ProfessionalRoofing August 2010 35

The 2010 edition of ASCE 7, “Minimum Design Loads for

Buildings and Other Structures,” became available in May

and replaces the 2005 standard. ASCE 7, issued by the American

Society of Civil Engineers (ASCE), provides information that

allows designers to determine various types of loads and load

combinations on buildings.

The wind load provisions in the new edition have changed

dramatically from those in the 2005 publication. In some in-

stances, loads on a given building are higher, and in other cases,

they are lower. The 2010 changes are the most significant since

the dramatic changes of the 1995 edition. The 2010 edition will

be referenced in the 2012 editions of the International Building

Code and International Residential Code.

Following is an overview of the additions and changes pertain-

ing to calculating wind loads on roof assemblies only. I do not

address changes that only pertain to primary structural elements

such as beams, columns, shear walls and diaphragms that provide

support and stability for overall buildings.

ASCE 7’s significant changes pertain to calculating wind loads on buildings

by Thomas L. Smith, AIA, RRC

36 August 2010 www.professionalroofing.net

The maps

In the 2005 edition, basic (design) windspeeds are presented in one map (see Figure 1). In the continental U.S., basicwind speeds range from 85 to 150 mph.These speeds are for peak gusts measuredat 33 feet above grade in Exposure C(open terrain). In the 2010 edition, there are three maps.

One map is for buildings in Category I(buildings representing a low risk to life inthe event of failure, such as a greenhouse).The second map is for buildings in Cate-gory II, which include most residential,commercial and industrial buildings. Thethird map is for buildings in Categories IIIand IV, which include buildings housing a large number of people and buildingsproviding critical community services(such as schools, hospitals, and fire andpolice stations). The 2010 maps are “strength design”

maps, meaning they map winds that have

a much longer mean recurrence intervalthan the 2005 mapped speeds. As can beseen in Figure 2, the speeds for CategoryII buildings range from 110 to 180 mph(as with the 2005 edition, these are alsofor peak gusts at 33 feet above grade inExposure C). For Category II buildings,the 85 mph from 2005 is equivalent to the 110 mph from 2010. In the 2005 edition, an importance fac-

tor was used in the wind load calculationequation to account for the building cate-gories. The importance factor reducedloads for Category I and increased themfor Categories III and IV. Because the2010 edition provides separate maps forthe various building categories, the impor-tance factor was eliminated. The influenceof building category can be seen by com-paring Category II wind speeds of 110 to180 mph (see Figure 2) to the CategoriesIII and IV wind speeds of 115 to 200 mph(see Figure 3). To compensate for the change to strength

design maps, changes were made to the load

combination criteria provided in Chapter 2. In the 2005 edition, a load factor of 1.6 wasused for strength design. This was changedto 1.0 in the 2010 edition. Except in thehurricane-prone region along the AtlanticOcean and Gulf of Mexico, wind pressurescalculated in accordance with ASCE 7’s2005 edition should be about the same asthose calculated in accordance with its 2010edition. When using allowable stress design(which almost always is the case for roofsystems), a load reduction factor is used asdelineated in Chapter 2.The other significant mapping change

occurs in the same hurricane-prone region.By comparing Figure 1 with Figures 2 and3, you will notice the location of the con-tour lines has changed. The boundaries between hurricane-prone and non-hurricane-prone regions generally haveshifted toward the coast. As can be seen, in some areas, the shift is quite significant.

Figure 1: ASCE 7’s 2005 edition provides basic design wind speeds in one map.

ProfessionalRoofing August 2010 37

For more information regarding ASCE 7’s 2010 changes, information about an ASCE webinar regarding changes to the standard and a link toan Applied Technology Council guide detailing how to apply the standard’s provisions, log on to www.professionalroofing.net.

Hurricane coast

Before the 1998 edition of ASCE 7, a nar-row strip of land along an ocean’s coastwas included in Exposure D conditions.However, research during the 1990s indi-cated that during hurricanes, oceans wererough and more approximated ExposureC conditions. So the 1998 edition speci-fied that land along hurricane coastlinesbe designated as Exposure C, which sub-stantially reduced the wind load. Subsequent research has shown surface

roughness over oceans during hurricanes is consistent with Exposure D. As a result,ASCE 7’s 2010 edition requires use of Ex-posure D along hurricane coastlines (seethe photo). Therefore, buildings near a

The basic wind speed becomes 115 mph. Using the simplified procedure, thestrength design uplift load is 23.8 psf inthe field of the roof. Using the load reduc-tion factor of 0.6 from Chapter 2 for al-lowable stress design (ASD), 23.8 psf x 0.6= 14.28 psf, which becomes the ASD load.So 14.28 psf x 2 (safety factor) = 28.56(slightly lower than the 29.2 psf derivedfrom the 2005 edition). As with the 2005procedure, a system that had an FM 1-60rating still would be sufficient.When working with roof systems via

ASD with the 2010 edition, it is impor-tant to apply the 0.6 load reduction factorand apply the safety factor (typically 2) tospecify an appropriate laboratory uplift-resistance rating.

The location of the contours that occurbetween the boundary line and coast alsohas shifted.As a result, some areas that previously

were in a hurricane-prone region no longerare in one. Also, the contour shifts will re-sult in reduced design wind loads on manybuildings. All the Atlantic/Gulf of Mexico contour

lines are driven by hurricane winds. The2010 change in contour location is basedon new, more complete analysis of hurri-cane characteristics.

Determining resistance

ASCE 7 addresses wind loads, not resist-ance. However, it is important to considerhow the changes to strength design mapsaffect resistance determination. Whenusing the 2005 edition, wind-uplift loadsare calculated for the different roof zones(field, perimeter and corner). For low-slope roof assemblies, the field-uplift de-sign pressure commonly is calculated andthen multiplied by a safety factor of 2 toobtain the minimal level of resistance asystem would need to have achieved during laboratory testing. For example, consider a mechanically at-

tached membrane system on a 30-foot-tallCategory II building in a suburban expo-sure (Exposure B) in the center of the U.S.(90-mph wind zone). Using the simplifiedprocedure (in Figure 6-3 of the 2005 edi-tion), the design uplift load is 14.6 poundsper square foot (psf ) in the roof ’s field:14.6 psf x 2 (safety factor) = 29.2 psf. Sofor the roof ’s field, a system would beneeded that resisted a test pressure of at least 29.2 psf. Therefore, a system thathad a rating of FM 1-60 would be suffi-cient because such a system would havesuccessfully resisted 60 psf during testing. Now, consider the same building, but

analyze it using ASCE 7’s 2010 edition.

Figure 2: Category II wind speeds in ASCE 7’s 2010 edition

coast (as defined by ASCE 7) typically willhave significantly higher wind loads underthe 2010 edition versus the 2005 edition.Determining a building’s exposure some-

times is difficult. Further guidance for exposure assessment and interpolating be-tween exposure categories is provided in thestandard’s commentary. The commentary isnot part of the standard, but it is boundwith it and consists of explanatory andsupplementary material designed to assistin applying the standard’s requirements.

Wind-borne debris region

A wind-borne debris region was added to the 1995 edition. The wind-borne debris region occurs within a portion ofthe hurricane-prone region. The 2005 edition requires that glazing in buildings

in Categories II, III and IV be impact-resistant or protected with an impact-resistant covering (an exception is stipu-lated for certain buildings taller than 60feet). In the 2005 edition, the extent of the

wind-borne debris region is the same forCategories II, III and IV. However, this no longer is the case. In 2010, the wind-borne debris region occurs within 1 mileof the coast where the basic wind speed isequal to or greater than 130 mph or wherethe basic wind speed is equal to or greaterthan 140 mph. Because Categories III andIV’s contours for 130- and 140-mph windsextend much further inland than CategoryII’s contours, the wind-borne debris re-gion is much larger for some Category IIIbuildings and all Category IV buildingscompared with Category II buildings.

The figures’ shaded areas show the wind-borne debris regions. The 2010 wind-bornedebris region for Category II buildings isfairly consistent with the 2005 criteria, butin 2010, there is a significant reduction inhow far inland the region extends fromthe coast around Jacksonville, Fla., theFlorida panhandle and North Carolina. During hurricanes, a majority of the

wind-borne debris-induced glazing dam-age occurs within the wind-borne debrisregion shown in Figure 2. However, dam-age investigations have shown sporadicdamage inland of the region. Because ofthe critical nature of services provided bybuildings in Category IV and some inCategory III, ASCE 7’s 2010 edition in-corporates more conservative criteria forsuch buildings.

Simplified procedure

The simplified procedure, which providestabulated pressure values, was limited tobuildings up to 60 feet high in ASCE 7’s2005 edition. Tables now are provided forbuildings up to 160 feet high. Users of thesimplified procedure should have a goodunderstanding of ASCE 7. Although it iseasy to determine design pressures fromthe tables, it also is easy to incorrectly determine a value if there is lack of understanding.

Rooftop equipment

Further research has been conducted regarding loads on rooftop equipment,which has resulted in refinements in the2010 edition. See Sections 29.51, 30.11and C29.6 for more information.

Risk category

Criteria regarding building categories areprovided in Chapter 1 in the 2005 and2010 editions. In 2005, the categorieswere referred to as “Occupancy Cate-gories.” For each of the four categories,specific occupancies were defined (for

38 August 2010 www.professionalroofing.net

Figure 3: Categories III and IV wind speeds in ASCE 7’s 2010 edition

example, a building where more than 300people congregate in one area). In 2010,the categories are referred to as “Risk Cat-egories,” but more important, specific oc-cupancies are not listed. For example, RiskCategory III says: “Buildings and otherstructures, the failure of which could posea substantial risk to human life.”The lack of specificity in building cate-

gory selection (which has a large effect oncalculated loads) may present liability ex-posure to designers. I encourage designersto carefully review ASCE 7’s Section C1.5.1and discuss it with building owners so appropriate risk categories are selected.

Although building codes stipulate build-ing categories, with the 2010 change inChapter 1, in some instances it may beprudent and appropriate to select a highercategory than what the building code requires.

Chapter reorganization

In ASCE 7’s 2005 edition, wind load pro-visions were provided in Chapter 6. The2010 edition provides wind load provi-sions in six chapters. Chapter 26 has gen-eral requirements, Chapters 27–29 addressmain wind-force resisting systems, Chap-ter 30 addresses components and cladding(which include roof systems), and Chapter31 addresses wind tunnel procedures. It

will take awhile to become familiar with the new organization,

but the new organization should facilitate the

standard’s use.

End result

Determining design wind loads is a keyfactor in ensuring good high-wind per-formance of roof assemblies. Unless windloads are appropriately determined anddesigned for, blow-off problems can be expected. ASCE 7’s wind load provisions are rela-

tively easy to use for those familiar withthe standard; however, it is just as easy fora novice to make serious errors. First-timeusers of the standard should become fa-miliar with it before determining windloads. To purchase ASCE 7’s 2010 edition, go

to www.asce.org/product.aspx?id=2147487569.

Thomas L. Smith, AIA, RRC, is president of TLSmith Consulting Inc., Rockton, Ill., and amember of the ASCE 7 Task Committee onWind Loads.

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Buildings along hurricane coastlines previously were considered to be in Exposure C. However, ASCE 7’s 2010 edition states they now are consideredto be in Exposure D, which has significantly higher loads.

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