Design of floors for vibration quiz


Please answer the following 10 multiple choice questions, then click 'submit' to check the result. The pass mark for a CPD certificate is 8 out of 10, and you may retake the quiz as many times as you wish, but the questions will vary! Please note that one, two, three or all of the possible answers presented for each question may be right, and to gain a mark for that question all correct answers must be identified.

Good luck

Design of floors for vibration

Which of the following descriptions accurately describes primary beam mode?

The primary beams form nodal lines about which the secondary beams vibrate as simply supported members. The slab is assumed to be continuous over the secondary beams and so a fixed-ended boundary condition is used.
The primary beams vibrate about the columns as simply supported members, and the secondary beams and slab are taken to be simply supported
The primary beams vibrate about the columns as simply supported members, and the secondary beams and slab are taken to be fixed-ended
The primary beams vibrate about the columns as fixed-ended members, and the secondary beams and slab are taken to be simply supported

Human perception to vibration depends on the direction of incidence of the acceleration relative to the human body. In the basicentric coordinate system, the z-axis corresponds to the direction of the human spine. In which of the situations below is the human body more sensitive to vibration?

Walking (z-axis perpendicular to the floor)
Lying down (z-axis parallel to the floor)

An engineer has designed a floor plate to a response factor of 7. The floor comprises secondary beams at 3m centres acting compositely with a 140mm composite deck. The secondary beam has a frequency of 4.5Hz. The main contractor is having difficulty in getting materials into what is a congested site, and has asked if it would be acceptable to replace the composite deck with a lightweight timber joist floor. He submits calculations that demonstrate that the same beam acting non-compositely is perfectly adequate in terms of strength, and achieves the same frequency of 4.5Hz due to a reduction in dead load of 75%. How should the engineer respond?

As long as the beam is satisfactory in terms of strength and the beam frequency remains the same or higher, then that proposal is perfectly acceptable.
Whilst your calculations demonstrate that the beam can support the change in loads, and that both the beam and system frequency are satisfactory, I have concerns about the vibration performance of the floor. The dead load of the floor construction has been reduced by 75%. Newton’s second law (acceleration = Force/Mass) suggests a four fold increase in the accelerations due to this significant reduction in dead load. The response factor has gone up from 7 to 28, which is well in excess of current practice.
No, the drawings clearly state the form of construction required.
Your proposal is acceptable provided you can offer a saving to the Client and accept design liability for your proposals.

Which statement below does NOT describe a characteristic of a low frequency floor?

Has a fundamental frequency greater than 3 Hz but less than 10Hz
Exhibits a near resonant response
Both steady state and transient response must be checked
Has a fundamental frequency greater than 10Hz

Where is the magnitude of damping of most importance?

Floors with low frequency
Floors with low frequency
Resonant floors
Floors subject to group activities

When assessing the frequency of a floor, which elements of the floor must the designer check?

The slab
The secondary beam
The primary beam
All of the above, both individually and collectively

What is a reasonable frequency range caused by walking?

2 Hz to 3 Hz
1 Hz to 2 Hz
2.8 Hz to 3.2 Hz
1.8 Hz to 2.2 Hz

According to SCI’s P354, “Design of Floors for Vibration: A New Approach”, the recommended response factor or multiplying factor for an office is?


When assessing vibrations in a floor, appropriate acceptance criteria can be referenced from where (more than one may apply)?

BS 5950: Structural use of steelwork in building
BS 6472: Guide to evaluation of human exposure to vibration in buildings
SCI publication P354 “Design of Floors for Vibration: a New Approach”
BS 6399: Loadings for buildings

For a traditional composite floor system, comprising a slab continuous over a number of secondary beams that in turn are supported by primary beams, what two mode shapes should the designer consider if analysing by hand?

Slab mode
Secondary beam mode
Simply supported beam mode
Primary beam mode