19 - Design of Timber Floor Joists.pdf

Table 2: Common sizes of timber floor joists
Sawn depth
(h) mm
Sawn width
(b) mm
Exposure conditions
Timber is a hygroscopic material that
absorbs/releases moisture from/to its
surrounding environment depending on the
amount of moisture in that environment. As
the strength and stiffness properties are
dependent on the moisture content, it is
necessary to account for the environment
around the timber. Three service classes
have been defined. Examples of typical
environments and the respective service
class are:
• Service class 1 – intermediate floors, warm
roofs, internal and party timber frame walls
• Service class 2 – ground floors, cold roofs,
timber frame walls that are against the outer
skin of cladding and all other instances
where the timber is protected from direct
exposure to water
• Service class 3 – external, fully exposed
Load duration
The strength of a piece of timber is
dependent of the duration of the load. The
longer the duration of the load the higher the
strength of timber that must be provided in
order to resist that load. To this end there
are a number of factors that are applied to
the characteristic properties of the timber
as defined in Table 1. The UK National Annex
to Eurocode BS EN 1995-1-1 classifies load
durations as follows:
• Permanent – more than 10 years, e.g. selfweight
including finishes
• Long term – 6 months to 10 years e.g.
storage loading
• Medium term – 1 week to 6 months e.g.
imposed floor loads
Machined depth
(h) mm
Machined width
(b) mm
150 25 145 22
175 38 170 35
200 47 195 44
225 63 220 60
250 75 245 72
Table 3: Values of k mod for solid timber joists
• Short term – less than 1 week e.g.
snow loads, maintenance access and
accidental loads
• Instantaneous – fractions of a second e.g.
wind, impact and explosive loads
The figures given in Table 3 provide the
values for the factor k mod , which is the factor
that is applied to the strength properties of
timber and is based on the imposed load
(variable action) duration. Note that in the
case of load combinations the load condition
with the shortest time period defines the
value of k mod . When designing timber
elements it is important to check for all
conditions and to design the element based
on the most critical. These conditions along
with their respective k mod values are:
• Permanent loads with k mod = 0.6
• Permanent loads + long term loads
with k mod = 0.7
• Permanent loads + long term loads +
medium term loads with k mod = 0.8
• Permanent loads + long term loads +
medium term loads + short term loads
with k mod = 0.9
• Permanent loads + long term loads +
medium term loads + short term loads +
instantaneous loads with k mod = 1
For timber floor joists within a building the
typical critical condition is the imposed
floor load with the self-weight of the joists
and super-imposed dead load. This load
condition results in a value of k mod of 0.8
as it is subject to an imposed load, which
is defined as medium term, as well as
self-weight.
Service class Permanent Long term Medium term Short term Instantaneous
1 & 2 0.6 0.7 0.8 0.9 1.10
3 0.5 0.55 0.65 0.7 0.9
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Deformation
The elastic properties of a timber structure
depend on the moisture content of the
timber and consequently the deflection
will be dependent on the service class.
To take this into account the factor k def is
applied to the elastic modulus properties
of the timber. Table 4 defines these values.
Table 4: Values of k def for solid timber joists
Service
class
Enhancement due to shallow
member size
The way that the grading rules for structural
timber work, means that for joists less than
150mm in depth, some enhancement of the
strength is allowed.
To reflect this, a modification factor k h is
applied to bending strength of the timber. If a
member is less than or equal to 150mm deep
and has a material density of less than 700
kg/m 3 , then k h factor is defined thus:
150 . 02
kh
= a k
h
1 2 3
k def 0.6 0.8 2.00
or 1.3 whichever is the lesser.
For all members that are greater than
150mm deep, the value of k h is taken to
be 1.0.
(1)
Load sharing
Timber floor joists are generally placed at
fairly close centres with decking/boarding
across them which will distribute load
between the joists. To account for this,
the modification factor ksys is applied to
characteristic strength properties of the
timber joist which enhances its resistance to
bending and shear stress. Provided the floor
boards/boarding has staggered connections
and are continuous over at least two spans,
the value of ksys is taken to be 1.1. In all other
instances the value of ksys is taken to be 1.0.
Lateral torsional buckling of
timber joists
In most instances the risk of lateral torsional
buckling affecting a floor joist is not present.
This is due to the existence of a floor
finish that the joists are supporting acting
as a restraint. In the rare condition where
the compression face of floor joists is not
restrained against bending induced torsional
rotation, then the factor k crit is applied to
the bending capacity of the joist. For more