Steel Buildings in Europe

Part 10: Technical Software Specification for Composite Beams 10 - 31 6.5 Serviceability limit states 6.5.1 General For the serviceability limit states, there is no stress limitation in the buildings. The limit states are:  The deflection of the beam  The natural frequency of the beam that is derived from the deflection. 6.5.2 Inertia of the composite beam The deflection is estimated from the combination of actions under consideration and from the stiffness of the composite beam. The stiffness depends on the second moment of area of the composite section that is determined using a modular ratio n between the structural steel and the concrete. As stated by EN 1994-1-1 § 5.4.2.2(11), the deflection in buildings under both permanent actions and variable actions is calculated using: n = 2 E a / E cm For the estimation of the natural frequency, the deflection has to be calculated using the short term modular ratio: n = E a / E cm The position of the elastic neutral axis is calculated from:         A b h h n n Ah b h h h h h y / / 2 / / 2 p f eff p f p f eff el        The second moment of area of the composite cross-section is calculated from:           2 el p f p f eff 2 el 3 p f eff y eq / 2 / 2 12 y h h h n b h h A y h n b h h I I            Note that: b eff is the effective width at mid-span. For a plain slab, h p = 0. 6.5.3 Deflections General The deflection can be calculated at the various key points along the beam for each combination of actions under consideration. Then the maximum value can be derived. The deflection should be calculated for each variable load case, Q 1 and Q 2 , and for each SLS combination of actions, either characteristic or frequent combination depending on the National Annex. When the beam is fully propped at the construction stage, the deflection under the self-weight (steel profile and concrete) is calculated with composite action.