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DEVELOPMENT OF THE PORTLAND CEMENT ASSOCIATION’S FIRST CONSENSUS STANDARD
PCA 100 – 2007
PRESCRIPTIVE DESIGN OF EXTERIOR CONCRETE WALLS FOR ONE- AND TWO-FAMILY DWELLINGS

Joseph J. Messersmith, Jr., P.E.
Coordinating Manager Regional Code Services
Portland Cement Association

INTRODUCTION

Over the many centuries that building codes have existed, empirical requirements have evolved for constructing buildings with masonry and wood framing.  During the latter half of the twentieth century, empirical requirements in codes were gradually being replaced with prescriptive provisions based on rational design procedures, primarily due to concerns about building damage caused by high winds and earthquakes.  Because concrete was not used extensively in the construction of above-grade walls in one- and two-family dwellings, empirical requirements never found their way into most building codes.  Therefore, when constructing exterior concrete walls with insulating concrete forms was first introduced in this country approximately 30 years ago, contractors desiring to use this system for one- and two-family dwellings were required by local building codes to have the building’s structure designed by a registered design professional.  The additional cost of the design service put concrete walls at a competitive disadvantage compared to other wall systems that did not need to be engineered because of prescriptive code provisions.

BACKGROUND

In response to the need for prescriptive requirements to construct homes with above-grade concrete walls, the Portland Cement Association (PCA), National Association of Home Builders (NAHB), and the United States Department of Housing and Urban Development (HUD) sponsored a test program to gather data that could be used to develop prescriptive design solutions for homes constructed with concrete walls cast in insulating concrete forms.  The NAHB Research Center (RC), with guidance from a broad-based steering committee, completed the first edition of Prescriptive Method for Insulating Concrete Forms in Residential Construction1 in 1998.  The prescriptive designs were based on ACI 318-952 and test results.  The loads and forces used in the designs were based on ASCE 7-95, except for wind loads which were based on the 1994 Standard Building Code4.  Although the method could be used to construct homes in all areas of the United States for wind design, it only applied to low and moderate risk seismic areas (Seismic Zones 0, 1 and 2 or Seismic Design Categories A, B and C).  This meant that in areas of high seismic risk (Seismic Zones 3 and 4 or Seismic Design Categories D and E), engineered designs were still required. 

Concurrent with the development of the prescriptive method for concrete walls, the International Code Council was developing the International Residential Code (IRC).  PCA introduced a proposal, which was accepted, which transcribed most of the provisions in the prescriptive method into the first edition (2000) of the IRC5.  Because of changes ongoing in ACI 318 and ASCE 7, the prescriptive method had to be updated to comply with later editions of these two standards since they are adopted by the IRC.  With the support of PCA, NAHB and HUD, the update was prepared by NAHB RC, again with guidance from a broad-based steering committee and supplemented by additional testing.  This effort culminated with the publication of Prescriptive Method for Insulating Concrete Forms in Residential Construction Second Edition (Revised)6 in 2002.  This edition complied with ACI 318-997 and ASCE 7-988.  The principles changes in the second edition were:

1. provisions for use of 60,000 psi reinforcing steel were added,
2. wind design provisions were revised to agree with ASCE 7-98, including changing from fastest mile wind speeds to 3-second gust wind speeds,
3. seismic hazard classification was changed from seismic zone to seismic design category to agree with ASCE 7-98, and
4. provisions for Seismic Design Categories D1 and D2 (Seismic Zones 3 and 4) were added.

The revised prescriptive provisions in the second edition of the method were then transcribed into the 2003 edition of the IRC.  Except for a few relatively minor changes, the provisions in the 2006 IRC remained unchanged from the 2003 IRC.

About the same time the second edition of the prescriptive method was being prepared, the American Concrete Institute (ACI) Committee 332 on Residential Concrete Work was in the process of converting ACI 332.1, Guide to Residential Concrete Construction, into a consensus standard containing mandatory requirements.  In the process they wanted to incorporate the provisions for above-grade walls of the second edition of the prescriptive method into the ACI standard since the ACI 332.1 guide did not address this subject.  While this had the support of ACI Committee 332, the ACI Technical Activities Committee decided that since some of the prescriptive provisions were based on testing and did not meet the full requirements of ACI 318, the provisions should not be incorporated into ACI 332, Requirements for Residential Concrete Construction.  Since PCA had hoped to standardize the prescriptive procedures through a consensus process, such as ACI’s, it was decided that PCA would get American National Standards Institute’s (ANSI) approval to develop standards in accordance with a consensus procedure within PCA approved by ANSI.  Once “Procedures for the Development and Maintenance of Portland Cement Association Standards” was approved by ANSI, prospective candidates were contacted to determine if they were interested in being a member of the National Standards Development Committee (NSDC).  To assure balance according to ANSI, the NSDC is composed of individuals representing producers, users and general interests in the proportions stated in the approved procedures.  The NSDC formed a Residential Subcommittee (RS) consisting of some members of the NSDC and others with specific interest and expertise to oversee the technical development of this standard.  Although not required, the RS was also balanced and operated under the same rules required of the NSDC.  However, the NSDC has final approval authority before any standard is issued.

Over a period of approximately two and one-half years, the second edition of the prescriptive method was updated to comply with ACI 318-059 and ASCE 7-0510, and published in February 2008 as Prescriptive Design of Exterior Concrete Walls for One- and Two-Family Dwellings – PCA 100-200711.  Most of the remainder of this article will focus on the key advantages of PCA 100 compared to the two editions of the prescriptive method.   

COMPARISON OF PCA 100 TO PREVIOUS PRESCRIPTIVE METHODS

Scope

The scope of the two editions of the prescriptive method focused on concrete walls cast-in-place in insulating concrete forms (ICFs).  Forms with three different cross-sections were recognized: flat, waffle-grid and screen-grid.  This scope was expanded in Section R612 of the IRC, which indicated that the provisions for flat ICF walls could also be used to design and construct conventionally formed walls with flat surfaces.  In PCA 100, the three wall cross-sections still apply; however, the provisions apply to any forming method, removable or stay-in-place, and forming material as long as the wall is cast-in-place and the wall’s cross-section complies with the dimensional limitations given in the standard.  

Grade of Steel Reinforcement

The first edition of the prescriptive method was based on Grade 40 reinforcement. The second edition was revised to recognize the use of Grade 60 steel; however, the tabular values continued to be based on Grade 40 and, where applicable, adjustments were given in footnotes to the tables if Grade 60 steel was to be used.  Since Grade 60 reinforcement is available in all portions of the country and generally specified for commercial construction projects, reinforcement schedules in PCA 100 are based on Grade 60 steel, or both grades are covered.  For tables based on Grade 60 alone, Table 2.3 is provided that shows alternate bars spacings for Grades 40 and 50.

Worst Case Scenarios

By their nature, prescriptive methods cannot cover all situations that may be encountered.  In order to keep the provisions to a manageable and user-friendly size, the scope of the provisions and the cases to be considered need to be limited.  This results in worst-case conditions being used as the bases of the designs.  Since the typical building does not have characteristics that represent all the worst-case assumptions, the design solutions given by most prescriptive methods are conservative for most buildings.  PCA 100 provides design solutions for more conditions in order to reduce the amount of conservatism and thus provide a more economical solution for the typical case.  Two examples are illustrated below.

In the two editions of the prescriptive method, vertical reinforcement in foundation walls was based on the bars being located at the centerline of the wall thickness.  While bars are frequently located in the center of the wall in residential construction, it is more economical if the reinforcement is proportioned to be located as near to the inside face of the wall, or as far as possible from the backfill as cover requirements will permit.  Therefore, Table 3.12 of PCA 100 for flat basement walls gives the vertical steel required based on the bars being placed near the inside face of the wall.  The following example illustrates the advantage of the new table. 

Assume an 8-inch flat basement wall that is 9 feet high and supports 8 feet of backfill with a design lateral soil load of 60 psf/ft.  With the reinforcing steel in the center of the wall Table 3.7 requires No. 6 bars at 23 inches on center.  However, for these same wall conditions Table 3.12 shows that the No. 6 bars can be placed at 29 inches on center if placed near the inside face of the wall. This represents a 20% reduction in the amount of steel required.

In the two editions of the prescriptive method, the length of solid wall (shear wall) required to provide resistance to lateral wind forces was based on a building with a mean roof height of 35 feet and an unsupported wall height within each story of 10 feet.  Many houses, especially those that are one story, are considerably less than 35 feet in height, and 10-foot high ceilings are used infrequently.  In PCA 100, the length of solid wall required for wind resistance in Tables 5.1A, 5.1B and 5.1C continues to be based on a mean roof height of 35 feet and a wall height within each story of 10 feet; however, reduction factors for shear wall length are provided for mean roof heights less than 35 feet and for wall heights less than 10 feet in Tables 5.2 and 5.3, respectively.  To illustrate the reduction for mean roof height, for a one-story building with a mean roof height of 15 feet located in wind exposure category C terrain, Table 5.2 shows that the reduction factor is 0.84, which means that the solid wall length shown in Table 5.1A can be reduced 16%.  Taking advantage of the reduction factors, where applicable, will permit the length of solid wall and/or amount of vertical reinforcement at the ends of solid wall segments to be reduced from what would be required for the worst-case scenario.

Above-Grade Walls

The prescriptive methods provided tables that gave the amount of vertical wall reinforcement required to resist out-of-plane wind forces for 4- and 6-inch thick flat above-grade walls.  In PCA 100, tables have also been provided for 8- and 10-inch flat walls.

In the prescriptive methods the basic wind speed and exposure category were used to determine a design wind pressure.  Then the design tables were entered with the design wind pressure to determine the required vertical wall reinforcement.  For determining vertical wall reinforcement in PCA 100, Tables 4.1, 4.2 and 4.3 are entered directly with the basic wind speed and exposure category (see Figure 1 for example).  The new tables save a step and make it less likely that in going from one table to another, the wrong design wind pressure will be used. 

Vertical wall reinforcement tables in PCA 100 offer design solutions for most conditions for 4-inch nominal flat walls, and 6-inch waffle- and screen-grid walls.  Under the previous prescriptive methods, design by a registered design professional was often required for higher wind speeds for these wall types and thicknesses, thus defeating the purpose of the prescriptive method.

Table 4.4, new to PCA 100, provides design solutions for above-grade walls constructed continuous with the stem-wall where no lateral support is provided by the slab-on-ground.  Since no lateral support is provided at the juncture of the stem wall and slab-on-ground, the wall must be designed to span vertically from the footing, or soil capable of providing lateral support, to the floor or roof above the first story where the wall is laterally supported.

Solid Wall Segments for Resistance to Lateral Wind Forces

For determining required solid wall (shear wall) lengths to resist lateral wind forces, the previous prescriptive provisions had three sets of tables for flat, waffle- and screen-grid walls, with each set consisting of three tables, for a total of 9 tables.  New Tables 5.1A, 5.1B and 5.1C have been developed for PCA 100. They are: Table 5.1A for one-story or the top story of two-story dwellings for wind perpendicular to ridge, Table 5.1B for first story of a two-story dwelling for wind perpendicular to ridge, and Table 5.1C for the same conditions described in Tables 5.1A and 5.1B, but for wind parallel to the ridge.

The new tables give more flexibility for determining required solid wall lengths.  They are based on worst-case scenarios (i.e., mean roof 35 feet, unsupported wall height 10 feet, and the wall’s design strength based on a 6-inch screen-grid wall without horizontal shear reinforcement).  However, to mitigate the effects of the worst-case conditions assumed, three additional tables provide factors for buildings with a mean roof height less than 35 feet, walls with unsupported wall heights less than 10 feet, and walls with design strengths greater than assumed. These factors, where applicable, are used to reduce the required solid wall length for buildings that do not conform to one or more of the three worst-case conditions assumed.

The new solid wall length tables are entered directly with the basic wind speed and wind exposure category (see Figure 2 for example), whereas, in the previous provisions, the basic wind speed and exposure category were used to determine the velocity pressure, and then the design tables were entered with the velocity pressure. The new tables save a step and make it less likely that in going from one table to another, the wrong velocity pressure will be used.

Previous tables for solid wall length for wind perpendicular to ridge were based on a building with an endwall length parallel to the wind of 30 feet, and a table note gave adjustment factors for endwall lengths of 45 and 60 feet. The new tables have solutions for endwall lengths of 15, 30, 45 and 60 feet, which makes it less likely the important information in the previous table note will be missed or applied incorrectly.  This format also provides more flexibility and provides design solutions for buildings with endwall lengths less than 30 feet.

Solid Wall Segments for Resistance to Lateral Seismic Forces

New Tables 5.5A, 5.5B and 5.5C have been developed for PCA 100 that are used to determine required solid wall (shear wall) lengths for resistance to lateral seismic forces.  They are: Table 5.5A for one-story or top story of two-story for endwalls and sidewalls, Table 5.5B for first story of two-story for endwalls and sidewalls, and Table 5.5C for first story of two-story for endwalls and sidewalls where the second story exterior walls are constructed of light-framed construction. Each table applies to all three types of walls.  The previous prescriptive provisions had one table for all wall types that gave the required solid wall length as a percentage of the length of the wall.

Since the new solid wall length tables give actual lengths, versus the percentage of wall length required to be solid, as in the previous prescriptive methods, a way of considering building plan geometry had to be devised.  Since the exterior concrete walls of a house contribute a significant percentage of the building’s mass, and since the standard limits the maximum plan aspect ratio to two, a parametric study showed that assuming the maximum aspect ratio of two did not yield overly conservative results compared to a house of the same area with an aspect ratio of one.  In fact, regardless of the area considered, the perimeter of a rectangle with an aspect ratio of two is only 6% greater than the perimeter of a square of the same area.  Therefore, the tables for determining solid wall lengths in PCA 100 are entered with the area of the building within the exterior walls projected onto a horizontal plane (see Figure 3 for example).

The new seismic tables give more flexibility for determining the required solid wall lengths.  They are based on worst-case scenarios (i.e., unsupported wall height 10 feet, maximum weight of interior and exterior wall finish of 13 psf, and the wall’s design strength based on a 6-inch screen-grid wall without horizontal shear reinforcement). To mitigate the effects of the worst-case conditions assumed, three additional tables provide factors for buildings with unsupported wall heights less than 10 feet, interior and exterior wall finishes weighing less than 13 psf, and walls with design strengths greater than assumed, such as where horizontal shear reinforcement is provided. These factors, where applicable, are used to reduce the required solid wall length for buildings that do not conform to one or more of the three worst-case conditions assumed.

Solid Wall Segments at Corners

One provision of the previous prescriptive methods that interfered with freedom of design layout was the requirement that a minimum two-foot length of solid wall had to occur at the ends of all walls (i.e., usually at corners).  It is common to have window or door openings closer than this to a corner.  While the genesis of this requirement is unknown, it may have originated from the requirement of editions of ACI 318 prior to 2008 that not less than two No. 5 bars had to be provided around all window and door openings and such bars had to extend the distance beyond the corners of the openings required to develop the bar but not less than 24 inches.  Extending the bar to develop it is logical; however, requiring that the extension be not less than 24 inches does not recognize that smaller bars with hooks can be developed in much less than 24 inches.  For example, a No. 5 Grade 40 or Grade 60 bar with a standard hook can be developed in 7 or 11 inches, respectively, in typical situations.

This inconsistency was brought to the attention of ACI Committee 318 and after they thoroughly researched the background of the provisions, the 2008 edition of ACI 318 has been revised to simply require that the bar be extended beyond the corner of the opening to develop the yield strength, fy, of the bar in tension.  PCA 100 has implemented this new provision, thus offering more freedom in the layout of openings in walls with respect to corners.  

Lintel Tables

Tables for lintels in load-bearing walls in the previous methods were based on a maximum roof clear span of 32 feet, despite the fact that the scope of the provisions applied to roof spans of 40 feet.  Similar tables in PCA 100 have been revised to apply to roof clear spans of 40 feet, thus providing prescriptive design solutions for larger homes where design was previously required.  In addition, separate tables for roof clear spans of 32 feet are provided in order to offer more economical solutions for buildings with shorter roof spans.

In the previous methods, pairs of tables gave allowable spans of lintels based on either a No. 4 or No. 5 bar in the bottom of the lintel.  The allowable spans were based on Grade 40 reinforcement with a note allowing spans to be increased, if appropriate, where Grade 60 steel was used in lieu of Grade 40.  The new tables give allowable spans for Grade 40 and Grade 60 reinforcement for three different bar sizes (Nos. 4, 5 & 6) and one and two bars. These choices offer more flexibility in selection of reinforcement and prescriptive solutions where none were previously available.  See Figure 4 for example. 

Since stirrups are typically not required the entire length of a lintel, in the second edition of the prescriptive method, separate tables gave the length in the center portion of the span where stirrups could be omitted.  In PCA 100, this information is providing in the same table as the allowable spans, thus eliminating the need to go from one table to another, with the possibility of making a mistake.

In the previous methods, the possibility that wind uplift forces acting on the roof may control the design of a lintel rather than gravity loads was not considered.  This condition is addressed in PCA 100 by Tables 7.19 – 7.25 that give allowable spans for each wall type (flat, waffle & screen-grid) based on the wall thickness and roof uplift forces (see Figure 5 for 6-inch Flat Wall Lintel).  From Table 7.1A the uplift forces acting on the roof are determined for the range of building geometries covered by the standard.  After determining the allowable span of a lintel subjected to gravity loads, the same lintel must be checked for uplift forces to determining if the allowable span of the lintel, when subjected to uplift forces is equal to or greater than that permitted for gravity loads.

Connections Between Concrete Walls and Light-Framed Floors and Roofs

The previous methods contained prescriptive details of connections between concrete walls and floors and roofs constructed with wood framing members.  Because Appendix D, Anchoring to Concrete, was added to ACI 318-02, significant revisions were required to these details in PCA 100. In addition, new details were included for connections between concrete walls and cold-formed steel framed floors and roofs.

OTHER FEATURES OF PCA-100

The details for connecting concrete walls to light-framed floors and roofs use generic connectors such as anchor bolts.  However, the details provided are not suitable for the entire range of wind and seismic forces covered by the standard.  In order to facilitate the use of proprietary connectors and the design of alternate connection details, design forces are given in Appendices A and B for all possible loading combinations.  The forces in Appendices A and B are for use with allowable stress design procedures (ASD) and load and resistance factor design (LRFD), respectively.

A commentary is provided in Appendix C that gives background information on many provisions of the standard.  Assumptions used in the development of the tables are given in the commentary.  To facilitate use of the commentary, section numbers are keyed to the section number of the provisions to which they relate.

To illustrate the use of the provisions, an example problem is given in Appendix D.  The example focuses on the differences in the design of a house that is sited where only design for wind forces is required (e.g., Seismic Design Category (SDC) A), and a similar house that is sited so it is subjected to the same design wind speed, but assigned to SDC D1, thus requiring seismic design.

PCA 100 AND THE IRC

Two code changes were submitted to the 2006 IRC to update the code’s provisions for below-grade walls (RB116-07/08) and above-grade walls (RB171-07/08) based on PCA 100.  Both changes were approved.  Therefore, the provisions in the 2009 IRC will be based on PCA 100. In addition, PCA 100 will be referenced for situations not covered by the IRC as explained below.

The partial transcription of PCA 100 into the 2009 IRC was primarily based on the amount of material in PCA 100.  As described earlier in this article, the provisions in the standard are greater than in the previous methods.  It was felt that ICC staff, and possibly others with an interest in the code, would not be receptive to significantly increasing the amount of material in the IRC.  To minimize the impact of the new provisions on the IRC, it was decided to limit the scope of the transcribed provisions for wind design to areas where the basic wind speed and exposure categories do not exceed 130B, 110C and 100D.  This allowed the lintel tables of PCA 100 for wind uplift forces to be left out of the IRC since uplift does not govern the design of lintels in these areas. 

The IRC does not require seismic design for detached one- and two-family dwellings and townhouses assigned to SDC A or B, and detached one- and two-family dwellings assigned to SDC C.  Therefore only townhouses assigned to SDC C and all dwellings assigned to SDC D have to be designed to resist seismic forces.  The 2009 IRC will not have prescriptive seismic design provisions for concrete walls for townhouses assigned to SDC C, and all dwellings assigned to SDC D.  However since PCA 100 has prescriptive seismic designs for dwellings assigned to SDC C and D, it will be referenced in the 2009 IRC.  Therefore, PCA 100 will be referenced for the design of dwellings where the basic wind speeds and exposure categories exceed 130B, 110C and 100D, and for the seismic design of townhouses assigned to SDC C and all dwellings assigned to SDC D.

SUMMARY

A prescriptive method for designing exterior walls of concrete cast in insulating concrete forms was developed to level the playing field with competitive building systems.  When time came to update the second edition of the method, the consensus standards development procedures of PCA approved by ANSI were employed.  The resulting standard, Prescriptive Design of Exterior Concrete Walls for One- and Two-Family Dwellings – PCA 100-2007, offers many improvements over the previous methods; namely more conditions covered, and provisions for use of cold-formed steel floor and roof framing members.  Two code changes updating the provisions of the 2006 IRC based on PCA 100 have been approved, and some provisions of PCA 100 will be incorporated into the 2009 IRC by transcription, while others will be adopted by the 2009 IRC referencing PCA 100.

REFERENCES

1.  Prescriptive Method for Insulating Concrete Forms in Residential Construction. Portland Cement Association, Skokie, Illinois. 1998.

2.  Building Code Requirements for Structural Concrete (ACI 318-95) and Commentary (ACI 318R-95). American Concrete Institute, Farmington Hills, Michigan. 1995.

3.  Minimum Design Loads for Buildings and Other Structures, ASCE 7-95. American Society of Civil Engineers, Reston, Virginia. 1995.

4.  Standard Building Code, 1994 edition. Southern Building Code Congress, Birmingham, Alabama. 1994.

5.  International Residential Code, 2000 Edition. International Code Council (ICC),Falls Church, Virginia. 2000.

6.  Prescriptive Method for Insulating Concrete Forms in Residential Construction Second Edition (Revised). Portland Cement Association, Skokie, Illinois.  2002. 

7.  Building Code Requirements for Structural Concrete (ACI 318-99) and Commentary (ACI 318R-99). American Concrete Institute, Farmington Hills, Michigan. 1999.

8.  Minimum Design Loads for Buildings and Other Structures, ASCE 7-98. American Society of Civil Engineers, Reston, Virginia. 1998.

9.  Building Code Requirements for Structural Concrete (ACI 318-05) and Commentary (ACI 318R-05). American Concrete Institute, Farmington Hills, Michigan. 2004.

10.  Minimum Design Loads for Buildings and Other Structures, including Supplement No. 1, ASCE/SEI 7-05. American Society of Civil Engineers, Reston, Virginia. 2005.

11.  Prescriptive Design of Exterior Concrete Walls for One- and Two-Family Dwellings, PCA 100-2007. Portland Cement Association, Skokie, Illinois. 2008.

Table 4.1. Minimum Vertical Reinforcement for Flat Above-Grade Walls

Figure 1

Table 5.1A. Unadjusted Length of Solid Wall, TL, Required in Each Exterior Endwall for Wind Perpendicular to Ridge
One Story or Top Story of Two-Story

Figure 2

Table 5.5A. Unadjusted Length of Solid Wall, TL, Required in Each Exterior Endwall and Sidewall for Seismic Resistance
One Story or Top Story of Two-Story

Figure 3

Table 7.11. Maximum Allowable Clear Spans for 6-inch Thick Waffle-Grid Lintels in Load-Bearing Walls
Maximum Roof Clear Span of 40 feet and Maximum Floor Clear Span of 32 feet

Figure 4

Table 7.20. Maximum Allowable Clear Spans for 6-inch Nominal Thick Flat Lintels in Top Story Walls Subject to Roof Uplift Forces

Figure 5

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