Steel beams. Deflection.

The active deflection in steel beams is calculated in the following way:

  • The instantaneous deflection due to the self weight of the floor slab and screed is not considered by the program, only that due to the partitions and live load.

    active_defl = DL instant_defl · % partition load w.r.t. total DL + LL instant defl

  • The differed deflection is not considered, as it does not make sense to in the case of a steel beam.

Note: The % partition load with respect to the total dead load is defined in the beam options, whose default value is set at 30%.

Example:

Instantaneous deflection due to dead load = 0.937 cm

Instantaneous deflection due to live load = 0.363 cm

Partitions % = 30%

Active_defl = 0.937 · 0.3 + 0.363 = 0.644 cm

Only the partitions percentage affects the calculation of the active deflection. The introduced creep coefficients have no effect. Neither do the long term creep coefficients. Therefore the long term deflection in steel beams coincides with the total instantaneous deflection due to the dead load plus the live load.

We consider the relative active deflection to be limited to L/400, as in the case of concrete beams, because the deflection to limit is that which damages the partitions. The same can be said for the absolute deflection limit of 1cm.

Nonetheless, other deflection limits can be established (for example L/500 for the total deflection, etc.)

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