PRELIMINARY BUILDING CHECK
CONTROL POINTS PLACEMENT
The builder and construction crew must know the proposed building-placement location before construction can begin. Numerous municipal regulations and by-laws stipulate requirements such as minimum setback distances of proposed underground garages and buildings from property boundaries. Early research with the architect, professional planner and client will bring these matters to light and allow all parties to proactively address issues, if required. As Urban Construction surveyors, we review architectural plans, in detail, to verify drawing dimensions, strategize where and how many construction “control points” may be required, and confirm property-boundary bars on the site. Each site is unique, depending on the nature of the construction, size of the project and height of the buildings. For example, the job may entail a multi-block townhome complex or a 50-storey high-rise condominium building.
The establishment of fixed construction control points surrounding a building site is critical for the ongoing accuracy of the construction layout. What is a construction control point? It is a retraceable point on the surface of the earth with known coordinate values (X, Y, Z). Construction sites undergo significant earth and material movement due to the use of construction vehicles and equipment, but construction control points are solid, stable points, usually established outside the construction zone, which provide permanent stability and a reliable frame of reference.
The reconnaissance step includes establishing and verifying the legal site boundaries, and assessing whether there are additional factors for consideration—such as a sub-surface subway tunnel, significant trees, adjacent conservation lands, or anything on, under or surrounding the site—that may affect the construction of the building and the installation of construction control points. Determining precise coordinate values in the network of construction control points is essential to the positioning of the building construction and for reference. As buildings rise, their mathematical integrity becomes more important and more difficult to maintain. The nature of the project dictates the density of the network of construction control points needed.
INTEGRATION & MODELLING
2-D to 3-D MODELLING
Once the Urban Construction surveyor receives working drawings of the building from the architects, structural engineers and shoring consultants, the complex design information is digitally modelled and integrated into the coordinate network system established on the construction site during stage 1 (Site Assessment and Reconnaissance). These drawings may also include site grading, servicing and landscaping, and, once coordinated, they allow everything to be computed within the same coordinate system—from the position of soldier piles and caissons to the grid lines and structural elements of the foundation, underground garage and building.
Mathematical computations are carried out to ensure that all drawings provided by each design professional are consistent with all the other design information. It is critically important to resolve non-coordination of, and inconsistencies in, the design drawings. This early review of consultants’ drawings will catch any potential discrepancies at an early stage, avoiding costly repercussions for the client. All professional consultants must work together to ensure that the drawings meet the level of reliability required for modern building construction. For example, good architects will leave a reasonable construction tolerance for how close the building can be to the property line (if the client permits it). A tolerance of a few inches on architectural drawings can prevent future condominium-registration delays. The Urban Construction surveyor’s role is to compute accurate digital models of the proposed foundation and building design—based on the architectural and other design drawings—and generate physical, three-dimensional construction coordinate points, so that the two-dimensional representations on the designs can be actualized in 3-D in the digital model and on the ground.
When the design drawings from all necessary disciplines have been coordinated, we then need to coordinate with the client to establish our involvement in the future construction layout on the site. Low-rise developments (townhouses) have different requirements from those of high-rises, and a five-storey building is not nearly as complicated as a 50-storey structure. Krcmar’s role can vary greatly, from positioning the excavation shoring system to determining the placement of caissons and piles to laying out the construction grid lines at ground level and the upper levels as construction progresses.
NEIGHBOURING BUILDING IMPACT ASSESSMENT
The first thing any construction crew has to do on a site is excavate, and they have to know where and how deep to dig the hole, as well as where to place the shoring piles to hold back the dirt. As surveyors, we lay out a shoring system, and as excavation reaches the bottom, we can also provide grid lines and a footing layout so construction can begin. If the building is near a water line, the caissons put in place have to be stable.
We can also provide clients with as-built information as construction progresses, to catch any deficiencies early and correct them as the building rises. For example, with a high-rise building, we recommend surveying the concrete slab edges of the building and the elevator core every three to five floors, to ensure that the building is as horizontally and vertically accurate as possible. The accurate placement of concrete structures is the starting point for consultants of other disciplines: the installation of a high-speed elevator requires a perfectly vertical elevator shaft with minimum tolerance for error, and curtain-wall sections and windows are built to exacting dimensions. All construction parameters, such as concrete crush or settlement (when the weight of new concrete causes previously poured concrete to settle), must be taken into consideration. Even a few centimeters can make a huge difference when it comes to curtain-wall sections, windows and elevator shafts being installed, as they require exact fits.
From grade and up, continually ensuring the building’s horizontality and verticality every few floors is critical to the construction progress. If the elevator core is not within tolerance, for example, the units cannot be installed. And if the distance between floors is not within tolerance, the curtain-wall window sections cannot be installed. As surveyors, we designate where everything goes, and then we check to make sure that all guidelines are being adhered to.
Errors can happen in numerous ways (e.g., when surveying very high floors, putting grid lines in place and estimating the effect of concrete crush). By checking every few floors, calculations can be fine-tuned and corrections can be made, so that construction can move forward. The complexity of this step is exemplified by The Absolute (“Marilyn Monroe”) Towers in Mississauga, Ontario, which have a curved shape. This structure features a complete spiral every three to five floors, and, during the time of construction, the weight of the concrete was not evenly distributed. Consultants needed to understand the tilt component of this distribution to determine what was happening to the building as it rose, so our role as surveyor was particularly important.
Typically, at the end of construction, an as-built survey of the super-structure is prepared for a number of reasons: it may need to be submitted to a financial institution before they release funds to the developer, or the municipality may require a copy to ensure that the structure meets zoning and other requirements before it is registered. Urban sites are often so tight that architects have to work with absolute minimums. A building may be permitted to extend right to the property line, but never over it, so in dense urban areas, such as the City of Toronto, every centimetre counts. As surveyors, we understand that problem spotting is as important as problem solving. By proactively confirming compliance early on, there will be less chance of a delay—or of ending up with a condominium that does not meet requirements and subsequently needing to submit an application for relief from the municipality.