Answer: D. Steel will yield first
An under-reinforced section means steel will yield first.
An under-reinforced section means sections where the yield strain of the steel occurs at loads lower than the load at which the failure strain of the concrete occurs.
- The occupants have enough time to escape before the section falls because severe bending and beam breaking occur before steel yields. Therefore, the yielding of steel in an under-reinforced section does not indicate that the structure has failed.
- The collapse in the under-reinforced section is caused by the concrete reaching its ultimate failure strain of 0.0035 before the steel achieves its failure strain, which is substantially greater at 0.20 to 0.25.
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