Updated: May 22, 2020
When sitting in a potential customer's office during discussions about a future building project, one topic is usually at the forefront of everyone's mind... WIND SPEED. Buildings, however, are not designed to withstand wind speed, but rather corresponding WIND PRESSURE.
How do we convert wind speed to wind pressure? With NOAA releasing a projection this week showing a "unusually active" hurricane season, we thought a step-by-step process of what wind speed means was worth discussing.
All building codes in the United States, including the Florida Building Code derive their wind speed maps from the ASCE-7: Minimum Design Loads for Buildings and Other Structures. This document is released every few years with the latest version being ASCE 7-16 corresponding in a 2016 release date.
In many countries, local building codes typically refer back to the IBC, the International Building Code. The IBC in turn sites the ASCE-7 document for typical gravity design loads and lateral load calculation procedure. Each country and/or municipality sets the design wind speed that all new construction projects must be designed to withstand.
Converting Wind Speed to Wind Pressure
With the roll out of ASCE 7-10, the American Society of Civil Engineers fundamentally changed the formula for converting wind speed to wind pressure. This became evident when comparing wind maps in the United States...
The highlighted wind speeds above show the design wind speed in South Florida increasing from 150 MPH to 180 MPH between 2005 and 2010... what happened??
Between 2005 and 2010 the ASCE switched from "service based design maps" to "strength based design maps". This follows the increasing prevalence of LRFD, Load and Resistance Factor Design, over ASD, Allowable Strength Design. The difference between LRFD and ASD is a topic for another day but the gist is that LRFD is based on more variables producing a more accurate result and in turn, a more efficient design.
Setting that point aside, we will continue converting the wind speed to wind pressure. This is done through equations 27.3-1 (ASCE7-10)
The wind pressure is a product of the velocity squared as well as a host of other project specific variables. From this point, further calcs are done to map the whole building taking into consideration roofs and walls, corners/sides/middle of plane, height of building, uplift considerations, etc.
For customers in the United States, this really does not matter. The entire country is mapped in great detail and the design professionals design to the precise wind speed and code applicable in that municipality. Things are not always as simple for our customers in the Caribbean...
Why Does The Change from ASCE7-5 to ASCE7-10 Matter?
In the absence of clear-cut code compliance and enformcent, we often hear from our customer "Design my building to a wind speed of X MPH". Sometimes that's easier said than done ¯\_(ツ)_/¯.
Several countries including the Cayman Islands and the Turks & Caicos Islands utilize IBC 2009. Believe or not, this ties back to ASCE 7-05. However, because of Miami's cultural significance in all aspects of Caribbean life, several of our customers know that Miami's design wind speed is 180 MPH... so we have, on occasion, been asked to design their structure to 180 MPH. The pitfall is that we must used the local code for building department approval. A 180 MPH design speed utilizing IBC 2009 is extremely conservative and would prove to be cost prohibitive for the building owner. At this point we do a verbal rendition of this blog; which is to try to explain how one building designed to 150 MPH and another designed to 180 MPH are virtually the same. It's confusing!
The good news is that our computer software includes a database of many revisions to the code. We can provide a building designed to IBC 2009, 2012, or 2015 with the same amount of effort. Caribbean Structural Systems is well-versed in the various codes in use throughout the Caribbean and we always work with our customers to ensure we have produced a building they can feel safe in.
With the COVID-19 Pandemic reducing construction capabilities throughout the islands (and the CSS office), we intend to turn this into a series. In the coming weeks we will discuss
How design codes are set (what happens when a hurricane's winds exceed the design code?)
A more detailed look at how wind speed is converted to wind pressure