19 - Design of Timber Floor Joists.pdf

Table 5: Deflection limits for
solid timber joists
Use Deflection limit
Brittle finishes
e.g. plasterboard,
ceilings, walls
No brittle finishes
e.g. roofs
Accidental
e.g. during a fire
Span/250
Span/150
Span/20
(1+k def ). The instantaneous deflection
for imposed (variable) load is increased by
a factor (1+ψ 2,1k def ), assuming only one
variable load. The sum of these deflections,
less any precamber, provides the overall
deflection due to bending.
There is also the issue of deflection due to
shear. This is rarely a critical condition, but it
is prudent to add 10% to the overall deflection
to arrive at a final value to allow for it.
Additionally floor joists should be checked
to ensure that their vibration performance is
acceptable, this can be of concern with joists
which span is in excess of 4.0m.
www.thestructuralengineer.org
Eurocode 0.
Applied practice
BS EN 1995-1-1 Eurocode 5: Design of
Timber Structures – Part 1-1: General –
Common rules and rules for buildings
39
Timber joist span tables
It is possible to refer to timber joist span
tables for sizing of joists, such as those
included in Table 7 of BS 8103:3 2009. These
tables however are limited to imposed loads
of 1.5 kN/m 2 .
Alternatively it is possible to use the
Eurocode 5 Span Tables, that have been
developed by TRADA. These have a similar
set of tables, but again are limited to imposed
loads of no greater than 1.5 kN/m 2 .
BS EN 1995-1-1 UK National Annex to
Eurocode 5: Design of Timber Structures –
Part 1-1: General – Common rules and rules
for buildings
BS EN 336 2009: Structural Timber –
Sizes, permitted deviations
BS EN 338 2009: Structural Timber –
Strength classes
BS 8103:3 2009: Structural Design of Lowrise
Buildings Part 3: Code of practice for
timber floors and roofs for housing
Glossary and
further reading
Hygroscopic – Moisture absorbing.
Modification factors – Factors that
are applied to the characteristic material
properties of timber elements.
Precamber – A forced deformation of a
member that reduces the effect of deflection
following the application of loads.
Further Reading
The Institution of Structural Engineers/
TRADA (2010) Manual for the design of
timber building structures to Eurocode
5 London: The Institution of Structural
Engineers/TRADA
TRADA (2009) Eurocode 5 span tables
for solid timber members (3rd ed.) High
Wycombe: TRADA
Porteous, J. and Kermani, A. (2007)
Structural Timber Design to Eurocode 5
Chichester: John Wiley & Sons