Jacking Of The BRB’s On Wilshire Grand

Last Monday and Tuesday we pre-compressed the upper buckling restrained braces (BRB’s) between levels 70 – 73 in four equal increments to a total compressive force of 1050 kips (1.05 million pounds) per brace.  We performed this work slowly over two days to provide the structure adequate time to respond to these loads.  We measured the lengths of these braces after jacking and the BRB’s shortened by approximately .316 inches or 8mm.  Over the next 50 years as the core wall shrinks (shortens) due to drying shrinkage of the concrete and concrete creep, these braces will stretch approximately .30” until the force in the braces becomes zero.  This will provide the braces with additional tensile strains for the maximum considered earthquake and 100 year and 1700 year wind events.

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