Crown Sydney Construction Surveying Case Study
Surveying Sydney’s tallest tower.
The Crown Hotel Resort was a once in a lifetime project with a design, size and construction technique that had never been seen before in Australia. CMS Surveyors were engaged by Lendlease to provide full survey services throughout the construction of the Wilkinson-Eyre designed building.
The Crown Hotel Resort was a 71 storey build at a cost of $1.2 Billion. From a surveying perspective, the size and design of the building lead to high spatial complexity from the beginning, and then with the added complexity of building movement as predicted by the structural engineers. For CMS, our challenge was to capture and interpret this movement, and more importantly for our client, manage the risk of structural failure.
Project complexities
The design of the Crown Hotel Resort is unique. The concrete structure including the columns and slabs were not aligned from floor to floor. The slab edges were curved, and the columns were not vertical. The entire building was designed with a horizontal torsional twist and an inclined façade. Put simply, everything about this building is complex.
Crown Sydney is Sydney’s tallest building construction to date. In effect it was 3 different sites in one:
- GF to L4 podium containing the restaurants, gaming floors, pools and tennis court.
- L6 to L23 containing the 6-star hotel.
- Levels 29-69 containing the apartments and villas.
In terms of design, there were many challenges:
- all columns had a 4 degree pitch off vertical
- horizontal slabs were not aligned vertically
- the building design led to major structural movement
- this movement required a comprehensive survey monitoring regime
The Surveying Team
The combination of the buildings size and design resulted in many spatial challenges and required significant planning and coordination of staff by CMS management. At peak times, the survey team consisted of the following:
- Structural Monitoring – 1 surveyor
- Formwork and Lift Core – 2 surveyors
- Hotel Fit out – 1 surveyor
- Façade Grids – 1 surveyor
- Basement – 1 surveyor
- Apartment Fit out – 1 surveyor
- Podium/Restaurants/Gaming rooms – 1 surveyor
Staff on the project included Stuart Soltau, Andrew Cummins, Andrew Irwin, Roland Christinat, Fred Pall, Hamish Roberts, Archie Moutavelis, Damian Edwards, Brian Whiting, Siva Sivasothy BSc Eng (Hons), Dan Luff, Jaime De Andrade, Jordan Diaz, Harry Hobday, Liam Bennett, and Kord Koelmel.
It should be noted that The Crown Hotel site required a lot of set out on a level never seen before. The 6-star nature of the hotel rooms and the high-end finishes of the apartments dictated this.
Additionally, as the site was situated on reclaimed land on the Sydney Harbour foreshore, the basement excavation was particularly hazardous. CMS were required to survey the basement extent as it progressed down three floors. We also had to check the movement of the plunge columns and Barrette pile foundations holding up the 71 story structure. It was extremely noisy, hot, humid, muddy and dark. In addition to the hazmat suit, our surveyors also had to wear normal PPE i.e. helmet, gloves, protective glasses, earplugs and steel toed gum boots These conditions continued for 12 months.
Technical excellence to deliver solutions
The unconventional design of the Crown Hotel project led to major structural movement. Our client and the structural engineers required accurate survey data which captured this movement on a weekly basis.
CMS Surveyors were responsible for delivering a comprehensive monitoring regime which could quantify this shift in the structure. These measurements provided our client a window in which to view what was really happening week to week as the 2 year construction cycle rolled along.
The monitoring regime was twofold:
- Optical monitoring using total station theodolites
- Automated digital monitoring using tiltmeters, strain gauges and extensometers.
The engineers and builder relied on this monitoring survey data for informed analysis of the concrete slabs, columns and lift cores. This analysis was important. It was crucial to ensure that the concrete slabs would not crack from the forces of vertical deflection and torsional rotation. Such an event would lead to the major repercussion of ongoing structural defects. These defects would have had the potential to create serious financial uncertainty over the entire project.
The two main structural shifts that CMS needed to measure were;
- Vertical slab deflection,
- Horizontal movement and rotation of the lift cores and columns.
SLAB DEFLECTION
Slab deflection is caused by the differential shortening or compaction of the concrete columns and lift core. The survey data relating to the slab deflections needed to be on Australian Height Datum. This was important so the design team at Lendlease could set the finished floor level of the apartments.
It was not possible to use a resection of external control to measure slab heights as this would not provide the required precision. Additionally, we could not use a tape measure as this would take too long and be inaccurate over such a tall structure. A technical solution was sought to accurately measure the shortening of the concrete structure over the entire building height of 275m and as such, determine the actual RL of the deflecting slabs.
By using a digital extensometer, CMS could accurately relate our slab deflection surveys on multiple floors back to a datum on the ground floor. An extensometer is an instrument used to measure the deformation of a material under stress.
The amount of shortening of the concrete captured by the extensometer was transferred via data loggers to digital gateways in the stair core, then from the gateways via a wireless connection to a web portal. CMS could log onto the portal at any time and see how much compaction of the concrete was taking place in real time. This allowed us to link our survey data on the high-rise slabs back to a ground floor datum.
This solution to the problem of how to measure concrete compaction over such a large structure accurately and quickly by the use of digital extensometers was an important new innovative procedure for CMS Surveyors.
HORIZONTAL ROTATION
The amount of rotation of the core was determined by setting up our theodolite on an adjoining 51 storey building. This enabled us to take measurements of the lift core from Level 5 to Level 69. This data enabled the engineers to quantify how much to pre-set the core in a position offset to design. If the cores were not pre-set, the rotational movement would push the cores into the slab floor space, thereby compromising the detailed fit out of the apartments.
The amount of rotation in the slabs was monitored by taking measurements to nails placed in the slab the day after pouring. By setting our instrument up on the edge of the building, we could use our external control and measure the horizontal position of the nails. These measurements were done at particular stages of construction, essentially every 4 floors. This methodology of repeatedly returning to the slabs below and taking survey measurements allowed us to capture the compounding building movement over time. The recorded movement was up to 108mm on Level 51 over a 10 week period. You can see the detailed PDF here.
Clearly, this amount of rotational movement over a short period of time was of critical importance to the structural engineers. Our monitoring surveys enabled them to clarify whether the recorded building movement aligned with their design model and predicted movement tolerances. The successful capture and monitoring of this building movement in a methodical and precise manner by CMS Surveyors was an important part in this analysis.
BUILDING MOVEMENT AS A FUNCTION OF TIME
Another complex problem to overcome was the fact that the building performed differently according to on-going site activities and construction cycles. Essentially, the movement in the structure was a function, not only of shape and design, but also sheer mass and even more critically, time.
When the structure was at Level 43, our monitoring data indicated that the rate of horizontal building movement was slowing. Up to this point, the building was consistently moving to the East. After Level 43, the building began to straighten and then bend back on itself to the West. The engineers did not predict this to happen until about Level 60.
This unfolding development was of great concern to the team at CMS. We needed to ascertain why this was happening. Although our client had deep trust in CMS Surveyors’ abilities in high rise construction surveying, this unpredicted building movement required an explanation.
After much consultation with our senior surveyors on site, we concluded that something must have happened to interrupt the movement of the building to the east. Knowing that the movement behaviour was a function of design and time, we decided to analyse the site activities around the time of the L43 construction stage and look for any anomalies.
By checking the progress of the concrete pouring cycles in our site diaries, we realised the consistent four day pouring cycle had changed. In fact, after the pouring of the L43 slabs, there was a 20 day cessation of all concrete pours. This was due to the change in the lift core wall thickness requiring removal of part of the lift core jump form. The resultant exclusion zone on the slabs below meant all concrete pouring activities had to cease until the re configuration of the lift core jump form was complete. The fact that the concrete pours had stopped for 20 days meant that the force applied to the structure was less, thereby causing the building to stop moving to the east and in fact straighten.
Agreeing that our analysis of the building movement was correct, our client instructed the engineers and all stakeholders involved with the installation of the glass façade to a meeting. It was decided that CMS would continue to survey the movement of the slab edges all the way to the final occupied floor at Level 69. See PDF document concerning Proposed Adjusted control shift.
This information of the slab edge position determined the final setout of the glass façade thereby ensuring its successful installation without lost time or delays to construction.
For the construction surveyor, the issues in relation to capturing building movement are difficult to overcome. The complex design and sheer size of Sydney’s tallest building provided unique challenges for the team at CMS Surveyors.
Establishing a comprehensive survey monitoring regime over a two year timeline enabled CMS to prepare a detailed and precise data set document for our client. This data contributed to informed decision making by the engineers and provided certainty to our client, Lendlease.
OTHER NOTABLE CHALLENGES
- Summer bushfires in 2020 resulted in several weeks of lost time due to site closures. The
environment was considered unsafe as a result of poor air quality and smoke. - Covid 19 restrictions limited the number of workers in the site lifts. This caused delays in
accessing the high-rise floors. - The position and aspect of the site meant it was exposed to strong westerly winds for many
weeks on end in winter and north easterly winds in summer. This inhibited the set out of the
façade and the structure on the top leading decks.
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