Timber Design

Question 1

What are joists?

How far are joists spaced apart?

Answer 1

Joists are rectangular timber beams between floorboards.

They are typically spaced 400 and 600mm apart.

Question 2

What Eurocode supports timber?

Answer 2

EC5

Question 3

Where does most structural timber come from?

Answer 3

Softwood (coniferous trees)

Question 4

What is permissible stress (f) equal to in timber elements?

Answer 4

Characteristic strength (Fk)

X

Partial factor for safety

Question 5

When is a timber design deemed acceptable?

Answer 5

When permissible stress > actual stress

Question 6

What do the following subscripts represent?

m
t
c
v
0
90
y
z
d
k

Answer 6

Bending, tension, compression, shear, direction parallel to grain, direction perpendicular to grain, bending about y-y, bending about z-z, design value, characteristic value

Question 7

How is total ultimate load calculated in timber?

Answer 7

Total ultimate load per square meter of flooring

X

Spacing of Joists

= total ultimate load per metre length of joist

Question 8

How is actual bending stress calculated?

Answer 8

Max bending moment (e.g. WL^2/8) divided by section modulus (Z)

Question 9

How is section modulus (Z) calculated?

Answer 9

Z = I / Y

I= moment of inertia (2nd moment of area)

Y= distance from edge to neutral axis

Question 10

What is section modulus (Z) for a rectangle?

Answer 10

( b x h^2 ) / 6

Question 11

How is strength class determined?

Answer 11

Using tables T1 and T2 and the grade of wood in the question

Question 12

How do you find Fmk?

Answer 12

Using table T9 and the strength class of the timber

Question 13

How is load duration determined?

Answer 13

Using table T7 and the example of loading in the question.

For example, imposed load would be imposed floor loading.

Question 14

How is service class determined?

Answer 14

Using table T6 and the surroundings of the timber.

I.e an upper floor would be service class 1, an intermediate floor.

Question 15

How is Kmod determined?

Answer 15

Using the material type, load duration and service class in combination with table T8.

Question 16

How is Kh determined?

If the beam is deeper than 150mm, what is the depth of joist factor (Kh)?

Answer 16

Using table T3

1.0

Question 17

How is the lateral stability factor, Kcrit, obtained?

If joists are connected to floorboards, what value would Kcrit be, and why?

Answer 17

Using table T4

1.0, because they are prevented from moving laterally by the supports.

Question 18

How is the Load sharing factor, Ksys, obtained?

If the joists are less than 610mm apart, and there are at least four joists, Ksys= ?

Otherwise, Ksys = ?

Answer 18

Using table T5

1.1 if conditions are met, otherwise 1.0

Question 19

How is the partial safety factor, Ym, obtained?

Answer 19

From table T10, using the material in the question.

Question 20

How is actual max shear force calculated?

How is the actual max shear stress in a rectangular section calculated?

What are the units of max shear stress?

Answer 20

WL/2 if conditions are met

1.5Vd/bh

N/mm^2

Question 21

How is Fvk obtained for permissible shear stress?

Answer 21

Using table T9 in combination with the strength class looking for shear strength parallel to the grain.

Question 22

What value is Kv typically?

What is Kv

Answer 22

Kv is the notch factor

It is equal to 1 if there is no notching

Question 23

How is actual bearing stress calculated?

What is the end reaction?

What is bearing area equal to?

Answer 23

End reaction / Bearing Area

End reaction = Max applied shear force, typically = WL/2

Bearing area= bearing length x joist width

Question 24

What is Fc,90,k and how is it obtained?

Answer 24

Characteristic, Compressive strength perpendicular to grain, obtained from table T9 using strength class

Question 25

What is Kc,90 and how is it obtained?

Answer 25

Bearing strength modification factor

Obtained from table T11 using bearing length and joist depth

Question 26

In deflection checks for timber joists, are partial factors used to calculate, W, the total service load?

Answer 26

No, W is calculated using unfactored loads

Question 27

Where is E, stiffness property, obtained from?

Answer 27

Table T9, using strength class

Question 28

How is ‘I’ obtained for actual deflection?

Answer 28

(b x h^3) / 12

Question 29

How is M calculated for shear deflection?

Answer 29

WL^2/8 typically

Question 30

How is ‘A’ calculated for midspan deflection due to shear?

Answer 30

width x depth of joists

Question 31

What is permissible deflection equal to?

Where is this found?

Answer 31

L/250

Table T12 - Deflection limits

Question 32

How is deflection checked?

Answer 32

By ensuring that actual deflection is less than permissible deflection

Question 33

How is lateral restraint tested?

Answer 33

By ensuring that the actual depth/breadth ratio is less than the permissible depth/breadth ratio

Question 34

What is the actual depth/breadth ratio?

Answer 34

Depth of joist / breadth of joist

Question 35

Where and how is the permissible depth/breadth ratio found?

What is the typical value

Answer 35

Table T13 by describing the lateral support

Typically 5:1 and therefore = 5

Question 36

What is the service class for a timber upper floor?

Answer 36

Service Class 1 (intermediate floor)