Volume 25, Issue 6 - November/December 2011

Guest Book

Growing Awareness
Changes in ASCE 7-10 to Affect Hurricane-Prone Regions
by Rick De La Guardia

The American Society of Civil Engineers (ASCE) 7 is the design standard which has been adopted by the Florida Building Code

(FBC) to determine the applicable structural loading of its buildings and structures. The standard provides the basis to determine design windloads, define hurricane prone regions and specify impact protection requirements of glazed openings for wind-borne debris regions. There are dramatic changes in the organization and philosophy of the new edition, ASCE 7-10, compared to ASCE 7-05 and previous editions.

A Closer Look
Before discussing the changes to the actual design windload and the revised wind-borne debris regions, it is necessary to understand the standard’s organizational changes. The biggest change in the new edition is how the information on windload is presented. The new edition utilizes six chapters to disseminate the information that was previously encompassed in one chapter. It places the general requirements in chapter 26 with the remaining five chapters covering the new modified design procedures. The design procedures have been revised from three previously “allowed” design procedures (simple, analytical and wind tunnel) into numerous “permitted” design procedures separated into two categories: Main Wind Frame Resisting Systems (MWFRS) and Components and Cladding (CC).

In the Details
The biggest philosophical change is in how the buildings are categorized and the emphasis placed on impact protection of glazed openings. Previous editions categorized buildings by occupancy with adjustments to the windload based on an importance factor associated with a particular building due to its occupancy. These occupancy categories and their importance factors did not address the impact protection of the individual buildings in question. Instead of utilizing the occupancy and importance factors of each building to determine impact requirements, the previous editions of the standard utilized a single wind speed per region to identify locations requiring impact protection. The new edition has devised new wind speed maps categorized by risk. Each new category is assigned different wind speeds that increase in speed based on increasing risk to the particular building in question. These new risk categories are similar to the old occupancy categories, but what the new edition has done is effectively eliminate the importance factor when considering the effects on windload. There are now three different wind speed maps based on the new risk categories. Risk Category I (buildings and other structures that represent a low risk to human life in the event of failure), Risk Category II (all buildings and other structures except those listed in Risk Categories I, III and IV) and Risk Category III & IV (buildings and structures, the failure of which could pose a substantial risk to human life and buildings and structures designated as essential facilities).

It is also necessary to understand the distinction between wind speed, velocity pressure and design windload. The design windload, which is based on the velocity pressure and wind speed, among other factors, is the basis for design. A dramatic increase in the wind speed does not necessarily mean a dramatic increase in the velocity pressure or in the ultimate design windload. The determining factor for an increase or decrease in design windload ultimately will be determined in the equations and variables of each of the new design procedures. Whereas the increase in wind speeds may appear to be dramatic on the surface, what the new code has actually done is revamp its philosophical basis for protection.

“The new edition of the standard has devised new wind speed
maps categorized by risk.”

Another radical departure from the previous edition is the emphasis on impact protection based on risk and not location. Whereas the definition of wind-borne debris region remains the same with respect to location, the wind speed limits used to consider if the region is in the wind-borne debris region now depend on the risk categories. The new limit to determine if the area is in the wind-borne region, regardless of its proximity to the coast, is 140 miles per hour (mph) up from 120 mph. The biggest difference is that, whereas before the wind-borne debris region covered all buildings within a specific location, now it is possible for buildings next to each other in a particular location to have different impact protection requirements based on their risk categories. The new risk factors will affect both windload and impact protection requirements.

ASCE 7-10 will go into effect in March of 2012 when the new edition of the Florida Building Code is expected to be adopted.

Rick De La Guardia, A.M. ASCE, is the president of DLG Engineering Inc. in South Miami, Fla.


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