Masonry Magazine June 1999 Page. 20
Through the efforts of the Structural Engineers Association, the ACI Southern California Chapter and the Masonry Institute of America, a research program was conducted to test 20 masonry walls ranging from 3 1/2 inches thick up to 12 inches thick. The walls had h/t ratios from 30 to 52. All these walls were in excess of the building code allowances. The research program wanted to extend and violate the limitation and show that walls could capably perform in a safe ductile manner. The walls were built at a construction yard and by means of a loading frame shown below, loaded both vertically with a line load equivalent to ledger load and a lateral load equivalent to a wind or seismic load.
Loading
Frame
Air Bag
Test Specimen
Vertical Load
Vertical Load
on to
Lodge Angle
Pin Connection
Table 1 shows the result of this research program for the masonry walls. As the increments of the lateral load increased, deflections were measured, several specimens were unloaded and reloaded to check on a cycling effect. This research program on which a report was delivered at the Structural Engineers Convention showed that masonry walls had tremendous capabilities. The Committee analyzed all the results and developed two parameters of design. One was strength which has been the standard of design for all reinforced masonry structures and the other was performance in which the lateral out of plane deflection of the walls were limited so that in the event of a lateral force they would not be deflected to such a degree they were no longer useful. In fact the limitation was so severe that most of the time the deflection was within the elastic range. Comparing this qualification of deflection to the test results deflections in Table 1 it is miniscule compared to the deflection actually developed in the test program. So for the first time we had two parameters of design one was strength and the other was performance.
The major consideration was that the h/t limitation was completely eliminated. All that would have to be done was to conform to the conditions of strength and deflection and the wall could be acceptable. This opened up the door to a whole new design technique called strength design. Strength design assumes that the materials are working at their full capabilities and thus can be designed for load factors and for yield strength criteria. This was a complete divergence from what was done previously for working stress design. It was a breakthrough for masonry systems. The Structural Engineers Association of Southern California, the American Concrete Institute Southern California Chapter and the Masonry Institute of America must be recognized for their forward thinking in establishing this technique of design with criteria of strength and performance. Walls now can be designed and built, if tall thin walls are needed. H/'ts of 30, 35 and 40 are not uncommon. In fact walls have been designed with h/t up in the 50 range.
James Amrhein next to tall slender wall with a height of 35 feet. This means an 8 inch CMU wall could span vertically 33 feet 4 inches. Twice the previous limitation. Why build tall thin walls when we have been building walls 13 inches
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