Autodesk
by: Phil Russo
Revit Structure
Using Formulas for Engineering Calculations BACKGROUND INFORMATION Working for an engineering firm in central Florida has unique requirements that may not exist in other states. One of these requirements is to supply building departments with design pressures for openings. These are positive and negative pressures whose values are based on several pieces of design criteria. These include, but are not limited to, wind speed, exposure, and opening size. The location of the opening plays a part in the design pressures as well. An opening located closer to the corner of a structure usually has higher design pressures associated to the opening. ASCE (American Society of Civil Engineers) has code-related documentation for minimum design loads for buildings and other structures. In this documentation you can find charts of standard design pressures to use. These charts give you values covering different opening sizes, but not all needed.
Figure 1: Sample values from ASCE 7-05 minimum design loads for buildings and other structures.
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THE THOUGHT PROCESS Based on this information, the problem we faced was how to automate these values into our Autodesk速 Revit速 projects. The manual method was obvious; we could have our engineers figure out the design pressures and manually enter them as text values. Another solution would be to add the values in the door and window families themselves so it can be extracted into a schedule. Once you think about that solution you come to a conclusion rather quickly. All the iterations of wind speed, window sizes, etc. would make this extremely difficult to manage. Finally, we decided the best path was to create a formula that would calculate these pressures for us. THE FORMULA The goal of an engineer or any design professional is to show the client value. Design pressures for windows and doors are values that a manufacturer must meet so it can receive product approvals. These product approvals are required during the permit review process. With this in mind, the charts provided by ASCE are through a prescriptive method using a formula to generate the design pressures. Figure 1 shows a positive design pressure of 17.2 for an opening size of 20 square feet and a positive design pressure of 16.1 for an opening of 50 square feet. Typically if an engineer is providing design pressures for an opening that falls in between 20 March 2011