November 6, 2008
Changes coming in 2009 for concrete structures
By CARY KOPCZYNSKI
Cary Kopczynski & Co.
Recent updates made by the committee that regulates concrete building codes will soon mean changes both locally and globally for those who design and build concrete structures using ACI 318.
Every three years, the American Concrete Institute’s building code committee updates the code to which concrete buildings, bridges, and other structures are designed and built. These changes are then adopted by the International Building Code, which is the document followed by the United States and several other countries as the standard building code.
ACI Committee 318 issued its latest round of changes earlier this year. The IBC will incorporate the changes in 2009, and building jurisdictions will adopt the new code sometime thereafter. The committee’s changes deal with a number of design and construction issues, ranging from connectors on concrete buildings to seismic stability.
One of the significant changes involves durability specifications for concrete.
The committee has moved away from requirements that directed designers and contractors to follow specific “recipes” when developing concrete mixes. Until this change, prescriptive requirements had to be met in terms of concrete mix ingredients. Now, when a project requires a particular level of freeze-thaw resistance, sulfate resistance or other durability quality, the specifier can simply request durability levels that the mix must meet for each category.
How concrete mix designers achieve these performance specifications is left up to them. This change gives mix designers alternative ways of achieving required performance levels and encourages innovation. It allows for the creative use of cementitious materials, aggregates, admixtures and other products without the restraint of having to provide specific quantities of ingredients that may or may not be appropriate or necessary for a particular job.
Seismic design methodology
Another committee change reorganizes and clarifies the seismic design requirements for concrete structures. This change eliminates the confusion that previously existed in the application of ACI seismic design criteria.
ACI 318 previously specified seismic design requirements in a manner consistent with the seismic zones used by the Uniform Building Code. With the replacement of the Uniform Building Code by the International Building Code, the methodology outlined by ACI 318 has been updated to reflect the widely accepted approach used by structural load committees of the American Society of Civil Engineers and other organizations.
In addition to seismically designing structures for the anticipated severity of a particular region’s ground motions, the ASCE and other documents incorporate additional requirements that impact seismic performance and affect the need for seismic safety. Since buildings are designed for different occupancies, these differing occupancies play a role in determining the seismic design category for the structure. For example, buildings and other structures designated as essential facilities, like hospitals, are required to be functional in a seismic event as opposed to a typical office building. In these cases, a higher seismic design category is warranted in the hospital to protect human life.
Stronger steel rebar
The new code changes also reflect the recognition of different types of reinforcing steel that can be used in concrete. Specifically, ACI 318 now approves the use of 100 ksi high-strength rebar in certain seismic applications.
The change means that steel rebar can now keep up with the newer, stronger concrete mixes. With concrete strengths reaching 15,000 psi and higher in some of today’s high-rise towers, in seismic areas it becomes increasingly difficult to physically fit enough Grade 60 rebar ties the industry’s “work horse” steel into columns and shear walls to provide sufficient seismic confinement. The allowance for the use of steel with strengths up to and including 100 ksi will alleviate such confinement issues; reducing the quantity of rebar necessary and allowing increased spacing between ties. It will reduce rebar congestion and lighten the work load for contractors, who typically find placing these ties time consuming.
Grade 75 steel rebar with a yield strength of 75,000 psi is also expected to see more seismic use. In columns and shear walls where 100 ksi rebar is unwarranted, Grade 75 steel, which has been available for many years, can provide similar benefits of reduced quantity and increased tie spacing.
The new ACI 318 updates also include a revised section addressing connectors and the methods of installation used in concrete construction. This section details the requirements for so-called “post-installed fasteners” in concrete structures. Prior to the development of these requirements, little consistency existed in the way that designers and contractors dealt with this issue. The design data came primarily from manufacturers of the connectors themselves, much of which was adequate but some of which wasn’t. The new code section is very helpful, since it codifies and standardizes a topic not dealt with by prior codes.
ACI Committee 318 includes 43 members, primarily from the United States but with representatives from several foreign countries. While nearly half of the membership is comprised of structural engineers, the committee also includes university professors, general contractors, government representatives and other industry leaders. The group meets twice a year to discuss concrete-related additions and changes to the code.
The ACI is one of many industry organizations that write code for the IBC. The American Institute of Steel Construction and American Institute of Timber Construction are among the other groups that lend their expertise to the global building code-writing process.
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