Before (left) and after (right) building forms based on evaluation and analysis. Note that the second building form is taller and the top has a larger rotation angle with respect to the base.
The objective of this analysis process is to optimize the building form to have the largest amount of interior floor area while also being the more aerodynamic and having the lowest wind load acting on the structure as possible.
With this new building being constructed near the waterfront in San Francisco, wind loads for a tall, slender building in San Francisco’s otherwise shorter skyline are a critical detail to be considered in the design. If the building is expected to handle the large wind loads by itself, it would require the structure to be designed shorter and wider to increase stiffness. However, if the design is able to be optimized by rotating the structure in relation to the usual wind direction and the structure’s width is able to be optimized by thinning the shape where wind speeds are expected to be largest, then the building form can be more slender and designed the desired way.
The overall node logic to perform this evaluation is similar to that done in Module 5 with the exception of the two new custom nodes.
The pink region begins with the inputs for the evaluation, including the top height and top rotation values to be iterated during the analysis. Other values that can be modified include the maximum expected wind speed at the base and top of the structure, along with the average inter-story height for creating the slope of the wind profile shape.
The inputs are then directed into the blue region, where the evaluation with the custom nodes occurs. The green region creates an initial organizational framework for the output of the evaluation results. Lastly, the results of the analysis are sent to the purple region where the data is exported to Excel.
The first custom node serves to evaluate the proximity of another building in relation to the new proposed building form for the purpose of estimating how the other structure will protect the building against wind gusts.
The node begins with the input values on the left. The regions on the top colored in orange, purple, and pink all serve to organize the output and update the Revit model for the evaluation. The primary analysis occurs in the regions on the bottom colored in green and darker pink. The green region panelizes the surface using a UV grid to create rectangular panels on the building form. Centerpoints are then created for each panel and a normal vector is created extending from each centerpoint.
The darker pink regions begin by creating a vector from an object that is placed in the Revit model in the location of a nearby structure. Using the normal vectors from the building form and the vector from the object, the second darker pink region then calculates the directness of the two vectors and outputs a value between zero and one. These values are summed and the result is saved.
For this node, a lower directness value is desired as it indicates that the majority of the building faces away from the wind direction and that the building in proximity is able to block most of the wind. Ultimately, a large value would indicate that the building form is like a sail in the wind, while a low value indicates a more aerodynamic shape.
The second custom node evaluates the aerodynamics of the building form by evaluating different building widths and floor areas to try to create the slenderest building form that still maximizes the floor area inside of the building.
The node again begins with the input values on the left. Similar to the first node, the orange, purple, and pink regions on the top half serve to organize the output and update the Revit model for each iteration. The evaluation primarily occurs in the blue region at the bottom. This region contains two more custom nodes. The first inputs the building form from Revit and outputs the mass floor areas of each story level. This is then fed to the second node, which takes the mass floor areas and the input max/min wind speeds. The inside of this node is shown below:
The node begins with the previously mentioned mass floor areas and information input in the initial Dynamo workflow. This is then led into a code block which creates a series of linearly increasing wind velocities along the height of the building based on the input ground level and roof level wind speeds and the average story height in the building. This series is then input into another code block which multiplies the mass floor area by the wind velocity at the appropriate level. The values are then summed among all stories and the value is saved for that iteration.
The analysis was performed for building forms from 600 ft to 750 ft tall in 50 ft increments and top rotations from 60 degrees to 120 degrees in 30 degree increments. The results are summarized in the table below.
While the values of gross floor area and gross surface area denote actual values in the units of square feet, the values titled “Wind Blockage” (first node output) and “Direct Wind” (second node output) are currently dimensionless values. To utilize these values, an optimization scheme must be created.
The easiest way to execute this is to assign a scaling factor based on the importance of each design factor. These scaling factors are decided based on engineering judgement and what the goals for the structure’s design are. For this scenario, the goal is to maximize floor area and the wind blockade from surrounding buildings while minimizing the surface area of the structure’s exterior (save construction/material cost) and minimizing the surface area that directly faces the direction of the wind (prevent structure from acting like a sail). In addition to this, each value must be initially modified to be on the same magnitude as the other values. To do this, each factor is divided by an appropriate factor of 10 to result in a value with a single leading digit to be similar in magnitude to that of the other factors. The scores are then summed together, with a higher value indicating a better design option. The resulting table is shown below.
Based on the results of the optimization scheme, the three recommended design shapes are shown in gold, silver, and bronze. As can be seen, the buildings with the tallest heights are the most aerodynamically efficient while also providing the most interior floor area. This makes sense as the taller structures are able to be more slender, which allows for better air flow around the structure while also providing the most floor area due to its taller roof height. In addition, a top rotation of 60 degrees sheds wind the best in all evaluations while a structure with a larger rotation top rotation tends to catch the wind more.
Ultimately, the recommended design option would be a 750 ft tall structure with a 60 degree top rotation relative to the base.