Introduction of Floor Vibration for Steel Structures ENCE710 – Advanced Steel Structures C. C. Fu, Ph.D., P.E.

Department of Civil & Environmental Engineering University of Maryland College Park, MD

Background

    The first criteria in designing floor for service ability starts nearly 180 years ago. Tredgold (1828) wrote that girders over long spans should be made “deep” to avoid the inconvenience of not being able to move on the floor without shaking everything in the room.

Traditionally, soldiers "break step" when marching across bridges to avoid large, potentially dangerous, resonant vibration.

 The example of the millennium bridge.

A traditional stiffness criterion limits floor deflection due to live load = span/360. This limitation has limited success in controlling floor vibration.

Resonance has been ignored in the design of floors and footbridges until recently.

  Dynamic amplification.

Rhythmic activities, such as aerobics and high-impact dancing, can cause serious floor vibration problems due to resonance.

Types of Dynamic Loading

    (a) Harmonic load (Machine) (b) Periodic load (Dancing) (c) Transient load (Walking) (d) Impulsive load (Jumping)

Dynamic Resonance

Factors affecting the dynamic amplification: damping, ω and ω n

Peak Acceleration for Human Comfort for Vibrations

Acceptance criteria for peak floor acceleration with frequency ranges from 4 Hz to 8 Hz.

   Office (0.005 g).

Gym (0.05 g) ~ 10 times office acceptance.

Shopping mall (0.015 g) ~ 3 times office acceptance.

Acceptance criteria for peak floor acceleration increases outside the frequency range from 4 Hz to 8 Hz.

Dynamic Force – Human Activities

resonance response function

Response to Sinusoidal Force

The time-dependent repeated force can be represented by the Fourier series

Design for Peak Floor Acceleration

(Eq. 2.2) (Table 4.1) (Eqs. 4.2, 4.3a, b, 4.4) (Eq. 4.1)

Natural Frequency of Floor System

Combined mode

Floor Evaluation Calculation Procedure transformed slab moment of inertia per unit width effective width for joist effective panel weights for joist

Floor Evaluation Calculation Procedure effective width for beam effective panel weights for beam

Floor Evaluation Calculation Procedure equivalent panel weight damping ratio acceleration limit 5.7 kips per in.