (MSR)- Each wood member is subjected to a non-destructive test which is highly automated. This lumber is limited • to 2inch. or less in thickness. • Please Note: That thickness refers to the smaller cross-sectional dimension of a piece of wood, and width refers to the larger dimension.

  6. B. Species • Structural Lumber is selected from a group of species that have been tested for various parameters: • Hardwoods- are broad leafed and called deciduous trees • Softwoods - are narrow needlelike leaves (i.e evergreens) known as coniferous trees most structural lumbers come from softwoods (i.e. Douglas fir, southern pine)

  7. C. Wood Properties that effect the Strength of the Lumber • (Density, Moisture Content, & Wood Defects) • Density- Cellular Makeup • Cells walls in trees are made up of cellulose and are bound together • by lignin: • Each ring is considered the annual ring, which exist only in areas with growing seasons. • Large, large thin walled cells are springwood (during active growth) and the smaller thick walled cells (during slow growth) are summerwood. • The summerwood is stronger and denser and gives the wood better strength • Heartwood is considered the inactive cells at the inner darker portion of the tree and Sapwood is the active and inactive lighter portion which stores and supplies food to the tree. • The strength is the same but the sapwood is susceptible to penetration of wood preserving chemicals

  8. Moisture Content- Moisture content of the wood effects it strength • and it susceptibility to shrinkage • Equilibrium Moisture Content (EMC) is the wood’s balanced • moisture at a given atmospheric humidity and temperature(It is important to install wood members with MCs close to the EMC) • If framing lumber is surfaced with a MC greater than 19% it is graded (S-GRN) or maximum19%(S-DRY). If the maximum MC is 15% it is graded (MC 15). The maximum is done by seasoning the wood several percentage point lower than the max.(Large woods are not produced in DRY state) • Note: Interior wood is more critical for changes due to moisture (i.e. doors, strip flooring, etc.) than exterior members

  9. Growth Defects and characteristics -knots, checks, shakes, splits, slope of grain (p.4.26-4.28 text) knots- are locations in wood where limbs were growing. They effect the slope of the grain and produce stress concentrations within the member. They are critical when located in tension zone of the member checks- are splits perpendicular to rings and are radial cracks. They are caused by volume change due to loss of moisture shakes- are splits tangential to the annual ring and develop in the standing tree. They represent complete separation throughout the thickness of the member. slope of the grain- is the deviation of the wood fibers from a line that is parallel to the edge of a piece of lumber

  10. D. Lumber Sizing • Dressed Lumber - Is lumber that will be surface to a net size, which is less than the nominal size. • Lumber is surfaced for smooth surfaces and uniform size. (i.e. nominal size is 2x4, standard net size is 1.5x3.5) • Note: For your design calculations use the standard net size!!! • ex. S4S means surfaced four sides, S2S1E means surfaced 2 sides and 1 edge (seePage 16 WWPA for net sizes and P.19 for surfacing nomenclature) • Rough Sawn Lumber- Lumber is surface about an 1/8 of inch for large timber to maintain its texture • Full Sawn Lumber - Has the same finish size as the specified size.

  11. E. Beam Nomenclature Width of beam is the dimension parallel to the neutral axis. The depth is the dimension perpendicular to the width. “A load applied to the narrow face of the beam” is it strong axis or the XX axis. Board Sizes - 1 to 1.5 inch thick (see P.4.31 in Text or WWPA) 2 in and wider Dimension Lumber - 2 to 4 in thick 2 in and wider Timbers - 5 in and thicker 5 in and wider

  12. Class Sample Problem - Design of Wood member in Bending and Shear A 3x10 , #1 Grade, Hem-Fir wood joists having a moisture content of 11% with a span of 22 ft. are to be used to support a roof. The expected life of this structure is 10 years and the joist has no splits. Assuming that the uniform load (DL +LL) on this wood joist is 64 #/ft, will this section satisfy this loading? If not what could we use? From the Structural Analysis in Bending DL + LL= 64 #/ft S(3x10) = 35.65 in3 (P. 16 WWPA) 22’

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