A man had been in his new inner city apartment on the 10th floor for just under six months. And every morning brought the same unsettling experience.
He lay in bed staring at the alarm clock. It was 5:17am. It was late this morning. Then it happened.
A pause, and then it happened again. He knew what would come next. A high pitched keening, then the sound of gnashing and grinding teeth, followed by a satisfied exhalation of breath. Then the farewell: “thump, thump,” followed by a pause and another “thump, thump.”
Every morning, between 5:00 and 5:30, the same thing. He had talked to his neighbours and to the building manager. No one else in the building had the same problem.
Finally, in desperation, he engaged a paranormal investigator to spend the night in the bedroom (sitting in chair!) who happened to look down out of the window just as the latest ‘event’ occurred. Only then could the cause be established.
The bedroom in question was laid out with the bedhead right next to one of the building’s structural columns. This column, which extended right through the building from the top floors down to the service bay in the basement, was right next to waste bin alcove in the basement. Every morning between 5 and 5:30, the garbage removal contractor would arrive and swap the bins, with the thumping sounds being caused by the truck driving over the speed humps in the loading bay.
By a freak of vibrational physics, the sounds were being transmitted up the building column and amplified at around level 10, right next to the sleep deprived tenant!
Ok, the bit about the paranormal investigator is a fiction, but the rest is true and actually happened in a Southbank High Rise in Melbourne.
What other surprises are in store for those who decide to live high above the grounds?
It has been reported that some apartments in many of the high rise apartments recently built in Melbourne are experiencing creaking sounds within the walls and ceilings.
Whilst the fear of paranormal activity is not identified as a possible cause of these noises, there is concern that these noises are associated with the building structure and its safety against collapse.
It is almost impossible to retrospectively identify the cause of these noises without detailed forensic investigation. However, in most cases, noises occur in a cyclical nature, and generally with specific weather events. The case of the spooky garbage truck described above is an unusual case.
Noises that occur in a cyclical nature suggest a repetitive movement of one object against another.
These noises may be due to two elements without adequate allowance for differential movement. Possible sources include:
- partition wall header tracks which are fixed to slab soffits but which do not have adequate differential movement allowance for studs (this was the cause of noises in a Southbank residential tower designed by others and built about 10 years ago)
- inadequate differential movement allowance between prefabricated components such as bathroom pods and the structure
- movement between adjoining ductwork elements which have not been adequately fixed to each other
- out of balance fans within ductwork touching ductwork
- loose debris left in an area where fluctuations in air pressure occur
- referred building vibration from a remote source
- some section or element of the facade rubbing against the structure when the structure sways under wind loads.
As unsettling as these noises are, they are not due to poor structural design.
According to a quote attributed to Confucius, “the green reed which bends in the wind is stronger than the mighty oak which breaks in a storm.”
Tall buildings sway in response to lateral loads from wind, and in extreme situations, from earthquakes. The amount of sway is a function of the stiffness of the structure. The two parameters addressed in the design process are the actual sway at the top of the building and the maximum sway acceleration of the building as it responds to the wind or earthquake load
Residential tall buildings – indeed all tall buildings – are designed to limit sway acceleration to limits acceptable to human perception.
Whilst for very tall slender buildings, the total sway at the top of the building may be of the order of 500 millimetres, the incremental movement within the structure over short distances is so small that the generation of noise due to the structure, rather than the non-structural elements attached to the structure is highly unlikely.
To address structure movement causing damage to non-load bearing attachments, Australian Standard AS1170.0 provides some guidance in limiting structure sway movement. Table C1 gives the suggested side sway at the top of a column as height/500.
As an example of how this relates to tall buildings, structural analysis for the 568 Collins Street tower established a serviceability sway, under a one-in-20-year wind event, of 375 millimetres. Over the building height of 226 metres, this gives a height over drift ratio of H/602.
The Australian Standard limit of H/500 translates to a maximum drift of 452 millimetres over the full 226-metre building height, significantly more than actual 375 millimetres.
So the next time you hear a groan, or creaking, or a thump in your new high rise apartment, you don’t have to worry about calling a structural engineer. Whether you call Ghostbusters is up to you.