Salesforce Tower

High-rise Survey System

San Francisco, CA

On the Salesforce Tower project in San Francisco crews at BKF Engineers are optimizing precision and efficiency of survey during core wall construction using Leica GNSS and sensemetrics cloud connected THREAD devices, automating sensors and streaming real-time positions and analytics to field crews on their mobile devices via the sensemetrics software platform – providing a simple and seamless workflow while working above the city skyline. 

High-rise buildings are subject to strong external deflection effects caused by wind, exposure to sunlight, and unilateral loads. Such effects are a particular challenge during construction, as each of these effects cause at least temporary loss of vertical alignment, from which all survey and layout are based. Sources for effects on vertical alignment during construction can be categorized by the following:

Daily Movements
Concrete surfaces exposed to the sun will expand when compared to those on the opposite side of the building. This will result in the building moving “away” from the sun. Mathematical modeling of solar effects on the structure indicate that with a temperature differential of ten degrees celsius a movement of up to 150mm at the top of a concrete high rise structure is possible over a six hour period. Considering that control for concrete formwork needs to be set during the day when solar effects will be at a maximum, this can be a very challenging problem for survey crews to manage throughout their daily tasks.

Dynamic Movements
Building resonance induces movement in the tower with periods of as little as 10 seconds up to 15 minutes. Additionally, wind drag, and crane loads induce random movements with periods of 5 to 15 minutes. These compounded dynamic effects cause the building to move from the theoretical vertical axis and oscillate about an offset position. An successful survey program has to be designed to tolerate this movement and allow construction to proceed in a continuation of the alignment of the previous levels regardless of these dynamic effects.

Long Period Movements
Uneven Settlement manifests as the load on the foundation increases. If the settlement is uneven this will cause a corresponding tilt in the tower structure. Construction Sequence can also cause the center of mass of the building to move causing a corresponding movement in the structure.

Despite all of these effects on the structure, construction must progress in such a way that the building is aligned as planned. sensemetrics, in partnership with Leica Geosystems provide a GNSS based sensor solution that is integrated with the climbing formwork system to provide continuous tracking of the core wall throughout the construction process. It also provides real-time survey control for project surveyors and trades layout crews to precisely position components at the working level without the need to tie to external control points off structure, greatly increasing the productivity of all survey processes at elevation.

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