Timber Design Flashcards
What are joists?
How far are joists spaced apart?
Joists are rectangular timber beams between floorboards.
They are typically spaced 400 and 600mm apart.
What Eurocode supports timber?
EC5
Where does most structural timber come from?
Softwood (coniferous trees)
What is permissible stress (f) equal to in timber elements?
Characteristic strength (Fk)
X
Partial factor for safety
When is a timber design deemed acceptable?
When permissible stress > actual stress
What do the following subscripts represent?
m
t
c
v
0
90
y
z
d
k
Bending, tension, compression, shear, direction parallel to grain, direction perpendicular to grain, bending about y-y, bending about z-z, design value, characteristic value
How is total ultimate load calculated in timber?
Total ultimate load per square meter of flooring
X
Spacing of Joists
= total ultimate load per metre length of joist
How is actual bending stress calculated?
Max bending moment (e.g. WL^2/8) divided by section modulus (Z)
How is section modulus (Z) calculated?
Z = I / Y
I= moment of inertia (2nd moment of area)
Y= distance from edge to neutral axis
What is section modulus (Z) for a rectangle?
( b x h^2 ) / 6
How is strength class determined?
Using tables T1 and T2 and the grade of wood in the question
How do you find Fmk?
Using table T9 and the strength class of the timber
How is load duration determined?
Using table T7 and the example of loading in the question.
For example, imposed load would be imposed floor loading.
How is service class determined?
Using table T6 and the surroundings of the timber.
I.e an upper floor would be service class 1, an intermediate floor.
How is Kmod determined?
Using the material type, load duration and service class in combination with table T8.
How is Kh determined?
If the beam is deeper than 150mm, what is the depth of joist factor (Kh)?
Using table T3
1.0
How is the lateral stability factor, Kcrit, obtained?
If joists are connected to floorboards, what value would Kcrit be, and why?
Using table T4
1.0, because they are prevented from moving laterally by the supports.
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 = ?
Using table T5
1.1 if conditions are met, otherwise 1.0
How is the partial safety factor, Ym, obtained?
From table T10, using the material in the question.
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?
WL/2 if conditions are met
1.5Vd/bh
N/mm^2
How is Fvk obtained for permissible shear stress?
Using table T9 in combination with the strength class looking for shear strength parallel to the grain.
What value is Kv typically?
What is Kv
Kv is the notch factor
It is equal to 1 if there is no notching
How is actual bearing stress calculated?
What is the end reaction?
What is bearing area equal to?
End reaction / Bearing Area
End reaction = Max applied shear force, typically = WL/2
Bearing area= bearing length x joist width
What is Fc,90,k and how is it obtained?
Characteristic, Compressive strength perpendicular to grain, obtained from table T9 using strength class
What is Kc,90 and how is it obtained?
Bearing strength modification factor
Obtained from table T11 using bearing length and joist depth
In deflection checks for timber joists, are partial factors used to calculate, W, the total service load?
No, W is calculated using unfactored loads
Where is E, stiffness property, obtained from?
Table T9, using strength class
How is ‘I’ obtained for actual deflection?
(b x h^3) / 12
How is M calculated for shear deflection?
WL^2/8 typically
How is ‘A’ calculated for midspan deflection due to shear?
width x depth of joists
What is permissible deflection equal to?
Where is this found?
L/250
Table T12 - Deflection limits
How is deflection checked?
By ensuring that actual deflection is less than permissible deflection
How is lateral restraint tested?
By ensuring that the actual depth/breadth ratio is less than the permissible depth/breadth ratio
What is the actual depth/breadth ratio?
Depth of joist / breadth of joist
Where and how is the permissible depth/breadth ratio found?
What is the typical value
Table T13 by describing the lateral support
Typically 5:1 and therefore = 5
What is the service class for a timber upper floor?
Service Class 1 (intermediate floor)
