Wind Load Comparison between CFD and ASCE 7 for Rooftop shapes
Welcome back to our blog series on the phenomenon called Wind Load CFD Modeling and how it affects civil and mechanical engineers. Wind Load is the force that blowing wind exerts on any device or structure that extends above ground level. After an initial introduction to the factors that affect wind load on an object, I compared three different sets of wind load calculation methods using three simple objects, hypothetically placed on a 100-foot-tall building. Starting with generic drag equation for the first equation, added two modification coefficients called “gust factor” and “exposure coefficient” for the second equation. I used the model for rooftop objects for the third equation, explained in ASCE 7: Minimum Design Loads for Buildings and Other Structures. In general, the three different equations predicted the same relationship between the wind load on the objects. The cylinder should experience the lowest load, due to the smoother airflow path if offered for the wind around it. The square objects, with their sharper corners and larger areas for turbulence behind them, both should experience higher wind loads. The diagonal square should see the highest force, as it has the largest projected cross section (i.e. the two-dimensional shape of the object perpendicular to the direction of the wind).
Calculation results in the last blog entry showed some large discrepancies between the wind load results from the three different equations. Prevalent for the square tower: additional gust factor, exposure coefficient predicted a wind load 70% higher than the drag coefficient, ASCE 7 predicted a value 150% higher.
CFD Modeling Setup
Figure 1: Wind load calculation example structures