Wood Beams Flashcards
What type of stress is produced in a wood beam where it rests on the support?
Compression perpendicular to graining (bearing)
Glue laminated lumber (glulam)
structural engineered wood product comprising a number of layers of dimensioned lumber bonded together with durable, moisture-resistant structural adhesive
Where does the maximum shearing stress occur in a rectangular beam section?
At neutral axis
How does the increase in the slope of grain along a wood member’s length affect its strength?
An increase in slope decreases strength.
Because of the effect of grain, wood beams have lower allowable stresses in ______ ?
shear (longitudinal)
In visually graded lumber, what is the meaning of species?
“Species” actually refers to a group of wood species having similar mechanical
properties.
Give some examples of factors which affect visual wood grade.
Splits, knots, grain, and warping
Machine Evaluated Lumber (MEL) uses x-ray technology t measure what?
wood density
Machine Stress Rated (MSR) lumber uses pressure rollers to measure what?
E modulus, by measuring load and deflection
What is a typical application for MSR lumber?
Glulam timbers.
What is the difference in meaning of a 2x8 vs. an 8x2?
orientation - 2x8 would be edgewise and 8x2 is flatwise orientation.
What is the smallest timber size?
5x5
Post & Timber and Beam & Stringer are size designations of what wood categories?
timbers
What are the three size categories of sawn lumber?
boards, dimensioned lumber, and timbers
What is the meaning of S4S?
surfaced four sides, finished or dressed lumber
Which of these wood defects is NOT due to curing: split, check, shake?
shake
Moisture content in a living tree can be as high as what % of the dry wood by weight?
200%
At what point of drying will wood begin to shrink
when the moisture content becomes less that the fiber saturation point
What’s the oldest “engineered wood product” used in the US?
plywood
list examples of engineered wood products
plywood, glulam, oriented strand board (OSB) I-Joist, Laminated veneer lumber, parallel strand lumber (PSL)
Is determining the moment capacity of a beam a design procedure or analysis?
analysis
In finding the moment capacity of a beam, what is the actual stress set equal to?
the actual stress is set equal to the factored allowable stress
which of the following grading stamps represents the strongest wood ( 10:2, 11:1, 12:3)
B. 11:1
Which procedure is more likely to require iteration (cycling through the problem again) - analysis or design?
design
after a bending member has been chosen to resist shear and moment based on strength, what serviceability criteria should be checked?
deflection and end bearing
what is a reasonable range for the aspect ratio b/d of a wood member?
1:3 to about 1:7. This is roughly a 2x6 to a 2x14
what factor limits the practical use of beams beyond an aspect ratio of 1:7?
lateral stability or lateral torsional buckling - they flip out sideways
in beam design, what value is generally used to select a section size
the section modulus Sx
under what conditions is shear likely to control beam design
shear is more critical in short spans with heavy loads
how does shear being critical impact the section dimension
the width b should generally be increased, which gives a thicker section with more area and lower aspect ratio in the range of 1:1 to 1:2
what code is used in the united states for the design of wood structures
the national design specification for wood construction (NDS)
what design method is used in the NDS for the analysis and design of wood
both allowable stress design (ASD) and load and resistance factor design (LRFD) are covered in the NDS
what is the main difference between ASD and LRFD
the main difference is how the factor(s) of safety are applied
In determining the allowable bending stress values with the NDS, what type of factors are used?
In the NDS tabulated values are multiplied by “adjustment factors” which adjust the stress for a particular application.
Cd - load duration factor Cm - moisture factor Cl - Beam stability factor Cf - size factor Cfu - Flat use factor Cr - repetitive member factor Ci - Incising Factor
Based on values used for the load duration factor, will wood carry a higher stress for short term loads (eg wind loads) or long term loads (eg occupancy loads)
wood can carry a higher level of load for a short duration
What are the 3 methods of grading wood members?
visual grading, machine evaluated lumber, machine stress rated
For what cross sectional shape is 1.5V/A = VQ/lb?
1.5V/A applies to rectangular sections
For dimensioned lumber sections, are the actual dimensions larger or smaller than the nominal dimensions
the actual dimensions are always a little smaller due to shrinkage and surfacing
in the design of structural members we generally refer to the three S’s as design criteria. what are the 3 criteria for design
strength, stability, and serviceability
what adjustment factor accounts for stability in wood beams
Cl stability factor
which is more critical in the design of wood beams: horizontal shear or vertical shear
horizontal shear (parallel to the grain)
what consideration can be made regarding shear loads in the vicinity of a support
Loads may be neglected within the d distance of a support
In a safe beam analyzed using ASD, which will be larger - the allowable bending stress F’b or the actual bending stress fb
the allowable stress F’b is greater
for which orientation are beams inherently more stable, edgewise or flatwise
flatwise
which axis designation is used for the stronger (stiffer) axis
x-x
for a typical wood beam, what approach is used in design regarding shear and moment
one typically designs for moment and checks the shear is safe
which of the following width to depth ratios represents a well proportioned cross section for a laminated timber beam
1:4
true or false: bearing stress should taken into account in the design of wood beams
true
list defects that weaken the strength of wood
checks, splits, and shakes
the formula used for shear stress in wood sections is 1.5V/A. this is ______.
exactly equal to VQ/lb
when calculating maximum flexure stress with fb = M*c/I in rectangular wood beams, what value is used for c
c = d/2
How are the basic tabulated stress values for wood listed
tabulated stress values in NDS are listed by species, grade, and size classification
What are checks?
Checks are separation of the wood occurring across or through the rings of growth, usually as a result of seasoning
What are splits?
Splits are separation of the wood through due to the tearing apart of wood cells
What are shakes?
Shakes are lengthwise separation of the wood along the grain, usually occurring between or through the rings of annual growth. it is a naturally occurring defect
