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Structures 2 > Wood Beams > Flashcards

Wood Beams Flashcards

1
Q

What type of stress is produced in a wood beam where it rests on the support?

A

Compression perpendicular to graining (bearing)

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2
Q

Glue laminated lumber (glulam)

A

structural engineered wood product comprising a number of layers of dimensioned lumber bonded together with durable, moisture-resistant structural adhesive

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3
Q

Where does the maximum shearing stress occur in a rectangular beam section?

A

At neutral axis

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4
Q

How does the increase in the slope of grain along a wood member’s length affect its strength?

A

An increase in slope decreases strength.

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5
Q

Because of the effect of grain, wood beams have lower allowable stresses in ______ ?

A

shear (longitudinal)

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6
Q

In visually graded lumber, what is the meaning of species?

A

“Species” actually refers to a group of wood species having similar mechanical
properties.

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7
Q

Give some examples of factors which affect visual wood grade.

A

Splits, knots, grain, and warping

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8
Q

Machine Evaluated Lumber (MEL) uses x-ray technology t measure what?

A

wood density

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9
Q

Machine Stress Rated (MSR) lumber uses pressure rollers to measure what?

A

E modulus, by measuring load and deflection

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10
Q

What is a typical application for MSR lumber?

A

Glulam timbers.

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11
Q

What is the difference in meaning of a 2x8 vs. an 8x2?

A

orientation - 2x8 would be edgewise and 8x2 is flatwise orientation.

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12
Q

What is the smallest timber size?

A

5x5

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13
Q

Post & Timber and Beam & Stringer are size designations of what wood categories?

A

timbers

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14
Q

What are the three size categories of sawn lumber?

A

boards, dimensioned lumber, and timbers

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15
Q

What is the meaning of S4S?

A

surfaced four sides, finished or dressed lumber

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16
Q

Which of these wood defects is NOT due to curing: split, check, shake?

A

shake

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17
Q

Moisture content in a living tree can be as high as what % of the dry wood by weight?

A

200%

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18
Q

At what point of drying will wood begin to shrink

A

when the moisture content becomes less that the fiber saturation point

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19
Q

What’s the oldest “engineered wood product” used in the US?

A

plywood

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20
Q

list examples of engineered wood products

A

plywood, glulam, oriented strand board (OSB) I-Joist, Laminated veneer lumber, parallel strand lumber (PSL)

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21
Q

Is determining the moment capacity of a beam a design procedure or analysis?

A

analysis

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22
Q

In finding the moment capacity of a beam, what is the actual stress set equal to?

A

the actual stress is set equal to the factored allowable stress

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23
Q

which of the following grading stamps represents the strongest wood ( 10:2, 11:1, 12:3)

A

B. 11:1

24
Q

Which procedure is more likely to require iteration (cycling through the problem again) - analysis or design?

A

design

25
Q

after a bending member has been chosen to resist shear and moment based on strength, what serviceability criteria should be checked?

A

deflection and end bearing

26
Q

what is a reasonable range for the aspect ratio b/d of a wood member?

A

1:3 to about 1:7. This is roughly a 2x6 to a 2x14

27
Q

what factor limits the practical use of beams beyond an aspect ratio of 1:7?

A

lateral stability or lateral torsional buckling - they flip out sideways

28
Q

in beam design, what value is generally used to select a section size

A

the section modulus Sx

29
Q

under what conditions is shear likely to control beam design

A

shear is more critical in short spans with heavy loads

30
Q

how does shear being critical impact the section dimension

A

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

31
Q

what code is used in the united states for the design of wood structures

A

the national design specification for wood construction (NDS)

32
Q

what design method is used in the NDS for the analysis and design of wood

A

both allowable stress design (ASD) and load and resistance factor design (LRFD) are covered in the NDS

33
Q

what is the main difference between ASD and LRFD

A

the main difference is how the factor(s) of safety are applied

34
Q

In determining the allowable bending stress values with the NDS, what type of factors are used?

A

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
35
Q

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)

A

wood can carry a higher level of load for a short duration

36
Q

What are the 3 methods of grading wood members?

A

visual grading, machine evaluated lumber, machine stress rated

37
Q

For what cross sectional shape is 1.5V/A = VQ/lb?

A

1.5V/A applies to rectangular sections

38
Q

For dimensioned lumber sections, are the actual dimensions larger or smaller than the nominal dimensions

A

the actual dimensions are always a little smaller due to shrinkage and surfacing

39
Q

in the design of structural members we generally refer to the three S’s as design criteria. what are the 3 criteria for design

A

strength, stability, and serviceability

40
Q

what adjustment factor accounts for stability in wood beams

A

Cl stability factor

41
Q

which is more critical in the design of wood beams: horizontal shear or vertical shear

A

horizontal shear (parallel to the grain)

42
Q

what consideration can be made regarding shear loads in the vicinity of a support

A

Loads may be neglected within the d distance of a support

43
Q

In a safe beam analyzed using ASD, which will be larger - the allowable bending stress F’b or the actual bending stress fb

A

the allowable stress F’b is greater

44
Q

for which orientation are beams inherently more stable, edgewise or flatwise

A

flatwise

45
Q

which axis designation is used for the stronger (stiffer) axis

A

x-x

46
Q

for a typical wood beam, what approach is used in design regarding shear and moment

A

one typically designs for moment and checks the shear is safe

47
Q

which of the following width to depth ratios represents a well proportioned cross section for a laminated timber beam

A

1:4

48
Q

true or false: bearing stress should taken into account in the design of wood beams

A

true

49
Q

list defects that weaken the strength of wood

A

checks, splits, and shakes

50
Q

the formula used for shear stress in wood sections is 1.5V/A. this is ______.

A

exactly equal to VQ/lb

51
Q

when calculating maximum flexure stress with fb = M*c/I in rectangular wood beams, what value is used for c

A

c = d/2

52
Q

How are the basic tabulated stress values for wood listed

A

tabulated stress values in NDS are listed by species, grade, and size classification

53
Q

What are checks?

A

Checks are separation of the wood occurring across or through the rings of growth, usually as a result of seasoning

54
Q

What are splits?

A

Splits are separation of the wood through due to the tearing apart of wood cells

55
Q

What are shakes?

A

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

Structures 2 (10 decks)

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