Design Considerations for Reinforced Concrete Masonry Walls Regarding Moment Magnification

Currently, regardless of the height of a reinforced masonry wall, engineers are required to consider moment magnification when using strength design provisions. An attempt at finding reasonable h/r limits for consideration of moment magnification is explored. Moment-curvature diagrams were developed using a spreadsheet to define the behavior of 8 inch and 12 inch concrete masonry walls. A series of models were created with axial load, out-of-plane load, height, reinforcing, and wall width as variables. The models were loaded out-of-plane in a finite element program until failure. The results were organized to produce the moment magnification of each wall. Results were summarized to determine height to radius of gyration (h/r) ratio limits for consideration of moment magnification.

For walls with axial loads over 5% of the compressive strength times the cross sectional area, practical limits were not found. The limits corresponded to short walls that would not aid engineers in design. For those walls with axial load of 5% and less, walls with h/r ratios of 24 or less would not have to consider the effects of moment magnification, walls with h/r ratios between 24 and 40 would use a simplified method of increasing the static moment by 10%, and walls with h/r ratios over 40 would have to consider current moment magnification procedures.