By Jerry J. Barbera, P.E., and Donald R. Scott, P.E., S.E.
A practitioner’s guide for the IBC and ASCE 7
It’s been nearly 50 years since the American Society of Civil Engineers (ASCE) published a seminal article on wind loading in its proceedings, entitled "Wind Forces on Structures." Data, equations, and charts from that article and other research were incorporated into the wind provisions contained in the then-American National Standards Institute (ANSI), A58.1-1972 Standard for Minimum Design Loads for Buildings and Other Structures. In 1985, ASCE took over the duties of maintaining that document, which has now been revised eight times, including the present 2005 edition. The publication of the next edition is not expected until 2010, and it will be referenced in the 2012 International Building Code (IBC).
That A58.1-72 Standard was considered much too complicated for practical wind design of structures and was not accepted, especially in the Western and Southern states. In 1976, the International Conference of Building Officials (ICBO) set up a special task group to come up with more simple methods of calculating wind forces. The other two organizations, Building Officials and Code Administrators and, to a lesser extent, the Southern Building Code Congress International, eventually adopted edited versions of ANSI and ASCE standards, adding simplifications and containing regional variations of those standards. With the formation of the International Code Council (ICC), the concept of one common code for the United States and beyond resulted in wind provisions contained in ASCE 7. In the 2006 IBC, all wind design provisions were removed and replaced with a reference to ASCE 7-05. To assist building officials in their administrative duties, the IBC still reprints general, load, load combinations, environmental, and some design requirements.
Wind provisions in the IBC
Even though the technical aspects of wind design are now in ASCE 7, it may come as some surprise to code users that nearly three dozen diverse 2006 IBC provisions control aspects of wind design. Engineers, architects, and code officials need to be aware of and follow them as needed. For a list of IBC provisions that affect or supplement ASCE 7 requirements, see Table 1: IBC provisions that affect or supplement ASCE 7 requirements below.
Major ASCE wind design procedures
ASCE 7-05 has a Method 1, Simplified Procedure that was first printed in the 2000 IBC. Its coefficients were developed from research done for the Metal Building Manufacturers Association in the 1980s using a modern wind tunnel that accounted for the boundary layer. The coefficients presented do not represent "true" wind pressures found on the building, but envelope the pressures that will determine the requirements for the maximum structural actions. A popular design procedure, Method 1 coefficients come from combining projected windward and leeward coefficients shown in Method 2’s Figure 6-10 data and multiplying the result by 0.00256(V2) (Kz), with Kz determined at h = 30 feet and Exposure B to normalize the coefficients; after calculation, the results are adjusted by Method 1 Figure 6-3 for actual conditions. Method 1 is restricted to regular-shaped "simple diaphragm buildings," has many constraints on its applicability, and is restricted to 60 feet maximum in height.
The more robust design procedure is Method 2, Analytical Method, which applies for most "rigid" structures of all heights and shapes. But it can even design "flexible" structures with some limitations. Reportedly, most practicing engineers use this method.
A significant benefit for Method 2 designs is its applicability for a large variety of structures. It provides pressure coefficients for myriad shapes and heights of any type of regular-shaped buildings and numerous structures. Compared with Method 1, internal pressure coefficients related to enclosure classification have to be determined and then combined algebraically with external coefficients, both of which can be positive or negative relative to the exterior and interior building surfaces. In most wind designs of the main wind force resisting systems (MWFRS) of enclosed buildings, any internal pressure can be ignored because the pressure pushes or pulls equally on all walls and can be added out. However, roofs will be directly affected by internal pressure and it must be accounted for. And one-story metal "rigid frame" or all partially enclosed buildings always need to account for internal pressures on all surfaces.
Method 2 requires a complicated formula to calculate these pressures for MWFRS and their components and cladding. Here is one way ASCE Equations (6-12), (6-19), and (6-23) could be expressed, reflecting its most rigorous and robust form:
pasce = 0.00256 V2 Kd Iw [Kz Kzt (G)(Cp) ± Kzi Kzti (GCpi)] (RSM Eq. 1-1)
This equation groups all of the variables that can directly affect the pressure coefficient values inside the brackets. Most terms can be found in ASCE 7 Sections 6.3, 6.5.12.2, and 6.5.12.4 for MWFRS and components and cladding. But the terms pasce, Kzi, and Kzti are not in ASCE 7: pasce distinguishes RSM Eq. 1-1 from ASCE equations; Kzi and Kzti help describe the location for measuring velocity pressure qi on buildings. One will have to read the definitions of qi in Equations 6-17, 6-19, and 6-23 numerous times before understanding all combinations between where on the building they are measured relative to positive or negative internal pressures in enclosed or partially enclosed buildings.
Another difficulty with the equation stems from the fact that coefficients (G) from Section 6.5.8 and (Cp) from Figure 6-6 are separate terms multiplied by each other, while the internal pressure coefficients, (GCpi) are single, combined terms from ASCE 7 Figure 6-5. When determined, both products have to be laboriously combined algebraically. A common mistake is to "add" internal and external pressures together in terms of the "±" in RSM Eq. 1-1 without accounting for relative local directions of their vectors. (See Figure 1, which demonstrates how they physically need to be "added.") Fortunately, the definitions allow for a conservative determination of qi.
ASCE 7 divides up (RSM Eq. 1), into three discrete terms to efficiently calculate pressures:
* q = velocity pressure = 0.00256 V2 Kd I Kz Kzt;
* (G)(Cp) = gust-effect factor (virtually always 0.85 for rigid buildings) times external pressure coefficients (contained in ASCE Figures 6-6 through 6-17 depending on the structure); and
* GCpi = combined gust-effect factor/internal pressure coefficient (from ASCE Figure 6-5).
Accounting for the distribution of the velocity pressure as previously explained, this can be expressed as: p = q [(G)(Cp) ± (GCpi)], which could be further theoretically reduced to p = q [Net GCp]. But the user has to remember that "q" is defined as a variable term qz on windward walls and a constant term, qh on the windward and leeward roofs, as well as leeward and side walls.
Calculating [net GCp] for walls is not too difficult except that there will be two cases: one for what we call "Ballooning" (internal pressure pushing outward) and one for "Deflation" (internal pressure pulling inward); see Figure 1.
This difficulty begins when accounting for net pressure roof coefficients because of the following reasons: There are two conditions of roof slopes (<10 degrees and > 10 degrees); there are three divisions of relationships between L/h values (where L is measured in the direction of the wind and h is the mean roof height) for external roof pressures; these roof coefficients must also account for ballooning and deflation; real buildings will never be exactly at one of the h/L values or roof slopes given in the chart, and four-way interpolation needs to be performed; there are variable windward and leeward side values; there are eight footnotes; and there are three modifications for low-slope roofs that need to be accounted for! This complication comes in part from Figure 6-6, which is quite daunting to understand and apply.
Simplifications
The Structural Engineers Association of Washington (SEAW) Wind Engineering Committee has published a two-volume SEAW Commentary on Wind Code Provisions and a one-volume SEAW’s Handbook of a Rapid-Solutions Methodology of Wind Design.
Figure 2 comes from the Commentary and illustrates how the last part of ASCE 7 Figure 6-6 could be presented. As one can see from the modified excerpt of ASCE 7 Figure 6-6, it removes most, if not all, of the uncertainty of the footnote, terminology, and variable coefficients. Note we reversed the height-to-length relationship in Figure 6-6 to L/h to demonstrate the physical shape of the building so that users clearly see what is represented.
The Handbook has a plethora of easily understood charts, tables, equations, and helpful tips. The impetus for them came from work the SEAW, Structural Engineers Association of California (SEAOC), and Structural Engineers Association of Oregon (SEAO) did in the late 1980s and into the ’90s with ASCE, and in the ICBO and ICC code-change processes. SEAW is still making efforts to provide more practicality and ease in determining code or standard requirements. It built upon the pioneering work of a group set up by the ICBO to simplify the then-ANSI A58.1-’72 mentioned at the beginning of this article. Its major simplification is to move the coefficients around in Equation (6-12), et al, so that in its most common form, it will only have three variable coefficients:
prsm = qs, Kz, and Crsm (RSM Eqt. 2-1), where, prsm = design wind pressures to be used in determination of wind loads on buildings and structures or their components and cladding, in pounds per square foot (psf) from the Rapid-Solutions Methodology, qs = wind velocity pressure or 0.00256 V2 in psf, which can be made into a graph. Kzt = velocity pressure exposure coefficient evaluated at height z, feet. (ASCE 7 Table 6-3), which can also be made into charts, and Crsm = RSM net-pressure coefficient = ~ Kd [(G)(Cp)—(GCpi)] = ~ 0.85 [(0.85)(Cp)—(GCpi)], which can also be graphed out into useful visual aids.
In its most robust form, it would be, prsm = qs Kz Crsm [Iw Kzt], where our symbol, Iw = Importance factor from ASCE 7 Table 6-1 values and Kzt = comes from ASCE 7 Figure 6-4. Even though those two coefficients are not usual, they only have one maximum value for any building site.
These Crsm values can be calculated and plotted for everything in ASCE 7. As mentioned previously, ASCE 7 Figure 6-6 has roof charts with coefficients for the three categories of h/L and numerous footnotes and asterisks. Many hours of study are needed to completely understand how to apply them. Figure 3 shows a graphical representation of that part of the figure. We smoothed out the lines since the raw net data was somewhat scattered, but made it on the conservative side of the data. The handbook has dozens of useful charts, including those for wind-speed-up effects and MWFRS coefficients, which also include some open structures, components and cladding, and problem solutions.
Future plans for the wind provisions
The ASCE 7 Wind Force Task Committee plans to study its Wind Chapter 6 to see if it can be restructured to provide a clearer and more efficient document to help the user understand the wind load provisions. This is similar to the way the seismic provisions were grouped into separate chapters in the 2005 edition of the ASCE 7 document. Further procedures are being developed to provide wind design information for more elements of a building, such as attached canopies.
SEAW, SEAOC, and SEAO have joined together to propose another simplification to the ICC code development process for the last cycle of the 2006-2009 IBC change process. It will provide, if approved, text to set up another alternate method with a series of tables containing conservative coefficients for enclosed and partially enclosed buildings, along with an equation similar to RSM Eqt. 2-1 and tables of wind velocity pressures. It is very similar to the ’97 Uniform Building Code (UBC) wind design equation and is intended to check quickly if wind base shear controls over seismic base shear. But engineers in seismically active areas should remember that wind components and cladding pressures may control over seismic ones, especially in areas with high winds or wind-speed-up effects. Engineers in high-wind areas should note that depending on the actual level of seismic forces, they could control over wind in the longitudinal direction. At the very least, the minimum seismic detailing could apply, depending on the seismic design category at the site.
Conclusion
The authors appreciate the opportunity to discuss simplifications to wind design standards and codes. From a practitioner’s point of view, it makes sense to produce them as clear and intuitive as possible to avoid misunderstandings of their intent and possible errors in design.
Jerry J. Barbera, P.E., is the assistant building official of the Airport Building Department at Sea Tac Airport in Seattle. He is also the editor of the SEAW’s Commentary & Workbook. Donald R. Scott, P.E., S.E., is vice president/director of engineering for PCS Structural Solutions in Tacoma, Wash., as well as the chairman of SEAW’s Wind Engineering Committee and co-chairman of the ASCE 7Task Committee on Wind Loads.
Figure and chart captions
Figure 1: Illustration of how internal and external pressure coefficients are combined to achieve net pressures
Figure 2: One way to show roof coefficients in ASCE 7 Figure 6-6 for Ө < 10 degrees so that the application and distribution of coefficients is more clear.
Modified excerpt from ASCE 7 Figure 6-6
Copyright ASCE
Figure 3: Copy of RSM Figure 3-4B showing net enclosed building pressure coefficients in graphic form
Table 1: IBC provisions that affect or supplement ASCE 7 requirements
1. 101.2 Scope, Exception; IRC is alternative design methods including wind for 1- & 2-Family Dwellings & Townhouses.
2. 104.11 Alternate materials, design and methods of construction; Allows tested, innovative, technological design
3. Table 1604.3 Serviceability; Deflection limits for finishes and construction including wind.
4. 1604.5 Occupancy categories; Some technical differences w/ ASCE 7 Table 1-1 & Sec. 1.4-1.—IBC controls.
5. 1604.8 Anchorage; Rules controlling uplift/sliding forces from wind—IBC controls.
6. 1605.2.1 Load combinations strength/ LRFD methods—Some differences with ASCE Sec. 2.3.2—IBC may control.
7. 1605.3.2 Alternate basic load combinations. Provisions not in ASCE 7—formerly in UBC.
8. 1609.1.1 Determination of wind loads, Exceptions. Specific alternates and additional standards to ASCE 7.
9. 1609.5.1 Roof deck; Must resist chapter 16 wind pressures
10. 1609.5.2 Roof coverings; General & cladding pressure requirements.
11. 1609.5.3 Rigid tile. Unique IBC Uplift moment equation and prescriptive details.
12. 1612.5 Flood hazard areas—high-velocity wave action. Req. for lateral wind/flood loads on all building components
13. 1403.3 Structural; Walls and openings - ASCE 7 for veneer pressures.
14. 1404.8 Plastics; Various cladding systems must use ASCE 7.
15. 1405.13 Vinyl siding; Two-stage loading > 100 mph.
16. 1405.15—1405.17 Fiber cement siding; Design/details for wind pressures.
17. 1407.4 Structural design; Metal Composite Materials must use ASCE 7.
18. 1504.1 Wind resistance of roofs; Must resist chap. 16 wind pressures.
19. 1504.2 Wind resistance of clay and concrete tile. Must resist Chapter 16 wind pressures.
20. 1504.3 Wind resistance of non-ballasted roofs; Must resist chap. 16 wind pressures.
21. 1504.4 Ballasted low-slope roof. Requires complying ballast Section. 1504.8 & SPRI RP-4.
22. 1504.5 Edge securement for low-slope roofs; It has to comply with SPRI ES-1.
23. 1504.6 Physical properties; cyclic roof wind load testing for flexural response to wind.
24. 1504.8 Gravel and stone; Strict control of rock size on roofs in Hurricane-Prone Regions.
25. 2404.1 Vertical glass; Uniform wind pressure loading equation.
26. 2404.2 Sloped glass; Load combinations for wind design.
27. 2404.3.1 Vertical wired glass; References 2404.2 w/ special loading.
28. 2404.3.2 Sloped wired glass; References 2404.2 w/ special loading.
29. 2404.3.3 Vertical patterned glass; References 2404.2 w/ special loading.
30. 2404.3.4 Sloped patterned glass; Specific loading equations.
31. 2404.3.5 Vertical sandblasted glass; Specific loading equations.
32. 2404.4 Other designs; Alternate design procedures up to registered design professional
33. 2606.5 Structural requirement; Wind loads on light-transmitting plastics in walls and roofs.
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