Revisions to ASCE 7-10In 2010 a major revision to ASCE 7 Minimum Design Loads for Buildings and Other Structures, commonly referred to as ASCE 7-10, was published. This revision supersedes ASCE 7-05, ASCE 7-02 and ASCE 7-98. With respect to wind loads, ASCE 7-05, ASCE 7-02 and ASCE 7-98 were very similar, and have been the reference standards for all major building codes for almost a decade. ASCE 7-10, on the other hand, incorporates numerous editorial and fundamental changes that demand attention. ASCE 7-10, has already been adopted by the 2012 International Building Code and 2010 Florida Building Code leaving little time for the fenestration and building products industry to plan for its implementation. This Information Bulletin will explain the major changes to ASCE 7-10 and o�er guidance on using ASCE 7-10 in conjunction with product testing programs.

August 2012

Major Changes to ASCE 7-10 Wind Load ProvisionsThere are three obvious and signi�cant changes in ASCE 7-10. First, editorially, ASCE 7-10 was reorganized to a multiple-chapter format. The wind load provisions have been moved and expanded from Chapter 6 to Chapter 26 to Chapter 31. The change was implemented to present a better organized wind load methodology for the structural design community. Provisions for the main wind force resisting system (MWFRS), other structures, and components and cladding (C&C) are presented in separate chapters dedicated to each topic.

Second, previous versions of ASCE 7 used a single wind speed map with importance factors for each building risk category. ASCE 7-10 now incorporates three wind speed maps that vary by risk category (separate maps are provided for the following risk categories: I, II, and III & IV). Additionally, the wind speeds are now based on strength design/load and resistance factor design in place of previously used allowable stress design. While at �rst glance, the wind speed maps in ASCE 7-10 suggest the design pressures required are much higher than the previous edition, a 0.6 factor is applied to correlate strength design to allowable stress design of ASCE 7-05.

Furthermore, the provisions for both MWFRS and C&C have been expanded to include new optional analyses techniques. Each analysis type now includes tables showing step-by-step procedures for conducting an analysis.

August 2012

Finally, ASCE 7-10 has subtly shifted the Exposure Category for hurricane prone regions from Exposure C to Exposure D. In ASCE 7-05, Exposure D speci�cally excluded hurricane prone regions, but now that exclusion has been removed. ASCE 7-10 advises that Exposure D continues 600 feet or 20 times the building height into Exposure B and Exposure C conditions. This is generally interpreted that Exposure D is to be applied within 600 feet of the shoreline in coastal regions. ASCE 7-10 continues to employ the Wind Directionality Factor, Kd. This factor provides a minor adjustment to wind loads when they are combined with other load types (i.e. dead load, live load, snow load). It is not appropriate to use this factor with calculated wind loads for most door, window and cladding products as the only load they are subjected to is wind load. Exceptions would be curtain wall mullions with axial dead load or sloped glazing with snow load and dead load.

Strength Design vs. Allowable Stress DesignStrength design and allowable stress design have both been recognized by ASCE 7 as long as the two design philosophies have been prevalent. Traditionally, loads established by the standard as nominal loads are factored based on their certainty of occurring to be used for strength design. For example, dead loads which can be established with high con�dence have a strength design load factor of 1.2 and live loads which can be established with less con�dence have a strength design load factor of 1.6. Prior to ASCE 7-10 the wind speed maps provided nominal wind loads and a strength design load factor of 1.6 was prescribed for wind loads.

ASCE 7-10 wind speed maps now produce ultimate design loads that are directly applicable to the strength design method and the strength design load factor for wind is 1.0. To use ASCE 7-10 calculated pressures for the allowable stress design method, the calculated pressures are factored by 0.6 to convert the ultimate wind load to a nominal wind load. Note that 0.6 = 1/1.6. Straightforwardly, the major change in wind load determination between ASCE 7-10 and ASCE 7-05 is the order in which strength design wind loads are determined.

Product Testing and Windborne Debris RequirementsPhysical testing of most building components and assemblies, including windows, has traditionally and continues to use an allowable stress design methodology. Typically, products are tested to a design pressure where serviceability limits (de�ection, operability, etc.) are checked and then to a test load with a speci�c factor of safety where elastic behavior is veri�ed. The factor of safety for allowable stress design testing must not be confused with the 1.6 strength design load factor as di�erent product responses are expected at the respective test loads. That is, elastic behavior is expected at allowable stress design test load whereas component failure would be expected at the strength design test load. Thus, in comparing wind pressures established by ASCE 7-10 to product performance, one must verify which design philosophy was used to establish the wind pressures and convert those design wind pressures to allowable stress design pressures, if necessary.

For impact resistant products, the pressures prescribed for the cyclic loading program should match the product design pressures established by allowable stress design. However, the actual de�nition of wind-borne debris regions and missile levels are still based on the Basic Wind Speed. With the new Basic Wind Speed maps of ASCE 7-10, the de�nition of wind-borne debris was modi�ed as shown in Table 1 and Table 2. And, it is stipulated that the Building Category II Basic Wind Speed map shall be used for all Building Category III buildings except health care facilities.

An impacted test unit

August 2012

The wind zones that establish the particular missile level have been adjusted as shown in Table 1 and Table 2. Currently, there is a signi�cant disconnect between ASCE 7-10 and ASTM E1996. ASTM Task Group E06.51 on Impact Resistance is currently revising ASTM E1996 and changes addressing both ASCE 7-10 and ASCE 7-05 are expected in the next published version of the standard.

ASTM E1996 Wind Zones

Table 1. Wind-borne Debris Region and ASTM Wind Zone within 1 Mile of Coast

ASCE 7-05

ASCE 7-10

2010 FBC /2012 IBC

Standard Wind-borne Debris Region

Vasd ≥ 110 MPH

Vult ≥ 130 MPH

Vult ≥ 130 MPH

Wind Zone 1: 110 MPH ≤ Vasd < 120 MPHWind Zone 3: 120 MPH ≤ Vasd ≤ 140 MPHWind Zone 4: Vasd > 140 MPH

Wind Zone 1: 130 MPH ≤ Vult < 140 MPHWind Zone 3: Vult ≥ 140 MPH

Wind Zone 1: 130 MPH ≤ Vult < 140 MPHWind Zone 3: 140 MPH ≤ Vult ≤ 160 MPHWind Zone 4: Vult > 160 MPH

ASTM E1996 Wind Zones

ASTM E1996 Wind Zones

Table 2. Wind-borne Debris Region and ASTM Wind Zone more than 1 Mile of Coast

ASCE 7-05

ASCE 7-10

2010 FBC /2012 IBC

Standard Wind-borne Debris Region

Vasd ≥ 120 MPH

Vult ≥ 140 MPH

Vult ≥ 140 MPH

Wind Zone 1: 110 MPH ≤ Vasd < 120 MPHWind Zone 2: 120 MPH ≤ Vasd < 130 MPHWind Zone 3: 130 MPH ≤ Vasd ≤ 140 MPHWind Zone 4: Vasd > 140 MPH

Wind Zone 1: 130 MPH ≤ Vult < 140 MPHWind Zone 2: 140 MPH ≤ Vult < 150 MPHWind Zone 3: Vult ≥ 150 MPH

Wind Zone 1: 130 MPH ≤ Vult < 140 MPHWind Zone 2: 140 MPH ≤ Vult < 150 MPH Wind Zone 3: 150 MPH ≤ Vult ≤ 160 MPHWind Zone 4: Vult > 160 MPH

ASTM E1996 Wind Zones

August 2012

Sample CalculationsSome practical calculated wind pressures are presented to show how ASCE 7-10 a�ects wind loads for components and cladding. In all cases a mean roof height of 30 feet, component area of 15 ft2 and corner zone (Zone 5) was assumed. ASCE 7-10 allowable stress design values are calculated by multiplying the strength values by 0.6.

ConclusionASCE 7-10 is a signi�cant update and reorganization of the ASCE 7 standard. The wind speed maps and calculation methods have been revised to make strength design the primary design philosophy. Design pressures calculated by strength design should be converted to allowable stress design pressures for comparison to product testing and certi�cation design pressures. Existing product test reports or product certi�cations are valid for use with ASCE 7-10 determined pressures. Requirements for ASTM E1996 Wind Zones with respect to ASCE 7-10, 2010 Florida Building Code and 2012 International Building Code are speci�ed within the documents while ASTM E1996 is being revised.

717.764.7700 . jreed@archtest.com . www.archtest.com

Mr. Joseph Reed has continual engineering experience since 1988 and joined Architectural Testing in 2002. He is a Professional Engineer licensed in seven states. He has Master of Science in Civil Engineering from Lehigh University.

About The Author Joe Reed , P.E. Director - Engineering Services

Allowable Stress Design: a method of proportioning structural members such that elastically computed stresses produced in the members by nominal loads do not exceed speci�ed allowable stresses. Also commonly referred to as Working Stress Design.

Strength Design: a method of proportioning structural members such that the computed forces produced in the members by the factored loads do not exceed the member design strength. Also commonly referred to as Load and Resistance Factor Design.

Nominal Load: The magnitude of the loads speci�ed in ASCE 7-05 for dead, live, soil, wind, snow, rain, �ood and earthquake.

Main Wind Force Resisting System (MWFRS): an assemblage of structural elements assigned to provide support and stability for the overall structure.

Components and Cladding (C&C): elements of the building envelope that do not qualify as part of the MWFRS.

Basic Wind Speed, V: Three-second gust speed at 33 ft (10 m) above the ground in Exposure C.

Vasd: Basic Wind Speed used for allowable stress design.

Vult: Basic Wind Speed used for strength design

Table 3. Sample Calculations

Building MethodBasic Wind

SpeedDesign

Pressures

York, PAExposure BCategory II

ASCE 7-05ASCE 7-10, Strength

ASCE 7-10, ASD

+16.7/-22.0 psf

York, PAExposure BCategory III

ASCE 7-05ASCE 7-10, Strength

ASCE 7-10, ASD

Miami, FLExposure CCategory II

ASCE 7-05ASCE 7-10, Strength

ASCE 7-10, ASD

Miami, FLExposure DCategory II

ASCE 7-05ASCE 7-10, Strength

ASCE 7-10, ASD

Vasd=90 MPH

Vult=115 MPH +16.4/-21.6 psf+ 27.3/-36.0 psf

Vasd=90 MPH

Vult=120 MPH

+19.2/-25.4 psf

+17.8/-23.5 psf+ 29.7/-39.2 psf

Vasd=146 MPH

Vult=175 MPH

+61.6/-81.4 psf

+53.1/-70.1 psf+ 88.5/-116.9 psf

Vasd=146 MPH

Vult=175 MPH

N/A

+62.8/-83.0 psf+104.7/-138.3 psf