As a structural engineer, your heart sinks when you see one of your designed buildings make headlines for all the wrong reasons, as a property owner its devastating to have your home keep you up at night and drive you crazy.
Creaking, groaning, popping and any number of weird and wonderful noises often produce a head scratch moment for Structural Engineers and builders alike.
While the causes for Creaky or Noisy Building syndrome can be varied and complex, we first must define what this phenomenon is…
“What is Creaky Building Syndrome Anyway?”
Creaky Building Syndrome is when a building produces un-wanted nuisance noise which are at volume levels readily audible by the buildings occupants and users. While these nuisance sounds can vary, the most common is that of a continuous “Creak” which sounds very much like and old timber ship swaying on the ocean.
Here is an example of such a sound recoded in the Burj Khalifa in Dubai…
“Creaky building syndrome is when a building produces un-wanted nuisance noise which are at volume levels readily audible by the buildings occupants and users”
As a Structural Engineer it is interesting to audibly track the buildings period of motion in the recording above; as the building moves to the end of one cycle, a brief pause in sound can be detected before starting up again as the building moves in the opposite direction (I’m sure the buildings occupants aren’t as impressed with this as I am)
These sounds are generally created by movement of the structure, this movement can cause the structural elements of the building (beams, slabs and shear walls to influence and interact with the non-structural elements of the building). Can we stop structures from moving? Generally not! But the usual cause is a special kind of movement, this may explain why the dreaded Creak is more evident in high-rise structures, so its time we dig a little deeper…
Reason 1: Vortex Shedding
Ok we will try not to get too technical here… vortex shedding occurs when wind flows past a non aerodynamic object (noting that majority of buildings are square or rectangular in nature). The interaction between the air flow and the blunt object causes vortices to form in the wake of the object. The “shedding” of these vortices can form in alternating patterns on opposite sides of the building.
Where a vortex is formed, this produces a low pressure pocket in that location adjacent to the building, this low pressure pulls the building into the location of this vortex. The low pressure cycles occurring on alternate sides of the building causes it to have a tendency to sway side-to-side in a perpendicular direction to the direction of the wind hitting it.
Below is an animation of what this may look like if we could see the flow of wind around a building (this is a plan view representation and in this instance our building shape is circular, wind flow is coming from the left of screen, vortices can be seen forming in an alternating pattern in the wake of the building to the right. While not shown here, the movement of our circular building under these conditions would be in an up-screen down-screen direction).
The effects of vortex shedding become more and more significant the more slender and tall a building is (this is the reason why high-rise structures appear to be more effected by Noisy Building Syndrome) .
The reason for this is, taller structures, when set into motion by a wind gust, have large periods of motion. That is, when a gust of wind comes and goes, the structure may be swaying for several seconds after the wind gust has disappeared. Very tall buildings can have a natural period of 10-15 seconds or more! This means on a very windy day a tall building will continue to sway side-to-side between wind gusts and may not have a chance to come to rest and can be in motion for several hours at at a time (or all day!).
This leads to the second reason for the dreaded building creak…
Reason 2: Separation Between Structural and Non-Structural elements.
With our tall building swaying in the breeze all day, any elements within the building which are required to slide against one another under the buildings motion don’t have the opportunity to dissipate the build-up of heat. Heat build-up causes friction build-up. The increased friction between two surfaces can cases them to “grab” against each other. The quick succession of these surfaces grabbing and releasing from each other over and over is what causes this creaking (or ticking) sound.
Heat is the friend of friction, this is the reason why drag cars perform a spectacular burnout display before zooming down the quarter mile, not just to impress the crowd (although I’m sure this is a welcome secondary effect).
Lets focus on a particular detail of a non-structural partition wall to see this in action. It is common in Australia to construct partition walls from a cold formed metal stud with plasterboard cladding (or drywall for those playing at home in the US). In order to separate the stud wall from the structural slab/beam above, a gap is provided to allow the structure to deflect vertically. In order to hold the studs into place, a top track section is provided which clamps the top of the studs (this is shaped like a channel).
- Structural Slab/Beam
- Cold formed steel stud
- Continuous Top track (also cold formed steel)
- Deflection/gap allowance zone between slab/beam and studs
- Plasterboard (drywall) cladding
The top tack is connected directly to the structure so any movement of the structure will cause the top track to move with it. The studs within the top track are free to rotate and move to ensure vertical load is not passed from the slab/beam to the stud wall. While this detail is great at ensuring no unwanted load is passed to the stud wall, it does introduce an obvious metal-on-metal friction surface as the studs are free to rotate and move within the top track.
This is a significant smoking gun for the cause of the dreaded Creaky Building Syndrome.
This detail works fine in low-rise structures. When a gust of wind hits a shorter building, sure it will move and deflect, but it will come to rest far sooner than a tall or very tall building, therefore allowing the heat to dissipate and giving this detail a better chance of quietly doing its job.
Reason 3: Excessive Building Movement
Ok so I said earlier that we can’t stop buildings from moving, however that doesn’t mean Structural Engineers can ignore building movement altogether!
Creaky Building Syndrome can simply be caused by excessive movement of the building itself, greater than code allowances.
While this is usually a much more rare cause for Noisy Building Syndrome, Structural Engineers aren’t perfect and can sometimes get it wrong.
There may be a few reasons for this:
- The Engineer doesn’t account for the buildings behaviour if it is irregular in nature
- Wind parameters are incorrect or a wind tunnel test is not recommended by the structural engineer where it should have been.
- Over reliance on computer modelling without sanity proof checking with simplified approaches.
- Inadequate checking and third party peer review
“ok so how can we prevent our buildings from being noisy?”
Many building contractors would regard this quite literally as the multi-million dollar question.
The global engineering and construction industry has learned a lot over recent decades how to reduce the occurrence of Creaky Building Syndrome, however it still rears its ugly head from time to time.
There are some measures that can be used to assist with the battle against Noisy Building Syndrom…
Reduce the Effects of Vortex Shedding
Engineers and Architects have tools at their disposal which can assist in reducing the effects of vortex shedding, the most powerful being modification to the buildings shape and form.
The aim of the game is to disrupt the formation of the vortices and this is achieved by using a building form with a vertical irregular arrangement. This disrupts the flow of air along the buildings edges and prevents large vortices from forming up the buildings height. Here are a few great examples of building forms which would help fight the battle against Creaky Building Syndrome…
Better Connection Detailing Between Structural and Non-Structural Elements.
In order to try and solve the issue of heat and friction build-up with details such as the one pondered earlier of the non-structural partition wall; victory against the Creaky Building Syndrome can be achieved by reducing the friction as much as possible. Lets face it, builds have and always will move, the solution is to try and reduce the friction, not eliminate the movement altogether.
I have seen systems used in Australia for this very junction which have been relatively successful. I have seen this system used quite effectively on a number of high-rise buildings in Melbourne.
It provides a plastic clip which, when introduced between the steel top track and steel stud, takes away the metal-on-metal contact and replaces it with a lower friction and heat dissipating alternative.
Increased Awareness and Expertise Amongst Structural Engineers
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