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IBC Vibration checks for composite floors

new in v. 18.0

Highlights

SCIA Engineer v18.0 deals with a serviceability limit state that often dominates the design of composite beams: unaddressed floor vibration issues often interfere with human comfort in composite floor systems.
A quick and sufficiently accurate verification for walking-induced floor vibrations is offered based on established design methods from the United States.
 The dynamic response of the floor is predicted assuming full composite action between beams and slab and an increased Young's modulus of concrete. The interaction between primary and secondary composite beams is taken into account.
Acceleration levels are compared with tolerance levels for human comfort, taking into account the function of the floor.

 

Floor vibration checks for composite beams according to the 2016 AISC Design Guide 11 are available in SCIA Engineer since Patch 1 of version 17.1.

The implemented method controls floor vibrations due to walking excitation and has been used in the US structural engineering practice in its current logic and similar form since 1997. The method is based on scientific research and has been verified by long years of engineering experience. It is applicable to any simply-supported I-beam (American, European and other profiles).

These verifications follow a check logic, rather than requiring modal analysis: this simplifies and greatly speeds up the control of vibrations, giving a good estimate of the actual response of the floor(s).

Serviceability considerations for lighter floors

Floors are lighter nowadays. The first reason for this is the use of stronger materials, which let designers reduce the height of load-bearing elements in the floor. On top of that, modern design methods are more efficient in that they accommodate safety levels by making use of advanced statistical models, which, in overall, results in lower safety factors in strength considerations.

The second reason is the generally lower load on floors. Paperless offices are significantly lighter, so is modern furniture. Open floor layouts with movable partition walls do not offer the damping provided by solid permanent walls.

All this results in floor vibration problems and composite floors are particularly sensitive, because the beneficial effects of shear connection between steel beams and concrete slab is utilised in ultimate limit state verifications. Therefore, it is common for vibration due to human activity to be the governing serviceability concern in composite floor design.

What represents the floor response?

Natural frequency is probably the most important quantity in the floor to characterise vibration behaviour. SCIA Engineer uses deflections to derive natural frequencies per beam indirectly: deflection is a good indicator of stiffness and mass. The deflections are calculated assuming full composite action between beams and slab and an increased modulus of elasticity of concrete, to reflect the material response in a dynamic context.

Another floor property that needs to be estimated is damping: governed by the presence of nonstructural elements such as ceilings, partitions and furniture, damping is not something that can be calculated. In SCIA Engineer, the user defines damping coefficients based on floor function: a paper office or a shopping mall, values of damping can range between 1% and 5% of critical damping. Values based on good engineering practice are given in the AISC Design Guide 11.

The check itself

 The vibration verifications in SCIA Engineer follow a check logic (as described in the referenced AISC publication): using the predicted natural frequencies and damping, the maximum amplitude of acceleration due to the reference walking excitation is calculated. The position of the beam in the floor plan is obtained automatically from the 3D model, as is the spacing between beams.

Interactions between secondary and primary beams in the floor are correctly managed; as a result, the more flexible (lower frequency) response of secondary beams is captured.

As walking frequencies range between 1.6 and 2.2 Hz, and noticeable floor vibrations can be induced by up to the forth harmonic of the step frequency, only floors with fundamental natural frequencies lower than 9 Hz are checked. Floors with higher natural frequencies are assumed to be insusceptible to discomforting vibrations, as no resonant build-up can happen in this case.

Tolerance limits

The serviceability criterion in this case is the tolerance that people have to vibration in the floor. Tolerance varies depending on the floor function, and human comfort has different requirements in a dancing hall, in the office, and at home. The user defines the floor function per model or specifically per beam and SCIA Engineer derives the correct tolerance limit on accelerations and uses it in the check. Manually inputted values are also possible here.

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