An explanation of ICC Code Change Proposals S84 and S85

In the article "Simplifying wind provisions: A practitioner’s guide for the IBC and ASCE 7," published in the November 2007 issue of Structural Engineer, Don Scott and I gave some practical advice for users of the International Code Council’s (ICC) 2006 International Building Code (IBC) and of the American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE 7-05) for determining wind pressures on structures. At the end of it, we mentioned that the Structural Engineers Association’s of Washington, Oregon, and California along with other engineers in the country were completing code text for a simplified method for wind design to ASCE 7. This article explains the history and details of the method.

The current proposed methods would be published in the 2009 IBC if passed through the 2007-2008 ICC Code Development process. That process begins on Feb. 17, 2008, and continues through March 1 in Palm Springs, Calif. These two specific proposals will be heard sometime between Feb. 24 and Feb. 27.

The original motivation for these proposals was to provide a simplified way to obtain the wind forces on a structure to engineers who design for areas of the country where wind forces do not govern the design of structures other than, perhaps, low-rise, light-framed buildings. Typically these structures would be located in areas where earthquake design controls. However, since we started our efforts, we have heard from engineers across the country that are eager to see simplification incorporated into the building code process.

The proposed alternatives

Now that these proposals have been submitted to the ICC, we would like to discuss the merits of this effort for practical simplification of codes and standards. These alternatives are not intended to supplant ASCE 7, Chapter 6; rather, it is hoped that eventually they would be incorporated into ASCE 7.

To that end, the same procedure is being submitted to ASCE 7 as it goes through the IBC process. But since the next edition of the Standard will not be published until 2011, it could not be included as an approved alternative until the 2012 IBC is adopted. Since nearly all jurisdictions in the United States do not adopt the IBC without state and/or local legislative hearings, the simplification would not appear until mid-2013—at the earliest.

Alternately, if one of the two IBC code change proposals is adopted, the engineering community would have the option to use the simplified design procedure in 2010, instead of waiting five years from now to use it.

Details of the proposed method

Essentially, there are four methods for the design of buildings and structures for wind loads in ASCE 7 as follows:
* For buildings not higher than 60 feet—Method 1: Simplified-Low Rise, and Method 2: Analytical Procedure for Low-Rise (on which Method 1 is based); and
* For all buildings and structures—Method 2: Analytical Procedures for All-Heights, and Method 3: Wind Tunnel Design.

The model for the code text and tables of coefficients shown in the sidebar below originated from the work done by the Wind Committee of the Structural Engineers Association of California (SEAOC) in 1979-82 for the Uniform Building Code (UBC). They were simplifications of Method 2: Analytical Procedure for All-Heights (referred to in the rest of the document as ASCE 7’s All-Heights method). They were adopted and published in the 1982 through 1997 UBC, and had still been used in California, Oregon, Hawaii, Puerto Rico, and other localities until early 2008. The proposed new simplified coefficients are in full compliance with ASCE 7-05. In fact, in a few cases, they are slightly conservative, when compared with the "All-Heights" method. They were—and the proposals we are submitting still are—just an algebraic manipulation of the equations in the All-Heights method. Following is what the proposed equation looks like, and it applies to both the main wind force resisting system (MWFRS) and the components and cladding (C&C):
        P_net = q_s K_z C_net [I K_zt],

where, q_s = wind velocity pressure in pound/square foot (lb/ft² or N/m²), values of which are provided in a convenient table in the proposal as shown in the sidebar below; K_z = the velocity pressure-exposure coefficients from ASCE 7 Table 6-3; C_net = net-pressure coefficients based on K_d [GC_p—(GC_pi)], providing the simplifications; I = Importance Factor from ASCE 7 Tables 1-1 and 6-1; and K_zt = the topographic factor for wind speed-up effects, also found in ASCE 7, when necessary.

In the manipulation of C_net, the gust factor and directionality factor are taken as constants. The second part of the simplification is that the internal pressures are not included in the simplified equation; this means that the method is only applicable to a structure where the internal pressures cancel out, but that includes the vast majority of the structures that engineers design. Finally, the simplification combines coefficients for roof slopes where there are only minor differences between the C_p values; as a result, there are fewer categories for the designer to consider. We also reduced the number of roof slopes because the different coefficients for roof angles, the need for interpolation between different roof slopes, and height-to-width ratios of the building as a whole are three of the main reasons for the complications in ASCE 7.

The work of the various involved proponents produced the following two proposals, which were submitted to the IBC:
* S84—07/08, 1609.1.1, 1609.6 (New), submitted by Edwin Huston, representing NCSEA; and,
* S85—07/08, 1609.1.1, 1609.6 (New), submitted by James E. Lai, representing SEAOC.

Both code change proposals can be reviewed by downloading the Volume 1 grouping of S70 - S147 at: www.iccsafe.org/cs/codes/2007-08cycle/ProposedChanges/index.html. Illustrative excerpts from the proposed code language and tables are included in the sidebar below.

The tables resemble the UBC wind tables, but contain more information than those tables provided. Unlike the UBC, the proposed coefficients cautiously include only the maximal positive and negative pressures for given ranges of wall and roof slopes. Coefficients are also given for a limited number of non-building structures such as fences, towers, chimneys, and signs. Both proposals are limited to buildings 100 feet (30.5m) in height, with a height-to-least-width ratio (h/L) of 4 or less. Both ICC proposed alternates are based on ASCE 7’s All Heights Method, because it offers the greatest range of heights, shape, and conservatism for determining wind pressures on structures.

The proposals are projected to be placed in IBC Chapter 16, but only because the proponents involved with the issue designed them as a code "placeholder" to use until they eventually appear in ASCE 7 (as was done in 2000, when the ASCE 7 was undergoing many changes, but before they could be included in ASCE 7).

Since the proposals were submitted to the ICC in late August 2006, discussion has occurred between the proponents and other prominent wind engineers about the limitation of 100 feet and an h/L ≤ 4. These discussions are likely to result in a minor floor modification to at least one of the proposals at the ICC hearings to take into account the fact that one of the frequency equations in the commentary of ASCE 7 indicates that a 100-foot-tall building could be flexible.

Examples

We have done a trial design summary for a seven-story building to illustrate how the procedure works. The problem solutions contain the exact design done by ASCE 7’s All Height Method, and then the same design by the simplified proposal. Only the loading and the resultant base shear, roof uplift, overturning moments, and some components and cladding pressures have been given. View the Design Examples below to see the details of how these pressures were derived.

The seven-story example comes from The SEAW Commentary on Wind Code Provisions, Volume 2—Problem Appendices, copyright 2004 by the Structural Engineers Association of Washington, which will be published in a new edition in early 2008. All rights are reserved. In addition to looking at the MWFRS values, the C&C values were calculated and compared using the two approaches.

The bottom line—The two procedures are within about 5 percent of each other for the MWFRS base shear and within 7 percent for overturning moments, they are basically the same for the C&C design wall pressures, and within 8 percent to 9 percent of parapet wall pressures; see Table 1. The design procedure is exactly the same, but the Simplified method’s time effort is about one-third to one-half the time using ASCE 7’s-All Heights, depending on a designer’s familiarity with both procedures. The results are close enough for comparison purposes and are not out-of-line for design loadings.

Conclusion

The proposals introduced represent the thoughts, ideas, and simplifications of scores of practicing engineers over nearly three decades. We hope the proposals are eventually incorporated into the ASCE 7 Standard. We believe that this simplification will make life much easier for many, if not most, engineers in the United States and beyond. We are hopeful that the engineering community will be fully supportive of the effort, because both the IBC and ASCE 7 will be strengthened by the proposed simplified design procedure.

Jerry J. Barbera, P.E., the editor of the "SEAW Commentary on Wind Code Provisions" and "Handbook of a Rapid-Solutions Methodology for Wind Design." He can be reached at codes_knowledge@comcast.net.


 SIDEBAR: Selected excerpts from ICC Proposal S84—07/08, 1609.1.1, 1609.6 (New):

NOTE: The material below is only a small portion of the whole proposal, but it provides the majority of the relevant information for the illustrations. Also, note that we have removed the underlines in order to make it easier to read—but also note that these items are subject to change in the code development process. They are published with permission of the International Conference of Building Officials, copyright 2007. All rights are reserved to the ICC.

1. Revise as follows:
1609.1.1 (Supp) Determination of wind loads. Wind loads on every building or structure shall be determined in accordance with Chapter 6 of ASCE 7 or provisions of the Alternate All-Heights Method in Section 1609.6. [NOTE: More text follows in the proposal.]

2. Add new text as follows:
1609.6 Alternate All-Heights Method. The alternate wind design provisions are simplifications of the ASCE Standard 7, 2005 Edition, Method 2-Analytical Procedure.

1609.6.1 Scope: As an alternate to ASCE 7 Section 6.5, the following provisions may be used to determine the wind effects on regularly shaped buildings, or other structures which meet all of the following conditions:
* The building or other structure is less than 100 feet in height, with a height to least width ratio of 4 or less.
* The building or other structure is not sensitive to dynamic effects.
* The building or other structure is not located on a site for which channeling effects or buffeting in the wake of upwind obstructions warrant special consideration.

1609.6.1.1 Modifications. The following modifications shall be made to certain subsections in ASCE 7: Section 1609.6.3. Symbols and Notations that are specific to this section are used in conjunction with the Symbols and Notations in ASCE 7 Section.6.3.

1609.6.4.1 Design equations. When using the Alternate All-Heights Method, the Main-Wind-Force-Resisting System (MWFRS) and Components and Cladding of every structure shall be designed to resist the effects of wind pressures on the building envelope in accordance with Equation (16-36).
P_net = q_s K_z C_net [I K_zt] (Equation 16-36)

1609.6.4.2.4 Application of Wind Pressures

1609.6.4.2.4.1 General. When using the Alternate All-Heights Method, wind pressures shall be applied simultaneously on, and in a direction normal to, all building envelope wall and roof surfaces.

1609.6.4.2.4.2 Components and Cladding. Wind pressure for each component or cladding element is applied as follows using C_net values based on the effective wind area, A contained within the zones in areas-of-discontinuity of width and/or length "a", "2a," or "4a" at: corners of roofs and walls; edge strips for ridges, rakes, and eaves; or field areas on walls or roofs as indicated in figures in Table 1609.6.2(2) in accordance with the following:
* Calculated pressures at local discontinuities acting over specific edge strips or corner boundary areas.
* Include "field" (zone 1, 2, or 4, as applicable) pressures applied to areas beyond the boundaries of the areas-of-discontinuity.
* Where applicable, the calculated pressures at discontinuities (zones 2 or 3) shall be combined with design pressures that apply specifically on rakes or eave overhangs.

Links to the following tables:

Table 1609.6.3.1(1)—Wind Velocity Pressure (q_s) at Standard Height of 33 feet (10,058 cm)^{1, 2, 3}

Table 1609.6.3.1(2)—Net Pressure Coefficients, C_net^1,2,3 (Part 1)

Table 1609.6.3.1(2)—Net Pressure Coefficients, C_net^1,2,3 (Part 2)



DESIGN EXAMPLES

Design Examples for "A simplified wind design alternative: An explanation of ICC Code Change Proposals S84 and S85" By Jerry J. Barbera, P.E. (Article published in the February 2008 issue of Structural Engineer)

Figure 1-1 Typical Floor and East and West Frame Elevation Plan, Example Problem (Not to Scale)

It is for:
* A seven-story office building as shown in Figure 1-1
* Fundamental Period = 1.0 seconds perpendicular to the long side.
* Structure is not susceptible to dynamic effects, and is considered to be enclosed.
* Steel Braced Frames for resisting lateral forces of equal rigidity on all four sides.
* Parapet is 3’—0" high all around exterior of building.
The criteria are:
* Basic wind speed 85 mph
* Wind Exposure Category B
* K_d = 0.85, K_zt = 1.0, and I = 1.0


Problem # 1A: Determine the MWFRS for applicable wind pressures:

Q. What are the base shear and overturning moment for winds in the transverse direction (north/south) as shown on Figure 2A using ASCE 7-All-Heights Procedure?

A. The following is the distribution of the wind pressures, ignoring uplift on the roof. These are the results for the classic ASCE Method 2-All-Heights procedure. The details are available in Problem 1: Wind Loading of a 7-Story Office Building Using ASCE 7 Procedures and Critera.

Base Shear = 163 K; Overturning Moment about the center of the Building = 7,400 K-ft.


Problem # 1B

Q. What are the base shear and overturning moment for winds in the transverse direction (north/south) as shown on Figure 2B using the Proposed IBC Simplified All-Heights procedure?

A. The following is the distribution of the wind pressures, ignoring uplift on the roof. These are the results for the Proposed IBC Simplified All-Heights procedure: The details are located in Problem 2: Wind Loading of a 7-Story Office Building Using IBC Alternate Procedures and Critera.

Base Shear = 171 k; Overturning Moment about the center of the Building = 7,943 k-ft.

As can be seen in the distributions, the Simplified Procedure is conservative (using Proposed IBC Equation 16-36 and data in Proposed Table 1609.6.3.1(2)—Net Pressure Coefficients, (C_net), because it has a simpler formula to use as demonstrated by the length and scope of the design calculations.

Problems # 2A and # 2B: Determine the C&C for applicable wind pressures:

Q2. What are the C&C pressures for design of exterior wall elements 4 and 5 for the 7th floor and for the parapets using (a). The ASCE 7 procedure and (b). The Simplified IBC procedure.

A.2a. The ASCE 7 pressures are as follows: For the 7th Story, 11’-6" long mullions at 5-0" oc., with an effective wind area of 44 sq.ft.:

Zone 4: p = -15.9 psf and Zone 5: p = -29.2 psf

For the 3’-0" tall parapet mullions at 5-0" oc., with an effective wind area of
< 20 sq.ft.:

Zone 4: p = -45.6 psf and Zone 5: p = 58.4 psf

A.2b. The IBC Simplified pressures are as follows: For the 7th Story, 11’-6" long mullions at 5-0" oc., with an effective wind area of 44 sq.ft.:

Zone 4: p = -15.8 psf and Zone 5: p = -28.7 psf

For the 3’-0" tall parapet mullions at 5-0" oc., with an effective wind area of < 20 sq.ft. For the 3’-0" tall parapet mullions at 5-0" oc., with an effective wind area of < 20 sq.ft.

Zone 4: p = -49.9 psf and Zone 5: p = 63.3 psf