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

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

2
4
6
8

In respect of Vibration Dose Values (VDV’s), which statements are correct?

VDV’s provide a mechanism for taking account of the intermittent nature of vibrations
VDV’s can be used for operating theatres in hospitals
VDV’s are generally used when calculated response factors are within the limits of multiplying factors for continuous vibrations
Guidance on the use of VDV’s can be found in BS 6472 and P354

Which of the following statements are true?

The response factors or multiplying factors given in BS 6472 are “for a low probability of adverse comment”
Multiplying factors are for exposure to continuous vibration
Multiplying factors are for a 16 hour exposure period during the day
Multiplying factors are for an 8 hour exposure period during the night

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

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 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

Why do we calculate the natural frequency of a floor system? (tick all that apply)

To avoid resonant behaviour of the floor plate
To ensure that any dynamic effects do not induce loads greater than the static loads for which the floor plate has been designed
To ensure that the system frequency is sufficiently greater than the forcing or walking pace frequency
To obtain a qualitative prediction of the serviceability performance of the floor plate

Which of the following statements is incorrect? Human perception to vibration:

Depends on the frequency of the vibration
Depends on the direction of vibration relative to the spine
Is higher for intermittent vibrations
Is higher with vibrations having a frequency between of 4 and 8 Hz

The frequency of an element or system is inversely proportional to the square root of the deflection. On what loading is the deflection based in the UK?

Dead load + 30% of the imposed load
Dead load + 10% of the imposed load
Dead load only
Dead load + imposed load

A designer has conservatively assessed a floor plate for an office of 18m x 30m. The structural grid is 9m x 7.5m and the plans show a central corridor running the length of the building. The target response factor is 8, but initial calculations indicate a response factor of 9. Which of the following strategies could the designer use to potentially reduce the initial response factor calculated?

The source of excitation (the corridor) is close to the column line, and so a µe value less than 1 is possible
Replace normal weight concrete for the composite slab with lightweight concrete
Reduce the walking path and resonance build-up by introducing doors in the corridor
If the axis of vibration is known use the appropriate frequency weighting curve