a < ^ 1 (mm) for floor spans > 4000 mm (4.48)

where t is the span of the floor joist in mm.

In NA.2.6.2 of the UKNA to EC5 the deflection a of the floor under a 1 kN point load at mid-span on a floor having a design span t mm may be evaluated using the following equation:


kdist gives the proportion of the 1 kN load supported by a single joist. The value to be used is:


where kstrut = 1 or, where single or multiple lines of strutting are used in the floor structure and installed in accordance with the requirements of BS 5268, Structural Use of Timber (NA.4.1 in the UKNA to EC5), kstrut = 0.97; (EI)b is the flexural rigidity of the floor perpendicular to the joists in N mm2/m (see Notes relating to (EI)b (box below) for the factors to be taken into account when evaluating this rigidity); s is the joist spacing in mm.

teq is the equivalent span of the floor joists (mm); for simply supported joists, teq should be taken to be the span t in mm; for end spans of continuous joists, teq should be taken to be 0.9t in mm; for internal spans of continuous joists, teq should be taken to be 0.85t in mm.

kamp is an amplification factor that takes into account the effect of shear deflections in the case of solid timber and glued thin webbed joists or joint slip in the case of mechanically jointed floor trusses, and is: 1.05 for simply supported solid timber joists, 1.10 for continuous solid timber joists, 1.15 for simply supported glued thin webbed joists, 1.30 for continuous glued thin webbed joists, 1.30 for simply supported mechanically jointed floor trusses, 1.45 for continuous mechanically jointed floor trusses. EIjoist is the bending stiffness of a joist in N mm2, calculated using the mean value of the modulus of elasticity of the joist.

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