Urban densification in many cities has resulted in transit corridors very close to buildings

If a train passes nearby and nobody feels it, is it still a problem?

Aamna Arshad Acoustics & Vibration Consultant
Aamna Arshad

Aamna Arshad is an Acoustics and Vibration Consultant with experience working on numerous vibration control and mitigation studies, as well as architectural and mechanical acoustic design. She has experience in vibration analysis, interior acoustics, FEM modelling, multi-channel data acquisition and research. Aamna is currently in the process of applying for her Professional Engineering License (P.Eng).

Imagine this, you’re sitting in a hospital located not too far away from a set of tracks and a train passes by. You didn’t feel it, but the MRI did.

The image is blurry, schedules are pushed back, and the costs to both the patient and facility can be significant. How can this situation be prevented?

An assessment of the site early in a project, so that the team can plan for mitigation measures before it’s too late.

How early is to early?

It’s never too early. Feasibility analysis can start with determining the location of the building.

It may become clear that rail vibration is too high making the site unsuitable or that very costly mitigation measures will be required. It’s best to assess the rail vibration early before things have gone too far.

What is vibration and how does it travel?

In this example, vibrations are generated by contact between the wheel, rail, and track support system. Aspects such as the speed of the train, roughness of the wheels, condition of the track and its elevation all affect how much vibration transfers to the ground.

The vibration then travels through the ground and the soil type, depth and layering can change its characteristics as it get closer to the receiver. Finally, vibrations reach the building where the foundation and, mass, and structural system all contribute to the reduction or amplification of vibration felt by people or equipment inside.

How can rail vibrations be reduced?

There are various strategies for rail vibration mitigation. These options can be divided into three main categories to reduce vibration:

  1. The first is at the source. At the track vibration can be reduced using rail dampers, resilient fasteners, rail grinding to reduce irregularities, and speed limits. However, these strategies are not usually an option for building owners as the tracks are owned and maintained by the government, city, or other parties.
  2. Another approach to vibration mitigation is along the path. As vibration spreads through the soil towards the building a wave barrier can be used to reduce its intensity. Wave barriers intercept waves as they travel through the ground and may be an open trench, or a trench filled with concrete or sheet piles. The depth of the trench directly contributes to its effectiveness and its efficiency at reducing vibration at specific frequencies (e.g., the lower the frequency of the vibrations, the deeper the required trench).
  3. Lastly, vibration mitigation may be considered at the receiver, which in most cases is the building. Full building base isolation while efficient is a costly solution. Structural modifications pertaining to the stiffness and mass of the building as well as strategic layouts can be more cost affective, particularly during early building concept design when materials have not yet been ordered.

What is the Feasibility Assessment Process?

The first step in doing a feasibility assessment is to measure the vibration from train activity across the site. It’s helpful to take measurements at a few different locations to quantify how vibrations are reduced as they travel through the ground.

Next, all factors which may further reduce or amplify the vibration as it enters the building need to be considered. This involves accounting for effects from the building’s foundation, floor-to-floor reductions, and any influence from the way the building naturally vibrates (structural resonances).

Based on whether the predicted vibrations in the building are expected to exceed the criteria, mitigation OR more detailed analysis options can be explored.

As an example, we recently worked on a healthcare project where the predicted vibrations were exceeding the criteria of the instruments inside the building. To truly capture the affects of structural vibration within the building itself, a finite element model was created to conduct a time history analysis with the measured train vibrations.

finite element model of a typical floor in the healthcare facility
finite element model of a typical floor in the healthcare facility

This allowed for altering the dynamics of the structure (stiffness and mass) to determine which configuration will be least affected by train vibration. Specifically, it gave the ability to explore other structural options, such as enlarging beams and columns or adding additional concrete to the floor slab in certain areas. For this project, the time history analysis showed that predicted vibration levels would be below criteria and that structural changes were not necessary.

So, if your building is going to be located close to a railway consider getting a vibration feasibility assessment. Getting this assessment after the fact could make your project more costly and complex than it needs to be.

Want to know more?

If you have any questions, or would like to discuss a project, our team would be happy to hear from you. Find out more