March 2000
Concrete lintels are used in Insulating Concrete Form (ICF) construction to transfer loads above window and door openings in walls. Current design methods typically require vertical shear reinforcement as well as horizontal tensile reinforcing steel to support bending loads. However, in many residential and light-construction situations, shear reinforcement may not be necessary to achieve adequate performance. Shear reinforcement is difficult to place in ICF forms, complicates the construction process, and creates difficulties in placing the concrete. Therefore, this study is intended to investigate the need for shear reinforcement and, if possible, develop an improved ICF lintel design methodology based on testing results.
In May 1998, initial experimental studies were reported on various short-span configurations of ICF lintels [1]. The purpose of the testing was to determine if more economical designs, i.e. eliminating shear reinforcement, adversely affected the performance of concrete lintels. According to the Building Code Requirements for Reinforced Concrete (ACI 318-99) [2], shear reinforcement was required for all ICF lintels in the Prescriptive Method for Insulating Concrete Forms in Residential Construction (Prescriptive Method) [3]. In all cases the tested capacity of the ICF lintels without shear reinforcement outperformed the ACI 318-99 predicted capacities. However, a general design approach was not developed since lintels with longer-spans were not considered.
Expanding upon the previous work, this test program focuses on the structural performance of longer-span concrete lintels without shear reinforcement. This report will evaluate the structural performance of ICF lintels that span approximately 12 ft (3.7 m) and revisit the results from the May 1998 testing program. Due to the longer-span lengths and minimal tensile reinforcing steel amounts, yielding under a bending load is expected to result in a predictable ductile behavior prior to shear failure for the longer-span ICF lintels.
The objectives of the testing and evaluation program are as follows:
- Verify the performance (i.e. strength and ductility) of longer-span, simply-supported beams without shear reinforcement and with minimal tensile reinforcing steel (i.e. less than the minimum tensile steel ratio required by ACI-318);
- Verify and improve methods of calculating the bending capacity of ICF lintels;
- Verify and improve methods of calculating shear capacity of ICF lintels that have tensile reinforcing steel, but no shear reinforcement; and,
- Develop design recommendations for concrete lintel or beam design for each of the major ICF systems that is reasonably simple, safe, and accurate.
Prepared for:
The U.S. Department of Housing and Urban Development
Office of Policy Development and Research Washington, DC
Portland Cement Association
Skokie, IL
National Association of Home Builders
Washington, DC
40 pages
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